# Endangerment and Cause or Contribute Findings for

Greenhouse Gases Under Section 202(a) of the Clean Air

Act:

EPA’s Response to Public
Comments

Volume 3: Attribution of Observed Climate Change

# Attribution of Observed Climate Change

U. S. Environmental Protection Agency Office of Atmospheric Programs Climate Change Division Washington, D.C.

### FOREWORD

This document provides responses to public comment s on the U.S. Environmental Protection Agency’s (EPA’s) Proposed Endangerment and Cause or Cont ribute Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act, published at 74 FR 18886 (April 24, 2009). EPA received comments on these Proposed Findings via mail, e-mail, and facsimile , and at two public hearings held in Arlington, Virginia, and Seattle, Washington, in May 2009. Copies of all comment letters submitted and transcripts of the public hearings are available at the EPA Do cket Center Public Reading Room, or electronically through http://www.regulations.gov by searching Docket ID EPA-HQ-OAR-2009-0171.

This document accompanies the Administrator’s final Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act (Findings) and the Technical Support Document (TSD), which contains the underl ying science and greenhouse gas emissions data.

EPA prepared this document in multiple volumes, with each volume focusing on a different broad category of comments on the Proposed Findings. Th is volume of the document provides responses to public comments regarding the attri bution of observed climate change.

In light of the very large number of comments received and the significant overlap between many comments, this document does not respond to each co mment individually. Rather, EPA summarized and provided a single response to each significant argumen t, assertion, and question contained within the totality of comments. Within each comment summary, EPA provides in parentheses one or more lists of Docket ID numbers for commenters who raised particul ar issues; however, these lists are not meant to be

exhaustive and EPA does not individua lly identify each and every commenter who made a certain point in all instances, particularly in cases where multiple commenters expressed essentia lly identical arguments.

Several commenters provided additional scientific litera ture to support their arguments. EPA’s general approach for taking such literature into consideration is described in Volume 1, Section 1.1, of this Response to Comments document. As with the comments, there was overlap in the literature received.

EPA identified the relevant literature related to the significant comments, and responded to the significant

issues raised in the literature. EPA does not indivi dually identify each and every piece of literature (submitted or incorporated by reference) that made a certain point in all instances.

Throughout this document, we provide a list of re ferences at the end of each volume for additional literature cited by EPA in our responses; however, we do not repeat the full citations of literature cited in

the TSD.

EPA’s responses to comments are generally provided immediately following each comment summary. In some cases, EPA has discussed responses to specifi c comments or groups of similar comments in the Findings. In such cases, EPA references the Findings rather than repeating those responses in this

document.

Comments were assigned to specific volumes of th is Response to Comments document based on an assessment of the principal subject of the commen t; however, some comments inevitably overlap multiple subject areas. For this reason, EPA encourages the public to read the other volumes of this document

relevant to their interests.



### iii

## TABLE OF CONTENTS

3.0 Attribution of Observed Climate Change to Increase in Greenhouse Gas (GHG)
Concentrations................................................................................................................. .......1

3.1 Degree of Anthropogenic Climate Influence ..........................................................1

3.2 Alternative Explanations of Observed Changes .....................................................20

3.2.1 Other Substances With Radiative Forcing Effects.................................25

3.2.2 Solar Irradiance .........................................................................................30

3.2.3 Existence of GHG Effect ...........................................................................36

3.2.4 CO 2 and Past Global Warming Episodes.................................................41



### iv

### v Acronyms and Abbreviations

ACRIM Active Cavity Radiometer Irradiance Monitor
AMO Atlantic Multi-decadal Oscillation
BC black carbon CCSP U.S. Climate Change Science Program
CH 3 I methyl iodide
CH 4 methane

cm centimeter
CO 2 carbon dioxide dms dimethyl sulfide
ENSO El Niño-Southern Oscillation EPA U.S. Environmental Protection Agency GCM general circulation model GHG greenhouse gases HITRAN High-Resolution Transmission Molecular Absorption hPa hectopascal IPCC Intergovernmental Panel on Climate Change

IR infrared radiation K degrees Kelvin

Ka kiloannum

Km kilometer LLGHG long-lived greenhouse gases LOSU Level of Scientific Understanding

mm millimeter MPI maximum potential intensity

N 2 O nitrous oxide

NAM Northern annular mode NASA National Aeronautics and Space Administration NCAR National Center for Atmospheric Research
NCEP National Centers for Environmental Protection NH Northern Hemisphere NOAA National Oceanic and Atmospheric Administration
PDO Pacific Decadal Oscillation PMOD Physikalisch-Meteorologisches Observatorium Davos ppm parts per million RF radiative forcing SRES IPCC Special Report on Emissions Scenarios TSD Technical Support Document
TSI total solar irradiance USGCRP U.S. Global Change Research Program
W/m 2 watts per meter squared WMO World Meteorological Organization3.0 Attribution of Observed Climate Chan ge to Increase in Greenhouse Gas (GHG)

Concentrations

Comment (3-1): A commenter (3747.1) argues that the information EPA used as basis for the Proposed Findings does not meet EPA’s information quality re quirements because the Technical Support Document (TSD) does not differentiate between anthropogenic and non- anthropogenic climate forcing variables.

Response (3-1): Please see Volume 1 of this Response to Comments document for EPA’s general response to the information quality concerns submitted during the public comment process. In addition, please see previous responses in this section regarding EPA’s consideration of the science on the attribution of observed climate change to increases in GHG concentra tions and other substances with radiative forcing

effects.

EPA appreciates the complexities of the issues raised by the commenter, however, we disagree with the

comment that the TSD does not establish a reasonable baseline by differentiating between anthropogenic and non-anthropogenic climate forcing variables. Ple ase see the Introduction and Sections 4 and 5 of the TSD for our discussion of this topic. Our treatment of this attribution issue is consistent with EPA’s Guidelines for Ensuring and Maximizing the Quality, Objectivity, Utility and Inte grity of Information Disseminated by the Environmental Protection Agency . We note that the Guidelines encourage risk assessments to analyze and consider ‘real world s ituations.’ EPA’s approach to and description of anthropogenic and non-anthropogenic climate forci ng variables is therefore consistent with the Guidelines

because the TSD describes the radiativ e forcing effects of both variables and includes an entire section on the attribution of observed climate change to anth ropogenic greenhouse gas emissions at the global and continental scale. Furthermore, the Introduction of the TSD describes this approach, therefore ensuring transparency of methods employed.

Comment (3-2): Several commenters state that specific aspects of th e attribution evidence summarized in the TSD do not support the Administrator’s endangerment finding.

Response (3-2): The specific issues that underlie these comments are addressed in the responses throughout this volume, and other volumes of the Response to Comments docum ent. With regard to the commenters' conclusion that the current science does not support an endangermen t finding with respect to attribution, we disagree

based on the scientific evidence before the Administra tor. See the Findings, Section IV.B, “The Air Pollution is Reasonably Anticipated to Endanger Both Public Health a nd Welfare,” for details on how the Administrator weighed the scientific evidence underlyi ng her endangerment determination in general, and with regard to the evidence of attribution in particular.

3.1 Degree of Anthropogenic Climate Influence

Comment (3-3): Numerous commenters (e.g., 0214, 0247, 0286, 0434, 0525, 0534, 0546, 0583, 0714.1, 1312, 2895, 3427.1, 3497.1, 3722R85, 4092, 7037) argue that anth ropogenic greenhouse emissions have no effect on the climate. A second large group of commenters (e.g., 0400, 0499, 0736, 2210.4, 2818, 2933, 4003, 3427.1, 3440.1, 3450.1, 3553.1, 3596.1. 3915) do not explicitly rule out the possibility of an



### 1anthropogenic influence on climate but state that it is likely to be minimal or insignificant, or that the current level of scientific understanding about climate and the carbon cycle involves too many uncertainties for the observed changes to be attribut ed to anthropogenic emissions with any degree of

confidence.

Response (3-3): We reviewed the arguments by the commenters that anthropogenic GHG emissions have minimal or no influence on the climate. While uncertainties exist, we disagree that anthropogenic GHG emissions have

little to no effect on climate or that scientific understanding is inadequate for attributing climate change to

human causes. We find these comments to be incons istent with the assessment literature, which is summarized in Section III of the TSD. Studies to detect climate change and attribute its causes using patterns of observed temperature change show clear ev idence of anthropogenic infl uence. On the basis of these studies, the Intergovernmental Panel on Climate Change (IPCC), the U.S. Climate Change ScienceProgram (CCSP) and the U.S. Global Change Research Program (UGSCRP) reports have reached highconfidence conclusions that anth ropogenic GHG emissions and warming of the climate system are causally linked. The 2007 IPCC Fourth Assessment Report concluded the following about the linkage between anthropogenic GHG emissions and the observe d warming: “Most of the observed increase in global average temperatures since the mid-20th centu ry is very likely [where very likely signifies a 90-99% probability the statement is true] due to the observed increase in anthropogenic GHG concentrations.” In 2009, the USGCRP reached an even stronger conclusion, unequivocally stating: “The global warming of the past 50 years is due primar ily to human-induced increases in heat-trapping gases.” Both of these findings are included in the TSD.

The IPCC, CCSP and USGCRP have also concluded that there is evidence of an thropogenic influence in other parts of the climate system, including ocean heat content, precipitation, and wind patterns. Comments pertaining to the degree to which changes in these parts of the climate system can be attributed to the observed increase in the atmospheric GHG concentr ation are covered in othe r responses within this section of the Response to Comments document.

As the TSD describes, the attribution of observed climate change to anthropogenic activities is based on multiple lines of evidence. The fi rst line of evidence arises from our basic physical understanding of the effects of changing concentrations of GHGs, natura l factors, and other human impacts on the climate system. The second line of evidence arises from indirect, historical estimates of p ast climate changes that suggest that the changes in global surface temperature over the last several decades are unusual (Karl et al., 2009). The third line of evidence arises from the u se of computer-based climate models to simulate the likely patterns of response of the climate system to different forcing mechanisms (both natural and anthropogenic). We received many comments on each of these lines of evidence. See our responses regarding how GHGs trap heat in Section 3.2.2; ou r responses related to indirect estimates of climate change over the last 2,000 years in Volume 2; our response related to the carbon cycle in response 3-21 and Volume 2; and our responses related to the co nsistency between observed changes in climate and computer model simulations in both this Volume and Volume 4.

Comment (3-4): Numerous commenters (e.g., 2253, 3205.1, 3330, 3432.1, 3569.1, 10394, 10499) posit the lack of correlation between global surface and satellite-derived temperature trends and GHG changes calls into question any cause and effect relationship. Sp ecifically, they note global GHG emissions have dramatically risen since 2000 and yet there has not b een a concomitant increase in global temperature. Some commenters point to other (prior) short periods of no temperature change or cooling to demonstrate this lack of direct correlation. Another commenter (3722) states that CO 2 concentrations do not correlate causatively with temperature records, presenting a quot e by Professor Philander (Philander, 1998) that “in



### 2contrast to the steady rise in the atmospheric c oncentration of carbon dioxide...temperatures have fluctuated erratically.”

Response (3-4): We reviewed the arguments by the commenters that there is no correlation, and no cause–and-effect relationship, between GHG concentrations and global temperature trends. Climate science research and assessments have clearly established the relati onship between warming and anthropogenic GHG emissions. However, the relationship is complex and non-linear. As discussed in the IPCC, CCSP, and USGCRP assessment reports, and summarized in the TSD, elevated atmospheric levels of GHGs are not the only determinant of changes in temperature at th e surface and in the troposphere; they act in addition to aerosols, land albedo changes, volcanoes, solar chan ges, and internal variability As such, examination of the relationship of shorter intervals (e.g., five to 10 years) can provide limited insight, and drawing conclusions from short time-scales is of limited value. Directly comparing global GHG emissions with global temperatures on decadal or shorter time-scales mu st consider all plausible variations and other existing non-linear inter-relationships. Both the IP CC and the TSD note that “difficulties remain in attributing temperature changes on smaller than cont inental scales and over time scales of less than 50 years,” and that with limited exceptions attribution at these scales has not yet been established. Hegerl et al. (2007) state:

Averaging over smaller regions reduces the natural variability less than does averaging over large regions, making it more difficult to distinguish between changes expected from different external forcings, or between external forcing and variability. In addition, temperature changes associated with some m odes of variability are poorly simulated by models in some regions and seasons. Furtherm ore, the small-scale details of external forcing, and the response simulated by models are less credible than large-scale features.

From a recent study cited by the USGCRP, it is true (for some datasets) if a linear trend is fitted to annual global surface temperature data for the period 1998 to 2008, there is no real trend, even though temperatures remain well above the long-term aver age (Easterling and Wehner, 2009). Climate over the

21 st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight coo ling in the presence of longer-term warming. We note there are other 10-year periods in the temperature r ecord that when extracted show no trend even though these periods are embedded within a longer period showing substantial overall warming (Easterling and Wehner, 2009). Therefore, it is clear that temperat ures do not rise monotonically despite the continuing

increase of GHG concentrations. Observations over such short periods examined in isolation may be misleading in the interpretation of th e longer-term trend in temperatures.

Over longer time periods and larger geographic scales , the attribution of recent change in part to CO 2 and other GHGs is clear. See Hegerl et al. (2007) and Ka rl et al. (2009) and the summaries of those two assessment reports in Section 5 of the TSD for discu ssions of the specific observed changes that have

been attributed to increased GHG concentrations , and the evidence for and confidence in those

attributions.

Therefore, we disagree with the commenters’ assertion that there is not a cause-and-effect relationship between GHG concentrations and global temperature trends.

Comment (3-5): Several commenters (e.g., 0169, 3432.1) argue th at because the observed warming has occurred in Northern Hemisphere and not the Southern Hemisphere , the warming is not globa l and inconsistent with anthropogenic attribution.



### 3Response (3-5): While the Northern Hemisphere has warmed more and at a faster rate than the Southern Hemisphere, the commenter’s statement that the Southern Hemisphere has not warmed is inaccurate. Warming trends in both hemispheres are clearly documented by the IPCC (Trenberth et al., 2007), and the most current data from satellite and surface-based records) indicate some warming in the Southern Hemisphere. Differences in Northern and Southern Hemisphere temperature tre nds are related to the different evolution of aerosol forcing and the greater thermal inertia of the large o cean surfaces in the Southern Hemisphere (Hegerl et al., 2007). The Northern Hemisphere has significantly more land mass than the Southern Hemisphere and because oceans tend to warm and cool more slowly th an land areas, continental temperatures have risen more quickly than have ocean temperatures. Therefore, the data show that warming is indeed global, and that the hemispheric pattern is consistent with scientific understanding.

Comment (3-6): A number of commenters (e.g., 3446.3, 3596.1, 3722) note that temperatures rose significantly from 1910 to 1940 before significant increases in GHGs, and just when GHG emissions began ramping up, the temperature cooled from the 1940s to 1970s. They sugg est the lack of temporal correlation between GHG trends and temperature trends calls into qu estion GHG attribution for the observed warming.

One commenter (3136.1) argues that natural variability should receive greater consideration in the TSD and questioned whether the changes that occurred prior to the past 30 to 50 years are attributable to anthropogenic climate change. The commenter argued that the behavior of the climate system from 1901 to the mid-1970s shows that predominantly natural va riability can be quite large over the United States

and stated that: “The TSD needs to be modified to discriminate between natural and anthropogenic warming.” The commenter further argued the TSD re presents all changes that have occurred since 1901 as a result of anthropogenic climate change and su ggests that the TSD should present the trends for 30 year-periods between 1900 and 1975 as represen ting predominantly natural variability.

Response (3-6): As stated in an earlier response, elevated GHGs are not the only determinant of changes in temperature at the surface and in the troposphere, though most of the observed increase in global temperatures since the

mid-20 th century has been attributed to the observed increase in GHG c oncentrations. Elevated GHGs act in addition to aerosols, land albedo changes, volcanoes, solar changes, and internal variability. A review of the literature shows that there are scientifically compelling explanations for the pattern of global temperature change over the past century. The information on attribution assessed by the IPCC, USGCRP, and CCSP, as summarized in the TSD, is cons istent with the observed temperature record and therefore does not call into question the evidence suppor ting attribution of most of the observed warming

since 1950 to increased GHG concentrations.

As the TSD notes, “[t]he IPCC (Hegerl et al., 2007) finds that anthropogenic GHG emissions were one of the influences contributing to temperature rise during the early part of the 20 th century along with increasing solar output and a relative lack of vol canic activity. During the 1950s and 1960s, when temperature leveled off, increases in aerosols from fossil fuels and other sources are thought to have cooled the planet. For example, the eruption of Mt . Agung in 1963 put large quantities of reflective dust into the atmosphere. The rapid warming since the 197 0s has occurred in a peri od when the increase in GHGs has dominated over all other factors (Hegerl et al., 2007).”

We have also added a paragraph to the TSD in or der to better describe the key conclusions of the assessment literature with respect to the relative role of internal variability for observed and projected climate change on multi-decadal time scales:



### 4Changes arising from internally generated vari ations in the climate system can influence surface and surface and atmospheric temperatures substantially; however, climate models indicate that global-mean unforced variations on multidecadal timescales are likely to be

smaller than the 20 th century global-mean increase in surface temperature (Karl et al.,2006). The IPCC reports that global mean and hemispheric scale-temperatures on multidecadal time scales are largely controlled by ex ternal forcing (Hegerl et al., 2007). Hegerl et al. (2007) note that, “many observed changes in surface and free atmospheric temperature, ocean temperature, and sea ice extent, and some large-scale changes in the atmospheric circulation over the 20 th century are distinct from internal variability and consistent with the expected response to anthropogenic forcing.”

As stated, on multi-decadal scales global and hemispheri c temperatures are largely controlled by external forcings, which means that over the long term, la rge changes in GHG concentrations will dominate any unforced temperature changes from natural variability.

Comment (3-7): Many commenters (e.g., 3215.1, 3330, 3446.1, 3596. 2, 4003) indicate an anthropogenic warming is missing in the vertical and horizontal profile of the atmo sphere in the tropics. They argue that the distinct human fingerprint of warming concentrated in the tropics between 30 degrees N and 30 degrees S and increasing with altitude to 10 kilometers (km) is th e mechanism for amplified warming but that this fingerprint exists only in the models and not in th e empirical science. They state that models predict significantly increasing warming with altitude up to 10 km and warming continuing beyond 15 km. Yet, they find except for surface temperatures, observations show limited warming that is statistically significantly less than the warming projected by the models. They indicate observations show a cooling with altitude beyond 13 km while the models still project a warming.

Response (3-7): EPA is aware of the emerging literature on this issu e and the challenges in identifying the anthropogenic fingerprint in the tropics. The TSD’s characterizati on of this issue is consistent with the assessment literature as well as the most recent studies, whic h find that when uncertainties in models and observations are properly accounted for, newer obser vational data sets are in agreement with climate

model results.

In light of this comment, EPA reviewed the assessment reports and newer literature on this topic. As one commenter notes, Christy et al. (2007) find discrepancies between surface and tropospheric temperature data in the tropics, and Douglass et al. (2007) report model results that are in disagreement with the observed trends. However, Haimberger et al. ( 2008) analyze weather balloon (radiosonde) records of tropospheric temperature data and find “....we note th at the temperature tre nds from RICH–RAOBCORE version 1.4 [a homogenized radiosonde record] are more consistent with trends from recent climate model runs than earlier radiosonde datasets. In the tropical upper troposphere, where the predicted amplification of surface trends is largest, there is no significant discrepancy between trends from RICH–RAOBCORE version 1.4 and the range of temper ature trends from climate models. This result directly contradicts the conclusions of a recent paper by Douglass et al. (2007 ).” They further note: “A robust warming maximum of 0.2–0.3K (10 yr)

− 1 for the 1979–2006 period in the tropical upper troposphere could be found in both homogenized radiosonde datasets.”

Another paper by Allen and Sherwood (2008) reports: “C limate models and theore tical expectations have predicted that the upper troposphere should be warming faster than the surface. Surprisingly, direct temperature observations from radiosonde and satellite da ta have often not shown this expected trend.



### 5However, non-climatic biases have been found in such measurements. Here we apply the thermal-wind equation to wind measurements from ra diosonde data, which seem to be more stable than the temperature data....Warming patterns are consistent with model predictions except for small discrepancies close to the tropopause.”

Finally, Santer et al. (2008) analyze differences in trends between observed surface and tropospheric temperature records, and also compare the observational trends with the models. Th ey conclude: “There is no longer a serious and fundamental discrepancy between modeled and observed trends in tropical lapse rates, despite [the Douglass et al., 2007] incorrect claim to the contrary.”

The TSD summarizes this issue and cites the conclusions of the latest major assessments. It states: “...an important inconsistency may have been identified in the tropics. In the tropics, most observational data sets show more warming at the surface than in the tr oposphere, while almost all model simulations have larger warming aloft than at the surface (Karl et al., 2006). Karl et al. (2009) claim that when uncertainties in models and observations are properly accounted fo r, newer observational data sets are in agreement

with climate model results.” EPA concludes that the TSD’s summary of the current state of the science on tropical tropospheric warming as reflected in the underlying assessment literature is accurate.

Comment (3-8): Several commenters (3187.4, 7031, 9877) argue that the recent plateau in ocean heat content (from 2003 to 2008) suggests anthropogenic warming is not occu rring because it indicates that the climate system is not accumulating heat. The lack of heat accumulation, they state, demonstrates a failure of the anthropogenic global warming hypothesis to account fo r natural climate variability, especially as it relates to ocean cycles. They claim that the recent trends in ocean heat content suggest the Earth’s energy budget is not out of balance owing to GHGs, in contr ast to the findings of Hansen et al. (2005).

