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 6: Impacts and Risks to 

Public Health and Welfare: 

Agriculture and Forestry
Impacts and Risks to Public Health 

and Welfare: Agriculture and 

Forestry 

U. S. Environmental Protection Agency 

Office of Atmospheric Programs 

Climate Change Division 

Washington, D.C. 
FOREWORD 

This document provides responses to public comments on the U.S. Environmental Protection Agency’s 

(EPA’s) Proposed Endangerment and Cause or Contribute 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 Docket 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 underlying 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. This volume of the document provides responses to 

public comments regarding agriculture and forestry. 

In light of the very large number of comments received and the significant overlap between many 

comments, this document does not respond to each comment individually. Rather, EPA summarized and 

provided a single response to each significant argument, 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 particular issues; however, these lists are not meant to be 

exhaustive and EPA does not individually identify each and every commenter who made a certain point in 

all instances, particularly in cases where multiple commenters expressed essentially identical arguments. 

Several commenters provided additional scientific literature 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 individually 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 references 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 specific 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 this Response to Comments document based on an 

assessment of the principal subject of the comment; 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 

6.0 Impacts and Risks to Public Health and Welfare: Agriculture and Forestry 

6.1 Agriculture ...................................................................................................................1 

6.2 Forestry.......................................................................................................................18 

Acronyms and Abbreviations 

AMO Atlantic Multi-Decadal Oscillation 

ANPR Advance Notice of Proposed Rulemaking 

ACIA Arctic Climate Impact Assessment 

CCSP U.S. Climate Change Science Program 

CO2 carbon dioxide 

ENSO El Niño-Southern Oscillation 

EPA U.S. Environmental Protection Agency 

EU European Union 

FACE Free-Air CO2 Enrichment 

IPCC Intergovernmental Panel on Climate Change 

GHG greenhouse gases 

NRC National Research Council 

PDO Pacific Decadal Oscillation 

ppm parts per million 

TSD Technical Support Document 

USDA U.S. Department of Agriculture 

USGCRP U.S. Global Change Research Program 

iv 
6.0 Impacts and Risks to Public Health and Welfare: Agriculture and Forestry 

6.1 Agriculture 

Comment (6-1):

Several commenters (e.g., 2972.1, 3136.1, 3283.1, 3311.1, 3347.1, 3394.1, 3596.2, 3596.3, 3722, and 

3747.1) state that the climate impacts evidence as summarized in the Technical Support Document (TSD) 

with respect to agriculture do not support the Administrator’s endangerment finding. The commenters 

conclude that it would be arbitrary and capricious to base the endangerment finding on what they believe 

are vague statements and selective information found in the TSD regarding impacts and risk to 

agriculture. 

Response (6-1):

The specific issues that underlie these comments are addressed in the responses throughout this volume, 

and other volumes of the Response to Comments document. With regard to the commenters’ conclusion 

that the current science does not support an endangerment finding with respect to agriculture, we disagree 

based on the scientific evidence before the Administrator. See the Findings, Section IV.B, “The Air 

Pollution Is Reasonably Anticipated to Endanger Both Public Health and Welfare,” for details on how the 

Administrator weighed the scientific evidence underlying her endangerment determination in general, and 

with regard to the agriculture sector in particular. 

Comment (6-2):

Several commenters (e.g., 3394.1, 3449.1, 3722, and 3747.1) request clarification of EPA’s conclusions 

regarding the net effects of climate change on agriculture (i.e., if EPA is concluding that the positive 

effects of climate change on agriculture will ultimately be outweighed by negative effects or that the 

positive effects on agriculture will ultimately be outweighed by negative effects on other sectors). At least 

one commenter asserts that the TSD presents specific findings of effects of climate change that may occur 

for some parts of the agricultural system in the United States, but does not provide sufficient information 

to judge the overall consequences and risks to U.S. agriculture posed by greenhouse gases (GHGs). Other 

commenters (e.g., 1594.1, 3136.1, 3236.1, and 4003) conclude that the scientific evidence indicates that 

positive benefits of global warming and rising atmospheric CO2 on agriculture will outweigh any adverse 

effects. 

The commenters identify the following specific issues regarding net (adverse and beneficial) effects of 

climate change on agriculture: 

1) A commenter asserts that the TSD’s discussion of positive and negative impacts is not consistent 

with the U.S. Climate Change Science Program’s (CCSP) 2008 report SAP 4.3 (2008e), in which 

the U.S. Department of Agriculture (USDA) concludes that most crops for which there are data 

will experience net productivity gains, despite certain projected negative impacts resulting from 

temperature increases, precipitation changes, and increased pest and disease activity. 

2) Some commenters argue that the TSD’s conclusions regarding net effects are not consistent with 

reports from the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National 

Assessment of the Potential Consequences of Climate Variability and Change (Reilly et al., 

2001), which all indicate an overall neutral to positive effect on agricultural output from 

increased CO2 and warmer temperatures. These commenters cite positive effects of climate 

change including increased aggregate yields of rain-fed agriculture by 5% to 20%, the direct 

effects of elevated CO2 levels on crop yields, the extension of forage production in to late fall and 

early spring, and the reduction in livestock deaths from warmer winter temperatures. 

1 
3) Some commenters indicate that there is no supporting factual information to conclude that 

increased CO2 will negatively impact crops, and that even the TSD acknowledges that research 

on the combined effects of elevated CO2 and climate on pests, weeds, and disease is still 

insufficient to conclude a net detrimental effect on U.S. and world agriculture. One commenter 

(5846) indicates that as the length of growing season increases, the line marking the higher 

latitude limits to crop growth moves towards the poles and killer frosts become less frequent; 

therefore, we can expect agriculture to thrive. 

4) Some commenters accuse EPA of “cherry picking” information about the potential for negative 

health impacts of plants like poison ivy or allergenic plants whose increased health and vigor 

leads to negative reactions in some people. The commenters argue that the net effects of rising 

CO2 levels will be positive for society because increased corn production greatly trumps 

increased poison ivy production. 

5) At least one commenter indicates that although the TSD repeats the IPCC and CCSP conclusions, 

it does not independently investigate livestock mortality rates in extreme heat or extreme cold, 

and therefore cannot explain the summary conclusion, particularly given that the TSD in Section 

4(c) conceded spring and winter show the greatest increases in temperature, and daily minimum 

temperatures show more warming than daily maximum temperatures. As such, it would be logical 

to conclude that fewer livestock deaths from warmer winter temperatures would entirely offset, if 

not outweigh, livestock loss during summer seasons. 

Response (6-2): 

See Section IV.B.2 of the Findings for the Administrator’s consideration of the various effects to food 

production and agriculture for purposes of making the endangerment finding to public welfare. Here, we 

respond to the specific issues raised by commenters. 

1. After careful review, we disagree with the commenters that the conclusions of the scientific 

assessment literature as summarized in the TSD are inconsistent with the CCSP’s 2008 report 

SAP 4.3 (2008e). For example, Section 9 of the TSD cites numerous conclusions directly from 

the CCSP SAP 4.3 report (Backlund et al., 2008a) regarding projected climate impacts on 

agriculture. The commenter may have misinterpreted the CCSP’s conclusions about the net 

effects of climate change on agriculture because, although Backlund et al. (2008a) acknowledge 

some benefits for grain and oilseed crops with increased CO2 and temperature, they also 

concluded that as temperature rises, particularly in the later half of the century, these crops will 

increasingly begin to experience failure, especially if climate variability increases and 

precipitation lessens or becomes more variable. Further, the TSD summarizes conclusions 

specific to crop production from the CCSP report that indicate the 30-year outlook for U.S. crop 

production is relatively neutral. However, the CCSP assessment also concludes the outlook for 

U.S. crop production over the next 100 years would not be as optimistic, if temperature continues 

to rise along with climbing CO2 concentrations (Hatfield et al., 2008). Therefore, we conclude 

that the TSD’s summary of both positive and negative climate change effects on agriculture is 

consistent with and appropriately reflects the conclusions of this major scientific assessment 

report. Importantly, we note crop response to changes in climate variables (irrespective of 

direction or magnitude) is but one aspect the Administrator considers in terms of evaluating 

impacts and effects on food production and agriculture both in the near term and over the longer 

term. 

2. We have carefully reviewed the TSD and the literature cited by the commenter and strongly 

disagree that the conclusions of the scientific assessment literature as summarized in the TSD are 

2 
Further, we note the TSD summarizes precisely the same findings referred to by the commenters. 

For example, the TSD indicates the IPCC finding of a projected increase in aggregate yields of 

rain-fed agriculture by 5% to 20%, but with important variability among regions (Field et al., 

2007). In addition, the TSD clearly states the IPCC finding that increased average warming leads 

to an extended growing season, especially for northern regions of the United States. Thus, we 

conclude that information as summarized in the TSD is consistent with the major assessment 

literature in terms of identifying the benefits to the agricultural sector. However, we do not agree 

that these reports and other evidence compiled in the TSD indicate that beneficial effects will be 

uniformly distributed, or uniformly dominant over adverse effects, as a result of climate change in 

the future, particularly over the long term. See additional responses to comments on specific 

impacts and effects to agriculture in this volume. 

3. The commenters assert that there is no supporting factual information to conclude that increased 

CO2 will negatively impact crops. Section 9 of the TSD summarizes the most recent body of 

scientific assessment literature regarding the likely adverse and beneficial effects of climate 

change on United States agriculture. With regard to adverse effects, the TSD summarizes 

multiple lines of evidence based on observations and research that indicate weeds, pests, and 

disease will positively respond to increasing levels of CO2 and warming temperatures, which 

present significant challenges to agriculture operations. For example, as described in the TSD, 

weeds respond more positively than most cash crops to higher temperatures and CO2 levels 

(Hatfield et al., 2008), which increase stress on crop plants and the need for pest and weed control 

(Karl et al., 2009). In addition, rising temperatures allow both insects and pathogens to expand 

their geographic ranges northward where they have been historically restricted due to colder 

temperatures (Karl et al., 2009). Furthermore, research has shown that the most widely used 

herbicide used in the United States (glyphosphate), loses its efficacy on weeds grown at CO2 

levels that are projected to occur in the coming decades (Wolfe et al., 2007). In addition, as 

summarized in the TSD, agricultural crops are sensitive to various climate-related variables (i.e., 

heavy precipitation, drought, extreme temperatures and other disturbances). We therefore 

conclude that the TSD’s summary of projected climate impacts (both positive and negative) on 

agriculture is based on scientific evidence and appropriately reflects the conclusions of the body 

of scientific literature. Section 9 of the TSD provides a balanced discussion of the projected 

adverse and beneficial impacts of climate change on agriculture. Regarding the comment that 

agriculture will thrive due to extended growing seasons in the Arctic and lack of killer frosts, we 

find that this assertion is unsupported by the science, which finds that both adverse and beneficial 

effects from climate change are likely. We note the commenter did not provide literature to 

counter the conclusions summarized in the TSD. 