Response (3-8): We have reviewed the assessment literature in light of these comments and disagree with the assertions made by commenters. Just as temperature will not necessarily increase monotonically with increases in GHGs (per response 3-6) neither will ocean heat cont ent on short time scales. Many of the same factors that influence global surface temperature in additi on to GHG forcing will also result in short-term variability in ocean heat content such as aerosol emi ssions (anthropogenic and/or volcanic), solar forcing, and internal variability in the climate system. EPA does not suggest that GHGs are the only factors that would influence the global energy budget, and hence ocean heat content. EPA agrees that internal variability likely plays an important role in the in terannual and interdecadal variability of ocean heat content, as indicated by IPCC (Bindoff et al., 2007). But as noted in Volume 2 of the Response to Comments document, the long-term trend in ocean heat content is indisputably upward, which is what we would expect given the anthropogenic heating from GHGs. The IPCC notes that ocean heat content is a critical variable for detecting the effects of the ob served increase in GHGs in the Earth’s atmosphere and for resolving the Earth’s overall energy balance (Bindoff et al., 2007)

Though the commenters refer to a recent plateau in ocean heat content, there are published papers which find the opposite, as mentioned in Volume 2 of the Response to Comments document. In fact, this work (von Schuckmann et al., 2009) indicates the global o cean accumulated (between the surface and 2,000 meter depth) 0.77 (plus or minus 0.11) watts per s quare meter of heat betw een 2003 and 2008, which is roughly consistent with the 0.86 (plus or minus 0.12) watts per square meter of heat (between the surface and 750 meter depth) accumulated between 1993 and 2003 as documented in Willis et al. (2004); and Hansen et al. (2005). These studies suggest the ocean has and continues to accumulate heat, contributing to an overall imbalance in the Earth’s energy budget, as further documented in two other recent studies by



### 6Trenberth et al. (2009) analyzing the period Ma rch 2000 to May 2004 and Murphy et al. (2009) (analyzing the period 1950–2004).

We have added the following text on this topic to Section 4(f) of the final TSD on this topic:

The thermal expansion of sea water is an indicator of increasing ocean heat content.

Ocean heat content is also a critical variab le for detecting the effects of the observed increase in GHGs in the Earth’s atmosphere and for resolving the Earth’s overall energy balance (Bindoff et al., 2007). For the period 1955 to 2005, Bindoff et al. (2007) analyze multiple time series of ocean heat conten t and find an overall increase, while noting interannual and inter-decadal variations. NOAA’s report State of the Climate in 2008 (Peterson and Baringer, 2009), which incor porates data through 2008, finds “large” increases in global ocean heat content since the 1950s and notes that over the last several years, ocean heat content has reached consistently higher values than for all prior times in

the record.

Thus, the TSD’s summary of the current state of th e science on ocean heat content as reflected in the underlying assessment literature is reasonable and sound.

Comment (3-9): Commenters (e.g., 3596.2, 7031) indicate trends in global precipitation cannot be attributed to anthropogenic forcing and an enhancement of the hydrological cycle. Several (3596.2, 5058R8) refer to a study by Gerten et al. (2008) who find precipitati on over the global land area has been highly variable

over the 20 th century, characterized by a large de gree of interdecadal fluctuation.

Response (3-9):

We have reviewed these comments and the referenced st udy, and we agree that a human influence has not been detected in global precipitation trends. We al so agree that there has b een significant interdecadal fluctuation in global precipitation as documented in Ge rten et al. (2008). Our review of Gerten et al. (2008) reveals they were not able to confirm an enhanc ement of the hydrological cycle in their analysis of precipitation in recent decades. Note that the IPCC (Hegerl et al., 2007), consistent with this finding,

concludes a human influence has not been detected in global-scale precipitation trend, and that this conclusion is reported in the final TSD.

However, the IPCC does find that the latitudinal patte rn of change in land precipitation and observed increases in heavy precipitation over the 20 th century appear to be consiste nt with the anticipated response to anthropogenic forcing (Hegerl et al., 2007), as is noted in the TSD.

A recent study by Zhang et al. (2007) supports and builds on IPCC’s assessment. The study finds:

Overall, we find that anthropogenic forci ng has had a detectable and attributable influence on the latitudinal pattern of large-scal e precipitation change over the part of the twentieth century that we were able to an alyse. Our best estimate of the response to anthropogenic forcing suggests that anthropogenic forcing has contributed approximately 50–85% (5–95% uncertainty) of the observed 1925–1999 trend in annual total land precipitation between 40° N and 70° N (62 mm [millimeters] per century), 20–40% of the observed drying trend in the nor thern subtropics and tropics (0° to 30°N; a decrease of 98 mm per century) and most (75–120%) of the moistening trend in the southern tropics and subtropics (0° to 30° S; 82 mm per century).



### 7Gerten et al. (2008), in fact, cite this study and conc lude “...the human impact on the global water cycle is both intensifying and diversifying.”

Therefore, EPA concludes that the TSD provides an accurate and sound summary of the state of the science by stating that latitudinal patterns in land pr ecipitation, observed increases in heavy precipitation events, and increased extremes of summer dryness a nd winter wetness are consistent with anticipated response to anthropogenic forcing.

Comment (3-10): Comments (e.g., 3161.1, 3394. 1, 3596.2) assert there is absolutely no indication that U.S. GHG emissions have produced an unusual situation when it comes to U.S. annual precipitation totals and/or trends. They also assert the TSD does not adequately review the topic of precipitation variability and its causes across

the United States.

Response (3-10): While attribution of precipitation trends at the regional scale (e.g., within the United States) is challenging, the TSD includes appropriate caveats di scussing the links between anthropogenic forcing and U.S. precipitation trend. In response to this co mment, we added the following language to the final TSD, which explains why attribu ting trends in precipitation is difficult at the regional scale:

As with temperature, attributing changes in precipitation and other climate variables to anthropogenic forcing at continental or smalle r scales is more challenging. One reason is that as spatial scales considered become smaller, the uncertainty becomes larger because internal climate variability is typically larger than the expected responses to forcing on these scales (Gutowski et al., 2008). For exampl e, there is considerable evidence that modes of internal variability (such as ENSO [ El Niño-Southern Oscillation] , PDO [Pacific Decadal Oscillation], and NAM [Nor thern Annual Mode]) substantially affect the likelihood of extreme temperature, drought s, and short-term precipitation extremes over North America (Gutowski et al., 2008).

Although, for the most part, we ag ree with commenters that a human influence on North American (and therefore the United States) precipitation amounts has not been formally attributed in the assessment literature (e.g., Clark et al., 2008), and this has been further reflected in the TSD (see Section 5[a]), certain characteristics of the observed changes in preci pitation over the United States have been linked to anthropogenic forcing in the assessment literature. As the TSD now notes, Karl et al. (2009) find that heavy precipitation events averaged over North Ameri ca have increased over the past 50 years at a rate higher than total precipitation increased, consistent with the observed increases in atmospheric water vapor, which have been associated with human-induced increases in GHGs. In addition, Clark et al. (2008) indicate the recent trend toward increased arid ity in the Southwest is consistent with model projections for that region under increased GHG forcing, though the TSD cites the cautions by Clark et al. that there is considerable natural variability in the hy droclimate of the Southwest and that there is no clear evidence to date of human-induced global climate change effects on North American precipitation

amounts. While the assessment literature has not fo rmally attributed North American precipitation changes to human-induced climate change, Clark (200 8a) did find that “both models and observations show a pattern of increasing precipitation north of 50°N and decreasing precipitation between 0-30°N” and conclude that anthropogenic influence on the spa tial distribution of global land precipitation has been

detected.



### 8The TSD adequately reviews the topic of precipitati on variability and its trends across the United States and that its summary of the current state of the sci ence as reflected in the un derlying assessment literature

is reasonable and sound.

Comment (3-11): Commenters (e.g., 3136.1) posit that drought that h as occurred in the United St ates has been mostly a result of the long-term variability in atmospheric/ocean cycle rather than anthrop ogenic forcing. They and another commenter (5058R29) reference a study by McCa be et al. (2004), who specifically examined the trends in the variability of drought frequency (as cap tured by the Palmer Drought Severity Index) across the United States and found that the majority of the long-term variability is explained by atmospheric/ocean cycles over the Pacific and Atlantic oceans (specifically, the Pacific Decadal

Oscillation and the Atlantic Multi-decadal Oscillati on). They, along with another commenter (3394.1), also reference a recent study by Seager et al. (2009) who find recent droughts in the southeastern United States do not show any indication of anthropogenic in fluences. A commenter (3394.1) contends that the discussion on drought suggests that water shortages ar e a problem in certain regions of North America because of causes unrelated to climate change.

Response (3-11): For most of the United States, we do not make any statements that link observed drought to anthropogenic forcing. The exception to this, as stat ed in response 3-10, is that the TSD does summarize the findings in the assessment literature that the recent trend toward incr eased aridity in the Southwest is consistent with model projections for that region under increased GHG forcing (Clark et al., 2008).

We reviewed the studies by McCabe et al. (2004) a nd Seager et al. (2009) and agree that they do not document evidence of anthropogenic influence on drought in the regions studied; however, Karl et al. (2009) indicate that increasing temperatures—which have been linked to anthropogenic forcing over North America—have made existing droughts more se vere and widespread than they otherwise would have been. Also note that even in cases where we cannot yet attribute current trends to changes in GHG concentrations, the signals of these changes are projec ted to become much larger as GHG concentrations

continue to increase.

Karl et al. (2009) find that drought, “related to re duced precipitation, increased evaporation, and increased water loss from plants, is an important issue in many regions, especially in the West,” and notes that the Southwest, in particular, is expected to experien ce increasing drought.” Though water shortages are also related to natural variability and human usage pattern s, by “changing the existing patterns of precipitation and runoff, climate change will add another st ress to existing problems” (Karl et al., 2009).

Comment (3-12): A commenter (3596.1) writes that EPA’s TSD suggests that trends in non-tropical (or extratropical)

storms are a reflection of extreme weather events cr eated by anthropogenic warming but that a literature review by Craig Idso (comment 3596.3) does not support EPA’s claim.

Response (3-12):

We have reviewed the comment and the referenc ed literature review, and we disagree with the commenter’s characterization of EPA’s assessment of this issue. The April 2009 TSD did not include a statement attributing trends in non-tropical storms to anthropogenic warming. Rather, we noted that “Karl et al. (2008) find that heavy precipitation events av eraged over North America have increased over the past 50 years at a rate higher than total precipitati on increased, consistent with the observed increases in



### 9atmospheric water vapor, which have been associated with human-induced incr eases in GHGs.” A change in heavy precipitation events (and its attribution) is a different issue from changes in non-tropical storms, in general. We agree with the commenter that the lite rature does not link trends in non-tropical storms to anthropogenic warming. To provide further summary of the conclusions of th e assessment literature on this topic, we have added a statement to the TSD from the IPCC, which states: “An anthropogenic influence has not yet been detected in extra-tropi cal cyclones owing to large internal variability and problems due to changes in observing systems (Hegerl et al., 2007).”

Comment (3-13): A number of commenters (e.g., 0591, 3432.1, 3596.2, 3679.1, 7031) suggest that trends in glaciers in certain regions/locations are inconsistent with a nd/or unrelated to anthropogenic warming. Some commenters link glacier loss to a recovery from cold c onditions prior to 1900 and/ or to internal variability (changing atmospheric and oceanic cycles) within the climate system.

Response (3-13): EPA’s summary is consistent with the assessment literature. The TSD notes that the IPCC (Hegerl et al., 2007) finds that anthropogenic warming has likely (where likely signifies a 66-90% probability the statement is true) “contributed” to the “widespread” retr eat of glaciers. This wording implies other factors besides anthropogenic forcing impact glacier trends and it is referring to glaciers collectively, rather than individually. As noted in responses in Volume 2 about glaciers trends, although most observed glaciers are retreating, there are exceptions, which are noted in the TSD.

The IPCC (Hegerl et al., 2007) found that widespread shrinkage in glaciers implied widespread warming as the probable cause, although in the tropics changes in atmospheric moisture might be contributing. Hegerl et al. also found that the correlation between temperature and net glacier balance indicates the primary role of temperature in forcing glacier fluctuations. In addition, the IPCC cites the study by Reichert et al. (2002), which finds recent glacier retr eat cannot be explained by internal variability, while the glacier advance during the “Little Ice Age” c ould be explained by internal variability.

While noting the primary role of temperature in causing glacier fluctuation (i.e., warming causing glacial retreat; Hegerl et al., 2007) and the large-scale correlation between tropical sea surface temperature anomalies and tropical glacier mass balance (Lemke et al., 2007), the IPCC (Lemke et al., 2007) finds the evidence suggests changes in atmospheric moisture have been the primary driver of the observed retreat of tropical glaciers. For example, studies (some cited by IPCC, some cited by commenters, and some new; e.g., Kaser et al., 2004; Cullen et al., 2006; Molg et al., 2008, 2009; Duane et al., 2008) attribute most of the retreat of slope glaciers on Mt. Kilamanjaro to a transition to a drier regime since the late 1800s. Shrinkage of the ice area on Kilimanjaro’s plat eau has been largely linked to solar radiation (Cullen et al., 2006).

In other words, at regional to local scales, partic ularly in the tropics, we acknowledge (based on our review of the assessment literature and the latest studies) that the evidence indicates other variables besides temperature are important for glacier mass balance trends; however, at global scales the body of

the literature remains consistent with the stat ement from the IPCC on the likely contribution of anthropogenic warming to glacial retreat, as summarized in the TSD.



### 10Comment (3-14): A commenter (3729.7) posits snowpack trends in th e U.S. Pacific Northwest are not consistent with anthropogenic warming but rather natural atmospheric-oceanic cycles (e.g., the Pacific Decadal Oscillation).

Response (3-14): We begin by noting that EPA did not assign a cause fo r the observed changes in U.S. western snowpack in the April 2009 TSD. Rather, we reported the overa ll declining trend in snowpack as stated in the assessment literature (Lettenmaier et al., 2008) whil e subsequently stating (also citing the assessment literature, IPCC, 2007b) that many observed changes in physical and biological systems are being affected by regional climate changes, particularly temperature increases (which are linked to anthropogenic forcing).

As we noted in our response in Volume 2, a numbe r of very recent studies have also documented the trend towards declining snowpack in the West, t hough some highlight significant decadal variability especially over smaller regions such as the Pacific Northwest (in the Cascades). These studies also evaluate attribution, and all suggest that anthropo genic forcing is contributing to trends in western snowpack, but to varying degrees.

We reviewed a recent study by Barnett et al. (2008), which finds that in the Western United States “up to

60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced.” A study by Pierce et al. (2008) concludes about half of the snowpack reductions observed in the West from 1950 to 1999 are the result of climate changes forced by anthropogenic GHGs, ozone, and aerosols.

Two recent studies attempt to attribute changes in a region confined to the Cascades (in the Pacific Northwest). Casola et al. (2008), while cautioning “the science of modeling the regional impacts of global warming is still in its infancy,” finds “global wa rming would have produced an 8%–16% decrease in the snowpack in the Cascades over the last 30 yr.” Th ey suggest most of that warming was caused by anthropogenic forcing but do not quantif y the percentage. A study submitted to Journal of Climate (Stoelinga et al., 2009, submitted) finds a 16% decline in snowpack in the Cascades for the period 1930–

2007 and conclude 80% is due to na tural variability (including the Pacific Decadal Oscillation, referred to by the commenter) with some or all of the remain ing loss attributable to anthropogenic warming.

Finally, we note that CCSP (Backlund et al., 2008b) finds: “There is a trend toward reduced mountain snowpack and earlier spring snowmelt runoff peaks acro ss much of the western United States. This trend is very likely attributable at least in part to long -term warming, although some part may have been played by decadal-scale variability, including a shift in the phase of the Pacific Decadal Oscillation (PDO) in the late 1970s.” Accordingly, while we agree with the comme nter that the PDO may play a role in the decadal variability of western snowpack, we do not agree th at the current science shows that anthropogenic warming is not a contributor. EPA has incorporated the findings from Backlund et al. into the TSD (Section 5), and note it is further supported by our review of the most recent literature.

Comment (3-15): Numerous commenters (e.g., 2750, 3291.1, 3411.2, 3446. 1, 3596.1, 3679.1, 3729.8, 7031) note Arctic ice has melted to a significant degree in the historic past in the absence of si gnificant anthropogenic GHG emissions. They note that although there has been a considerable temperature increase during the last decade, a similar increase and at a faster ra te occurred during the early part of the 20 th century (1920 to 1930) when carbon dioxide or other GHGs could not be a cause. Many relate sea ice loss to multi-decadal ocean cycles. Some refer to a paper by Chylek et al. (2009) that documents the major role of regional



### 11atmospheric/ocean circulation pattern changes on regi onal multi-decadal climate variability in the Arctic. One commenter suggests the possibility that sea ice loss has been recently accentuated by major undersea volcanism and the invasion of tundra shrubs and depo sition of soot from Asia. A commenter (3722) also provides an analysis by Dr. Akasofu that notes high variability in Arctic temperatures, noting that it is unlikely that CO 2 caused major temperature fluctuations before 1940.

Response (3-15):

The commenters’ claims are inconsistent with the assessment literature. The scientific assessment literature (notably the IPCC’s Fourth Assessmen t Report and the CCSP Synthesis and Assessment Product [SAP], Past Climate Variability and Change in the Arctic and at High Latitudes )—

as summarized in the TSD—concludes that the recent d ecline in Arctic sea ice is unusual and cannot be explained by natural variability alone.

The IPCC (Hegerl et al., 2007) states that the recent d ecline in Arctic sea ice is inconsistent with simulated internal climate variability and the response to natural forcings alone.

The CCSP (Polyak et al., 2009) further finds that:

The current decline of the Arctic sea-ice cove r is much larger than expected from decadal scale climatic and hydrographic [i.e., pertaining to the physical characteristics of water] variations (e.g., Polyakov et al., 2005; Steele et al., 2008). The recent warming and associated ice shrinkage are especially anomal ous because orbitally driven insolation has been decreasing steadily since its maximum at 11 ka [11,000 years ago], and it is now near its minimum in the 21 k.y. [21,000 years] precession cycle (e.g., Berger and Loutre, 2004), which should lead to cool summers and extensive sea ice.

Based on the IPCC assessment, the TSD st ates that anthropogenic forcing has likely contributed to the

recent decreases in Arctic sea ice extent while noting that Karl et al. (2009) add:

T he observed decline in Arctic sea ice has been more rapid than projected by climate models.

C lear linkages between rising GHG concentrations and declines in Arctic sea ice have been

identified.

Although rapid observed warming in the Arctic also occurred from approximately the late 1920s to the early 1950s (as described in Section 4(b) of the TSD and discussed further in Volume 2 of the Response to Comments document), we do not find this past warming suggests the warming in recent decades and related loss of Arctic ice is not significantly attri butable to anthropogenic forcing. In fact, the CCSP (White et al., 2009) states the following with respect to attribution of Arctic temperature trends:

In the instrumental data (Parker et al., 1994; also see Delworth and Knutson, 2000), the Arctic sections, particularly the North Atla ntic sector, show warming of roughly 1°C in

the first half of the 20 th century (and with peak warming rates of twice that average). The warming likely arose from some combination of volcanic, solar, and human (McConnell et al., 2007) forcing, and perhaps some o ceanic forcing. The warming was followed by weak cooling and then a similar warming in the latter 20 th century (roughly 1°C per 30 years) primarily attributable to human forcing with little and perhaps opposing natural forcing (Hegerl et al., 2007).

We note that the McConnell et al. (2007) paper cited by the CCSP (White et al., 2009) finds anthropogenic black carbon emissions have substantia lly contributed to surface forcing over the Arctic during and after industrialization. They write: “Beginning about 1850, industrial emissions resulted in a



### 12sevenfold increase in ice-core BC [black carbon] concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing.” So we concur with the comment that soot deposition has probably influenced Arctic sea ice tren ds. Other recent studies draw similar conclusions (e.g., Flanner et al., 2007; Strack et al., 2007; Quinn et al., 2008a, 2008b; Hegg et al., 2009).

We also acknowledge natural variability plays an import ant role in year-to-year changes in sea ice extent (as stated in Hegerl et al., 2007 and Karl et al., 2009) and we note this in the TSD.

We are aware of the study by Chylek et al. (2009) , which finds “Arctic temperature changes are highly

correlated with the Atlantic Multi-decadal Oscillati on (AMO).” While the study concludes the AMO is a potential “major” cause of Arctic temperature variation, it does not specifically quantify the anthropogenic contribution. This study is too new to have been assessed by the broader scientific community, but its finding that the AMO is an importa nt factor in Arctic temperature trends is not

inconsistent with IPCC’s assessment of the role of multi-decadal oscillation on regional climate, as we note in the following TSD statements:

“ The IPCC (Hegerl et al., 2007) cautions that di fficulties remain in attributing temperature changes on smaller than continental scales a nd over time scales of less than 50 years”

“ Changes arising from internally generated variati ons in the climate system can influence surface and atmospheric temperatures substantially.”

One commenter (3729.8) suggests that the encroachment of tundra shrubs is also contributing to recent declines in sea ice. IPCC (Fischlin et al., 2007) in dicates shrubs have been migrating poleward as the climate warms and are likely to decrease regional albe do (reflectivity) in the Arctic leading to a net warming effect in the future. We are aware of a modeling study by Strack et al. (2007) that finds a complete invasion of the tundra by shrubs would lead to a 2.2°C warming of 3 meter air temperatures. Rather than being a forcing of Arctic warming and sea ice loss, our review of the literature (e.g., Strum et al., 2005) indicates the encroachment of tundra shrubs into the Arctic acts as a positive feedback enhancing the warming initiated by GHGs, aerosols, and/or natural variability.

Regarding the potential effects of underwater volcanism on Arctic sea ice trends, we are not aware of any recent peer-review studies documenting such an effect or any discussion of such an effect in the

assessment literature. We note that New York Times writer Andrew Revkin queried a number of Arctic oceanographers and climate and ice experts on this issu e and he reported that: “They uniformly reject the idea that heat from the bottom—either from the gene ral geothermal activity beneath the seabed or the

occasional outbursts of lava or vents—could have a si gnificant impact on the veneer of floating, drifting ice on the surface.” For more information, see http://dotearth.blogs.nytimes.com/2008/07/01/whats-upwith-volcanoes-under-arctic-sea-ice/ . Also see the responses in Volume 4 on the relatively small magnitude of total geothermal heat compared to the radiative forcing imposed by changes in GHG

concentrations.

Comment (3-16): Numerous commenters (e.g., 0498, 3291.1, 3477, 3596.1, 4509) find that the observed trends in Antarctic temperatures and sea ice cast doubt on anthropogenic wa rming. They refer to research and datasets that indicate Antarctica is cooling and gaining ice.

Another comment (3596.2) indicates the TSD’s statemen t that models explain the observed warming over the Antarctic Peninsula is false and based on Figur e 3 in a study by Gillett and Thompson (2003). The comment argues that models underestimate the obser ved warming over the Antarctic Peninsula by a



### 13factor of three. This comment, in addition to a second comment (3136.1), also references Karpechko et al. (2008), stating that Karpechko et al. claimed that the Southern Annular Mode is far more influenced by ozone depletion than by global warming.

Response (3-16): Regarding the observed trends in temperatures and sea ice extent over the Antarctic, we refer readers to Volume 2 of the Response to Comments document. While the IPCC found that “The substantial anthropogenic contribution to surface temperature increases likely applies to every continent except Antarctica (which has insufficient observational covera ge to make an assessment) since the middle of the

20 th century (Hegerl et al., 2007),” the more recent USGCRP assessment found that there were human fingerprints in the pattern of changes in Antarctic surface temperatures, based on a study by Gillett et al. (2008). The Gillett et al. study, The Attribution of Polar Warming to Human Influence , stated quite clearly that updated land-surface temperature sets and modeling results led to the finding that “the observed changes in Arctic and Antarctic temperatures are not consistent with internal climate variability or natural climate drivers alone, and are directly attributable to human influence. Our results demonstrate that human activities have already caused significant warming in both polar regions...”

Additionally, Baldwin et al. (2007) also found that the literature showed that combined effects of stratospheric ozone depletion and GHG increases could explain the pattern of wa rming and cooling found

in Antarctica:

The observed trend toward the positive phase of the SAM [Southern Hemisphere Annular Mode] in December to May has been associat ed with a surface cooling of the Antarctic interior of ~1 K [degrees Kelvin], and a wa rming of the Antarctic Peninsula, the Scotia Sea, and the southern tip of South America (Thompson and Solomon, 2002) (Figure 5-8). A similar pattern of warming and cooling has been simulated in response to stratospheric ozone depletion (Gillett and Thompson, 2003) and combin ed stratospheric ozone depletion and GHG increases (Shindell and Sc hmidt, 2004; Arblaster and Meehl, 2006)

Regarding the comment (3596.2) that models underestimate the observed warming based on the results shown in Gillett and Thompson (2003) in Figure 3, whic h is also Figure 5-8 in Baldwin et al., we find that the commenters have misinterpreted this informatio n. The legend in the figure notes that the observed temperature trends are 32-year linear trends for the period December to May, whereas the modeled temperatures in that figure are perturbed minus control runs, averaged for the period December to February. Therefore, the trends are not comparable in the way in which the comment suggested. Further, Gillett, and Thompson explicitly stated that “The discrepancy between the simulated and observed trends during the months of April and May implies that the observed trends during this season are not

attributable to the prescribed ozone depletion.” This discr epancy was addressed by Shindell and Schmidt (2004), which was also included in the Baldwin assessment that the TSD relies on, who state:

Indeed, previous simulations driven by ozone depletion alone showed a strong DJF [December, January, February] respon se but could not reproduce observed MAM [March, April, May] trends in the lo wer troposphere [Gillett and Thompson, 2003]. Conversely, previous simulations driven by greenhouse gases alone showed a weaker DJF response than the observations suggest [Fyfe et al., 1999; Kushner et al., 2001]. Observations thus suggest, and previous modeling allows, a sizeable role for both ozone and greenhouse gases in the lower atmosphere.

Finally, Arblaster, and Meehl (2006), the third study cited by Baldwin et al., included solar and volcanic forcings and updated observational trends, and stated:



### 14The observational trends are calculated fro m the SAM [Southern Hemisphere AnnularMode] index of Marshall (2003) (updated online at http://www.nercbas.ac.uk/icd/gjma/sam.html) and show positiv e trends in all seasons and for the annual value, with the summer season and annual values having a large significant positive trend, and DJF and March–May (MAM) ha ving the largest relative positive seasonal trends, consistent with Thom pson and Solomon (2002). The model all-forcing ensemble reproduces this seasonality, obtaining significant trends of a similar magnitude to the

observations in both DJF and MA M seasons as well as annually.

Therefore, it is inappropriate to conclude that mode ls “underestimate warming by a factor of three” based on comparing a December to February result to a December to May result, from a figure from a 2003 paper that only looked at ozone changes without c oncurrent GHG or other forcing changes and which has been superseded by later papers included in the assessment. While, as stated in the IPCC and summarized by the TSD, attribution at smaller than continental scales is difficult, and the observational coverage in Antarctica is limited, we find that Baldwin et al . appropriately assessed the underlying literature in regards to model agreement with observations of patterns of warming and cooling, and the TSD appropriately reflects those conclusions. The Karpech ko et al. (2008) paper cited by one commenter is reasonably consistent with these results, stating that “[o]ur results demonstrate that both ozone depletion and GHG increases have played a role in the obser ved decrease of tropospheric geopotential height over the Antarctic. At the surface, their contributions are comparable while in the upper troposphere the ozone contribution dominates,” and Karpechko is also a co-author of the 2008 Gillett study cited by the

USGCRP.

Comment (3-17): A number of commenters (e.g., 0509, 0700.1, 1309.1, 1616.1, 1961, 3136.1, 3394.1, 3596.2, 3722, 10562, and 11000) argue that there is no relatio n between sea level and carbon dioxide (CO 2 ) emissions, or that it is highly uncertain. One commenter (3722) states that “t here is no consistent rate of sea level rise in the last century, much less is there any corr elation with the increase in human-caused CO 2 emissions.” Many of these commenters (e.g., 0509, 0700.1, 1309.1, 1616 .1, 1961, 3136.1, 3394.1, 3596.2, 3722, and 11000) argue that sea levels are rising not because of gr eenhouse-induced warming but due to other factors, including seabed shifting, land subsidence, atmospheri c circulation patterns, and solar-driven temperature effects of this past century.

Response (3-17):

These comments are inconsistent with the assessment literature. Regardi ng the primary causes of sea level rise, IPCC states (as discussed in Section 4(f) of the TSD):

“On decadal and longer time scales, global mean sea level change results from two major processes, mostly related to recent climate ch ange, that alter the volume of water in the global ocean: i) thermal expansion (Section 5.5. 3), and ii) the exchange of water between oceans and other reservoirs (glaciers and ice caps, ice sheets, other land water reservoirs - including through anthropogenic change in land hydrology, and the atmosphere). All these processes cause geographically nonuniform sea level change (Section 5.5.4) as well as changes in the global mean; some oceanographic factors (e.g., changes in ocean circulation or atmospheric pressure) also affect sea level at the regional scale, while contributing negligibly to changes in the global mean. Vertical land movements such as resulting from glacial isostatic adjustment (G IA), tectonics, subsidence and sedimentation

influence local sea level measurements but do not alter ocean water volume; nonetheless, they affect global mean sea level through their alteration of the shape and hence the volume of the ocean basins containing the water (Bindoff et al., 2007).”



### 15The IPCC concludes that “it is very likely that th e response to anthropogenic forcing contributed to sea level rise during the latter half of the 20 th century. Models including anthropogenic and natural forcing simulate the observed thermal expansion since 1961 reasonably well. Anthropogenic forcing dominates the surface temperature change simulated by models and has likely contributed to the observed warming of the upper ocean and widespread glacier retreat (H egerl et al., 2007).” Furthermore, IPCC finds that “Anthropogenic forcing is also expected to produce an accelerating rate of sea level rise (IPCC, 2007d).”

Section 5 of the TSD on Attribution of Observed Climate Change to Anthropogenic Greenhouse Gas

Emissions at the Global and Continental Scale furthe r discusses this issue, and the statement above from Hegerl et al. (2007) has been added to that section.

We agree that non-climate factors, such as subsidence and seabed shifting, can have a substantial effect on locally observed sea level rise, and we discuss this point in the TSD. For example, Section 4(g) of the TSD describes that sea level changes in the United St ates vary from about 0.36 inches per year of rise (10 mm per year) along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska (because land is rising) (Field et al., 2007). The TSD describes that the sea levels are rising along the Louisiana Coast by about 10 mm per year, which is greater than other rates in the United

States because of the additional affects associated wi th subsidence. Furthermore, new studies (Hu et al., 2009; Yin et al., 2009) that have not yet been review ed by the assessment literature, and thus not included in the TSD, suggest that warmer water temperatur es and increased melting of Greenland ice sheets could shift ocean currents in a way that would raise sea le vels off the Northeast by about 12 to 20 inches more than the average global sea level rise. However, the IPCC has concluded that non-climate factors have negligible impact on global rates of sea level rise (Bindo ff et al., 2007; Hegerl et al., 2007; Nicholls et al., 2007) and are most relevant for analyzing sea level changes at regional and local scales. Therefore, we disagree that sea levels are rising due solely to non- climate factors such as land subsidence. For reasons discussed in the following section of this volume (3.2), we have additionally c oncluded that the evidence does not support arguments that observed temperature change and related sea level rise are due solely or mostly to internal variability (e.g., changes in atmospheric circulation patterns) or solar-driven temperature effects

Given robust findings by the assessment literature that anthropogenic forcing contributes to sea level rise

we have determined that the TSD’s discussion of this issue is accurate and represents the best available

scientific information.

Comment (3-18): Numerous commenters (e.g., 3136.1, 3291.1, 3347.3, 3596.1, 3722) suggest observed trends in tropical cyclone activity (i.e., hurricanes and tropical storms) cannot be attributed to anthropogenic forcing. They refer to the Statement on Tropical Cyclones and C limate Change from the WMO [World Meteorological Organization] International Workshop on Tropical Cyclones, held November 2006 (WMO, 2006) and a number of very recent studies (e.g., Vecchi et al., 2008; Swanson, 2008; Knutson et al., 2008).

Response (3-18): We base our statements about the attribution of tropical cyclone activity to anthropogenic GHG forcing on the latest assessment literature, namely IPCC’ s Fourth Assessment Report and the CCSP SAP “Weather and Climate Extremes in a Changing Clim ate” (CCSP, 2008i, as referenced in the TSD). As summarized in the TSD, the IPCC (Hegerl et al., 2007) finds it is more likely than not that the anthropogenic influence has contributed to increases in the frequency of the most intense tropical cyclones, where more likely than not conveys a greater than 50% assessed likelihood. For the Atlantic basin specifically, the TSD cites the CCSP (Gutowski et al., 2008) finding that the evidence suggests a human contribution to recent tropical cyclone activity.



### 16We acknowledge the attribution of trends in tropi cal cyclone activity is challenging and have added cautionary language to this effect in the TSD. Specifically, we added the following text:

...the IPCC (Hegerl et al., 2007) cautions that detection and attribution of observed changes in hurricane intensity or frequency due to external influences remains difficult

because of deficiencies in theoretical understa nding of tropical cyclones, their modeling, and their long-term monitoring.

This caveat is in addition to the following statemen t in the TSD (in the final version as well the April 2009 version):

[Gutowski et al., 2008] caution that a confident assessment of human influence on hurricanes will require further studies using models and observations, with emphasis on distinguishing natural from human-induced ch anges in hurricane activity through their influence on factors such as historical sea surface temperatures, wind shear, and atmospheric vertical stability.

We are aware of the Statement on Tropical Cyclones and Climate Change (WMO, 2006) authored by participants of the WMO International Workshop on Tropical Cyclones, IWTC-6, and recognize that one of its conclusions was that “Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.” We note another conclusion in the stat ement was that “Given the consistency between high resolution global models, regional hu rricane models and maximum potential intensity (MPI) theories, it is likely that some increase in tropical cyclone intensit y will occur if the climate continues to warm,” and the final conclusion in the statement was, “Despite th e diversity of research opinions on this issue it is agreed that if there has been a r ecent increase in tropical cyclone activit y that is largely anthropogenic in origin, then humanity is faced with a substantial and unanticipated threat.” However, we defer to the more

recent statements in the IPCC and CCSP assessments for the discussion of this issue in the TSD.

Furthermore, the very recent assessment of Karl et al. (2009), which synthesizes some of the latest literature, draws conclusions consis tent with the 2008 CCSP assessment (CCSP, 2008i). For example, it reports that an increase in the maximum wind speeds of the strongest Atlantic hurricanes has been documented and linked to increasing sea surface temperatures, citing Elsner et al. (2008).

Karl et al. (2009) also cite recent studies on the cau ses of variability in Atlantic tropical cyclones which highlight the complexity of this issue. These include the studies referenced by commenters such as Vecchi et al. (2008), Swanson (2008) and Knutson et al. (2 008). These studies were cited in the context of the following discussion:

New evidence has emerged recently for other temperature related linkages that can help explain the increase in Atlantic hurricane activity. This includes the contrast in sea surface temperature between the main hurricane development region and the broader tropical ocean [Knutson et al., 2008; Swanson, 2008; Vecchi et al., 2008]. Other causes beyond the rise in ocean temperature, such as atmospheric stability and circulation, can also influence hurricane power. For these and other reasons, a confident assessment requires further study. [Gutowski et al., 2008]

In summary, the latest assessment literature integrat es key findings from the latest studies on this issue and is consistent with the very qualified discussi on of the linkage between anthropogenic forcing and tropical cyclone activity in our TSD.



### 17Comment (3-19): Some commenters (e.g., 0400, 0583, 0736, 0796, 3307.1) point to the facts that Earth’s climate has never been static and that over long time scales temperat ures have dropped and later increased significantly without any anthropogenic GHG emissions as evidence that anthropogenic emissions has little to no influence on climate. Commenter 3307.1, for example, states: “The Earth’s history demonstrates not only that there have been previous periods in our history th at are equally warm (or even warmer), but also that such warming can occur at a rate that is even more rapid than the current 20 th Century warming: All without any industrial emission of CO 2 .”

Response (3-19):

The fact that Earth’s climate has never been static and that in the past the Earth has experienced significant temperature change without anthropoge nic emissions of GHGs in no way contradicts the robust conclusion that the global average net effect of human activities since 1750 has been one of warming. The assessment literature takes natural clim ate variability and historical records of climate changes into account and still comes to conclusions su ch as “Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 ye ars,” “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations,” that the rate of increase in radiative forcing from the increase in anthropogenic GHGs is very likely unprecedented in mo re than 10,000 years, and finally that “Continued

GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.”

For EPA’s responses to comments on past periods of global warming and their relationship to GHG concentrations please see Section 3.2.4 of this volume.

Comment (3-20): Some commenters (e.g., 0525, 0546) argue that the idea that human activity can change climate runs counter to established scientific principles. Commenter 0525, for example, states: “The hypothesis that human activity can create global warming is extraordin ary because it is contrary to validated knowledge from solar physics, astronomy, histor y, archaeology and geology.”

Response (3-20): We disagree that “the hypothesis” that human activ ity can create global warming runs contrary to established scientific principles. The claim was not acco mpanied by any evidence and is inconsistent with both the assessment literature and fundamental physi cal understanding. As discussed throughout this document, there is extensive observational and theore tical evidence that human activity is exerting a warming influence on the global climate.

Comment (3-21): A number of commenters (e.g., 0736, 0796, 3553 .1) argues that we do not understand climate mechanisms well enough to establish GHGs as the primary driver of global warming with any degree of confidence. Some commenters (e.g., 0553) suggest that the carbon cycle, in particular, is poorly understood and that the anthropogenic contribution to climate change is therefore highly uncertain.



### 18Response (3-21): For a number of aspects of climate change, attributi on to increased GHGs has become clearer over time. For example, Hegerl et al. (2007) found the attribution of most of the warming in the last 50 years to increases in GHG concentrations to be “very likely,” compared to a judgment of “likely” in the Third Assessment Report (IPCC, 2001c). A number of other aspects of climate have also been attributed to human influences with some level of certainty, as su mmarized in Section 5 of the TSD. The attribution of

the increase in CO 2 concentrations to human emissions, in speci fic, is not in doubt—see Volume 2 of this Response to Comments document. Some uncertainties inhe rent to climate change attribution remain, and these uncertainties are acknowledged and discussed in both the Finding and the TSD, as well as throughout this Response to Comment document. Th e precise rate and magnitude of future climate change, for example, remain uncertain, and we do no t know exactly what fraction of the recent warming may be due to natural factors (although on the basi s of the IPCC, CCSP, USGCRP, and NRC reports, we are confident it is less than half).

We also acknowledge that there are a number of un certainties involved in fully understanding the carbon cycle. There exist, for example, uncertainties about the exact quantity of carbon contained in the ocean and terrestrial carbon reservoirs and the precise size of the fluxes between the various reservoirs. We disagree, however, that knowledge of the basic nature of the carbon cycle is lacking, or that the remaining uncertainties call into question our fundamental unders tanding of GHG forcing. The key mechanisms that drive the carbon cycle are well established in numerous scientific textbooks and reports (see, for example, CCSP, 2007), as are the mechanisms by which an thropogenic carbon emissions cause the amount of carbon contained in the atmosphere and ocean reservoirs to increase. The annual flux from fossil fuel combustion and cement manufacturing to the atmosphe re is well constrained, and the amount of carbon dioxide contained in the atmosphere is known w ith a high degree of precision (Denman et al., 2007; CCSP, 2007).

Comments about the level of certainty with resp ect to the scientific basis for this action are comprehensively addressed in Volume 1: Section 1.2 of this Response to Comments document.

Comment (3-22): A commenter (3291.1) objects that the IPCC underestimat es uncertainty. In particular, he objects that the “Level of Scientific Understanding” (LOSU) for radia tive forcings was raised since the Third Assessment Report without explanation. The commenter also not es that the LOSU for forcings from direct and

indirect aerosol effects are still estimated to be medium-low and low. Because the understanding of these forcings are low, the commenter argues that it is not valid to add these poorly understood forcings together with the better understood forcings in or der to come up with a total forcing estimate.

Response (3-22): Page 199 to 200 of Forster et al. (2007) in the IPCC Fourth Assessment Report discusses in detail the changes in LOSU for radiative forcing (RF) since the Third Assessment Report. Forster et al. note that the Third Assessment process of assigning an LOSU was “a subjective judgment of the estimate’s reliability.” In contrast, for the Fourth Assessmen t Report, Forster provides this description of an approach:

“Evidence” is assessed by an A to C grade, with an A grade implying strong evidence and C insufficient evidence. Strong evidence implies that observations have verified aspects of the RF [radiative forcing] mechanism and that there is a sound physical model to explain the RF. “Consensus” is assessed by assigning a number between 1 and 3, where 1 implies a good deal of consensus and 3 insufficient consensus. This ranks the number of studies, how well studies agree on quantifying the RF and especially how well



### 19observation-based studies agree with models. The product of “Evidence” and “Consensus” factors give the LOSU rank. These ranks are high, medium, medium-low, low or very low. Ranks of very low are not evaluated.

Similarly, Forster et al. discusses in detail the rationale for summing multiple forcings together to develop a probability density function for a net effect of all forcings. The discussion notes that the procedure used followed the example of Boucher and Haywood (2 001), giving individual RF mechanisms equal weighting “even though the level of scientific understanding differs between forcing mechanisms,” along

with several other issues that would introduce furthe r uncertainties. However, the IPCC states (and EPA concurs) that “Despite these caveats ... it remains extr emely likely that the combin ed anthropogenic RF is both positive and substantial” (Forster et al., 2007).

3.2 Alternative Explanations of Observed Changes

Comment (3-23): A very large number of commenters (e.g., 0230, 0245, 0315, 0368, 0400, 0435, 0521, 0630, 0639.1, 0657, 0659, 0700.1, 0716.1, 1217.1, 1309.1, 1309.1, 1468, 1519, 1544, 1613.1, 1924, 2888.1, 2929, 2992, 3136.1, 3160, 3160.1, 3205.1, 3281.1, 3281.1, 3440.1, 3446.1, 3446.2, 3535.1, 3535.2, 3596.3, 3602.1, 3627.2, 3707.1, 3722, 3722, 37222R13, 3722R24, 3722R49, 3722R51, 3722R55, 3722R85, 3729.1, 3769.1, 4003, 4206, 4244, 4395, 4632R22, 4632R48, 6712, 7025, 7026, 7037) argue that natural factors provide a sufficient explanation for most or all r ecent climate change. These commenters state that natural processes either are or could be the primary driver of the observed changes in climate discussed in the Proposed Findings and the April TSD, and they posit sp ecific alternative drivers other than anthropogenic GHG emissions. Some commenters focus solely on natura l external forcings (e.g., solar output), others solely on natural modes of internal variability (e.g., the El Niño-Southern Oscillation [ENSO]), and some suggest that climate change could be the result of interactions between the two.