4. We strongly disagree with the commenter that EPA has “cherry picked” information about 

negative health effects of certain plants resulting from climate change. Sections 7 and 9 of the 

TSD include information on both the positive and negative effects of climate change on 

aeroallergens, pollen production, plant toxicity, and agriculture. Therefore, we find that the 

TSD’s summary of projected climate impacts on agriculture was reasonable and appropriately 

3 
5. Regarding the comment that EPA did not conduct an independent investigation of livestock 

mortality rates in extreme heat or extreme cold, see Volume 1 for our response to comments 

alleging that EPA must conduct its own scientific assessment rather than relying on the existing 

assessment literature. The TSD relies on major synthesis documents of the IPCC and CCSP 

regarding the potential impacts to livestock production from climate change. For example, as 

indicated in the TSD, the CCSP concludes higher temperatures will very likely reduce livestock 

production during the summer season, but these losses will very likely be partially offset by 

warmer temperatures during the winter season. The commenter does not provide supporting 

scientific literature to support his claims that fewer winter livestock deaths would completely 

offset increased summer livestock mortality. As summarized in the TSD in Section 4(k), 

widespread changes in extreme temperatures have also been observed in the last 50 years. We 

conclude the TSD provides a balanced and accurate synthesis of the science regarding potential 

changes in livestock productivity and disagree with the commenter that observed trends in daily 

temperature alone, which do not represent projected changes in extreme conditions, provide 

evidence that livestock mortality will be offset or increase. 

Comment (6-3): 

One commenter (3394.1) states that Section 9 of the TSD exhibits significant bias because it does not 

attempt to quantify the relative impacts of positive and negative effects on agriculture. The Agency only 

presents quantitative information in instances where it supports an argument for negative climate impacts 

on agriculture. For example, the TSD cites the IPCC conclusion that “at ambient CO2 concentrations of 

550 ppm, crop yields increase under unstressed conditions by 10% to 25% for C3 crops, and by 0% to 

10% for C4 crops.” Yet, without reasoning, EPA dismisses this potential benefit of GHG emissions as 

small. 

Response (6-3): 

The TSD explicitly summarizes both beneficial and adverse effects to agriculture from changes in climate 

and provides quantification where such information is available. For example, as indicated in the TSD, 

the IPCC finds that moderate climate change in the early decades of the century is projected to increase 

aggregate yields of rain-fed agriculture by 5% to 20%. Based on our interpretation, the commenter 

appears to take issue with the word “small” in the following IPCC finding: [e]levated CO2 levels are 

expected to contribute to small beneficial impacts on crops. We note the preceding sentence indicates that 

there is still uncertainty about the sensitivity of crop yields in the United States and other world regions 

due to the direct effects of elevated CO2 levels. Thus, the IPCC uses the qualifier word “small” because 

multiple factors other than the direct effects of CO2 could lead to either positive or negative consequences 

for crop yields. We therefore disagree with the commenter that the TSD exhibits bias with respect to 

quantification of the relative positive and negative impacts on food production and agriculture. The 

TSD’s summary of the projected positive and negative effects of climate change is accurate, balanced, 

and appropriately reflects the conclusions of the body of scientific literature. 

See the Findings, Sections II.A.2, “Summary of Response to Key Legal Comments on the Interpretation 

of the Section 202(a) Endangerment and Cause or Contribute Test,” and IV.B, “The Air Pollution Is 

4 
Reasonably Anticipated to Endanger Both Public Health and Welfare,” for EPA’s response to comments 

on the general issue of quantifying the relative impacts of climate change. 

Comment (6-4): 

Several commenters (3283.1, 3347.1, and 3747.1) indicate that a net detrimental effect is not apparent 

from freshwater and marine fisheries nor have these effects in some cases been quantified. One 

commenter notes the conclusion cited in the TSD that indicates that cold-water fisheries will be 

negatively affected; warm-water fisheries will generally benefit, and the results for cool-water fisheries 

are mixed. 

Response (6-4): 

The summary in the TSD does not draw any conclusions regarding “a net detrimental effect” for fisheries, 

but presents the balanced view that some fisheries may be adversely affected whereas others may benefit 

as a result of human-induced climate change. 

Regarding quantification of effects, as described in the preceding response (6-3), the TSD summarizes 

both beneficial and adverse effects to fisheries from changes in climate and provides quantification when 

such information is available. See the Findings, Sections II.A.2, “Summary of Response to Key Legal 

Comments on the Interpretation of the Section 202(a) Endangerment and Cause or Contribute Test,” and 

IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger Both Public Health and Welfare,” for 

EPA’s response to comments on the general issue of quantifying the relative impacts of climate change. 

Comment (6-5): 

Several commenters (e.g., 0521, 0540, 0664, 0692, 0768.1, 1616.1, 2972.1, 3136.1, 3236.1, 3411.1, 

3449.1, 3596.3, and 4032) indicate that the carbon cycle and increased levels of CO2 in the atmosphere 

are and will continue to be beneficial for plant growth and agriculture. In support of this argument, the 

commenters make the following general assertions: (1) the carbon cycle in plants is a basic function for 

their survival and the majority of plant life prefers a much higher concentration of CO2 than currently in 

our atmosphere; (2) essentially all important agricultural crops exhibit enhanced growth under both 

increased CO2 concentrations and increased global temperatures; (3) CO2 is not a pollutant, but a required 

ingredient for plant growth; (4) studies have shown that growth of plant life is accelerating with an 

increase in CO2 in the environment. 

Many commenters (e.g., 1594.1, 0540, 0664, 0692, 0768.1, 1616.1, 2972.1, 3136.1, 3236.1, 3411.1, 

3449.1, 3596.3, 4032) also cite specific findings from the scientific literature as evidence of the beneficial 

effect of CO2 on plants and crop production. Some commenters note Dr. Craig Idso of the Center for the 

Study of Carbon Dioxide and Global Change has provided extensive documentation of the benefits of 

CO2 for plants, and at least one commenter submits a literature review by Dr. Idso, which the commenter 

claims provides scientific evidence of the beneficial effects of CO2 on plants (e.g., plant growth, wateruse efficiency, resilience to diseases, herbivores, and insects) and rebuts the TSD’s conclusions that harm 

to agriculture is likely. 

Response (6-5): 

We do not dispute the fact that CO2 (i.e., carbon) is a required ingredient for plant survival and growth, or 

that CO2 can have a stimulatory effect on plant growth. This has been well known for quite some time. 

Sections 3, 9, and 10 of the TSD summarize the scientific assessment literature on the stimulatory or 

fertilization effect of CO2 on plant growth and productivity. In addition, Section 9 of the TSD summarizes 

several findings from the assessment literature on overall plant growth in North America. The IPCC 

5 
(Field et al., 2007) cited a study by Nemani et al. (2003), which found that plant growth (measured by net 

primary productivity) increased from 1981 to 1998 across North America. See comment response (6-38) 

below for additional responses to comments on trends in net primary productivity. 

We reviewed the studies submitted by commenters related to the direct effects (i.e., growth, water-use 

efficiency, photosynthetic rates, and productivity) of CO2 on plants. Several of these same authors and 

studies are cited in the CCSP and IPCC reports. For example, the IPCC (Easterling et al., 2007) cites the 

work of Kimball et al. (2002) and the free-air CO2 enrichment experiments on crop response. In addition, 

the IPCC (Field et al., 2007) cites the study by Idso et al. (1987), which shows increases on dry matter 

production with elevated temperature and CO2, and the study by Woodward et al. (2004) regarding CO2

stimulation of plant growth. Further, we note the CCSP (Hatfield et al., 2008) provides a comprehensive 

evaluation of Free-Air CO2 enrichment (FACE) experiments studying crop response (e.g., photosynthesis, 

total biomass, grain yield, stomatal conductance, and evapotranspiration) to elevated CO2 concentrations. 

Therefore, we conclude that these studies do not provide additional information not already summarized 

in Sections 3 and 9 of the TSD. Furthermore, not all studies that have examined the effects of elevated 

CO2 concentrations on plant growth or crop productivity to date, including those submitted by 

commenters, have comprehensively included all relevant factors (e.g., CO2 increases, temperature 

increases, precipitation changes, weather variability changes, ground-level ozone changes, soil nutrient 

changes), which is why, for this sector in particular, it is important for EPA to rely primarily on the 

assessment literature that brings many individual studies together in order to gain a more comprehensive 

view of the impacts of elevated CO2 concentrations and climate change on United States agriculture. 

Section 9 of the TSD summarizes the conclusions of the assessment literature that there is still uncertainty 

regarding overall crop sensitivity to elevated CO2 levels when considered in light of other documented 

and projected climate impacts on agriculture. There, we summarize findings of the IPCC (Easterling et al. 

2007), which provide scientific evidence that adverse effects on crop yields due to droughts and other 

extreme events may offset the beneficial direct effects of elevated CO2 and moderate temperature 

increases in the near term. We describe the CCSP (Hatfield et al., 2008) finding that high temperatures, 

water and nutrient availability, extreme weather events (i.e., heavy downpours, droughts), enhanced pest 

and weed growth, and ozone exposure can significantly limit the benefits of the direct stimulatory CO2

response on plant growth. In addition, we summarize the CCSP conclusion that as temperature rises 

beyond a certain threshold in the future, grain and oilseed crops will increasingly begin to experience 

failure, especially if climate variability increases and precipitation lessens or becomes more variable 

(Backund et al., 2008). In addition, CCSP concludes that the marketable yield of many horticultural crops 

(e.g., tomatoes, onions, fruits) is very likely to be more sensitive to climate change than grain and oilseed 

crops. Therefore, we disagree with the comment that all important agricultural crops exhibit and will 

continue to exhibit enhanced growth in the future under both increasing CO2 concentrations and 

increasing global temperatures. 