Response (3-23): The claim that natural internal variability or known natural external forcings can explain most (more than half) of the observed global warming of the past 50 ye ars is inconsistent with the assessment literature and the vast body of science it represents. Based on analyses of widespread temperature increases throughout the climate system and changes in other climate variables, the IPCC has reached the following conclusions about external climate forcing: “It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing, and very unlikely that it is due to known natural external causes alone” (Hegerl et al., 2007). With respect to internal variability, the IPCC reports the following: “The simultaneous increase in energy content of all the major components of the climate system as well as the magnitude and pattern of warming within and across the different components supports the conclusion that the cause of the [20 th century] warming is extremely unlikely (<5%) to be the result of internal processes” (Heger l et al., 2007). As noted in the TSD, the observed warming can only be reproduced with models that contain both natural and anthropogenic forcings (IPCC, 2007d), and the warming of the past half cen tury has taken place at a time when known natural forcing factors alone (solar activity and volcanoes) would likely have produced cooling, not warming (Hegerl et al., 2007). See also other responses in this volume for EPA’s responses to comments on specific alternative warming mechanisms.



### 20Comment (3-24): Many commenters (e.g., 0245, 0400, 0435, 1309.1, 1519, 1544, 1924, 3205.1, 3627.2, 3729.1, 3769.1, 4003, 4632R48, 7025) argue that changes in solar activ ity are (or in a few cases that solar activity could be) the primary driver of both distant past and recen t changes (and possibly future changes) in climate. Commenter 3627.2, for example, states: “It is the sun’s influence that is responsible for the lion’s share of climate change during the past century and beyond” and additionally argues that “[c]osmic rays could provide the mechanism by which changes in solar activity affect climate.”

Response (3-24): The claim that changes in solar activity are the prin cipal cause of recent (1955 to present) climate change has received limited support from a small number of pee r-reviewed articles, many of which have been assessed in the IPCC and/or CCSP/USGCRP reports (d iscussed in Section 3.2.2), but this claim is inconsistent with the vast body of the science a nd the assessment literature. As the TSD notes, the IPCC Fourth Assessment Report estimates that changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12 (+0.06 to +0.30) watts per square meter (W/m 2 ) The combined radiative forcing due to the cumulative (1750–2005) increase in atmospheric concentrations of CO 2 , methane (CH 4 ), and nitrous oxide (N 2 O), on the other hand, is 2.30 W/m 2 with an uncertainty range of +2.07 to +2.53 W/m 2

. In addition, the IPCC (Solomon et al., 2007) reports th at continuous monitoring of total solar irradiance now covers 28 years and that “[t]he data show a well-established 11-year cycle in irradiance that varies by 0.08% from solar cycle minima to maxima, with no si gnificant long-term trend.” As noted above, the sum of solar and volcanic forcing in the past half century would likely have produced cooling, not warming. In addition, direct satellite measurements of solar output show slight decreases during the recent period of warming (Karl et al., 2009).

The fact that the observed pattern of atmospheric temp erature change is inconsis tent with well established scientific understanding of how the climate should respond to solar forcing provides additional evidence that changes in solar output are not the principal cause of recent changes in climate. If an increase in solar output were responsible for the recent warming, bot h the troposphere and the stratosphere would have warmed. By contrast, the observed pattern of atmosphe ric temperatures change has been characterized by warming in the troposphere and cooling in the stratos phere, a result predicted by models and consistent with a GHG forcing. For our responses to additiona l comments on solar irradiance see response 3-35.

As noted previously, some commenters argue that cosmic rays could provide the mechanism by which changes in solar activity affect climate. Howeve r, the IPCC (Solomon et al., 2007) assessed the science on this issue and concluded the following:

Empirical associations have been repor ted between solar-modulated cosmic ray ionization of the atmosphere and global aver age low-level cloud cover but evidence for a systematic indirect solar effect remains am biguous. It has been suggested that galactic cosmic rays with sufficient energy to reach the troposphere could alter the population of cloud condensation nuclei and hence microphysical cloud properties (droplet number and concentration), inducing changes in clou d processes analogous to the indirect cloud albedo effect of tropospheric aerosols and thus causing an indirect solar forcing of climate. Studies have probed various correlati ons with clouds in particular regions or using limited cloud types or limited time periods; however, the cosmic ray time series does not appear to correspond to global total cloud cover after 1991 or to global low-level cloud cover after 1994. Together with the lack of a proven physical mechanism and the plausibility of other causal factors affecti ng changes in cloud cover, this makes the association between galactic cosmic ray-indu ced changes in aerosol and cloud formation

controversial.



### 21The TSD appropriately summarizes this information by stating that empirical associations have been reported between solar-modulated cosmic ray ionizati on of the atmosphere and global average low-level cloud cover, but evidence for a systematic indirect solar effect remains ambiguous, and by noting that the lack of a proven physical mechanism and the plausi bility of other causal factors make the association between galactic cosmic ray-induced changes in aerosol and cloud forma tion controversial (Forster et al., 2007). For our responses to additional co mments on cosmic rays, see response 3-36.

Thus, the TSD comprehensively reviews the topic of solar activity and its effects on climate and its summary of the current state of the science as reflect ed in the underlying assessment literature is accurate

and sound.

Comment (3-25): Some commenters (3136.1, 3411.2, 3627.2, 3729.1, 4003, 4632R31) argue that modes of interannual variation in oceanic temperature and circulation such as the Pacific Decadal Oscillation (PDO), the El Niño-Southern Oscillation (ENSO), and the Atlan tic Multidecadal Oscillation (AMO) could be the proximate cause of most or all recent climate changes. Commenter 3729.1, for example, argues that the oceans “act as the flywheel of the climate system, providing the mechanisms to bring about the changes” and states: “with decadal scale smoothing the ocean mu ltidecadal indices and U.S. temperatures correlate with an r-squared of 0.85.” Commenter 4003 include s with their comment a document authored by Alan Carlin that states the following: “B y far the best single explanation fo r global temperature fluctuations appears to be variations in the PDO/AMO/ENSO.” A document included with comment 3411.2, a chapter by Joseph D’Aleo from the “Supplementary Analysis of the Independent Summary for Policymakers,” published by the Fraser Institute, states: “When you combine the two indices [the PDO and AMO], you can explain much of the temperature variation of th e past 110 years in US annual mean temperatures.”

One commenter (4632) submitted a peer-reviewed st udy (Tsonis et al., 2007) that suggests that synchronous states between four climate indices (PDO , ENSO, the North Atlantic Oscillation, and the North Pacific Oscillation) grow in strength and then abruptly shift to a new climate state, and that interactions between these indices could explain mu ch of the recent climate change. Commenter 3722 also submitted a peer-reviewed study pertinent to the re lative influence of natural variability: Stott et al., (2001).

Response (3-25): Claims that ENSO), PDO, AMO, and other known modes of internal climate variability can explain all or most of the changes in climate that have occurre d over the past century are inconsistent with the assessment literature, and commenters did not provide compelling evidence that the assessment literature has reached fundamentally flawed conclusions.

Although ENSO and other teleconnecti ons (a fixed spatial pattern with an associated index time series showing the evolution of its amplitude and change ) influence climate over interannual and multi-decadal time scales, and fluctuations in these phenomena can account for much of the interannual variability in the circulation and surface climate (as supported by the assessment literature), these phenomena cannot, by themselves, explain most of the changes in clim ate that have occurred over the past half-century, including in particular the long-term warming trend clearly evident in the temp erature record, as claimed by some commenters.

Karl et al. (2006) note that while changes arising fro m internally generated variations in the atmosphereocean-land-ice/snow climate system, such as equatorial sea-surface temperatures associated with ENSO,can influence surface and atmospheric temperature substantially, climate models indicate that global

### 22mean unforced variations on multidecadal times cales are likely to be smaller than the 20 th century globalmean increase in surface temperature. In addition, the IPCC reports that global mean and hemisphericscale temperatures on multi-decadal are largely contro lled by external forcing, noting that “many observed changes in surface and free atmospheric temperature, ocean temperature and sea ice extent, and some large-scale changes in the at mospheric circulation over the 20 th century are distinct from internal variability and consistent with the expected response to anthropogenic forcing” (Hegerl et al., 2007).

With respect to projections of future change in relation to natural variability, the TSD (April 2009 and final) notes that some of the most challenging asp ects of understanding and projecting regional climate changes relate to possible changes in the circulation of the atmosphere and the oceans, and their patterns of variability. However, the TSD also included the IPCC’s conclusions that recent advances in regionalscale modeling lead to higher confidence in proj ected patterns of warming and other regional-scale features, including changes in wind patterns, precipita tion, and some aspects of extremes of ice. The IPCC has reported that changes in natural patterns such as ENSO themselves might be related to climate change (Bindoff et al., 2007) and that the question of whether observed changes in ENSO behavior are physically linked to global climate change is “a research question of great importance” (Trenberth et al., 2007).

We note that commenter 4632 provided a recent peer -reviewed reference paper (Swanson and Tsonis, 2009) of relevance to the interplay between internal variability and external forcing. One of the paper’s main arguments is that climate shifts due to couplin gs between modes of climate variability have not only played a key role in recent climate change but could lead global mean temperatures to remain near current levels for an extended period of several decades. In light of the magnitude of expected GHG forcing, we find it highly unlikely that temperatures will remain near current levels for decades—and it bears mentioning that Swanson and Tsonsis state: “[I]t is purely speculative to presume that the global mean temperature will remain near current leve ls for an extended period of time.”

It is important to note that Swanson and Tsonis (20 09) do not interpret their findings in the same way the commenters have. In fact, they say:

“Finally, it is vital to note that there is no co mfort to be gained by having a climate with a significant degree of internal variability, even if it results in a near-term cessation of global warming. It is straightforward to ar gue that a climate with significant internal variability is a climate that is very sensitive to applied anthropogenic radiative anomalies [cf. Roe, 2009]. If the role of internal variability in the climate system is as large as this analysis would seem to suggest, warming over the 21 st century may well be larger than that predicted by the current generation of models, given the propensity of these models

to underestimate climate internal vari ability [Kravtsov and Spannagle, 2008].”

Thus, the conclusion they derive is that their findings could imply more, not less, severe climate

effects in the future.

Tsonis et al., (2007) do not discuss the relative role of anthropogenic warming in as direct a manner as

Swanson and Tsonsis but do state that the climate shif t they find was trigged by synchronous states in the PDO, ENSO, the North Atlantic Oscillation, and the North Pacific Oscillation “may be superimposed on an anthropogenic warming trend.”

The principal conclusion of the Stott et al. ( 2001) paper submitted by commenter 3722 is highly

consistent with the assessment literatu re conclusions on this topic. The authors state: “All the results presented here support the conclusions of T99 that an thropogenic factors are largely responsible for the warming in near-surface temperature observed since 1945.”



### 23Comment (3-26): Commenters submitted a number of works that claim that the recent warming is part of a cycle of temperature trends. One commenter (3722) submitted Kl yashtorin and Lyubushin (2003), stating that the 60-year cycle found in the paper can be used to e xplain recent warming and extended to show that the Earth is entering a cooling phase. This commenter states that Loehle et al. (2004) support a similar conclusion of imminent near-term cooling, and also submitted Davis and Bohling (2001). The commenter states that these cycle analyses and near-term cooling predictions are consistent with a 1998 peak in global average mean temperature. Other commenters submitted Unstoppable Global Warming Every

1500 Years by Fred Singer and Dennis Avery, which claims that a cycle of warming has dominated the earth’s temperature for the past 10,000 years, that ther e is evidence that this cycle has existed for a million years, and that this cycle explains most of the Earth’s warming since 1850.

Response (3-26): The predictions of imminent cooling based on cycl e analysis are not consistent with the assessment literature. These studies do not present any evidence for any negative external ra diative forcings that could be of the magnitude of th e positive forcing from increases in GHG concentrations. IPCC has shown that it is possible to explain previous temperature tr ends based on reconstructions of historical solar, volcanic, GHG, and orbital forcings, but the methods u sed to explain the previous changes cannot explain recent warming without the contribution of cha nges in GHG concentrations due to anthropogenic emissions. Specifically, historical reconstructions of solar and volcanic forcing have been used as inputs to model simulations; these simulations explain much of the last 1,000 years of temperature change, but the recent warming cannot be explained by the same na tural forces that explained previous temperature changes (Jansen et al., 2007). Similarly, when forced by changes in solar forcing due to orbital parameters, coupled climate models and Earth System Models of Interm ediate Complexity were both able to capture reconstructructed regional temperatur e and precipitation changes (Jansen et al., 2007).

We note that the submitted references do not use rigo rous attribution studies. Klyashtorin and Lyubushin (2003) compared correlations of trends in world fuel consumption to temperature, rather than the more appropriate comparison of some kind of reconstruction of radiative forcing based on actual greenhouse gas concentrations. The book by Singer and Aver y, which has not been peer-reviewed, summarizes literature that looks at historical cycles, but relies on changes in solar irradiance or cosmic rays to explain how these cycles could be operating in the present. S ee response 3-24 and 3-36 in this volume for an analysis of the role of cosmic rays, and response 3-35 fo r an analysis of the role of solar irradiance in the past 50 years. Davis and Bohling put recent oxygen- 18 isotope changes as recorded in Greenland ice cores in the context of past cha nges in the last 10,000 years, finding that recent changes have been unusually rapid but not unprecedented. However, wit hout examining the causes of the previous changes, this does not present evidence that the recent changes are natural in origin, nor does it counter the evidence that large increases in GHG concentrations will lead to larg e increases in temperature in the future. Loehle (2004) tries to explain historical da ta using time-series models based on cosine functions, and shows that a large number of the time series models that fit the historical record both explain recent warming and project cooling over the next century. Ag ain, as the models presented in the paper include no physical variables and are merely statistical fits, they are not credible alternatives to physics-based

models.

With respect to the contention by the commenter that 1998 was the peak in global average surface temperature, please see responses on gl obal average temperature records in Volume 2 of the Response to

Comments document.



### 24Thus, these theories are not appropriate for inclusion in the TSD, and the summary in the TSD of the current state of the science as reflected in the underlying assessment literature is accurate and sound.

Comment (3-27): Commenters 3136.1 asserts that the primary cause of recent warming in Alaska is the PDO, not human GHG emissions. Commenter 3627.2 made a similar point with regard to glacial extent in Alaska. A reference document provided by commenter 5058 (Har tmann and Wendler, 2005) did not explicitly claim that all observed warming in Alaska can be attributed to the PDO but suggested that “a significant amount of the warming trend seen throughout Alaska during the last half of the twentieth century is largely a result of the sudden shift [in the PDO] in 1976,” and al so stated: “[t]he cooling trend throughout much of Alaska since 1977, though not statistically significant, is in contrast to some theories regarding the atmospheric warming in an increasing greenhouse gas environment.”

Response (3-27): The TSD does not specifically attribute recent temper ature changes in Alaska to human GHG emissions, though the TSD summarizes the findings of the recent USGCRP assessment (Karl et al., 2009) on Alaska, stating that “Over the past 50 years, Alaska’s annual average temperature has increased by 3.4°F (1.9 C)
and winters have warmed by 6.3°F (3.5 C), which is more than twice the rate of the rest of the United States These observed changes are consistent with climate model projections of temperature increases in
Alaska of 3.5 to 7°F (1.9 to 3.9 C) by mid-century.” This does not el iminate a role for the PDO, as the assessment literature notes that attributing changes in temperature to anthropogenic forcing at continental or smaller scales is challenging. One reason is th at as spatial scales considered become smaller, the uncertainty becomes larger because in ternal climate variability is typically larger than the expected responses to forcing on these scales (Gutowski et al., 2008). We also note that short-term, local regional variability occurs on top of long-term, global trends. Fo r more on attribution of Alaska temperature trends to different causes by Hartmann and Wendler (200 5), see Volume 4 of the Response to Comments

document.

We specifically discuss Alaska temperature trends in Volume 2 of the Response to Comments document, and note that data are conflicting about Alaska temp erature trends since 1977. For a response on glacial trends, see the response in Volume 2 of the Response to Comments document.

3.2.1 Other Substances With Radiative Forcing Effects

Comment (3-28): A few commenters (3283.1, 3596.1, 3596.2) cite recent brown and black carbon (BC) research (e.g., the Ramanathan and Carmichael, 2008, estimate of 0.9 W/m 2 , and work by Nagashima et al., 2006) as both a reason that GHG effects may have been overestimated and as a possible area for reduction rather than

GHGs. Commenter 3596.1 estimates that this means th at GHG impacts have been overestimated by 25% (based on an IPCC radiative forcing impact of BC of 0.2 W/m 2 in the IPCC). Similarly, a commenter (3316.1) questions the reliability of models, suggesting that they missed a major climate change factor.



### 25Response (3-28): First, we note that the effect of GHGs is in large part determined by the cl imate sensitivity and that climate sensitivity estimates are determined in a nu mber of ways, only one of which is likely to be affected by changes in our understanding of black carbon. The one methodology from the IPCC Fourth Assessment Report, Ch. 9.6 (Hegerl et al., 2007) for determining climate sensitivity that might be sensitive to a changing estimate for black carbon would be the methodology that uses 20 th century historical temperature patterns to constrain climate sensitivity estimates. Estimates based on paleoclimate data, response to individual volcanic events, or th eoretical approaches would be insensitive to new estimates of black carbon forcing. Even for the estimates based on 20 th century data, net aerosol forcing is often included as an uncertain input (e.g., Forest et al., 2008) and therefore the new Ramanathan and Carmichael estimates might only imply a different a llocation of positive and negative contributions within the total net aerosol forcing estimate, rather than an y change in allocation of warming attribution between aerosols and GHGs. Indeed, Ramanathan and Carmichael estimate a net aerosol forcing of -1.4 W/m 2

, which is actually a larger cooling influen ce than the IPCC net estimate of -1.2 W/m 2 from direct and cloud albedo effects, so if anything, changing the estim ates of the strength of the aerosol forcing effect used as an input to climate sensitivity studies to match the Ramanathan and Carmichael data might increase the estimates of climate sensitivity rather than decrease them, although this effect is likely to be small. In any case, our examination of the met hods for estimating forcing for various substances

concludes that the new literature on black car bon does not imply a reduced forcing for GHGs.

The change in the estimate of current BC forcing co uld also affect future projections, inasmuch as any change in BC emissions (either increasing or decreasi ng) would have a larger effect than previously estimated. However, given that the total GHG forc ing in the reference projections from CCSP 2.1a (CCSP, 2007b) range from 6.4 W/m 2 to 8.6 W/m 2 by the end of the century, th e uncertainty of less than 1

W/m 2 due to BC forcing will not fundamentally change the results. On a technical note, the IPCC estimate for radiative forcing from BC was 0.34 W/m 2 for the direct effect and 0.1 W/m 2 for the snow albedo effect, for a total of 0.44 W/m 2 , not 0.2 W/m 2 , as one commenter claimed (0.2 W/m 2 refers to the direct BC contribution from fossil fuels alone).

Regarding the comment that BC could be a possible area for emissions reductions, please see Volume 1: Section 3, Role of Adaptation and Emissions Mitigat ion Considerations, for EPA's response to comments regarding the relevance and implications of mitig ation measures in the context of this action.

Comment (3-29): One commenter (0373.1) proposed that stratospheric ozone depletion due to CFCs better explains the observed temperature pattern than does CO 2 , referencing a study by Ashworth.

Response (3-29) : Many attribution studies already incl ude stratospheric ozone depletion as a component of their analysis (e.g., Forest et al. 2008). The best explanations of historic temperature patterns arise from inclusion of stratospheric ozone depletion in addition to, not in replacement of, increasing GHG concentrations. Indeed, the science suggests that the global impact of d estruction of stratospheric ozone is estimated to be a negative forcing (Forster et al., 2007). We note th at the Ashworth study submitted by the commenter has not been peer-reviewed, and we have determined that the assertion regarding the role of stratospheric ozone depletion is not valid.



### 26Comment (3-30) : A commenter (3476.6) notes that there are a number of “first-order” anthropogenic radiative forcings that are not GHGs, namely aerosols (direct, indirect, and snow-albedo effects), n itrogen deposition, land use change, albedo effects, and water evaporation from land use and irrigation. Additionally, CO 2 has an effect on plant growth changing water transpiration. The commenter claims that regional climate change will be dominated by regional forcings, not by gl obal GHG forcings. Another commenter (3283.1) also objects that warming is the result of non-GHG anthr opogenic factors as well as GHG factors but that the EPA’s endangerment finding seems to place all of the focus on anthropogenic GHG emissions. Several other commenters (3295.1, 3722, 4395) argue that va riations in the amount of water vapor in the atmosphere are a principal driver of climate change.

Response (3-30) : The assessment literature di scusses all of these additional forcings, and they are summarized in the TSD. As stated in Forster et al. (2007):

While most current GCMs [general circula tion models] incorporate the trace gas RFs, aerosol direct effects, solar and volcanoes, a few have in addition incorporated land use change and cloud albedo effect. While LLGHGs [long-lived greenhouse gases] have increased rapidly over the past 20 years and contribute the most to the present RF (refer also to Figure 2.20 and FAQ 2.1, Figure 1), Figure 2.23 also indicates that the combined positive RF of the GHGs exceeds the contributions due to all other anthropogenic agents throughout the latter half of the 20 th century.

Both the IPCC and CCSP agree that these effects ca n be locally important. CCSP (2008e) examines the effects of climate change on agriculture, land resour ces, water resources, and biodiversity in the United States and concludes that global climate change e ffects will be superimposed on and modify those resulting from land use and land cover patterns in ways that are as of yet uncertain. Karl et al. (2009) find that the anthropogenic land use ac tivities such as cutting and burning forests, replacement of natural vegetation with agriculture and cities, and large- scale irrigation can cause local (and even regional) warming or cooling, but that globa lly the net effect of these changes has probably been a slight cooling over the last 100 years. Karl et al. also discuss the possibility that the Dust Bowl of the 1930s was the result of an interaction between poor agricultu ral practices removing native vegetation that had maintained soil moisture, and variations in ocean te mperatures that disrupted wind patterns and therefore rainfall. While these local and regional variations can lead to differing trends in the short term, in the long term the continued accumulation of GHGs will lead to a global forcing that will exceed the short-term, local variability.

The TSD has been updated to address water evaporati on from land use. We have added the following statement to Section 4a: “Irrigation and deforesta tion both have small, poor ly understood effects on humidity, in opposite directions, and the IPCC concl uded that radiative forcing from these sources of tropospheric water vapor was smaller than their non-ra diative effects (such as evaporative cooling).” On the other hand, water vapor changes resulting from ch anges in the global climate are significant, and are included in the models as a feedback (but not a forcing). For further de tail on water vapor, see the responses on the percent contribution in the atmosphe re in Volume 2, and on the contribution of emissions from irrigation and combustion in Volume 9, of this Response to Comments document.

Comment (3-31): Commenter (3722) discusses the profound uncertainty invo lved in determining the interaction of water with the climate system (including clouds) and st ates that this needs to be properly acknowledged.



### 27Response (3-31): Randall et al. (2007) address the issue of uncertainties involved in the interaction of water with the climate system, stating that:

Substantial progress has been made in unde rstanding the inter-model differences in equilibrium climate sensitivity. Cloud fee dbacks have been confirmed as a primary source of these differences, with low clo uds making the largest contribution. New observational and modeling evidence strongly supports a combined water vapour-lapse rate feedback of a strength comparable to that found in General Circulation Models (approximately 1 W m –2 °C -1 , corresponding to around a 50% amplification of global mean warming).

Therefore, the assessment literature finds that obser vations ands models are consistent in terms of describing the interaction of water and climate fee dbacks, though uncertainties remain. Uncertainties due to clouds are analyzed in the assess ment literature and discussed in the TSD in the context of aerosols and contrails (Section 4a), and the determination of climate sensitivity and model projections (Section 6b). For example, Box 6.3 in the TSD summarizes the ap proach of the assessment literature in assessing cloud uncertainties in the process of determining the likely range of equilibrium cl imate sensitivity. We have reviewed the TSD in light of this comment and find that its summary of the i ssue accurately reflects the

scientific literature.

Comment (3-32): Commenter (3679.1) requests that the TSD address the natural heat vent over the tropics.

Response (3-32):

We have reviewed the literature on the natural heat vent over the tropics, a theory that increased warming will lead to changes in cloud cover in the tropics that will lead to cooling (e.g., a negative feedback). We found, however, that several recent pa pers have indicated that cloud feedback in the tropics may actually be positive (Clement et al., 2009, Su et al., 2008). A st rong negative feedback is also inconsistent with historical variability. Climate sensitivity estimates and uncertainty in cloud feedbacks in general are addressed in more depth elsewhere in this volume and in Volume 4.

Comment (3-33): A commenter (3679.1) requests that the TSD address c ooling from plant emissions of carbonyl sulfide, iodine, and dms (dimethyl sulfide) emissions from marine organisms in the oceans and other natural aerosol emissions. The commenter also requests that the TSD address increased photosynthesis from diffuse light. Another commenter (3596.3) reviewed studies from 1990 to 2002 regarding dms and carbonyl sulfide, and cites Smythe-Wright et al. (2006) regarding methyl iodide feedbacks. The

commenter also notes that natural aerosols should contri bute significantly to radiative forcing uncertainty.

Response (3-33): We have reviewed the literature on these aerosol typ es. We do find that there may be some feedbacks from climate change on natural aerosol emissions, as addressed in the Section 8 (Health) of the TSD, but the climate-relevant impacts of these feedbacks are expected to be small in comparison to direct anthropogenic forcing changes. For example, Denman et al. (2007) found that dimethyl sulfate feedbacks represent a “small negative climate feedback to gl obal warming,” estimating the negative climate feedback due to natural production of dimethyl sulf ate from marine organisms as resulting in –0.05 W/m 2

,



### 28two orders of magnitude smaller than the averag e projected forcing changes from the IPCC Special Report on Emissions Scenarios (SRES) and CCSP emissions scenarios (CCSP, 2008d).

The Smythe-Wright et al. (2006) paper concludes, b ased on extrapolations from laboratory experiments, that a shift away from microalgae towards the methyl iodide producing Prochlorococcus due to warming of the oceans might lead to an increase of 15% in global methyl iodide production, leading to cooling through indirect cloud effects. This would therefore be another small negative feedback. However, other papers indicate that the total ocean emissions of met hyl iodide is smaller than the Smythe-Wright estimate

of Prochlorococcus methyl iodide production alone (Sive et al., 2007), and while Wang et al. (2009) find a correlation between methyl iodide production and te mperature they also note that “While the global ocean is an important source of atmospheric methyl iodide (CH 3 I), the major producers of CH 3 I within the ocean remain unclear.” Denman et al. (2007) also stated: “Thus, a hitherto undiscovered remote ocean source of iodine atoms (such as molecular iodine) must be present if iodine-mediated particle formation is to be important in the remote marine boundary laye r.” The overall literature on methyl iodide does not make the case for a significant negative feedback.

The most recent paper submitted with regards to car bonyl sulfide was by Aydin et al. (2002), who found that preindustrial variations of carbonyl sulfide recons tructed from Antarctic ice co re data were small in comparison to the anthropogenic increase in the last century. The only other paper more recent than 1994 that was submitted was by Kuhn and Kesselmeier (2000), who found that soils were actually more likely to be sinks for carbonyl sulfide than sources, in contrast to earlier research. Therefore, we find that the scientific literature in this area is still uncertain, and that these effects are likely to be small in comparison to changes in anthropogenic forcings.

With regard to increased photosynthesis from diffuse light , this phenomenon is related in large part to the “dimming” caused by aerosol particles. Trenberth et al. (2007) report that global “dimming” was not global nor had it continued after 1990. Denman et al. (2007) addressed this phenomenon and stated that “an increase in diffuse light at the expense of direct light may promote leaf carbon assimilation and transpiration.” However, in our review of the assessment literature, we find no indication that this effect is large enough to materially change future projections of climate.

Comment (3-34): Commenter (3701.1) requests that EPA analyze Miskolczi's theory (Miskolczi 2007) that water vapor

balances out CO 2 forcing. Commenter 3535 submits a statement by Miskolczi that claims that the National Oceanic and Atmospheric Administrati on (NOAA) National Centers for Environmental Protection (NCEP)/National Center for Atmospheric R esearch (NCAR) reanalysis data show a slight decrease in global average absolute humidity in the past 61 years, which compensates for increases in GHGs. Another commenter submits Paltridge et al . (2009), which also found decreasing absolute humidity at high altitudes from the NCEP data. Some (0798, 2982) cite Miskolczi (2007), which theorizes that water vapor will condense or evaporate as needed to maintain a constant greenhouse effect, citing a finite atmosphere used in calculations and observe d decline in upper atmosphere humidity as validating factors. A number of other commenters (3323.1, 4003, 4041.1, 4932.1, and 5158) state that the lack of observed constant humidity levels are contrary to anthropogenic global warming theory and the IPCC computer models.

Response (3-34): The hypothesis that increased CO 2 forcing will lead to a counterbala ncing decrease in water vapor is highly speculative, and is not supported by the vast body of scientific literature. Miskolczi claims that the greenhouse effect should maintain a balance, so that every increase in a GHG should lead to a corresponding decrease in water vapor (and vice versa), effectively implying a climate sensitivity of zero.



### 29A climate sensitivity of zero is completely incompatible with historical temperature variations, as it would imply an unchanging climate in direct contrast to historically recorded temperatures changes on all timescales. Miskolczi also claims that “On global scale, however, there can not be any direct water vapor feedback mechanism, working against the total energy balance requirement of the system. Runaway greenhouse theories contradict to the energy balan ce equations and therefore, can not work.” This demonstrates a lack of understanding of feedback m echanisms in the climate (see response in Volume 4 for a discussion of runaway climate).

Several commenters also cite evidence for decreasing absolute humidity, in contrast to the IPCC conclusions (cited in the TSD) that “[a]lthough surf ace specific humidity globally has generally increased after 1976 in close association with higher temper atures over both land and ocean, observations suggest that relative humidity has remained about the same overall, from the surface throughout the troposphere (Trenberth et al., 2007).” The data from the NOAA NCEP/NCAR reanalysis for humidity has been questioned in other papers (Soden et al., 2005) (especia lly for the pre-satellite period), and a Dessler et al. (2009) review also contradicts this data. Even Paltrid ge et al. (2009), which relied on the NCEP reanalysis data, recognized that “[i]t is accepted that radiosonde -derived humidity data must be treated with great caution, particularly at altitudes above the 500 hP a [hectopascal] pressure level.” Falling absolute humidity during a period of warming is also difficult to reconcile with theoretical understanding, model results, and historical temperature trends. The analysis in the IPCC (Trenberth et al., 2007) stated: “Due to instrumental limitations, long-term changes in wa ter vapour in the upper troposphere are difficult to assess,” but nonetheless concluded: “To summarise, the av ailable data do not indicate a detectable trend in upper-tropospheric relative humidity. However, th ere is now evidence for global increases in uppertropospheric specific humidity over the past two decades, which is consistent with the observed increases in tropospheric temperatures and the absence of any change in relative humidity.” No trend in uppertropospheric relative humidity, and evidence for increases in specific humidity, are consistent with model predictions that relative humidity should stay fairly constant, implying increasing absolute humidity with increasing temperature, and therefore a positive fee dback (see response on Volume 4 for more responses on relative humidity predictions in models).

3.2.2 Solar Irradiance

Comment (3-35): A number of commenters (e.g., 0670) argue that the sun is the primary driver of global temperature changes. Several commenters (3323.1, 4003, 4041.1, 4932.1, and 5158) referred to a new 2009 paper by Scafetta and Willson suggesting that the IPCC used faulty solar data in dismissing the direct effect of solar variability on global temperatures. Commenters also cite other research by Scafetta and others that suggests that solar variability could account for up to 68% of the increase in Earth’s global temperatures. One commenter (1616.1) attributes 0.14°C of the wa rming since 1950 to increased solar irradiance, and another 25% of warming since 1979, as in Scafetta and West (2006) (3596.1). Another commenter (7031)

states that the correlation between solar variations su ch as sunspots and global climate has been pointed out by several scientists, such as Scafetta and West (2008). A number of specific climate-related regional phenomena have been related by commenters (e.g., 3596.1) to solar variability, such as sea surface temperature, floods, droughts, monsoons , and North Atlantic drift ice.

Response (3-35):

We have reviewed the comments and the literature s ubmitted and have de termined that changes in solar irradiance are not a sufficient explanation for recent climate change. The contention that direct solar variability can explain recent warming is not supported by the bulk of th e scientific literature. As the TSD notes, the IPCC Fourth Assessment Report estimates that changes in solar irradiance since 1750 are

estimated to cause a radiative forci ng of +0.12 (+0.06 to +0.30) W/m 2 , or approximately 5% of the



### 30combined radiative forcing due to the cumulative (1750–2005) increase in atmospheric concentrations of

CO 2 , CH 4 , and N 2 O (2.30 W/m 2 with an uncertainty range of +2.07 to +2.53 W/m 2 ). The natural 11-year cycle of solar irradiance has a magnitude of less than 2 W/m 2 at the distance of the Earth—which, once corrected for albedo and distribution over the surface area of the planet, is a magnitude of less than 0.35

W/m 2 . In addition, Karl et al. (2009) state that “if most of the observed temperature change had been due to an increase in solar output rath er than an increase in GHGs, Earth’s atmosphere would have warmed throughout its full vertical extent, including the stratosphere. The observed pattern of atmospheric temperature changes, with its pronounced cooling in th e stratosphere, is therefore inconsistent with the hypothesis that changes in the Sun can explain the warming of recent decades. Moreover, direct satellite measurements of solar output show slight decrease s during the recent period of warming.” A number of

other recent studies also show results that contrast w ith the interpretation that solar variability is driving recent warming. Both Lockwood and Fröhlich (2008) and Lean and Rind (2009) show that the solar contribution to warming in recent decades has been small or negative, consistent with the IPCC attribution of most of the warming in recent decades to anthropogenic GHGs.

The attribution of components historical climate cha nge to solar activity involves a number of issues. The first is the actual reconstruction of historical solar activity: even for the last three decades there is some controversy, as is evident in the differences between Scafetta and Willson (2009), which uses a total solar irradiance composite from the Active Cavity Radiom eter Irradiance Monitor (ACRIM) analysis of satellite data, and Lockwood and Fröhlich (2008), which uses a composite based on the PhysikalischMeteorologisches Observatorium Davos (PMOD) analys is of satellite data. These two composites don’t even agree on the sign of the solar irradiance trend over this time period. Lockwood and Frolich analyze

both datasets and find that the ACRIM dataset is incons istent with methods of historical reconstructions that have shown correlations between historical solar activity and climate. Krivova, Solanki, and Wenzler (2009) also find no evidence of an increase in total solar irradiance (TSI) from 1986 and 1996 using an analysis based on magnetograms. Scafetta and Willson, on the other hand, claim that the PMOD approach requires a correction of the data from the earth radiation budget (ERB) system on the NIMBUS7 satellite, and this correction has been rejected by one of th e scientists on the NIMBUS team (D.V. Hoyt, personal communication to Scafetta, 2008). Neither dataset shows an increase of solar irradiance between the minima of 1986 and 2008, which w ould be required in order to e xplain warming over that period. Therefore, reconstructions of recent solar variability do not agree, but in one case show no trend, and in the case of the Lockwood and Fröhlich reconstruction the solar contribution during this period would have been a cooling, not warming, influence.

The second issue is that in order for solar irradiance to be a major driver of recent warming, there must be an amplification effect that is active for solar irradiance that is not active for forcing due to GHGs. Studies such as Scafetta (2009) often rely on a signifi cantly different factor for solar irradiance than is used for GHG climate sensitivity. Additionally, the Scafetta study relies on a “slow lag” solar response and the timescale chosen has itself been the subject of dispute. The climate sensitivity for this slow lag response used by Scafetta is 0.46° K/Wm -2 . Note that this is compared to the total solar irradiance: therefore, the effective sensitivity to the average solar irradiance according to Scafetta would be (4*0.46)/0.7 = 2.6° K/Wm -2 . This can be compared to a climate sens itivity range of 2 to 4.5, or about 0.5°

K/Wm -2 to 1.2° K/Wm -2 . Additionally, Scafetta claims that solar variability accounts for most of the recent warming and that GHG sensitivity is on the low end of the range: this means that Scafetta is effectively claiming that sensitivity to solar variability is on the order of five times the sensitivity to forcing by GHGs, without a good mechanism to explai n this extreme difference. Although it is not impossible that there are differences between so lar and GHG induced chang es, the evidence for an amplification of the magnitude needed to explain r ecent warming is weak. For example, while Meehl et al. (2009) find an amplification of the solar cycle variability is needed to explain certain patterns of tropical Pacific climate response, the authors note: “T his response also cannot be used to explain recent global warming because the 11-year solar cycle has not shown a measurable trend over the past 30 years.”



### 31Moreover, the sensitivity needed is nowhere near as large as the Scafetta sensitivity, and the behavior explained is geographically localized, which is diffe rent from a global increase in sensitivity. Therefore, the evidence for an amplification of the magnitude needed to explain recent warming is weak.

Some other authors also show some correlations be tween solar variability and regional trends. Eichler et al. (2009) find a strong correlation between solar act ivity (as reconstructed by carbon-14 and beryllium10 proxies) and temperatures in the Siberian Altai region. However, the authors note that “underlying physical processes are still not yet understood” in te rms of amplifying a weak solar signal (in terms of radiative forcing) in order to see larger effects, and al so that “[i]n large spatial scale hemispheric or global reconstructions the solar signal may therefore even va nish” because the “main effect of solar forcing is presumably on location, routes, and stability of atmo spheric pressure systems, which all act on regional

scales.” The conclusion of the Eichler work is that wh ile solar activity was a main driver for temperature variations in the Altai region preindustrially, during the industrial period they found that only CO 2 concentrations show a significant correlation with th e temperature record. They did find agreement with the northern hemisphere (NH) temperature reconstructi on of Scafetta and West (2007) in that they found that only up to approximately 50% of the observed globa l warming in the last 100 years can be explained by the sun. Note that this conclusion provides 50% as an upper limit to the explanatory power of solar variability, and this is for the full century. Therefor e, for the last 50 years, this conclusion is still consistent with the IPCC (2007b) statement that “[m]ost of the observed increase in global average temperatures since the mid-20 th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

Therefore, to summarize: attempting to attribute late-20 th century temperature change mainly to solar variability requires choosing a specific solar dataset, assuming a simplified model with different “fast” and “slow lag” responses based on timescales from a controversial paper, and assuming that the climate system is several times more sensitive to changes in solar irradiance (or other, non-radiative changes in the sun) than it is to changes in GHG forcing. All three of these assumptions are counter to the

conclusions of the IPCC and CCSP assessments and not viewed as established conclusions in the

literature. While science in this area will continue to evolve, our review did not uncover any compelling alternatives to the science represented in the assess ment literature, and summarized in the TSD.

Comment (3-36): Many commenters (0153, 0245, 0509, 0591, 1017.1, 1187, 2953, 3722, 3729.1) claim that temperature is better correlated with solar activity patterns than with greenhouse forcing, some of whom reference

researchers such as Svensmark or Shaviv that attrib ute the mechanism not to solar irradiance but rather solar wind or solar-magnetic flux (2917, 3205.1, 3324.1, 4632, 5058) and interactions with cosmic rays seeding low-lying clouds (0542, 06 46, 0798, 1616.1), or length of solar cycles (0543) or sunspots (1219.1).

One commenter (7031) indicates that solar impacts on climate have received scant research attention and are minimized in the IPCC Fourth Assessment Report (2007a) and the climate model community, even though the IPCC authors rank the level of scientific understanding of solar-climate interactions as very low. The assumption is that variations in TSI are th e only significant solar impact on global climate. The commenter also posits:

Recent studies have shown strong correlati ons between solar-modulated cosmic ray

fluxes and low-level cloud cover and its sub sequent impact on global temperatures. Experimental verification of a cosmic-ray cloud seeding mechanism was recently completed by Svensmark et al. [1997], and the CLOUD (Cosmics Leaving OUtdoor



### 32Droplets) experiments at CERN (the European Organization for Nuclear Research) over the next few years will provide definitive measu rements of cloud seeding by cosmic rays.

The commenter concludes it is clear that solar variati ons have much larger impacts on global climate than what is estimated based solely on TSI variations.

One commenter (3446.2) requests that the TSD include a rigorous presentation of sunspot activity and temperature over the past century, and notes objection to the lack of sunspot discussion in Karl et al. (2009). Another (3397) requests more discussion of solar activity as a climate forcer.

Response (3-36): The contention that cosmic rays could provide the me chanism by which changes in solar activity affect climate is not supported by the literature. Solomon et al . (2007) address this topic, noting that “the cosmic ray time series does not appear to correspond to globa l total cloud cover after 1991 or to global low-level cloud cover after 1994.” More recent research continues to question the ability of this mechanism to play a significant role in climate change. Pierce and Adam s (2009) use calculations to show that potential impacts on clouds from cosmic rays and “conclude that the hypothesized effect is too small to play a significant role in current climate change.” Erlykin et al. (2009) found that the evidence showed that

connections between solar variation and climate were mo re likely to be mediated by direct variation of insolation rather than cosmic rays, and concluded: “Hence within our assumptions, the effect of varying solar activity, either by direct solar irradiance or by varying cosmic ray rates, must be less than 0.07 ◦ C since 1956, i.e. less than 14% of the observed gl obal warming.” Carslaw (2009) and Pittock (2009)

review the recent and historical literature in this fi eld and continue to find that the link between cosmic rays and climate is tenuous, though th ey encourage continued research.

The CLOUD experiments at CERN are interesting res earch but do not provide conclusive evidence that cosmic rays can serve as a major source of cloud seeding. Preliminary results from the experiment (Duplissy et al., 2009) suggest that though there was so me evidence of ion mediat ed nucleation, for most

of the nucleation events observed the contribut ion of ion processes appeared to be minor. These experiments also showed the difficulty in main taining sufficiently clean conditions and stable temperatures to prevent spurious aerosol bursts. There is no indication that the earlier Svensmark experiments could even have matched the controlled conditions of the CERN experiment. We find that the Svensmark results on cloud seeding have not yet been shown to be robust or sufficient to materially alter the conclusions of the assessment literature, esp ecially given the abundance of recent literature that is skeptical of the cosmic ray-climate linka ge reviewed in the previous paragraph.