See the Findings, Section IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger Both Public 

Health and Welfare,” for EPA’s response to comments on the Administrator’s conclusions regarding the 

net effects of climate change on agriculture. See also Volume 1, Section 1.4, “Consideration of Net 

(Adverse and Beneficial) Effects,” for our response to comments on the general issue of evaluating net 

effects in the context of this action. Lastly, see Section III.A., “The Science on Which the Decisions Are 

Based,” for our response to comments on the use of the assessment literature and our treatment of new 

and additional scientific literature provided through the public comment process. 

Comment (6-6): 

Several commenters (e.g., 2972.1, 3136.1, 3347.1, 3394.1, 3596.2, 4003, and 11459) state that past and 

future technological improvements have and will continue to push agriculture to even higher production 

6 
yields despite the challenges that a changing climate may present. These commenters suggest that EPA 

must consider technological advances in assessing the scientific evidence for an endangerment finding. 

Commenters provide a report by Alan Carlin (Carlin, 2009) that indicates that the TSD is incomplete with 

regard to observed trends in agricultural productivity that show that, despite decades of increasing 

anthropogenic GHGs, crop yields have increased. As evidence of this positive trend in production yields, 

commenters submit the following information: 

1) A commenter quotes the version of the TSD released with the Advance Notice of Proposed 

Rulemaking (ANPR): “For the past 30, 50, 100 years, agricultural yields have been increasing in 

the U.S., despite climate fluctuations and trends.” 

2) Commenters submit a figure depicting global average temperature and precipitation trends and 

corn and wheat yields over time (1895-2005) as evidence that crops yields have increased despite 

a variety of changes (climate, soil, and landscape, GHG levels, etc.). 

3) A commenter submits a figure containing three maps of harvested acreage of major U.S. food 

crops (wheat, corn, and soybeans) as evidence that U.S. major cash crops are grown across a large 

range of climate conditions, indicating their adaptability to climate change. 

At least one commenter states that although some more sensitive crops may be less productive under 

future climatic conditions, the U.S. agricultural and livestock industries are renowned for developing new 

strains and varieties of crops that prosper in different climatic conditions in the United States and 

throughout the world. Some commenters contend that herbicide-resistant and insect-resistant crop 

varieties help improve farmers’ ability to control for insects and weeds, and state that this discussion is 

absent from the TSD Section 9(d). Another commenter argues that genetically engineered crop species 

will continue to improve farmers’ ability to grow productive, healthy food crops in ample supply despite 

climate change effects, citing a reference from the Economic Research Service of the USDA on the 

widespread adoption of genetically engineered crops. 

Response (6-6): 

The commenters mention evidence of trends in observed increased productivity and refer to a number of 

technological improvements (e.g., genetically engineered crops) designed to reduce the impacts of climate 

change on agriculture. Regarding the potential for genetic selection or modification to alleviate climate 

change stressors on crops, Section 9 of the TSD states: “There are continual changes in the genetic 

resources of crop varieties and horticultural crops that will provide increases in yield due to increased 

resistance to water and pest stresses. These need to be considered in any future assessments of the 

climatic impacts; however, the genetic modifications have not altered the basic temperature response or 

CO2 response of the biological system (Hatfield et al., 2008).” The scientific literature is clear that the 

possibility of changes in the genetic resources of crop varieties does not preclude evidence indicating 

vulnerability and impacts to the agricultural sector from climate change. See our responses to other 

comments in this section (e.g., 6-2, 6-5) for our response regarding the positive and negative effects of 

future climate change on agriculture. 

With regard to the comment about considering technological advances in assessing the scientific evidence 

for an endangerment finding, see the Findings, Section IV.B, “The Air Pollution Is Reasonably 

Anticipated to Endanger Both Public Health and Welfare,” for our response to comments on how the 

Administrator weighed the scientific evidence underlying her endangerment determination. 

1) With regard to the statement quoted by the commenter from the version of the TSD released with 

the ANPR, we note that it was replaced by the discussion of crop yields and productivity in 

Section 9(a) of the April, 2009 version of the TSD, as well as the final TSD. To more accurately 

reflect the scientific assessment literature, the final TSD states: “Observational evidence shows 

that, over the last century, aggregate yields of major U.S. crops have been increasing (USDA, 

7 
2) We reviewed the submitted figures documenting higher production yields and harvested acreage 

over the last century and have determined that they are generally consistent with the underlying 

scientific literature of summarized in Section 9 of TSD. As indicated in the final TSD, 

observational evidence from Field et al. (2007) and USDA’s 2007 report Crop Production 

Historical Track Records (USDA 2007) indicates that, over the last century, aggregate yields of 

major U.S. crops have been increasing, although with significant regional and temporal variation. 

We disagree, however, that past trends are indicative of future production trends under a changing 

climate. See our responses to other comments in this section (e.g., 6-2, 6-5) for our response 

regarding the positive and negative effects of future climate change on agriculture. 

3) We note that agriculture in the United States is distributed across a range of climate conditions 

and note the distribution of crops and livestock. However, this does not preclude evidence 

indicating vulnerability of the agricultural sector to climate change. For example, increased 

climate variability or rapid changes in environmental conditions (i.e., extreme heat) can disrupt or 

reduce productivity regardless of climate conditions. See our responses to other comments in this 

section (e.g., 6-2, 6-5) for our response regarding the positive and negative effects of future 

climate change on agriculture. 

Comment (6-7):

A commenter (3394.1) notes that the TSD indicates that human system responses to climate change are 

more difficult to identify and isolate due to the larger role that non-climate factors play (e.g., management 

practices in agriculture and forestry, and adaptation responses to protect human health against adverse 

climatic conditions). The commenter notes this is not mentioned in Section 9, nor does the TSD address 

these difficult uncertainties in its actual analysis of agricultural effects. The commenter states that it is 

unclear whether any of these considerations factored into EPA’s assessment of this area of the science, 

and that accordingly, the proposed endangerment finding and the TSD fail to present a persuasive case in 

support of endangerment. 

Response (6-7): 

The science regarding the attribution of climate change to human activities is discussed in Section 5 of the 

TSD and our response to general comments on this issue is found in Volume 3 of this Response to 

Comments document. See Volume 1, Section 1.2 for our response to comments on the issue of 

differentiating between climate and non-climate drivers of impacts. We describe in Section 5(b) of the 

TSD the importance of considering climate variability and non-climate drivers (e.g., land-use change, 

habitat fragmentation) in order to make robust conclusions about the role of anthropogenic climate change 

in affecting biological and physical systems. Section 9 of the TSD discusses the number of ways in which 

crops, livestock, and fisheries may be affected by both elevated concentrations of CO2 and resultant 

climate change, based on the underlying assessment literature on this topic. EPA therefore disagrees with 

the comment that our evaluation of the agricultural sector did not include the appropriate considerations. 

See the Findings, Section IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger Both Public 

Health and Welfare,” for our response to comments on how the Administrator weighed the scientific 

evidence underlying her endangerment determination. 

8 
Comment (6-8): 

A number of commenters (e.g., 0329, 0664, 0692, and 4032) argue that warmer temperatures and 

increasing CO2 in the atmosphere may or will benefit agricultural production, food supply, and the 

prevention of widespread starvation due to longer growing seasons and CO2 fertilization effects. The 

commenters ask if EPA considered the short- and long-term consequences to food supplies if GHGs, 

particularly CO2 levels, are suppressed or even reduced to earlier levels. 

In support of this argument, one commenter (0768.1) references a study by Nemani et al. (2003) on the 

increase in net primary production from 1982 to 1999; a USDA report with no citation documenting an 

increase in worldwide grain production to approximately 90 million tons between 1990 and 1999; and a 

United Nations Food and Agriculture Organization 2004 report presenting statistics on world mortality 

from malnutrition. The commenter attributes the crop production increases to rising CO2 levels and 

temperatures and concludes that climate change will improve worldwide food production and decrease 

world mortality from starvation. 

Response (6-8): 

See Section 9 of the TSD and our responses to other comments in this section (e.g., 6-2, 6-5) for our 

response regarding the positive and negative effects of elevated CO2 concentrations and climate change 

on agriculture and food supply. We reviewed the submitted figures documenting higher production yields 

and harvested acreage over the last century and determined that they are generally consistent with the 

underlying scientific literature summarized in Section 9 of TSD. As indicated in the final TSD, 

observational evidence from Field et al. (, which states: “Observational evidence shows that, over the last 

century, aggregate yields of major U.S. crops have been increasing (USDA, 2007; Field et al., 2007), with 

significant regional and temporal variations..” We do not find scientific support in the literature submitted 

for the commenter’s claim that past trends are indicative of future production trends under a changing 

climate. Past trends reflect the impact of many variables in addition to climate, and the past impact of 

climate itself cannot be determined just by looking at the overall trend. 

The endangerment analysis does not consider policies that would result in lowering atmospheric GHG 

concentrations to levels below current levels; nor does the endangerment analysis assess policy scenarios 

that would stabilize atmospheric GHG concentrations at some future level higher than today’s. For our 

broader responses to these issues, see the Findings, Section III.C, “Adaptation and Mitigation.” We 

reviewed the study by Nemani et al. (2003) and note the authors’ findings of an increase in net primary 

productivity apply to vegetation of all types, and do not specifically address climate effects on crop 

species. In addition, the authors indicate the largest increase was in tropical ecosystems where Amazon 

rain forests accounted for 42% of the global increase in net primary production. Thus, we conclude this 

study does not address the regional variation and susceptibility of the U.S. agricultural sector to changes 

in climate. 

Comment (6-9): 

A commenter (3347.3) argues that the CCSP’s conclusions in Backlund et al. (2008a) as summarized in 

the TSD are unreasonably pessimistic and over emphasize potential negative effects. The commenter 

states that the CCSP report’s fundamental weakness is “a lack of significant consideration for the 

realization that agriculture is constantly changing, and farmers and their crops adapt to these changes.” 

The commenter makes the following points regarding specific CCSP conclusions summarized in TSD: 

1. Contrary to the conclusions regarding increased future crop failure as temperature rises, the 

USDA Agricultural Projections to 2018 predicts a general increase in crop yield per acre through 

2018. The commenter notes this includes projections for crops such as tomatoes, which are often 

9 
2. Contrary to the conclusions regarding the positive growth response of weeds to increasing CO2, a 

study by Ziska (2003) found that the positive growth response of weed species to rising CO2 

tapers off above a certain level. 