Therefore the TSD summary of the assessment literature on this issue is well founded: that the lack of a proven physical mechanism and the plausibility of other causal factors make the association between galactic cosmic ray-induced changes in aer osol and cloud formation controversial.

Comment (3-37): Several commenters cite good sun-climate agreemen t over various timescales, such as during the past millennium, including the Maunder and Dalton minima (3411, 3729.1, and 4003), or over the past 500 million years (4003). One commenter attributed the “snowball earth” event 2.3 billion years ago to cosmic ray fluctuations resulting from passing through the arm of the Milky Way (3205.1).



### 33Response (3-37): With respect to the claim that cosmic rays can explain the “snowball earth” event, and other cosmoclimatology claims by Svensmark (2006) and Shaviv (2003), more recent research has indicated

that there are no robust correlations between th e solar system’s passage through spiral arms and paleoclimate research (Overholt et al., 2009). We do not dispute that solar variability has likely had an influence on the millennial timescale, though Jansen et al. (2007) estimate that “the magnitude of the radiative forcing used in Chapter 9 for the Ma under Minimum period is relatively small (–0.2 W m –2
relative to today)” and in general that the “recons tructions of natural forcings (solar and volcanic) are uncertain for this period.”

Comment (3-38): A number of commenters (0219, 0425, 0468, 0591, 1579.1) mention the recent sunspot lull and concurrent low temperatures. Some (3159, 3265, 7026) worry that this lull is the first step towards a Maunder or Dalton minimum (3729.1) and cooling b ased on the projections of Landscheidt (0543), Archibald (0714.3), or Clilverd (3729.1). One comme nter requests that the TSD discuss the possibility of imminent cooling (4003).

Response (3-38): In response to commenters who noted the recent sunspot lull: while a lull in sunspots or solar irradiance may have a small cooling effect on climate, there is no evidence upon which to conclude that the sunspot lull will continue (as in a Maunder minimum). Even the radiative forcing from the Maunder minimum has been estimated to be only 0.2 W/m 2 , which is significantly less than the projected increased forcing from elevated GHG levels. The possibility of orbitally induced reglaciation is also addressed by Jansen et al. (2007), which found that “It is virtually certain that global temperatures during coming centuries will not be significantly influenced by a natural orbitally in duced cooling.” In addition, the IPCC placed the rate of current and projected warming in the context of past changes and found that the present warming is occurring at an unprecedented rate on the global scale, exceeding even that of glacial to interglacial transitions—indeed, according to Jansen et al. (2007), it is possible that the rate of global change projected to occur in the next cen tury would exceed “any comparable global temperature increase of the last 50 million years.” Therefore, it is extremely unlikely that over the next couple of decades that there will occur reductions in solar forcing of sufficient ma gnitude to counteract the projected warming due to anthropogenic GHGs.

We find no evidence that historical variations in sola r forcing, including the recent sunspot lull, are of comparable magnitude to the recent and continuing an thropogenic warming, and c onclude that there is no expectation of large scale near-term cooling. Thus, there is no need for the TSD to address this issue.

Comment (3-39): A few commenters (0717, 4003) noted that there is wa rming on other planets, mentioning Mars, Jupiter, and Pluto, and stated that this was evid ence for the solar cause of global warming.

Response (3-39): The commenters did not provide any peer-reviewed lit erature to support their argument. One indication that Mars is warming was a retreat of the South Polar Cap, but Colaprete et al. (2005) discuss the fact that the South Polar Cap is unstable, and that it is theref ore difficult to extrapolate short-term changes in the cap to a long-term global trend. Martian climate is also influenced by non- solar mechanisms such as positive feedbacks between albedo changes and changes in dust storms (Fenton et al., 2007). Therefore, it is neither clear that Mars is warming nor that th e warming is solar induced. The climate on Jupiter is



### 34dominated by the dynamics of the massive standing vortices on the planet (Marcus, 2004), and solar energy is a less significant contribution to the temperature of Jupiter than it is for Earth. It is also unclear whether the warming of Jupiter is global or regional.

The changes in Pluto’s atmosphere have been detailed in Elliot et al. (2007). The basic atmospheric expansion is well modeled by the frost migrations m odels of Hansen and Paige (1996), without requiring any solar effects beyond Pluto's seasonally changing sub-solar latitude. Because Pluto takes about 248 years to orbit the sun, Pluto’s seasons can be measure d in decades. Finally, Triton is the last commonly referenced “warming” planet. The last good measurements of Triton were in 1998 (Elliot et al., 1998), and the changes in Triton's pressure have been expl ained by the change in Triton's subsolar latitude uncovering polar icecaps to the sun as a result of Triton's obliquity.

Therefore, there is no indication that solar variability is the cause of recent warming on any solar system body. Additionally, the lack of recent observed trends in solar insolation (discussed in responses in this section of the Response to Comments document) makes it implausible that there would be such solarinduced warming trends on solar system bodies.

Comment (3-40): One commenter (3722) submitted Kärner (2002) on an alysis of the nonstationary climate system, emphasizing a dominant role for solar variability.

Response (3-40): Kärner (2002) used satellite data records to show that solar variability was more dominant than elevated

GHG concentrations. As Kärner relied on the Univers ity of Alabama–Huntsville (UAH) satellite dataset before the 2005 corrections which led to a significan t change in temperature trends, we find that the conclusions of the paper are obsolete and do not affect the conclusions of the TSD.

Comment (3-41): Several commenters (3250, 3283.1,) indicate that anot her phenomenon that complicates attribution is the fact that since the 1980s, the incident sunlight in North America and Europe has increased as solar dimming due to aerosols has abated, possibly causing the observed land temperature to increase. One of these commenters (3283.1) submitted Alpert et al. (2005), which studied global dimming over the period 1964-1989. According to the study, globally, there was considerable solar dimming in both rural and metropolitan areas. The study also finds:

I n urban locations the effect was 2.6 times larger.

S ince 1990, much of the Northern Hemisphere h as brightened. This change may be reconciled with at least changes in cloudiness, and atmosphe ric transmission. This is yet another good reason to suggest that the global instrumental temperatur e record may be in error and in a way that has not been adequately appreciated.

The authors conclude the following: “[T]he absence of dimming since the mid-1980s may profoundly

affect surface climate. Whereas the decline in solar energy could have counterbalanced the increase in down-welling long-wave energy from the enhanced gr eenhouse effect before the 1980s, this masking of the greenhouse effect and related impacts may no long er have been effective thereafter, enabling the greenhouse signals to become more evident during the 1990s.” The commenter concludes these data demonstrate the complexity of processes of the lower atmosphere and also the importance of considering various sources of information to provide evidence of a warming climate.



### 35Response (3-41): Although global dimming is a recognized phenomenon, and since passage of the Clean Air Act in 1970 (and the successive amendments) aerosol concentrati ons have decreased—which has led to modest increases in incident sunlight—this phenomenon h as been accounted for in the assessment literature already. The IPCC (Trenberth et. al., 2007) addressed the submitted studies (Alpert et al., 2005; Schwartz, 2005), finding that dimming varies geographically, with the greatest effect in large urban areas. The IPCC agrees that the trend of global dimming seems to have reversed in the early 1990s, possibly due to both direct and indirect (cloud interaction) effects of the reductions in anthropogenic aerosols. Dimming is discussed in Hegerl et al., Denman et al., Trenbert h et al., and CCSP 2.3 (CCSP, 2009a). The changes in anthropogenic aerosol emissions and associated uncer tainties are addressed in detail in the IPCC, CCSP, and UGSCRP attribution studies, which serve as the basis for the TSD’s discussion of attribution. Because these aerosol effects are already included in all the IPCC studies, we conclude that the scientific literature on global dimming does not alter any of the conclusions from the assessment literature, as

summarized in the TSD.

3.2.3 Existence of GHG Effect

Comment (3-42): A commenter (2818) requests that the derivation fo r radiative forcing be presented (mentioning the HITRAN [High-Resolution Transmission Molecula r Absorption] database). Another commenter (10071.2) requests the same data, claiming that infra red radiation only matters for the bottom 800 m of the atmosphere, and therefore the thickness of the full atmosphere should not be used to compute radiative forcing. The commenter suspects that th e forcing numbers are chosen to make the models produce the right results, and that CO 2 cannot cause global warming.

Response (3-42): Radiative forcing for gases is usually calculated within global circulation models using some form of line by line or other radiative code. The IPCC Third Assessment Report includes a discussion of these issues in Chapter 6.3 (Ramaswamy et al., 2001). The c itation for the commonly used approximate expression 5.35 ln(C/C 0 ) (the log of the current concentration of CO 2 over the pre-industrial concentration) is Myhre et al. (1998), which uses three different approaches to calculate radiative forcing (line by line, narrow band, or broad band). Myhre used the HITRAN database (information on that database can be found at http://www.cfa.harvard.edu/hitran// ). Forster et al. also include a discussion on radiative forcing calculations, where they find that “Collins et al. (200 6) performed a comparison of five detailed line-byline models and 20 GCM radiation schemes. The spread of line-by-line model results were consistent with the ±10% uncertainty estimate for the LLGHG RFs (long-lived GHG radiative forcings) adopted in Ramaswamy et al. (2001) and a similar ±10% for the 90% confidence interval is adopted here. However, it is also important to note that these relatively small uncertainties are not always achievable when incorporating the LLGHG forcings into GCMs. For ex ample, both Collins et al. (2006) and Forster and Taylor (2006) found that GCM radiation schemes could have inaccuracies of around 20% in their total LLGHG RF (see also Sections 2.3.2 and 10.2).” GISS Model E is also available publicly (http://www.giss.nasa.gov/tools/modelE/), and therefor e the commenter can easily acquire the actual radiative code for at least one major model, along with the documentation of how the radiative forcing equations are developed from theoretical principles and laboratory experiments, and tested against satellite measurements of atmospheric radiative flux es. We find that these radiative equations match the best available science, and are widely available for scrutiny.

The statement that infrared radiation only matters in the bottom 800 m of the atmosphere is consistent with the scientific literature. Radiative forcing is comm only report as top of the atmosphere forcing, not at



### 36800 m. CO 2 and other GHGs interact with infrared radiati on in the full tropospheric column (and indeed, in the stratosphere as well) both in reality and in the model code.

Comment (3-43): A commenter (3160.1) contends that long-wave infra red (IR) radiation from GHGs cannot heat the ocean as it can only penetrate 1 mm into the surface. The co mmenter also argues that increases in long-wave IR radiation in the upper troposphere at 220°K cannot im pact surface temperatures of 288°K due to the Second Law of Thermodynamics, and that therefore the application of “radiative forcing” concepts to

climate models is flawed.

Response (3-43): We disagree with the assertion by the commenter, which was not supported by any peer-reviewed literature. We find that long-wave IR radiation can and does heat the oceans despite a small direct penetration distance. Increasing the skin temperature of the ocean changes the rate of heat flux into or out of the ocean, thereby leading to ocean warming comp ared to the counterfactual without long-wave IR radiation. An empirical test of this logical assumption was recently performed by Millett (2006), who

showed that incident infrared radiation on the sea su rface modulated the heat flow from the ocean to the atmosphere in a way consistent with this theory. Similarly, increasing high-altitude IR will lead to increases in surface temperatures both due to direct radiative effects and decreased temperature differentials leading to a decrease in outward flux. Th is interaction of high-altitu de radiative interactions and surface temperatures is not in contradiction to the Second Law of Thermodynamics; rather, it is consistent with basic textbook physics, has been u sed throughout the scientific literature, and explains many observed features of the atmosphere (see also response 3-45 on thermodynamic impossibility). Therefore, both theory and experiment contradict the assertion of the commenter.

Comment (3-44): One commenter (3440.1) claims that the blackbody temperature of the Earth due to solar radiation would be 278.5° K, and that therefore the sum total of th e greenhouse effect is only 8.5° K. The commenter goes on to calculate a climate sensitivity parameter as nine-tenths of the surface temperature divided by the

solar irradiance and shows that their calculation of th is parameter is much less than the IPCC estimate. The commenter then uses another approach based on di viding the previously calculated 8.5 degrees by the

100 W/m 2 of greenhouse effect and again calculates a valu e of lambda much lower than the IPCC. The commenter requests that EPA undertake its own study of climate sensitivity, as a crucial parameter in determining endangerment.

Response (3-44): See Section III.A. of the Findings, “The Science on Which the Decisions Are Based” and Volume 1: Section 1, for our response to comments on the use of the assessment literature and our decision that it was not necessary for EPA to conduct an independent assessment of the science. We also find that this comment includes several calculation errors: for exampl e, it does not appear that the commenter took into account the reflective albedo of the Earth (about 0.3) when calculating the blackbody temperature of the Earth to be 278.5K: repeating the calculation with th at albedo yields the more standard 255K for the Earth’s blackbody temperature, leaving 32K for the to tal greenhouse effect. We are unclear as to why the commenter used the equation of blackbody temperatur e over total solar irradiance to calculate a climate sensitivity parameter, but the average W/m 2 solar irradiance on the surface of the Earth would be a more appropriate number to include in that equation than the total solar irradiance at noon at the top of the atmosphere (e.g., multiply by 1⁄4 for the Earth’s area, followed by 0.7 for the albedo effect). Using the commenter’s methodology (which is flawed) the resulting climate sensitivity parameter would be



### 370.9*(255/238) = 0.96° K/W/m 2 . As to the commenter’s calculation of climate sensitivity by diving the greenhouse temperature by the total forcing, as previously shown, the commenter was off by a factor of 4 in his or her calculation of the temperature, and becau se water and clouds are considered to be part of the feedback when calculating sensitivity, it is inappropria te to include them in the forcing side of the equation. CO 2 , CH 4 , N 2 O, and ozone together account for about 35 to 50 W/m 2 (Kiehl and Trenberth, 1997). Therefore, using the commenter’s methodology (which is flawed), 32/42 = 0.76° K/W/m 2 . Both

0.96 and 0.76 are close to the IPCC estimates. Theref ore, we find that the commenter has provided no convincing evidence to counter the assessments of climate sensitivity studies in the IPCC and subsequent

assessment reports.

Comment (3-45): A number of commenters believe that anthropoge nic global warming is impossible, many citing arguments made by Gerlich and T scheuschner (2009). Several commenters (e.g., 0430) note that the greenhouse effect is not like a real greenhouse. Seve ral claim that it is thermodynamically impossible because heat cannot be transferred from a cool substance to a warmer substance (0430, 2210.5): for example, blankets cannot make you warmer th an body temperature (1707, 0183.1,). Another thermodynamic argument for the impossibility of the greenhouse effect was proposed by two commenters (2210.3, 4509) citing Gerlich and Tscheuschner ( 2009) who states that the greenhouse effect as commonly formulated violates the Second Law of Thermodynamics. Another commenter (0711.1) requests evidence of any peer-reviewed climate change paper that does not rely on computer simulation. Another theory (2887.1) holds that long-wave radiation will cause increased evaporation of the surface ocean, negating any heat increase. One commenter (0535) submitted a non-peer reviewed paper providing a different explanation for the net energy budget of the Earth, with no role for warming by CO 2

.

Response (3-45): The evidence for the atmospheric greenhouse effect is well supported by the scientific literature. The objections raised by a number of commenters to the basic thermodynamics are without grounds. We are well aware that the greenhouse effect is not at all like a real greenhouse. However, the analogy of a blanket is a little bit better: and indeed, sufficien tly insulating blankets can cause overheating. GHGs (blankets) will, by reducing the rate of heat loss, raise the surface temperature of the Earth (body) until a new thermodynamic balance is achieved between inco ming solar radiation (internal body heating) and outgoing thermal radiation (in the case of a blanket, including convection and non-radiative processes). This process works regardless of whether the atmosphere (blanket) is cooler than the surface (body). We are aware of the paper by Gerlich and Tscheuschner, a nd we have determined that the conclusions of the paper are inconsistent with the well-supported lite rature regarding the mechanism of the greenhouse effect. For example, as a disproof of the greenhou se effect, the paper by Gerlich and Tscheuschner presents the example of a pot of water, noting that the bottom of the pot will be cooler if it is filled with water than if it is empty. Contrary to the assertion in the paper, the primary thermal effect of adding water to the pot is not a reduction in heat transfer, but ra ther an increase of thermal mass. We assert that a more appropriate example for the paper to have examined would have been the addition of a lid to a pot of water, which reduces the rate of heat loss, and leads to an increase of heating of the water compared to a case with no lid. The paper by Gerlich and Tscheuschner is also inconsistent with the scientific literature with regards to the interpretation of radiative balance diagrams and the assertion that there is no “mean temperature” of the Earth, in contrast to the hundr eds of peer-reviewed publications and many assessment reports which use both concepts.

Comment (3-46):



### 38Two commenters (0183.1, 1707) submitted links to a non peer-reviewed paper, “Climate Change and Long-Term Fluctuations of Commercial Catches - The Possibility of Forecasting.” The commenters claim that this paper shows that temperature is a function of rotational velocity of the Earth (e.g., length of day),

and the commenters also claim that CO 2 cannot warm the atmosphere because at 400 parts per million (ppm) it would need to reach hundreds of degrees in order to change the temperature of the whole atmosphere by a fraction of a degree. These commenters also claim that water vapor only serves to increase the heat capacity of the atmosphere, thus maki ng it more difficult to warm. They also claim that radiative energy is not additive (e.g., they claim that 101 W/m 2 + 100 W/m 2 = 101 W/m 2 ).

Response (3-46): We disagree with this comment and find that th e commenter misunderstands fundamental scientific processes. The rotational velocity of the Earth can have some effect on temperature; for example, at the extreme, a tide-locked planet will be very hot on the sun side and very cold on the dark side. However, there is no evidence that changes in the velocity have or could cause the observed changes of temperature over the last century.

We also disagree with the commenters’ analysis of radiative properties of CO 2 , which is in contradiction to basic scientific principles. Because CO 2 molecules can transfer absorbed heat to neighboring molecules through collisions, there is no need for them to reach hundreds of degrees Celsius in order to raise the average temperature. A similar argument holds fo r water vapor in the atmosphere: though water does have a high heat capacity, if the water wasn’t in th e atmosphere, it would be in the oceans and would still contribute to the thermal mass of the Earth system . Finally, radiation is i ndeed additive—two equal infrared sources will heat a target more than one source individually.

Comment (3-47): A commenter (2210.1) proposes several reasons that CO 2 does not absorb IR radiation; 1) because it is saturated during the daytime because of the sun, only a millionth of photons of outbound radiation would be captured; 2) because absorption by gases only causes electron excitation and re-ra diation, not heating; and 3) because experiments that show otherwise are cont aminated by heating of surfaces and heat transfer through conduction.

Response (3-47): There is compelling evidence for the absorpti on of IR radiation in the atmosphere by CO 2 as shown by experiment and theory. Satellites and ground-based instruments can measure atmospheric transmission spectra and detect the influence of different atmospheric components, and these measurements are inconsistent with the assertions by the commenter. Also, see response 3-42 describing the derivation of

radiation code in models.

Comment (3-48): A commenter (2818) states that GHGs cool the atmosphe re by 100°C, otherwise the Earth would have the same temperature as the maximum temperature on the moon (121°C). Another commenter (10071.2) claims that “You always hear that GHGs warm the surface by 33° C, but most sources forget to mention that they also cool the atmosphere by more than 100° C. If the atmosphere was just nitrogen, with no greenhouse gases, the temperature of atmosphere woul d be about 121° C (249° F), well above the boiling point of water. It is the greenhouse gases that cool the atmosphere.”



### 39Response (3-48):

The scientific literature is clear that GHGs have a warming effect on the temperature of the Earth. The existence of an atmosphere and oceans explains why the surface of the Earth does not reach the high temperatures reached by the sun-side of the moon; this is not a function of GHGs. The atmosphere and oceans also serve to keep the night -side of the Earth from reaching the –233°C that the moon reaches. The average temperature of the moon is actually less than the average temperature of the Earth. The average temperature of the Earth is 33°C higher than the theoretical blackbody temperature, and the difference is attributed mainly to GHGs. Venus (see response 3-49) is another example demonstrating that GHGs do not cool the surface. Therefore, we disagree with the assertion of the commenter that GHGs are a cooling

influence.

Comment (3-49): A commenter (2210.1) states that Venus is not an exam ple of the greenhouse effect but is merely warmer

because it is closer to the sun. Another comment er (2210.5) attributes Venus' warmth to higher atmospheric pressure because compression causes temper ature increases (for exampl e, this occurs when inflating a bicycle tire, due to the proportional relationship between pressure and temperature represented in the ideal gas law, pV=nRT, i.e., pressure times volume equals amount of gas times temperature times a constant), and that a 95% CO 2 atmosphere is actually cooler than a 100% biatomic atmosphere would be.

Response (3-49):

Venus is warmer than the Earth both because of the greenhouse effect and because of its distance to the sun; in contrast, Mercury is cooler than Venus d espite being even closer to the sun. Were Venus’ atmosphere to be transparent to radiation, then the surface temperature of Venus would be determined only by the blackbody radiation of the surface, and the pressure of the atmosphere would not change this equilibrium temperature. There is a large body of literature on Venus’ climate; one example is Bullock and Grinspoon (2001)—all of which show that CO 2 is a significant contributor to the planet’s warmth. Because volume is not held constant, it is not appr opriate to use the ideal gas law to determine the temperature on the surface of Venus based only on know ledge about its pressure. Therefore, the scientific literature shows clearly that the temperature of Venus is an example of a greenhouse effect, in contrast to the assertion by the commenters.

Comment (3-50):

The commenter states the CO 2 does not cause reflection because a boundary between heterogeneous substances is necessary for reflection or refraction, and because any heating of the air would cause it to rise and therefore cool as it expands: therefore, th e greenhouse effect can only occur in stationary atmospheres. Also, the commenter argues that gases only radiate at the upper boundary layer of the troposphere: below that, they only transfer energy through conduction and convection.

Response (3-50): The mechanism of warming by CO 2 is not reflection, but rather absorp tion of infrared radiation, followed either by radiation or collisional energy transfer mechanisms. The expansion and cooling resulting from the rise of warm gases is reflected in the temperature profile of the atmosphere (e.g., the adiabatic lapse rate). However, in the case of absorbing heat from an ex ternal source, even if the particular packet of gas that absorbed the heat rises and cools, there must be a corresponding packet of gas which sinks and warms, leading to warming of the atmosphere regard less. Therefore, the critiques of the commenter are

not consistent with scientific understanding.



### 40Comment (3-51): One commenter (3013) mentions Mars and states that even given the lower atmospheric pressure, his

calculation shows that it has twice as much CO 2 as Earth, and further claims that the National Aeronautics and Space Administration (NASA) found that the even with this high CO 2 concentration, the atmosphere

of Mars does not retain heat.

Response (3-51): The commenter did not provide any evidence for his cl aim that NASA has found that the atmosphere of Mars does not retain heat. Mars does have an atmosphere that is mainly carbon dioxide, but because of the low atmospheric pressure, it is only 16 times the quantity of carbon dioxide on Earth. At the same time, it receives less than 45% as much sunlight , so the increased radiative forcing from the CO 2 is not sufficient to make up for the decrease in solar in solation. Thus, the commenter’s claim that the atmosphere of Mars does not retain heat is not consistent with the scientific literature.

3.2.4 CO 2 and Past Global Warming Episodes

Comment (3-52): A large number of commenters (e.g., 0169, 0373.1, 0401, 0455, 0482.1, 0591, 0639.1, 0661, 0664, 0700.1, 0921, 1017.1, 1309.1, 1312, 1519, 1616.1, 1961, 2666, 2759, 2890.1, 2952.1, 2980, 3013, 3497.1, 3596.1, 3679.1, 3722, 3769.1, 3722R87, 4003, 4206, 4632R48, 5858, 6712, 7025, 10197) argue that atmospheric CO 2 concentrations were not the cause of past periods of global warming and are therefore not the cause of the current period of warming.

Numerous commenters (e.g., 0153, 0169, 0373.2, 0455, 0482.1, 0525, 0534, 0591, 0639.1, 0661, 0664, 1309.1, 1312, 1519, 2666, 2759, 2890.1, 2952.1, 2980, 3013, 3497.1, 3596.1, 3679.1, 3722, 3769.1, 4003, 3722R87, 4632R48, 9799.1, 10197) similarly state that increased CO 2 concentrations lag temperature change and that atmospheric CO 2 concentrations did not drive past periods of climate change but rather were an effect of temperature increases caused by other mechanisms. A document written by Alan Carlin and provided by commenter 4003, for example, stat ed: “Logic demands that cause must precede effect. Increases in air temperature drive increases in atmospheric CO 2 concentration, and not vice versa.”

One commenter (3596.1) stated, “Studies identify periods of time when CO 2 levels were two to four times higher than the current level (Pagani et al., 2005) , and these carbon dioxide spikes followed increases in temperature by hundreds or thousands of years. This contradicts EPA’s prime assertion that carbon dioxide and the other specified greenhouse gases drive warming.”

Response (3-52): Although the assessment literature (see, for example, Jansen et al., 2007) indicates that the primary initiator of past periods of warming over at least the past 3 million years was orbital forcing, which refers to changes in the seasonal and latitudinal distributio n of incoming solar radiation linked to regular

variations in the Earth’s orbit around the sun. CO 2 and GHGs were an important amplifier of these past periods of global warming. As stated in Jansen et al., “It is very likely that glacial-interglacial CO 2 variations have strongly amplified climate variations, but it is unlikely that CO 2 variations have triggered the end of glacial periods. Antarctic temperature star ted to rise several centuries before atmospheric CO 2 during past glacial terminations.” Jansen et al. note that the forcing from GHGs during the last glacial

maximum was 2.8 W/m 2 lower than the preindustrial era, compared to 3.2 W/m 2 of cooling from reflective ice sheets, and about 1 W/m 2 of cooling from dust, and 1 W/m 2 of cooling from vegetation changes. Therefore, GHGs were a significant contribu tor to the temperature difference between glacial



### 41maxima and interglacials. The reason why CO 2 can be an initiator in pr esent times when it was only an amplifier previously is that previously CO 2 concentrations changed only in response to climatic changes, but in the current period, CO 2 concentrations are changing due to human emissions. CO 2 serves as both a cause and an effect. Therefore, we disagree that CO 2 and other GHGs cannot be the cause of the current period of warming. Indeed, projected concentrations of GHGs may contribute as much forcing by the end of the century as the total difference in forcing be tween glacial and interglacial periods from all causes

combined.

In order to clarify the relationship between GHGs a nd temperature over geologic time, we have added a

text box to Section 5 of the TSD. It states the followi ng in regard to temperature changes over the past million years in relation to CO 2 concentrations:

According to the IPCC (Jansen et al., 2007): “The ice core record indicates that GHGs co-varied with Antarctic temperature over glacial-interglacial time scales, suggesting a close link between natural atmospheric GHG c oncentrations and temperature.” Evidence strongly suggests that the timings of the glacial-interglacial cycles are paced by the variations in the orbit of the earth; however , the large response of the climate system implies a strong amplification of the initial or bital forcing (Jansen et al., 2007). Jansen et al. (2007) conclude: “It is very likely that glacial-interglacial CO 2 variations have strongly amplified climate variations, but it unlikely that CO 2 variations have triggered the end of glacial periods.” Antarctic temperatures started to rise several centuries before atmospheric CO 2 during past glacial terminations. CO 2 (and other GHG) changes over glacial to interglacial transitions therefore contribute to, but do not initiate, the temperature changes seen.

Regarding the reference to Pagani et al. (2005): The abst ract of Pagani et al. (2005) states: “Our results demonstrate that pCO 2 ranged between 1000 to 1500 parts per million by volume in the middle to late Eocene, then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO 2 likely allowed for a critical expansion of ice sheets on Antarctica...” Therefore, Pagani et al. are effectively st ating that historically, high levels of CO 2 lead to sufficient warming to prevent Antarctic ice sheets from forming. This contradicts the implication of the commenter

that CO 2 cannot drive warming. Neither the underlying assessment literature nor the TSD’s summary of that literature concludes that carbon dioxide initiated, for example, the glacial-interglacial cycles, but rather that the science clearly shows that CO 2 served as a positive feedback during that period.

Comment (3-53): A number of commenters (e.g., 0401, 0921, 1017.1, 2750, 4206) argue that the concentration of CO 2 in the atmosphere and the Earth’s temperature are not curre ntly and never have been correlated, and many of these commenters provided graphs in support of that argument. Commenter 0373.2, for example, provided a graph that, according to the commenter, show s that there is “absolutely no correlation between

CO 2 concentration in the atmosphere and the Earth’s temperature.”

Commenter 3596.1 argues that “global temperatures and atmospheric CO 2 concentrations have not been associated over geologic time.” The commenter c ites a study (Rothman, 2002) that, according to the commenter, shows that for the majority of th e last half of a billion years atmospheric CO 2 concentrations have fluctuated between two and four time present value and stated that at this scale there is no

correlation between increased CO 2 and increased temperature. The commenter stated that EPA should address the geologic temperature record, which, accord ing to the commenter, seems to suggest that a

direct correlation between CO 2 and global temperature may not exist.



### 42Response (3-53): We firmly disagree that atmospheric CO 2 concentrations and the Earth’s temperature do not show correlations. The extensive evidence supporting our conclusion that recent (1950 to present) CO 2 concentrations are causally linked to the observed temp erature record is discussed in Section 3.1 of this volume of our Response to Comment document. EPA ’s responses addressing the correlation between CO 2 and temperature over geologic time are addressed in the response 3-52.

We note that several commenters provided graphs they claimed show that CO 2 and temperature were never correlated in the distant past. The origins of these graphs were generally unclear and no underlying data were provided. This issue has been extensively addressed in the assessment literature, and the commenters have not made a compelling case that atmospheric CO 2 concentrations do not affect the Earth’s temperature. The provided reference of Rothma n (2002) is not consistent with the reconstructions in the assessment literature that are based on multiple independent proxies. Royer et al. (2004) assessed Rothman’s methodology and found that the proxy method used by Rothman did not properly account for interactions of the proxy with terrestrial matte r from rivers, seawater temperatures, or oxygen concentrations, all of which contribute to the proxy signal.

Comment (3-54): Several commenters (e.g., 0339, 3440.1, 7026) note that CO 2 concentrations in the distant past have at times been much higher than at present. Commenter 0639.1, for example, stated: “There have been ice ages when carbon dioxide content has been high in the thousands of part per million.” Commenter 0661

stated that the current concentration of CO 2 is “dwarfed by not only peaks, but by long term concentrations as high as 7000 ppm, which did not cause an end to the Earth”.

One commenter (0541) notes that CO 2 was high during previous a ice age (the late Ordovician) and that therefore it cannot lead to warming; another commenter (3440.1) said that CO 2 was high during the Cambrian and Triassic when calcite and aragonite corals formed and therefore corals can survive acidification; and another (0639.1) asked if raising CO 2 by 100 ppm caused 1 degree warming, why 5000 ppm concentrations in the past didn't cause 40 de grees of warming and kill everything. Commenter 2914 argued that higher CO 2 levels existed in the past and the Ea rth has survived dramatic changes in atmospheric composition such as the Precambrian appear ance of oxygen, demonstrating that it is unlikely

that the world will end.

One commenter (11454.1) provides a quote from Heaven & Earth (Plimer, 2009) on historical CO 2 concentrations: “The current CO 2 content of the atmosphere is the lowest it has been for thousands of millions of years and life (including human life) has thrived at times when CO 2 has been significantly higher.” Another commenter (3394.1) objected that the statement that CO 2 concentrations exceed the natural range for the past 650,000 years is not supp orted by the cited Chapter 2 of Working Group 1.

Response (3-54): Although GHG concentrations in the distant past have substantially exceeded curr ent levels, the existence of high GHG concentrations in the very distant past does not demonstrate that there are not negative consequences of high concentrations in the present, as addressed in the assessme nt literature. First, the current atmospheric concentrations of CO 2 and CH 4 very likely exceed by far the natural range of at least the last 650,000 years (Jansen et al., 2009), and at least 800,000 years for CO 2 specifically (Karl et al., 2009) (see responses on historical CO 2 concentrations in Volume 2) meaning that concentrations as high as the present may not have existed during the time in which anatomically modern humans have existed (in contrast to the quote from Plimer, for which no evidence was provided). Second, as a consequence of these elevated concentrations, the combined rate of increase in positive radiative forcing due to CO 2 , CH 4 , and N 2 O is very likely unprecedented in at least the pa st 16,000 years (Jansen et al., 2007). Third, as



### 43discussed in Section 3.1, there exists robust ev idence that the current very high atmospheric concentrations of GHGs in the atmosphere are causally linked to recent climate changes.

It is important to note that past climates associated with present-day level GHG concentrations were much warmer than today. According to the IPCC, the Mid-Pliocene (about 3.3 million to 3 million years ago), for example, was characterized by a climate s ubstantially warmer than our own. Global mean temperatures for this time period are estimated by clim ate models to have been about 2 to 3°C (3.6 to 5.4°F) above pre-industrial levels, “providing an accessi ble example of a world that is similar in many respects to what models estimate could be the Earth of the late 21 st century” (Jansen et al., 2007). Furthermore, a very recent study published in Science (Tripati et al., 2009) indicates that CO 2 concentrations have not reached as high as current le vels for a sustained period in at least 15 million years. The study found that wh en the partial pressure of CO 2 was last sustained at levels similar to its current level during the Middle Miocene (approximately 14 million to 20 million years ago) temperatures

were 3 to 6  ̊ C warmer and sea level 25 to 40 meters higher than at present.

With respect to the comment that very high levels of atmospheric GHG concentrations did not “cause an end to the Earth,” we note that the “end to the Earth” is not an issue for this finding. Rather, this finding is

about whether elevated GHG concentrations are r easonably anticipated to endanger public health or

welfare.

Finally, for EPA’s response to comments on the relati onship between coral health and ocean acidification and past CO 2 levels please see Volume 7 of the Response to Comments document.

Comment (3-55): One commenter (3136.1) contends that “[t]his is not th e first time that the climate was this warm in the Holocene.” The commenter argues that a study by Kaufman et al. (2004) found that for 2,000 years, from 9,000 to 10,000 years ago, Alaskan temperatures averaged 3  ̊ F higher, and that there have been three similarly warm periods in Alaska between the years 0 to 300, 850 to 1200, and 1800 to present. The commenter further argued that “Thompson Webb III et al . (1998) found timings si milar to MacDonald et al (2000): northwestern and northeastern North America were more than 4  ̊ F warmer than the baseline from 7,000-9,000 and 3,000-5,000 years ago, respectively... The lack of historical perspective in the TSD reduces its credibility as a basis for an endangerment finding.”

Response (3-55): The IPCC (Jansen et al., 2007) assessed both the Kaufman and MacDonald studies referenced by the commenter. On the basis of these and other studies, IPCC found that there is evidence for local multicentennial periods warmer than the last decades by up to several degrees in the early to mid-Holocene. Although temperatures in some regions were warmer than present during earlier parts of the Holocene, the IPCC additionally noted that these local warm periods were very likely not globally synchronous and that that the tendency for high-latitude summer temper ature maxima to occur ear ly in the Holocene (8,000 to 10,000 years ago) points to a direct influence of orbital forcing. Jansen et al. (2007) find: “When forced by 6 ka [kiloannum] orbital parame ters, state-of-the-art coupled c limate models and EMICs [Earth System Model of Intermediate Complexity] capture reconstructed regional temp erature and precipitation changes...whereas simulated global mean temperatures remain essentially unchanged (<4  ̊ C; MassonDelmotte et al., 2005b), just as expected from the seasonality of the orbital forcing.”

In any case, the TSD summarizes the assessment science regarding the si gnificant uncertainties associated with large-scale surface temperature reconstructions. With specific regard to temperatures from earlier parts of the Holocene, we report the following in the TSD (see Box 5.1): “According to the IPCC (Jansen



### 44et al., 2007), current data limitations limit the ability to determine if there were multi-decadal periods of global warmth compared to the last half of the 20 th century prior to about 1000 years ago.”

Comment (3-56): Commenter (3535) submitted a reference to Soon et al. (2007) on “the secondary role of CO 2 and methane forcing.” According to the commenter, Dr. Soon asked that EPA consider several of his papers.

Response (3-56): The paper by Soon et al. allocates a secondary role to CO 2 and CH 4 in explaining glacial-interglacial responses. That GHG changes lagged the start of te mperature change during the glacial-interglacial transitions is fairly non-controversial: as described in response 3-52, changes in GHGs likely amplify changes initiated by snow and ice retreat triggered by orbitally caused changes in patterns of solar insolation. The paper by Soon offers no compelling counterargument to the standard explanation. Soon’s conclusion that “there is no quantitative evidence that varying levels of minor GHGs like CO 2 and CH 4 have accounted for even as much as half of the rec onstructed glacial-interglacial temperature changes” is

not inconsistent with the estimate in the IPCC chapter on paleoclimate, which calculates that CO 2 and

CH 4 accounted for a radiative perturbation of 2.8 W/m 2 for the last glacial maximum compared to 5.2

W/m 2 from other sources such as ice sheets, vegetation changes, and aerosols. Soon critiques Figure 6.5 from the IPCC for being misleading by showing CO 2 and CH 4 to be large compared to orbitally moderated global radiative influences, but the text of th e chapter is clear that the key orbital moderation is of seasonal hemispheric insolation not global insolation. Therefore, the paper by Soon does not change any conclusions summarized in the TSD.

Comment (3-57): A commenter (11454.1) provided a quote from Heaven & Earth (Plimer, 2009) that Little Ice Age ended in 1850, so temperature rise after that point is to be expected, and “the post-1850 temperature rise cannot be related to post-1850 industrialization and must be related to natural processes.” Other commenters (e.g., 2890, 3432.1, 3 446.1) made similar comments that the recent warming can be explained as being a recovery from the little ice age. One commenter (3722) provided an analysis by Dr. Akasofu that explained recent temperature rise by attributing a c onstant half-degree rise per century since 1800, and possibly since 1600, to natural causes. Dr. Akasofu st ated that the linear nature of the rise indicates natural variability.

Response (3-57): We disagree with the commenters that warming in th e past 50 years can be explained as a recovery from the little ice age. The IPCC addresses various reconstructions of temperature change in the last several centuries in Jansen et al. (2007). A pertinent chart is presented on page 477, showing various forcing reconstructions over time, and simulated NH (northern hemisphere) temperatures based on those forcings. The clear message is that temperature change is b ased both natural and anthropogenic factors. From 1850 to 1950, there was a decrease in volcan ic activity, an increase in solar ra diative forcing, and an increase in forcing due to long-lived gases from anthropogenic activities (presumably mostly land-use change and agriculture during that period). The combination of these three factors contributed to increasing temperature during that era. However, since 1950, as stat ed in the TSD, “it is very unlikely that the global pattern of warming observed during the past half century is due to only known natural external causes (solar activity and volcanoes) since the warming o ccurred in both the atmosphe re and ocean and took place when natural external forcing factors would likel y have produced cooling (Hegerl et al., 2007).” This conclusion is also based on spatial patterns of ch ange, and the concurrent c ooling of the stratosphere.



### 45The IPCC includes the caveat that “It is important to note that many of the simulated temperature variations during the pre-industrial period shown in Figure 6.13 ha ve been driven by assumed solar forcing, the magnitude of which is currently in dou bt.” However, there exis t reasonable explanations, which are consistent with natural forcing contribu ting significantly to the warming from 1850 to 1950, while there do not exist well-supported explanations that attribute the warming from 1950 to present mainly to natural changes. The concept of “recovery ” seen in a number of comments is not pertinent: over long timescales, the Earth’s climate system shifts in response to changes in forcing, whether anthropogenic or natural. What appears to be “rec overy” is actually the response to a new shift in these forcings. The time scale of such a response is expected to occur most quickly immediately after the shift in forcing, with diminishing changes over time as the system reaches a new equilibrium. Long-term unforced linear trends in temperature are not an expect ed feature of natural variability, contrary to the assertion by a commenter.

Thus, there is no well-supported evidence that the r ecent warming can be explained as a recovery from the little ice age.



### References

Akasofu, S.I. (2007) Is the Earth still recovering from the “Little Ice Age”? Intern ational Arctic Research Center, University of Alaska Fairbanks. http://www.iarc.uaf.edu/highlights/2007/ak asofu_3_07/Earth_recovering_from_LIA_R.pdf

.

Allen, R.J., and S.C. Sherwood (2008). Warming maximum in the tropical upper troposphere deduced

from thermal wind observations. Nature Geoscience 1:399-403.

Alpert, P., P. Kishcha, Y.J. Kaufman, and R. Sc hwarzbard (2005). Global dimming or local dimming? Effect of urbanization on sunlight availability. Geophys. Res. Lett.

, 32 , L17802,

doi:10.1029/2005GL023320.

Arblaster, J.M., and G.A. Meehl (2006). Contributions of external forcings to southern annular mode

trends. J. Climate 19 : 2896-2905.

Archibald, D. (2007). The Past and Future of Climate . Presented to the Lavoisier Group 2007 Workshop, Melbourne, Australia, 29-30 June, 2007

Ashworth, R.A. (2008). CFC Destruction of Ozone—Major Cause of Recent Global Warming!

Atomic and Molecular Physics Division, Harvard- Smithsonian Center for Astrophysics (2009). The

HITRAN database. http://www.cfa.harvard.edu/hitran/ . Accessed December 5, 2009.

Aydin, M., W.J. De Bruyn, and E.S. Saltzman (200 2). Preindustrial atmospheric carbonyl sulfide (OCS)

from an Antarctic ice core. Geophysical Research Letters 29. doi:10.1029/2002GL014796.

Barnett, T.P., et al. (2008). Human-induced changes in the hydrology of the Western United States.

Science 319:1080-1083.

Boucher, O., and J. Haywood (2001). On summing the components of radiative forcing of climate change. Clim. Dyn. 18:297-302.

Bullock, M.A., and D.H. Grinspoon (2001). The recent evolution of climate on Venus. Icarus 150:19-37.



### 46Carlin, A. (2009). Proposed NCEE Comments on Draft Technical Support Document for Endangerment Analysis for Greenhouse Gas Emissions under the Clean Air Act.

Carslaw, K. (2009). Atmospheric physics: Cosmic rays, clouds, and climate. Nature 460:332-333.

doi:10.1038/460332a.

Casola, J.H., L. Cuo, B. Livneh, D.P. Lettenmaier , M.T. Stoelinga, P.W. Mote, and J.M. Wallace (2008). Assessing the impacts of global warming on snowpack in the Washington. J. Climate 22:2758-2772.

CCSP (2007). The First State of the Carbon Cycle Report (SOCCR): The North American Carbon Budget and Implications for the Global Carbon Cycle . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [K ing, A.W., L. Dilling, G.P. Zimmerman, D.M. Fairman, R.A. Houghton, G. Marland, A.Z. Rose, a nd T.J. Wilbanks (eds.)]. Asheville, NC: National Oceanic and Atmospheric Administration, National Climatic Data Center. 242 pp.