3. Contrary to the conclusions regarding the future efficacy of glyphosate, the oft-cited study by 

Ziska and Goins (2006) does not obtain consistent results in all experimental trials of glyphosate; 

therefore, broad conclusions should not be drawn from this article. 

4. Contrary to the conclusions regarding increasing disease pressure, pathogens, and parasites, and 

overall effects on livestock production, there are substantial plant and animal breeding industries 

that regularly develop new cultivars to thrive under new conditions (Troyer, 2004). 

Response (6-9): 

We have carefully reviewed the comments and we do not see any evidence that Backlund et al. (2008a), 

as summarized in the TSD, is unreasonably pessimistic with regard to adverse effects to food production 

and agriculture. Our responses to the commenter’s specific points are as follows: 

1. The USDA Agricultural Projections to 2018 predicts a general increase in crop yield per acre 

through 2018. According to the USDA information, these projections assume normal weather and 

no further outbreaks of plant or animal diseases. However, we find this projection to be consistent 

with the CCSP, which notes that overall, the benefits of increasing CO2 concentrations over the 

next 30 years are projected to mostly offset the negative effects of temperature for most C3 crops, 

while the C4 crop yields are reduced by rising temperature (Hatfield et al., 2008). We note that 

the USDA projections cover a relatively short time frame (less than 10 years from present). In 

addition, regarding the commenter’s assertion that estimates given by IPCC (Reilly et al., 2001) 

are far less negative than the discussion in the TSD, we find that the commenter bases this 

assertion on near-term estimates of benefits and does not comprehensively discuss all relevant 

factors (e.g., CO2 increases, temperature increases, precipitation changes, weather variability 

changes, ground-level ozone changes, soil nutrient changes) in determining the overall risks and 

impacts to agriculture from climate change. Thus, EPA does not view the USDA projections or 

the commenter’s claims regarding the Reilly et al. (2001) study as inclusive of the collective risks 

and impacts associated with climate change projected to occur over time (over the next few 

decades and out to 2100). See response (6-2) above for additional information on our response to 

comments regarding Reilly et al. (2001). Thus, for these reasons, we disagree with the commenter 

that CCSP and the TSD are overly pessimistic and find that they accurately reflect the findings of 

the assessment literature on the overall risks and impacts to agriculture from climate change. 

2. Regarding the study referenced the commenter by Ziska (2003), we reviewed the study and note 

that it is consistent with research on the effect of climate change on competition between C3 and 

10 
3. Regarding the commenter’s point related to glyophosate efficacy, EPA reviewed the study by 

Ziska and Goins (2006) and note the study is consistent with the interpretation of the CCSP and 

as reflected in the TSD. The authors note that, overall, the data suggest that, depending on weed 

species (C3 vs. C4), elevated CO2 can increase weed biomass, decrease yields, and reduce 

glyphosate efficacy for Round-up Ready soybean (Ziska and Goins, 2006). In fact, we note this 

study was reconfirmed in the recent USGCRP report (Karl et al., 2009), which cites a recent 

study by Wolfe et al. (2007) that indicates future CO2 levels impact the efficacy of weed control 

from this herbicide. 

4. See the Findings, Section III.C, “Adaptation and Mitigation,” for our response to comments on 

the treatment of adaptation and mitigation in the Findings. 

Based on our review, we conclude that the findings of the CCSP 2008 report (Backlund et al., 2008a) are 

accurate and that the TSD’s summary of positive and negative effects of climate change on agriculture 

was reasonable and appropriately reflects the conclusions of the body of scientific literature. Regarding 

the commenter’s view that farmers and their crops will adapt to climate change, see the Findings, Section 

III.C, “Adaptation and Mitigation,” for our response to comments on the treatment of adaptation and 

mitigation. 

Comment (6-10): 

One commenter (0591) notes that, in the past, CO2 levels reached 1,000 parts per million (ppm) or 

greater. The commenter also states that temperatures during the Medieval Warm Period were significantly 

higher than they are now. The commenter states that since the Medieval Warm Period was one of the 

most flourishing times in human history, the likely benefits of climate change today include increasing 

crop yields in agriculture, barren lands becoming fertile, and warmer weather being easier to endure and 

more conducive to life than severe cold. 

Response (6-10): 

See Volume 4 of this Response to Comments document for EPA’s responses to comments regarding 

current and past CO2 levels in the atmosphere and temperatures during the Medieval Warm Period. The 

commenter provides no factual evidence upon which to base a conclusion that the Medieval Warm Period 

is analogous to present-day environmental conditions. The commenter provides no support for the 

assertions regarding increasing crop yields and barren lands becoming fertile. See our responses to other 

comments in this section regarding the effects of CO2 on agriculture (both adverse and beneficial). 

Comment (6-11): 

In reference to the TSD’s discussion of ground-level ozone effects on plant growth, one commenter 

(3411.1) states that, according to a study by Racherla and Adams (2006), climate change may actually 

reduce background levels of ozone, and, therefore, ozone would not greatly impact agricultural areas. 

11 
Response (6-11): 

We have reviewed the reference submitted by the commenter and note that this study, as well as a more 

recent study by Racherla and Adams (2008), were evaluated in the EPA (2009a) report, which is cited 

extensively in Section 8 of the TSD. The Racherla and Adams (2008) study found that, while global nearsurface tropospheric ozone is expected to decrease, summertime tropospheric ozone is expected to 

increase over North America. Section 8 of the TSD summarizes the following overall conclusions from 

the literature on this issue: (1) climate change is expected to decrease background ozone due to higher 

water vapor; (2) climate change is expected to increase regional and urban-scale ozone pollution due to 

higher temperatures and weaker air circulation; (3) ozone levels are expected to increase over substantial 

regions of the country at during summer months; and (4) the extent of changes in ozone levels in the 

future will vary at the regional and local level. Therefore, the TSD’s discussion of impacts to agriculture 

from increased ozone exposure (e.g., limiting the benefits of direct stimulatory CO2 response on plant 

growth) appropriately reflects the conclusions of the body of scientific literature. 

Comment (6-12): 

Commenters (3136.1, 3596.2) note that the TSD does not consider the implications of biofuel production 

on food and livestock production and assert that biofuels will be detrimental to the agricultural sector. A 

commenter (4032) indicates it is ironic that a contributing factor to the world’s food shortage is the 

millions of acres of farmland converted from food crops to biofuels meant to stop global warming. At 

least one commenter expresses concern that the production of feed for livestock will be adversely 

impacted by increased ethanol production. Commenters state that the European Commission admitted 

that, to meet European Union’s (EU) biofuel targets, they would eventually need almost all of the foodgrowing land in Europe. Two commenters (2972.1, 7037) submit a reference from Elock (2008) and the 

International Council for Science (2009) regarding biofuel production, which indicates the water 

requirements of biofuel-derived energy are 70 to 400 times larger than other energy sources such as fossil 

fuels, wind, or solar. Commenters also reference a study that indicates “The increase in corn production to 

support ethanol goals in the United States is predicted to increase nitrogen inputs to the Mississippi River 

by 37%” (Idso and Idso, 1994). 

On the other hand, at least one commenter (1594.1) speaks on behalf of the positive effects of increasing 

CO2 and indicates that rising atmospheric CO2 levels are helping to increase grain production, which is 

beneficial to the United States biofuel program and will have a positive effect on public welfare. 

Response (6-12):

These comments on biofuel production and agricultural food crops are not germane to the current 

endangerment and cause or contribute findings. Possible future policies or regulations related to 

alternative energy production or GHG mitigation approaches are outside the scope of this action. See 

Volume 11 of this Response to Comments document for our response to comments on the implications of 

the Findings for economic and related concerns. 

Comment (6-13): 

A commenter (3136.1) indicates that the TSD does not provide analysis of livestock production in the 

United States over the past few decades. In addition, at least two commenters (3136.1, 3347.3) reference a 

USDA Web site (http://www.ers.usda.gov/Briefing/baseline/livestock.htm) that provides agricultural 

baseline projections (2009 to 2018) for livestock and crop production. Another commenter disagrees that 

elevated CO2 will decrease C4 grasses and decrease the nutritional value of forage for animals, which in 

turn affects animal weight and performance. 

12 
Response (6-13): 

Section 9 of the TSD provides a summary of the vulnerability of the agricultural sector to climate change 

and how climate variability may affect U.S. food production and agriculture based on the conclusions of 

the scientific assessment literature. Past livestock production in the United States was reflected in the 

TSD in the context of observed and potential changes to productivity due to climate change. Regarding 

the referenced Web site, the commenter did not explain how an analysis of the livestock production in the 

United States over the past few decades or the USDA Web site on baseline livestock and crop production 

affect or alter the scientific findings summarized in the TSD. According to information on the USDA 

Web site, agricultural baseline projections are a neutral backdrop, or reference scenario, that provides a 

point of departure for discussion of alternative farm sector outcomes that could result under different 

domestic or international assumptions. These projections assume normal weather and no further outbreaks 

of plant or animal diseases. Thus, we have determined that these projections are not inclusive of the 

collective impacts associated with climate change projected to occur over much longer time-scales. 

Regarding the question of whether elevated CO2 will decrease C4 grasses and decrease the nutritional 

value of forage for animals, we note that the TSD does not state that forage quality will decrease. The 

TSD states there is limited information in the assessment literature regarding the specific effects on 

grasses and their nutritional quality. Section 9 of the TSD summarizes the IPCC (Easterling et al., 2007) 

and CCSP (Hatfield et al., 2009) conclusions that, based on expected vegetation changes and known 

environmental effects on forage protein, carbohydrate, and fiber contents, both positive and negative 

changes in forage quality are possible as a result of atmospheric and climatic change. Elevated CO2 can 

increase the carbon-to-nitrogen ratio in forages and thus reduce the nutritional value of those grasses, 

which in turn affects animal weight and performance. Thus, we conclude that the TSD’s summary of 

projected climate change effects on livestock appropriately reflects the conclusions of the body of 

scientific literature. 

Comment (6-14):

Commenters (3136.1, 3596.3) mention that the EPA leaves out information in the TSD that certain plants 

exhibit increased water-use efficiency under elevated CO2 conditions. Similarly, another commenter 

(3411.1) asserts that there is substantial observational evidence that the increased concentrations of 

atmospheric CO2 have led to hardier growth and better water-use efficiency of vegetation in and around 

desert areas. The commenter cites various journal articles as supporting evidence for his assertion (e.g., 

Feng, 1999; Cheddadi et al., 2001; Eklundh and Olsson, 2003). 