CCSP (2008). Reanalysis of Historical Climate Data for K ey Atmospheric Features: Implications for Attribution of Causes of Observed Change. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Randall Dole, Martin Hoerling, and Siegfried Schubert (eds.)]. Asheville, NC: National Ocean ic and Atmospheric Administration, National Climatic Data Center. 156 pp.

Christy, J.R., W.B. Norris, R.W. Spencer, and J.J. Hnilo (2007). Tropospheric temperature change since 1979 from tropical radiosonde and satellite measurements, J. Geophys. Res . 112: D06102.

doi:10.1029/2005JD006881.

Chylek, P., C.K. Folland, G. Lesins, M.K. Dubey, and M. Wang (2009). Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation. Geophys. Res. Lett . 36: L14801.

doi:10.1029/2009GL038777.

Clark, I., J. D’Aleo, C. Essex, C.D. Idso, O. Kärn er, M. Khandekar, W. Kininmonth, and R.C. Willson. (2009). Critical Topics in Global Warming: Supplementary Analysis of the Independent Summary for Policymakers. Fraser Institute.

Clement, A.C., R. Burgman J.R. Norris (2009). Observational and Model Evidence for Positive LowLevel Cloud Feedback, Science , 325 (5939), 460-464. 

### DOI: 10.1126/science.1171255.

Clilverd, M.A., E. Clarke, T. Ulich, H. Rishbeth, and M. J. Jarvis (2006). Predicting solar cycle 24 and beyond. SpaceWeather 4: S09005. doi:10.1029/2005SW000207.

Colaprete, A., et al. (2005). Albedo of the South Pole of Mars. Nature 435:184-188.

Cullen NJ, T. Mölg, G. Kaser, K. Hussein, K. Ste ffen, D.R. Hardy (2006). Kilimanjaro glaciers: Recent aerial extent from satellite data and new interpretation of observed 20th century retreat rates. Geophysical

Research Letters 33: L16502, DOI: 10.1029/2006GL027084. doi:10.1029/2005GL023740.

Davis, J.C., and G.C. Bohling (2001). The search for patterns in ice-core temp erature curves. In: L.C. Gerhard, W.E. Harrison, and B.M. Hanson, eds. Geological Perspectives of Global Climate Change. pp. 213-229.

Dessler, A.E., and S. Sherwood (2009). Atmospheric science: A matter of humidity. Science 323:1020-

1021.



### 47Douglass, D.H., J.R. Christy, B.D. Pearson and S.F. Singer (2007). A comparison of tropical temperature trends with model predictions. International Journal of Climatology 28:1693-1701.

Duane, W.J., N.C. Pepin, M.L. Losleben, and D.R. Hardy (2008). General characteristics of temperature and humidity variability on Kilimanjaro, Tanzania. Arctic, Antarctic, and Alpine Research 40:323-334.

Duplissy, J., et al. (2009). Results from the CERN pilot CLOUD experiment, Atmos. Chem. Phys.

Discuss . 9:18235-18270.

Easterling, D., and M. Wehner (2009). Is the climate warming or cooling? Geophysical Research Letters

36: L08706. doi:10.1029/2009GL037810.

Eichler et al. (2009). Temperature response in the Altai region lags solar forcing. Geophys. Res. Lett. 36:

L01808. doi:10.1029/2008GL035930.

Elliot et al. (2007). Changes in Pluto's atmosphere: 1988-2006. Astronomical Journal 134:1.



### Elliot, J. L., et al. (1998), Nature , 393, 765

Elsner, J.B., J.P. Kossin, and T.H. Jagger (2008). The increasing intensity of the strongest tropical cyclones. Nature 455(7209):92-95.

Erlykin, A.D., T. Sloan, and A.W. Wolfendale (2009). The search for cosmic ray effects on clouds. Journal of Atmospheric and Solar-Terrestrial Physics 71: 955-958.

Fenton, L.K., P.E. Geissler , and R.M. Haberle (2007) . Global warming and cli mate forcing by recent albedo changes on Mars. Nature 446:646-649.

Flanner, M.G., C.S. Zender, J.T. Randerson, and P.J. Rasch (2007). Present-day radiative forcing and climate response from black carbon in snow. J. Geophys. Res . 112: D11202. doi:10.1029/2006JD008003.

Forest, C.E., P.H. Stone, and A.P. Sokolov (2008). C onstraining climate model parameters from observed 20th century changes. Tellus 60(a):5, 911-920. doi:10.1111/j.1600-0870.2008.00346.x.

Gerlich, G., and R.D. Tscheuschner (2009) . Falsification of the atmospheric CO 2 greenhouse effects within the frame of physics. International Journal of Modern Physics B 23(3):275-364.

Gerten, D., Y. Luo, G. le Maire, et al. (2008). M odelled effects of precipitation on ecosystem carbon and water dynamics in different climatic zones. Global Change Biology 14(10):2365-2379.

Gillett, N.P., and D.W.J. Thompson (2003). Simulati on of recent Southern Hemisphere climate change.

Science 302:273-275.

Gillett et al. (2008). The Attribution of Polar Warming to Human Influence.

Haimberger, L., C. Tavolato, and S. Sperka (2008). Towards elimination of the warm bias in historic radiosonde temperature records—some new results fr om a comprehensive intercomparison of upper-air

data. J Climate 21(18):4587-4606. doi:10.1175/2008JCLI1929.1.

Hansen, C.J., and D.A. Paige (1996). Seasonal nitrogen cycles on Pluto. Icarus 120:247-265.

Hansen, J., et al. (2005). Earth’s energy imbalance: Confirmation and implications. Science 308:1431-

1435. doi:10.1126/science.1110252.



### 48Hartmann, B., and G. Wendler (2005). On the significance of the 1976 Pacific climate shift in the climatology of Alaska. Journal of Climate 18:4824-4839.

Hegg, D.A., S.G. Warren, T.C. Grenfell, S.J. Dohe rty, T.V. Larson, and A.D. Clarke (2009). Source

attribution of black carbon in arctic snow. Environ. Sci. Technol . 43:4016-4021. doi:10.1021/es803623f.

Hu, A., G.A. Meehl, W. Han, and J. Yin (2009). Tr ansient response of the MOC and climate to potential melting of the Greenland Ice Sheet in the 21st century. Geophys. Res. Lett. 36: L10707.

doi:10.1029/2009GL037998.

Kärner, O. (2002). On nonstationarity and antipersistency in global temperature series. J Geophys Res 107(D20):ACL1-1-1-11.

Karpechko, A. Yu., et al. (2008). Stratospheric in fluence on circulation changes in the Southern Hemisphere troposphere in coupled climate models. Geophysical Research Letters 35: L20806. doi:10.1

029/2009GL03 5354.

Kaser, G., D.R. Hardy, T. Mölg, R.S. Bradley, and T.M. Hyera (2004). Modern glacier retreat on Kilimanjaro as evidence of climate change: Observations and facts. International Journal of Climatology

24:329-339.

Kaufman, D.S., et al., 2004. Holocene thermal maximum in the Western Arctic (0 to 180W). Quaternary

Science Reviews , 23, 529-560.

Kiehl, J.T., and K.E. Trenberth (1997). E 

### arth's annual global mean energy budget

. Bull. Amer.

Meteor. Soc. 78:197-208.

Kuhn, U., and J. Kesselmeier (2000). Environmental variables controlling the uptake of carbonyl sulfide by lichens. Journal of Geophysical Research 105:26,783-26,792.

Klyashtorin, L.B., and A.A. Lyubushin (2003). On the coherence between dynamics of the world fuel consumption and global temperature anomaly. Energy & Environment 14(6):773-782.

Knutson, T.R., J.J. Sirutis, S.T. Garner, G.A. Vecchi , and I. Held (2008). Simula ted reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience , 1(6), 359-364.

Krivova, N.A., S.K. Solanki, and T. Wenzler (2009). ACRIM-gap and total solar irradiance revisited: Is

there a secular trend between 1986 and 1996? Geophys. Res. Lett. ,36: L20101.

doi:10.1029/2009GL040707.

Landscheidt, T. New Little Ice Age Instead of Global Warming? Schroeter Institute for Research in cycles of Solar Activity.

Lean, J.L., and D.H. Rind (2009). How will Earth's surface temperature change in future decades? Geophys. Res. Lett. 36: L15708. doi:10.1029/2009GL038932.

Lockwood, M., and C. Fröhlich (2008). Recent oppositely directed trends in solar climat e forcings and the global m ean surface air temperature. Proc. R. Soc. A 464:1367.

Loehle, C. (2004). Climate change: Detection and a ttribution of trends from long-term geologic data. Ecological Modelling 171:433-450.



### 49MacDonald, G.M., et al. (2000). Holocene treeline history and climate change across northern Eurasia. Quaternary Research 53:302-311.

Marcus, P. (2004). Prediction of a global climate change on Jupiter. Nature 428:828-831.

doi:10.1038/nature02470.

McCabe, G., M. Palecki, and J.L. Betancourt (200 4). Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proc. Natl. Acad. Sci. 101:4136-4141.

McConnell, J.R., R. Edwards, G.L. Kok, M.G. Flanne r, C.S. Zender, E.S. Saltzman, J.R. Banta, D.R. Pasteris, M.M. Carter, and J.D.W. Kahl (2007). 20th-century industrial black carbon emissions altered arctic climate forcing. Science 317:1381-1384.

Meehl, G.A, J.M Arblaster, K. Matthes, F. Sassi, and H. van Loon (2009). Amplifying the Pacific climate system response to a small 11 year solar cycle forcing. Science 325:1114-1118.

doi:10.1126/science.1172872.

Millett, P.J. (2006). Linking thermal skin gradients at the sea-surface to the radiative coupling of the atmosphere and ocean: A mechanism for heati ng of the oceans by atmospheric greenhouse gases. Eos

Trans

. AGU 87(36), Jt. Assem. Suppl.: Abstract A44A-02.

Miskolczi, F.M. (2007). Greenhouse effect in semi-transparent planetary atmospheres. Id ő járás 111:1-40.

Mölg, T., N.J. Cullen, D.R. Hardy, G. Kaser, and L. Klok (2008). Mass balance of a slope glacier on Kilimanjaro and its sensitivity to climate. International Journal of Climatology 28:881-892.

Mölg, T., N.J. Cullen, D.R. Hardy, M. Winkler, and G. Kaser (2009). Quantifying climate change in the tropical midtroposphere over East Africa from glacier shrinkage on Kilimanjaro. Journal of Climate

22:4162-4181. doi:10.1175/2009JCLI2954.1.

Murphy, D.M., S. Solomon, R.W. Portmann, K.H. Rosenlof, P.M. Forster, and T. Wong (2009). An observationally based energy balance for the Earth since 1950. J. Geophys. Res. 114: D17107.

doi:10.1029/2009JD012105.

Myhre, G., E. J. Highwood, K. P. Shine, and F. Stor dal (1998), New estimates of radiative forcing due to well mixed greenhouse gases, Geophys. Res. Lett. , 25(14), 2715–2718.

Nagashima, T., H. Shiogama, T. Yokohata, S. A. Crooks, and T. Nozawa (2006). The effect of carbonaceous aerosols on surface temperature in the mid twentieth century. Geophysical Research Letters

33: L04702. doi:10.1029/2005GL024887.

Overholt, A.C., et al. (2009). Testing the link between terrestrial climate change and galactic spiral arm

transit. ApJ 705:L101-L103.

Pagani, M., J. Zachos, K.H. Freeman, S. Bohaty, a nd B. Tipple (2005). Marked change in atmospheric carbon dioxide concentrations during the Oligocene. Science 309:600-603.

Paltridge, G., A. Arking, and M. Pook (2009). Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data. Theoretical and Applied Climatology. doi:10.1007/s00704-009-0117-x.

Pierce, D.W., et al. (2008). Attribution of declining western U.S. snowpack to human effects. J. Climate

21:6425–6444.



### 50Pierce J.R., and P.J. Adams (2009). Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophys. Res. Lett. 36: L09820. doi:10.1029/2009GL037946.

Pittock, B. (2009). Can solar variations explain varia tions in the Earth’s climate? An editorial comment. Climatic Change 96:483-487. doi:10.1007/s10584-009-9645-8.

Plimer, I. (2009). Heaven and Earth: Global Warming: The Missing Science. Ballan: Connor

Quinn, P.K., T.S. Bates, E. Baum, N. Doubleday, A.M. Fiore, M. Flanner, A. Fridlind, T.J. Garrett, D. Koch, S. Menon, D. Shindell, A. Stohl, and S.G. Warren (2008a). Short-lived pollutants in the Arctic: Their climate impact and possible mitigation strategies. Atmos. Chem. Phys . 8:1723-1735. http://www.atmos-chem-phys.net/8/1723/2008/

.

Quinn, P.K., T.S. Bates, E. Baum, T.C. Bond, J.F. Burkhart, A.M. Fiore, M. Flanner, T.J. Garrett, D. Koch, J.R. McConnell, D. Shindell, and A. Stohl (2008b). The Impact of Short-Lived Pollutants on Arctic

Climate. AMAP Technical Report No. 1. Oslo, Norway: Ar ctic Monitoring and Assessment Programme (AMAP).

Philander, S.G. (1998). Is the Temperature Rising? Princeton, NJ: Princeton University Press.

Reichert, B.K., L. Bengtsson, and J. Oerlemans (2002). Recent glacier retreat exceeds internal variability.

J. Climate 15(21):3069-3081.

Ramanathan, V., and G. Carmichael (2008). Global and regional climate changes due to black carbon.

Nature Geoscience 1(4):221-227.

Ramaswamy, V., et al. (2001). Chapter 6.3. In: F. Joos et al., eds. Climate Change 2001: The Scientific

Basis. Contribution of Working Group I to the Third A ssessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom, a nd New York, NY: Cambridge University Press.

Revkin, A. (2008). What’s up with volcanoes under Arctic sea ice? Dot Earth. http://dotearth.blogs.nytimes.com/2008/07/01/wh ats-up-with-volcanoes-under-arctic-sea-ice/ . Accessed December 5, 2009.

Rothman, D.H. (2002). Atmospheric carbon dioxide levels for the last 500 million years. Proceedings of the National Academy of Sciences USA 99:4167-4171.

Royer, D.L., R.A. Berner, I.P. Montañez, N.J. Tabor, and D.J. Beerling (2004). CO 2 as a primary driver

of the Phanerozoic climate. GSA Today 14(3):4-10.

Santer, B.D., et al. (2008). Consistency of modele d and observed temperature trends in the tropical troposphere. International J. Climatology. doi:10.1002/joc.1756.

Scafetta, N. (2009). Empirical analysis of the solar contribution to global mean air surface temperature change. Journal of Atmospheric and Solar-Terrestrial Physics. doi:10.1016/j.jastp.2009.07.007.

Scafetta, N., and R. Willso (2009). ACRIM-gap and tota l solar irradiance (TSI) trend issue resolved using a surface magnetic flux TSI proxy model. Geophysical Research Letters 36: L05701.

doi:10.1029/2008GL036307.

Scafetta, N., and B. West (2008). Is climate sensitive to solar variability? Physics Today March:50-51.



### 51Scafetta, N., and B.J. West (2007). Phenomenological reconstructions of the solar signature in the Northern Hemisphere surface temperature records since 1600. J. Geophys. Res. 112: D24S03.

Scafetta, N., and B.J. West (2006). Phenomenological solar contribution to the 1900–2000 global surface warming. Geophys. Res. Lett. 33: L05708. doi:10.1029/2005GL025539.

Schwartz, R.D. (2005). Global dimming: Clear-s ky atmospheric transmission from astronomical

extinction measurements. J. Geophys. Res. 110: D14210. doi:10.1029/2005JD005882.

Seager, R., A. Tzanova, and J. Nakamura (2009). Dr ought in the Southeastern United States: Causes, variability over the last millennium and th e potential for future hydroclimate change. Journal of Climate

22:5021-5045.

Shaviv, N.J. (2003). The spiral structure of the Milky Way, cosmic-rays and ice-age epochs on Earth. New Astronomy 8:39. doi : 10.1016/S1384-1076(02)00193-8

.

Shindell and Schmidt (2004). Southern Hemisphere climate response to ozone changes and greenhouse gas increases Geophysical Research Letters 31: L18209. doi:10.1029/2004GL020724.

Singer, F.S., and D.T. Avery (2008). Unstoppable Global Warming Every 1500 Years . Rowman and Littlefield Publishers, Inc.

Sive, B.C., R.K. Varner, H. Mao, D.R. Blake, O.W. Wingenter, and R. Talbot (2007). A large terrestrial source of methyl iodide. Geophys. Res. Lett . 34: L17808. doi:10.1029/2007GL030528.

Smythe-Wright, D., S.M. Boswell, P. Breithaupt, R.D. Davidson, C.H. Dimmer, and L.B. Eiras Diaz (2006). Methyl iodide production in the ocean: Implications for climate change. Global Biogeochem. Cycles 20: GB3003. doi:10.1029/2005GB002642.

Soden, B.J, D.L. Jackson, X. Huang, V. Ramaswamy, and M.D. Schwazrzkopf (2005). The radiative signature of upper tropospheric moistening. Science doi:10.1126/science.1115602.

Soon, W. (2007). Implications of the secondary role of carbon dioxide and methane forcing in climate change: Past, present, and future. Physical Geography 28:97-125.

Strack, J.E., R.A. Pielke Sr., and G.E. Liston (20 07). Arctic tundra shrub in vasion and soot deposition: Consequences for spring snowmelt and near-surface air temperatures. J. Geophys. Res . 112: G04S44.

doi:10.1029/2006JG000297.

Stoelinga, M.T., M.D. Albright, and C.F. Mass (2009) . A new look at snowpack trends in the Cascade

Mountains. J. Climate. Submitted.

Stott, P.A., S.F.B. Tett, G.S. Jones, M.R. Allen, W.J. Ingram, and J.F.B. Mitchell (2001). Attribution of twentieth century temperature change to natural and anthropogenic causes. Climate Dynamics 17:1-17.

Sturm, M., T. Douglas, C. Racine, and G. E. List on (2005). Changing snow and shrub conditions affect albedo with global implications. J. Geophys. Res . 110 : G01004. doi:10.1029/2005JG000013.

Su, H., et al. (2008). Variations of tropical uppe r tropospheric clouds with sea surface temperature and implications for radiative effects. J. Geophys. Res . 113: D10211. doi:10.1029/2007JD009624.

Swanson, K.L. (2008). Nonlocality of Atlantic tropical cyclone intensities. Geochemistry, Geophysics, Geosystems 9: Q04V01. doi:10.1029/2007GC001844.



### 52

### 53 Swanson, K.L., and A.A. Tsonis (2009). Has the climate recently shifted? Geophysical Research

Letters 36. doi:10.1029/2008GL037022.

Svensmark, H., and E. Friis-Christensen (1997) . Variation of cosmic ray flux and global cloud coverage—a missing link in so lar-climate relationships. J. Atmos. Sol.-Terr. Phys . 59:1225-1232.

Svensmark, H. (2006). Imprint of gal actic dynamics on Earth’s climate. Astron. Nachr . 327:866-870.

Trenberth, K.E., J.T. Fasullo, and J. Kiehl (2009). Earth’s global energy tropical temperature trends with model predictions. International J. Climatology. doi:10.1002/joc.1651.

Tripati, A.K., C.D. Roberts, and R.A. Eagle (2009). Coupling of CO 2 and ice sheet stability over major climate transitions of the last 20 million years. Science. doi:10.1126/science.1178296.

Tsonis, A.A., K. Swanson, and S. Kravtsov (2007). A new dynamical mechanism for major climate shifts. Geophysical Research Letters 34: L13705. doi:10.1029/2007GL030288.

Vecchi, G.A., K.L. Swanson, and B.J. Soden (2008). Whither hurricane activity? Science 322(5902):687-

689.

von Schuckmann, K., F. Gaillard, and P.-Y. Le Tra on (2009). Global hydrographic variability patterns during 2003–2008. J. Geophys. Res. 114: C09007. doi:10.1029/2008JC005237.

Wang, L., R.M. Moore, and J.J. Cullen (2009). Methyl iodide in the NW Atlantic: Spatial and seasonal

variation. J. Geophys. Res. 114: C07007. doi:10.1029/2007JC004626.

Webb, T., III, et al. (1998). Late quaternary climate change in eastern North America: A comparison of pollen-derived estimates with climate model results. Quaternary Science Reviews 16:587-606.

White, J.W.C., R.B. Alley, A. Jennings, S. Johnsen, G.H. Miller, and S. Nerem (2009). Past rates of climate change in the Arctic. In: Past Climate Variability and Change in the Arctic and at High

Latitudes .” A report by the U.S. Climate Change Program and Subcommittee on Global Change Research. Reston, VA: U.S. Geological Survey. pp. 247-302.

Willis, J.K., D. Roemmich, and B. Cornuelle (2004). Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales. J. Geophys. Res. 109: C12036.

doi:10.1029/2003JC002260.

WMO (2006, November). Statement on Tropical Cyclones and Climate Change. World Meteorological Organization (WMO) International Workshop on Tropical Cyclones, IWTC-6, San Jose, Costa Rica.

Yin, J., M.E. Schlesinger, and R.J. Stouffer (2009) . Model projections of rapid sea-level rise on the

northeast coast of the United States. Nature Geoscience 2:262-266.

Zhang, X., F.W. Zwiers, G.C. Hegerl, et al. (2007) . Detection of human influence on twentieth-century precipitation trends. Nature 448:461-465.