Response (6-14): 

We have updated Sections 3 and 9 of the TSD to include information regarding the effect of elevated CO2

levels on water-use efficiency and related biophysical processes in plants. We added to Section 3 a 

summary of the IPCC finding that increases in CO2 affects water use and water-use efficiency of plants. 

According to IPCC (Hatfield et al., 2008), elevated CO2 causes partial stomatal closure, which decreases 

conductance and reduces loss of water vapor from leaves to the atmosphere. Section 9 of the TSD 

describes the USGCRP conclusion that carbon dioxide makes some plants more water-use efficient and 

that this is a benefit in water-limited areas and in seasons with less than normal rainfall (Karl et al., 2009). 

See previous comment responses in this volume for our response regarding the projected adverse and 

beneficial effects of climate change on plants. 

Comment (6-15): 

13 
A commenter (2750) indicates that Section III.C.4 of the proposed action’s preamble is confusing. Which 

is it: “increased rain and yields of rain-fed agriculture, or, precipitation lessens? And how exactly is an 

increase in aggregate yields of rain-fed crops a harm?” 

Similarly, some commenters (3449.1, 3747.1) question the TSD’s discussion of agricultural risks and 

impacts from projected precipitation changes given the large uncertainties about projecting future 

precipitation levels. One commenter contends that the TSD makes conflicting statements regarding 

precipitation—on one hand describing very large uncertainty about precipitation projections and on the 

other hand, stating that global mean precipitation is expected to increase in the future. In addition, the 

commenter states that the TSD statement that “grain and oilseed crops will increasingly begin to 

experience failure if climate variability increases and precipitation lessens or becomes more variable” 

conflicts with its description of an increase in global mean precipitation. 

Response (6-15): 

Regarding the language in Section III.C.4 in the Proposed Findings, it appears that the commenter has 

linked two separate and distinct statements together and misinterpreted them. To clarify, the assessment 

literature as summarized in the TSD concludes that while global mean precipitation is projected to 

increase, precipitation is expected to become more variable in both frequency and amount across different 

regions of the United States. This issue is discussed in Section 6 of the TSD and our responses to 

comments regarding future projections of precipitation trends are provided in Volume 4 of the Response 

to Comments document. 

Section 9 of the TSD summarizes the scientific literature regarding the implications of precipitation 

changes on agriculture. It states: “Moderate climate change in the early decades of the century is 

projected to increase aggregate yields of rain-fed agriculture (water demand met primarily derived from 

precipitation) by 5 to 20%, but with important variability among regions. Future trends in precipitation 

are difficult to project but will be associated with strong regional and seasonal variation, which means 

some areas in the United States will continue to get wetter (e.g., Northeast and large parts of the Midwest) 

while some areas, particularly in the West, will become drier. Major challenges are projected for crops 

that are near the warm end of their suitable range or depend on highly utilized water resources [high 

confidence].” We therefore find no evidence that the TSD’s discussion of the effects of precipitation 

changes on agriculture conflicts with the finding that global mean precipitation will increase. 

In addition, Section 15 of the TSD summarizes the USGCRP’s conclusions regarding regional 

precipitation patterns in the United States. The assessment literature concludes that although there is still 

considerable variation and difficulty predicting precipitation changes at smaller spatial scales, changes in 

precipitation patterns will play a large role in determining the net impacts of climate change. The IPCC 

(Field et al., 2007) concluded that agriculture in areas projected to experience decreased precipitation will 

be challenged by restricted availability of water for irrigation and at the same time increasing water 

demand for multiple uses, including for irrigated agriculture and human and ecological uses. 

See the Findings, Section IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger Both Public 

Health and Welfare,” for our response to comments on how the Administrator weighed the scientific 

evidence underlying her endangerment determination in general, and with regard to the agricultural sector 

in particular. 

 

Comment (6-16):

Two commenters (3449.1, 3747.1) question the EPA’s conclusion of harm to agriculture from climate 

change by pointing to the fact that the TSD recognizes there is a lack of information on horticultural crops 

and CO2 and few reliable crop simulation models for use in climate change assessments. The commenters 

14 
conclude the evidence for impacts to agriculture from climate change is not strong enough to support an 

endangerment finding for public welfare. 

Response (6-16):

Section 9 of the TSD summarizes the most recent body of scientific assessment literature regarding how 

observed and projected climate change may affect U.S. food production and agriculture as a whole. 

Projected climate impacts on horticultural crops are just one aspect of the observed and projected 

agricultural impacts from climate change summarized in the TSD. As the commenter notes, the discussion 

of horticultural crops in the TSD states that there are relatively few published studies quantifying 

horticultural crop response to CO2 as compared to major grain and oilseed crops. However, the TSD also 

states that “The marketable yield of many horticultural crops is likely to be more sensitive to climate 

change than grain and oilseed crops because even short-term, minor environmental stresses can negatively 

affect visual and flavor quality (Hatfield et al., 2008).” We conclude that the TSD’s discussion of 

agricultural risks and impacts accurately reflects the conclusions of the assessment literature. 

See the Findings, Section IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger Both Public 

Health and Welfare,” for our response to comments on how the Administrator weighed the scientific 

evidence underlying her endangerment determination in general, and with regard to the agricultural sector 

in particular. 

Comment (6-17): 

One commenter (3475.1) states his support for the Findings, noting that California is particularly 

vulnerable to the impacts of climate change on agriculture, compared with the rest of the country. In 

support of this assertion, the commenter submits the following two papers from the California Climate 

Change Center as evidence that increasing drought and higher temperatures threaten agriculture in the 

state: Lobell and Field (2009), “California Perennial Crops in a Changing Climate,” and Lee et al. (2009), 

“Effect of Climate Change on Field Crop Production in the Central Valley of California.” 

Response (6-17): 

See Section III.A., “The Science on Which the Decisions Are Based,” for our response to comments on 

the use of the assessment literature and our treatment of new and additional scientific literature provided 

through the public comment process. We reviewed the submitted articles on California crop productivity 

and determined that they are generally consistent with the underlying scientific literature as synthesized in 

Section 9 of TSD. Lobell and Field (2009) find that the direct effects of temperature and rainfall on 

perennial crop yields in California will vary, but that cherries and almonds are likely be the most 

negatively affected by warming over the next decades. Lee et al. (2009) find that in the latter half of the 

century, crop yields of alfalfa, cotton, maize, rice, sunflower, tomato, and wheat are likely to be 

negatively affected by climate change, particularly cotton and sunflower. These findings are consistent 

with Section 15 of the TSD, which summarizes the conclusions of the USGCRP (Karl et al., 2009) 

regarding regional impacts on agriculture and states, “Much of the region’s agriculture may be negatively 

impacted by future warming, particularly specialty crops in California such as apricots, almonds, 

artichokes, figs, kiwis, olives, and walnuts.” 

Comment (6-18): 

A commenter (9051.1) references several figures from a 2006 United Nations Environment Program 

report 2006 (Kandji, 2006) entitled “Climate Change and Variability in the Sahel Region: Impacts and 

Adaptation Strategies in the Agricultural Sector” as evidence that over the same period that CO2 has 

supposedly been a “hazard,” agricultural productivity, vegetation, and global carbon productivity in 

15 
Africa has been increasing. The commenter also quotes the report indicating that aggregate food 

production has increased in the Sahel and many other parts of sub-Saharan Africa since the early 1980s, 

primarily driven by the continued expansion of the cultivated areas. The commenter contends that, if 

anything, increased CO2 is a benefit since it helps increase agricultural productivity and is definitely not a 

hazard to human health. 

Response (6-18): 

Section 9 of the TSD and our response to other comments in this volume summarize the most recent body 

of scientific assessment literature regarding how observed and projected climate change may affect U.S. 

food production and agriculture. The fact that agricultural productivity has increased in parts of subSaharan Africa does not preclude evidence indicating vulnerability and impacts to the U.S. agricultural 

sector from current and future climate change. The scientific literature is clear that the U.S. agricultural 

sector is experiencing the effects of, and will continue to be affected by, observed and future climate 

change. See our responses to other comments in this volume regarding specific projected positive and 

negative effects of future climate change on agriculture 

Comment (6-19): 

A commenter (3136.1) indicates the TSD mentions how California wine grapes are currently near climate 

thresholds and are likely to experience decreases in yields. The commenter indicates that EPA did not 

mention Oregon wines and how they may fare in the future. 

Response (6-19): 

Section 9 of the TSD mentions California wine grapes as an example of a crop that is currently near a 

climate threshold (i.e., a temperature above which the crop will fail) and is likely to experience decreases 

in yields, quality, or both, even under moderate climate change scenarios. The commenter did not identify 

any supporting literature regarding climate change effects on Oregon wine grapes, nor did the commenter 

provide any evidence that including information about Oregon wine grapes would change the TSD’s 

summary of the assessment literature’s conclusions regarding projected climate impacts on crop yields 

and productivity. 

Comment (6-20): 

According to a commenter (1616.1), Section III.C.1 of the Proposed Findings indicates that stimulated 

growth of crops is one of the adverse effects of increased warming and CO2. The commenter finds it 

difficult to see why this is a problem. 

Response (6-20):

EPA does not indicate that stimulated growth of crops due to elevated CO2 concentrations is one of the 

adverse effects of increased CO2 levels. See response (6-5) above for our response to comments regarding 

the direct stimulatory effects of elevated CO2 levels. See Section IV.B of the Findings for our response to 

comments on how the Administrator weighed the scientific evidence underlying her endangerment 

determination in general, and with regard to the agricultural sector in particular. 

Comment (6-21): 

A commenter (10939) indicates deep concern about the impact that the climate crisis will have on the 

environment, his ranching operation, and our economy, and describes in detail changes indicative of 

climate change effects on his range and in southeastern Montana. The commenter has been following the 

scientific discussion on climate change, and attended lectures and conversations with Steve Running, a 

16 
climatologist based at the University of Montana in Missoula and member of the IPCC. According to 

Running, the climate in southeastern Montana is already getting more arid, and if things continue as 

observed, a half a foot more rain a year to grow the same crops currently grown would be necessary. The 

commenter describes the hardship in making a living in agriculture in eastern Montana and cannot 

imagine it would even be possible in 50 years, when the effects of climate change are more fully 

expressed. 

Response (6-21): 

EPA reviewed the commenter’s observations about climate impacts in Montana and agrees that climate 

impacts are already occurring and will very likely continue to occur in the region. See Section 15 of the 

TSD for more information on agricultural impacts from climate change within the various regions of the 

United States. We also note that the TSD presents a synthesis of major assessment reports such as IPCC, 

and the work of Steve Running is represented in these reports. 

Comment (6-22):

A commenter (3248) submits an article by Costello et al. (2009) entitled “Managing the Health Effects of 

Climate Change” as additional scientific evidence in support of the Proposed Findings. 

Response (6-22):

We have reviewed the report and note that the information presented in Costello et al. (2009) with regard 

to food production and agriculture is generally consistent with or in some cases cite the same scientific 

assessments (Field et al., 2007) summarized in the TSD. Costello et al. (2009) find that “although 

agricultural productivity might increase in some regions as a result of global warming (almost entirely in 

the rich high-latitude countries, although Sahara greening might benefit west Africa), increases in extreme 

weather (e.g., drought, wildfire, flooding) events may damage crops and disrupt farming.” This is 

generally consistent with Section 9(c) of the TSD, which focuses on the United States and summarizes the 

literature regarding the adverse effects on crop yields due to droughts and other extreme events, which 

may offset the potential beneficial effects of elevated CO2, moderate temperature increases over the near 

term, and longer grower seasons. 

Comment (6-23):

Many commenters (e.g., 0362, 0731, 1807, 3383.1, 3642, 4184, 4249, 9786, and 10838) state their 

support for the Findings and describe various adverse effects that global warming will likely have on food 

production. According to the commenters, these include decreased crop yields, decreased food supply, 

increased application of pesticides, rising food costs and food shortages, and in worst case, famines. Some 

commenters (0725, 0803.1, 1318.1, 1543, 3104, 3601.1, 8516, and 8874) identify the following threats to 

food supply: (1) the impact of increasing ocean acidity and the collapse of marine ecosystems on 

fisheries; (2) crop impacts due to increasing viability of invasive pests; (3) impact of heat stress on farm 

animals and trees, which will decrease milk and maple syrup production; and (4) impacts of more intense 

periods of spring rain and flooding, along with longer summer droughts, on agriculture. A commenter 

(3400.1) from Washington State mentions that he is seeing increased water stress on agriculture. Some 

commenters (e.g., 3421, 4748, and 6894) express concern about the effects of extreme weather such as 

heat waves, droughts, floods, wildfires, and hurricanes have had or would have on the farming and fishing 

industries. 

Response (6-23):

We reviewed the comments provided and note they are generally consistent with the discussion of climate 

impacts on agriculture in Section 9 of the TSD, which summarizes the literature regarding the effects of 

climate change and associated changes in the frequency and magnitude of extreme events and 

17 
disturbances (i.e., extreme heat, droughts, wildfires, and insect pests) on crop yields, freshwater and 

marine fisheries, and livestock. 

6.2 Forestry 

Comment (6-24):

Several commenters (e.g., 3136.1, 3449.1, 3447.3, 3747.1, and 11166) state that specific aspects of the 

climate impacts evidence summarized in the TSD with respect to forestry do not support the 

Administrator’s endangerment finding. 

Response (6-24):

The specific issues that underlie these comments are addressed in the responses throughout this volume, 

and other volumes of the Response to Comments document. With regard to the commenters’ conclusion 

that the current science does not support an endangerment finding with respect to forestry, we disagree 

based on the scientific evidence before the Administrator. See the Findings, Section IV.B, “The Air 

Pollution Is Reasonably Anticipated to Endanger Both Public Health and Welfare,” for details on how the 

Administrator weighed the scientific evidence underlying her endangerment determination in general, and 

with regard to the forestry sector in particular. 

Comment (6-25):

A commenter (11453.1) submits a petition for EPA to resolve any dispute concerning the impact of 

elevated greenhouse gases on crop yields and forest growth to permit parties to present evidence 

supporting their conclusions, and to permit cross-examination of the competing experts offering that 

evidence. Absent such an approach, EPA cannot properly give credence to the vague claims of those who 

support a positive endangerment finding concerning crop yields and forest growth. The commenter claims 

the evidence supports the view that climate change has a net beneficial impact on forests. 

Response(6-25):

See Section I.C.3.e of the Findings for our response to the request to hold formal proceedings on the 

scientific evidence. See our responses to other comments in this section regarding the beneficial and 

adverse effects of climate change on U.S. forestry. 

Comment (6-26): 

Several commenters (e.g., 2750, 3136.1, 3347.1, 3394.1, 3449.1, 3722, 3747.1, and 7037) request 

clarification of or question what they believe to be EPA’s conclusions regarding the net effects of climate 

change on forestry (i.e., if EPA is concluding that the positive effects of climate change on forestry will 

ultimately be outweighed by negative effects or vice versa). For example, a commenter (2750) notes that 

EPA should explain how global warming conditions will lead to increases in wildfires and droughts yet 

overall forest growth will increase, as described in the TSD. Another commenter indicates that specific 

findings of climate change effects that may occur in some parts of the U.S. forestry sector do not provide 

sufficient information to judge the overall consequences and risks to U.S. forestry posed by greenhouse 

gases. The commenter states that the information presented in the TSD regarding forestry benefits 

resulting from climate change do not support the Administrator’s endangerment finding. 

Many commenters state that elevated CO2 levels and warming temperatures have produced and will 

continue to produce benefits for society, such as promoting increased plant and vegetation growth, forest 

products, and carbon storage capacity. Some commenters note there are numerous examples of forestry 

18 
benefits cited in the TSD and conclude that the positive benefits of climate change on forestry will 

outweigh any adverse effects. At least one commenter (3596.3) provides a literature review by Dr. Craig 

Idso of the Center for the Study of Carbon Dioxide regarding studies that the commenter asserts provide 

evidence not cited in the TSD of the positive benefits of elevated CO2 levels on plants. The commenter 

states that this literature provides scientific evidence that increasing atmospheric CO2 levels will increase 

plant (including tree species) productivity, water-use efficiency, resistance and resilience to disease, while 

reducing adverse effects of insect pest herbivory. 

Response (6-26): 

Section 10 of the TSD summarizes the findings of the assessment literature regarding both the adverse 

and beneficial impacts of climate change on the forestry sector. Here, we will first discuss the 

commenters’ concerns with respect to the TSD’s characterization of the science regarding the net effects 

of climate change on forestry, and then address the request for clarification regarding how the 

Administrator weighed the evidence. 

The assessment reports indicate that climate change can in some cases have a beneficial impact on forest 

growth and productivity. However, commenters fail to take account the variation in the impact climate 

change can have on forests and the source of this variation. Forests are complex, varied, and dynamic 

ecosystems, and there are many ways in which climate change can impact the circumstances relevant to 

the health of the widely differing forest ecosystems. As such, it is a serious oversimplification to conclude 

that climate change or increased CO2 concentration and temperature will result in increased growth or 

productivity of a forest. The diversity of forest ecosystems and the many ways in which climate change 

affects the environment of a forest mean there is not a single or simple answer to the question of how 

climate may impact forest growth, productivity, and overall health. The TSD and the underlying 

assessment reports reflect this complexity and provide a sound basis for assessing the varying impacts of 

climate change on forestry. 

The assessment literature recognizes the potential in the near term for increased forest growth and 

productivity in certain areas of the country. Increased CO2 concentrations can have a fertilizing effect, as 

rising CO2 will very likely increase photosynthesis for forests. However, the increased photosynthesis 

will likely only increase wood production in young forests on fertile soils, and new studies suggest that 

direct CO2 effects on tree growth may be lower than previously assumed (Easterling et al., 2007). 

Additionally, the initial increase in growth increments may be limited by competition, disturbance, air 

pollutants (primarily tropospheric ozone), nutrient limitations, ecological processes, and other factors, and 

the response is site- and species-specific (Easterling et al., 2007). The areas where forest growth increases 

could occur include areas where water is not an otherwise limiting factor. Nitrogen deposition and 

warmer temperatures have very likely increased forest growth broadly where water is not limiting and 

will continue to do so in the near future. Forest growth from climate change can also occur in regions 

where tree growth has historically been limited by low temperatures and short growing seasons, with 

climate change leading to increased temperature and longer growing season. However, increased growth 

in some areas or species can be at the expense of other species. Mountain forests are increasingly 

encroached upon from other species native to adjacent lowlands, while simultaneously losing high 

altitude habitats due to warming (Fischlin et al., 2007). For example, in Colorado, aspen have advanced 

into the more cold-tolerant spruce-fir forests over the past 100 years. 

Climate change, particularly changes in precipitation patterns and extreme weather events, can also 

adversely impact forest growth and productivity. As summarized in Section 10 of the TSD, the IPCC 

(Field et al., 2007) concluded that forest growth is slowing in areas subject to drought. For example, on 

dry south-facing slopes in Alaska, growth of white spruce has decreased over the last 90 years, due to 

increased drought stress. A combination of warmer temperatures and insect infestations has resulted in 

economically significant losses of the forest resource base in Alaska (Field et al., 2007). In semi-arid 

19 
forests of the southwestern United States, growth rates have decreased since 1895, correlated with 

drought from warming temperatures (Field et al., 2007). 

Forests are also significantly impacted by disturbances such as drought, storms, insect outbreaks, and 

wildfire. While in some cases a changing climate may have positive impacts on the productivity of forest 

systems, changes in disturbance patterns are expected to have a substantial impact on overall gains or 

losses. Wildfires and droughts, among other extreme events (e.g., hurricanes) that can cause forest 

damage, pose the largest threats over time to forest ecosystems. A climate-change-related increase in 

frequency or intensity of such disturbances is at least as important to ecosystem function as incremental 

changes in temperature, precipitation, atmospheric CO2 concentration, nitrogen deposition, and ozone 

pollution. 

As summarized in Section 10 of the TSD, disturbances partially or completely change forest ecosystem 

structure and species composition, cause short-term productivity and carbon storage loss, allow better 

opportunities for invasive species to become established, and command more public and management 

attention and resources. Climate change has very likely increased the size and number of forest fires, 

insect outbreaks, and tree mortality in the interior West, the Southwest, and Alaska, and will continue to 

do so. Several lines of evidence suggest that large, stand-replacing wildfires will likely increase in 

frequency over the next several decades because of climate warming (Ryan et al., 2008). General climate 

warming encourages wildfires by extending the summer period that dries fuels, promoting easier ignition 

and faster spread (Field et al., 2007). 

Following the assessment literature, the TSD also describes that insects and diseases are a natural part of 

forested ecosystems and outbreaks often have complex causes. The effects of insects and diseases can 

vary from defoliation and retarded growth, to timber damage, to wide-scale forest diebacks. For example, 

during the 1990s, Alaska’s Kenai Peninsula experienced an outbreak of spruce bark beetle over 6,200 

square miles (16,000 km2

) with 10% to 20% tree mortality (Anisimov et al., 2007). In addition, the TSD 

summarizes the following conclusions of the CCSP (Ryan et al., 2008) that 1) the ranges of the mountain 

pine beetle and southern pine beetle are projected to expand northward as a result of average temperature 

increases, and 2) increased probability of spruce beetle outbreak as well as increase in climate suitability 

for mountain pine beetle attack in high-elevation ecosystems has also been projected in response to 

warming. Insect life cycles, which are sensitive to climate change, can also be a factor in pest outbreaks. 

The assessment literature notes that many northern insects have a two-year life cycle, and warmer winter 

temperatures allow a larger fraction of overwintering larvae to survive. Recent warming trends in the 

United States have led to earlier spring activity of insects and proliferation of some species, such as the 

mountain pine beetle (Easterling et al., 2007). Climate change may also indirectly affect insect outbreaks 

by affecting the overall health and productivity of trees. For example, susceptibility of trees to insects is 

increased when multiyear droughts degrade the trees’ ability to generate defensive chemicals (Field, et al., 

2007). Warmer temperatures have already enhanced the opportunities for insect spread across the 

landscape in the United States and other world regions (Easterling et al., 2007). Climate change can shift 

the current boundaries of insects and pathogens and modify tree physiology and tree defense. An increase 

in climate extremes may also promote plant disease and pest outbreaks (Easterling et al., 2007). 

The impacts on a forest ecosystem from drought, storms, insect outbreaks, and wildfire are serious and 

can be very significant. Over a short period of time they can lead to dramatic and fundamental adverse 

changes in the health of the forest ecosystem, in contrast to the incremental, slow, and limited beneficial 

effect to forest growth that can occur in some areas and forests. 

Regarding the literature submitted by commenters, we find that these studies have not comprehensively 

included all relevant factors (e.g., CO2 increases, temperature increases, precipitation changes, weather 

variability changes, ground-level ozone changes, soil nutrient changes) for assessing the overall effect of 

20 
climate change on forestry. For this reason, it is important for EPA to rely primarily on the assessment 

literature, which integrates and synthesizes the results of many studies across multiple relevant disciplines 

in order to gain a more comprehensive view of the impacts of elevated CO2 concentrations and climate 

change on U.S. forestry. 

Regarding the specific comment on the dual effects of increased productivity under some circumstances 

and increased wildfires under others, we do not view these effects as inconsistent with the underlying 

science. The effect of gradual increases in productivity due to rising CO2 atmospheric concentrations and 

modest average temperature increases can be thought of as a change in average conditions, whereas the 

effect of increased risk of wildfires can be thought of as episodic events—the frequency and severity of 

which are altered by changes in underlying average conditions; therefore, these effects can coexist. Due to 

regional differences, where some regions are already more susceptible to wildfires, it is also possible to 

have both increases in wildfires in some regions yet overall increases in productivity in other regions, 

depending on local conditions and the rate and magnitude of climate change. Importantly, we note Field 

et al. (2007) indicates that the increase in overall forest growth will not be uniform, and that the effects of 

disturbances such as wildfire, insect outbreaks, and drought will also have regional variability. 

Regarding the comment that some impacts may occur in some parts of the United States and not others, 

additional regional information has now been brought into the TSD. Specifically, we included 

conclusions from the latest assessment from USGCRP, which reports that certain regions of the United 

States will be particularly susceptible to increased wildfire, including Alaska and the western United 

States (Karl et al., 2009). Changes in precipitation and weather extremes are also important 

considerations, accounting for part of the regional variability in forest response (Easterling et al., 2007). 

The CCSP (Ryan et al. 2008) concludes that forest productivity varies with annual precipitation across 

broad gradients and with interannual variability within sites. They state that if existing trends in 

precipitation continue: 

Forest productivity will likely decrease in the interior West, the Southwest, eastern portions of the 

Southeast, and Alaska. 

Forest productivity will likely increase in the northeastern United States, the Lake States, and in 

western portions of the Southeast. 

An increase in drought events will very likely reduce forest productivity wherever these events 

occur. 

We disagree with the comments that the existence of benefits or potential for benefits as a result of 

elevated CO2 concentrations and climate change does not allow for the conclusion that the evidence, 

when viewed in its entirety, shows current adverse effects and the potential for increasing risks. We have 

provided further clarification of the Administrator’s consideration of the forestry sector for the 

endangerment to public welfare finding in Section IV.B.2 of the Findings. See also Volume 1, Section 

1.4, “Consideration of Net (Adverse and Beneficial) Effects,” for our response to comments on the 

general issue of evaluating net effects in the context of this action. 

Comment (6-27):

A commenter (3722) states that as forests grow and expand their ranges, they sequester ever larger 

quantities of carbon, which helps reduce increasing concentrations of atmospheric CO2. As supporting 

evidence, the commenter cites information from a NASA Web site stating that “vast vegetated areas of 

the Northern Hemisphere, primarily from the boreal and temperate forests of North America and Eurasia, 

have been consuming and storing about one-quarter of CO2 emissions during the past 15 years.” Another 

commenter (3394.1) indicates that EPA ignores a singular benefit of increased forest growth: the 

21 
correlated increase in CO2 sinks. The commenter notes that neglecting these complex interactions skews 

the analysis of endangerment from GHG emissions and likewise feeds into what most likely are 

inaccurate projections of future GHG concentrations. 

Response (6-27):

EPA does not ignore the effects of carbon sequestration in forests, and Section 10(a) of the TSD clearly 

acknowledges that forest productivity has increased in the United States since the middle of the 20th

century. The purpose of the TSD, however, is to summarize the scientific assessment literature regarding 

how climate change and elevated CO2 may impact forests. Thus, the TSD describes the scientific findings 

with respect to forest productivity (Section 10[a]), wildfire and drought risk (Section 10[b]), forest 

composition (Section 10[c]), and insects and diseases (Section 10[d]). In each of these sections, the recent 

findings of the assessment literature are summarized, including both adverse and beneficial impacts and 

discussion of uncertainties. 

As summarized by the TSD, the literature indicates that there will be both gains and losses in carbon 

storage in U.S. forests as a result of climate change. How these changes in forest carbon, or more broadly 

in terrestrial ecosystem carbon, will act as either positive or negative feedbacks under climate change are 

generally taken into account in the future projections of climate change in the TSD. Response to 

comments on this latter issue is addressed in Volume 4 of this Response to Comments document, wherein 

we note that Meehl (2007) found “unanimous agreement among the coupled climate carbon cycle models 

driven by emission scenarios run so far that future climate change would reduce the efficiency of the 

Earth system (land and ocean) to absorb anthropogenic CO2. As a result, an increasingly large fraction of 

anthropogenic CO2 would stay airborne in the atmosphere under a warmer climate.” 

Comment (6-28):

A commenter (1594.1) indicates IPCC relies on predictions from models rather than factual scientific 

studies to evaluate impacts to forestry. The commenter contends that global warming and rising 

atmospheric CO2 levels have and will continue to benefit plant/vegetation growth, and that these benefits 

dwarf any adverse effects otherwise affecting humans. The commenter indicates there is abundant 

evidence to support this whereas factual evidence is lacking for the adverse effects. The commenter 

indicates that forests become more productive and create more biomass as a result of rising CO2 levels in 

the atmosphere. As supporting evidence, the commenter submits a scientific journal article by Norby et al. 

(2005) that finds that elevated CO2 levels resulted in a 23% increase in forest productivity within the 

study area. The commenter notes these are actual field studies, not computer model simulations. 

Response (6-28):

The commenter’s assertion that the IPCC and TSD rely solely on computer model simulations is 

incorrect. The TSD summarizes the latest conclusions of the assessment literature on climate change 

effects on forest productivity and growth from experimental field studies, observations data, and modeled 

projections. The TSD notes the general findings from a number of recent syntheses using data from 

FACE study sites that show North American forests will absorb more CO2 and might retain more carbon 

as atmospheric CO2 increases. Included in this series of studies is Norby et al. (2005), which the 

commenter references. Furthermore, the TSD also summarizes a number of studies reported by the IPCC 

(Field et al., 2007) that demonstrate (based on observed data) the connection between changes in U.S. 

forest growth and climate variables. 

As summarized in the TSD, in some areas, forest productivity will likely decrease due to increased 

increase climate variability and increases in disturbances (e.g., droughts, wildfires, pest outbreaks). In 

other words, the beneficial impact of elevated CO2 on forest growth and productivity is only one of the 

possible effects of climate change on forests. As addressed in the assessment literature and summarized in 

22 
the TSD, drought, pests, and wildfires will also affect growth and productivity. Further, forest 

productivity is known to be sensitive to changes in climate variables that may be modulated where water 

is limiting (Ryan et al., 2008). Similarly, the TSD also notes that young forests on fertile soils will 

achieve higher productivity from elevated atmospheric CO2 concentrations whereas older forests may not 

(Ryan et al., 2008). Therefore, the effects of productivity will not be uniform and will vary significantly 

by region. The commenter has not provided scientific evidence to support the assertion that the beneficial 

impacts of elevated CO2 on plant growth and productivity will “dwarf” the adverse effects. The 

assessment literature clearly indicates that the picture is much more complex, and the TSD accurately 

describes the multiple effects climate change may have on forests. 

Comment (6-29): 

Commenters (3347.1, 3347.3, and 3394.1) indicate that the Administrator fails to account for all of the 

benefits of climate change listed in the TSD in Section 10(a), and ignores the benefit of carbon sink 

potential with increased forest growth. A commenter (3347.3) indicates that the EPA does not present or 

consider the carbon storage potential associated with an increase in forest growth of 10% to 20%. The 

commenter indicates that the tone of the TSD ignores the scale of the net carbon sink of forests (some 256 

million tons of carbon [1 billion tons of CO2 per year]) as provided by King et al. (2008) and those 

provided provide by Joyce et al. (2001). 

 

Response (6-29): 

See the Findings, Section IV.B.2, for a description of how the Administrator evaluated the adverse and 

beneficial impacts of climate change on forests. In response to comments, this section was expanded and 

provides additional detail as compared to the Proposed Findings. See also Volume 1, Section 1.4, 

“Consideration of Net (Adverse and Beneficial) Effects,” for our response to comments on the general 

issue of evaluating net effects in the context of this action. See our response above (6-27) on the issue of 

the treatment of carbon storage potential in the Findings and in the underlying science summarized in the 

TSD. 

Comment (6-30): 

A commenter (3136.1) indicates that the TSD notes that “Bioclimatic modeling...suggests that, over the 

next century, tree species richness will decrease in most parts of the coterminous United States even 

though long-term trends (millennia) ultimately favor increased richness in some locations.” The 

commenter notes that, on the whole, there is no quantitative analysis given of the net change in richness, 

and therefore no support for the Proposed Findings. Another commenter (7020) indicates that the sections 

on forest productivity, wildfire and drought, forest composition, and insects and diseases need to be 

improved to quantify the impacts and risks. 

Response (6-30): 

With respect to tree species richness, the TSD reflects the state of scientific research and knowledge on 

this topic based on the major scientific assessment reports. We recognize that there is limited information 

on quantitative projections of species richness over the full range of tree species types. Section 15 of the 

TSD notes, “In the United States, some common forest types are projected to expand, such as oakhickory; others are projected to contract, such as maple-beech-birch. Still others, such as spruce-fir, are 

likely to disappear from the contiguous United States (Karl et al., 2009).” The TSD reports the most 

recent findings (both qualitative and quantitative) from the scientific assessment literature indicating how 

distributions of tree species and forest composition may shift or be modified as a result of climate change. 

Examples of where the TSD summarizes the scientific literature with quantitative estimates and 

uncertainty where available are included are provided below. The commenters’ contention that there is 

23 
“no quantitative analysis” is incorrect. Where quantitative information is available in the underlying 

literature, we have summarized it in the TSD. We note the commenters do not provide additional 

scientific literature on these issues. 

In Section 10(b) on forest productivity, the TSD notes: 

Overall forest growth in North America will likely increase modestly (10% to 20%) as a result of 

extended growing seasons and elevated CO2 over the next century but with important spatial and 

temporal variation. 

Forest growth appears to be slowly accelerating (less than 1% per decade) in regions where tree 

growth has historically been limited by low temperatures and short growing seasons. 

The length of the vegetation growing season has increased an average of two days per decade 

since 1950 in the conterminous United States, with most of the increase resulting from earlier 

spring warming. 

In Section 10(b) on wildfire and drought, the TSD notes: 

Since 1980, an average of about 22,000 km2

/year (8,500 mi2/year) has burned in wildfires, almost 

twice the 1920-1980 average of about 13,000 km2

/year (5,020 mi2/year). 

The forested area burned in the western United States from 1987-2003 is 6.7 times the area 

burned from 1970-1986. 

Disturbances such as wildfire and insect outbreaks are increasing and are likely to intensify in a 

warmer future with drier soils and longer growing seasons (very high confidence).

1

Earlier spring snowmelt has led to longer growing seasons and drought, especially at higher 

elevations, where the increase in wildfire activity has been greatest. 

In the southwestern United States, fire activity is correlated with El Niño-Southern Oscillation 

(ENSO) positive phases (La Niña) and higher Palmer Drought Severity Indices. El Niño events 

tend to bring wetter conditions to the southwest, enhancing the production of fine fuels and, La 

Niña events tend to bring drier conditions. Major fire years tend to follow the switching from El 

Niño to La Niña conditions due to buildup of material during wet years followed by desiccation 

during a dry year, whereas small fires are strongly associated directly with previous year’s 

drought. Other modes of atmospheric and oceanic variability are known to impact temperature 

and precipitation (Gutowski et al., 2008) and hence wildfire patterns and activity. 

In Section 10(c) on forest composition the TSD notes: 

Aerial photographs show increased shrub abundance in 70% of 200 locations. 

Along the Arctic to sub-Arctic boundary, the tree-line has moved about 6 mi (10 km) northwards, 

and 2% of Alaskan tundra on the Seward Peninsula has been displaced by forest in the past 50 

years. 

The pattern of northward and upward tree-line advances is comparable with earlier Holocene 

changes. 

Analyses of satellite images indicate that the length of growing season is increasing by three days 

per decade in Alaska. 

Furthermore, the TSD notes that evidence of shifts in tree species has been observed in the Green 

Mountains of Vermont where temperatures have risen 2°F to 4°F (4°C to 7 C) in the last 40 years and the 

 

1

 According to IPCC terminology, “very high confidence” conveys a 9 out of 10 chance of being correct. See Box 

1.2 for a full description of IPCC’s uncertainty terms. 

24 
ranges of some mountain tree species in this region have shifted to higher elevations by 350 feet (107 m) 

in the last 40 years. Tree communities were relatively unchanged at low and high elevations but in midelevation transition zones, the changes have been dramatic. Tree species suited to cold conditions in the 

Green Mountains declined from 43% to 18% while species suited to warmer conditions increased from 

57% to 82%. 

In Section 10(d) on insects and diseases, the TSD notes that recent warming trends in the United States 

have led to earlier spring activity of insects and proliferation of some species, such as the mountain pine 

beetle (Easterling et al., 2007). During the 1990s, Alaska’s Kenai Peninsula experienced an outbreak of 

spruce bark beetle over 6,200 square miles (16,000 km2

) with 10% to 20% tree mortality (Anisimov et al., 

2007). 

See the Findings, Sections II.A.2, “Summary of Response to Key Legal Comments on the Interpretation 

of the Section 202(a) Endangerment and Cause or Contribute Test,” and IV.B, “The Air Pollution Is 

Reasonably Anticipated to Endanger Both Public Health and Welfare,” for EPA’s response to comments 

on the general issue of quantifying the relative impacts of climate change. 

Comment (6-31): 

Several commenters (2972.1, 3136.1, 3596.2, 3722, and 7037) argue that large-scale circulation patterns 

and natural climate variations that occur over multiple decades (i.e., the Atlantic Multi-Decadal 

Oscillation (AMO), ENSO, and Pacific Decadal Oscillation (PDO))—are the primary drivers for wildfires 

and drought in the western United States rather than anthropogenic climate change. Another commenter 

argues that patterns in wildfires and drought frequency are a complex interplay between forest 

management practices, natural climate oscillations, and anthropogenic climate change, and that the TSD 

fails to take these issues into account. A commenter (2972.1) suggests that this indicates that it remains 

uncertain that wildfires will be more likely in the future. 

In addition, the commenters describe findings from the following studies, which they claim provide 

supporting scientific evidence for their argument. 

Schoennagle et al. (2005) conclude that ENSO and PDO variability affect drought-induced fire 

occurrence in the Rocky Mountain subalpine forests. There remains considerable uncertainty 

regarding the effects of CO2-induced warming at regional scales. 

Westerling and Swetnam (2003) conclude that drought and wildfire are associated with warm 

phases of ENSO and PDO in the Pacific Northwest and northern Rockies while the opposite 

occurs in the Southwest and southern Rockies. 

McCabe et al. (2004) conclude that wildfire patterns across the United States involve complex 

interactions with local and regional climate related to sea surface patterns in both the Pacific and 

Atlantic Oceans that are primarily driven by natural oscillations such as the PDO and the AMO. 

Drought and wildfire are associated with warm phases of ENSO and PDO in the Pacific 

Northwest and northern Rockies, while the opposite occurs in the Southwest and southern 

Rockies. 

Riano et al. (2007) conclude that there was no significant global annual upward or downward 

trend in burned area between 1981 and 2000 using NOAA-NASA Pathfinder and latitude was not 

determinative, as divergent fire patterns were encountered for various land cover areas at the 

same latitude. 

Kitzberger et al. (2007) conclude that sea surface temperature variability in the Pacific and 

Atlantic oceans (such as those associated with ENSO, PDO, and AMO) is a dominant factor in 

whether wildfires are widespread across the West, including recent wildfires. The key issue is that 

25 
Zhang et al. (2008) offer further evidence that the AMO cycles are driven by natural, rather than 

anthropogenic, forces as we are currently in the warm phase of the AMO and, given the very 

long-term nature of this Atlantic Ocean/climate phenomenon, we are likely to be in the AMO 

warm phase for quite some time to come. 

Response (6-31): 

See Volume 3 for EPA’s responses to comments broadly focused on the extent to which observed climate 

change can be attributed to the observed increase in atmospheric GHG concentrations. See Volume 2 for 

EPA’s responses to comments on observed changes in extreme events and precipitation. Here, we 

concentrate on the issues raised by the commenters on the effect of large-scale circulation patterns and 

natural climate variations on wildfires and drought in the western United States.S and the extent to which 

they are related. 

The scientific assessment literature summarized in the TSD supports the view that wildfire and drought 

patterns are a complex interplay between forest management practices, natural climate oscillations, and 

anthropogenic climate change. The TSD does not state that anthropogenic climate change is the primary 

driver of wildfire patterns; rather, the discussion in Section 10 of the TSD indicates that wildfire patterns 

involve multiple influencing factors but, despite these complicated relationships, there remains evidence 

that climate change has likely influenced forest fires in some regions and will continue to do so. With 

respect to the influence of natural climate variations such as the AMO, ENSO, and PDO on wildfire, the 

TSD includes the following IPCC conclusions (Easterling et al., 2007; Field et al., 2007): (1) ENSO 

events are likely to intensify with climate change, with subsequent changes in vegetation and water 

availability; (2) in the southwestern United States, fire activity is correlated with ENSO positive phases 

and higher Palmer Drought Severity Indices; and (3) higher temperatures in the future will likely extend 

fire seasons throughout the western United States, with more fires occurring earlier and later than is 

currently typical, increasing the total area burned in some regions. We have revised Section 10(b) of the 

final TSD to provide further clarification on climate variability and the relationship between fire activity 

and natural climate variations. 

We disagree with the commenters’ assertion that anthropogenic climate change is not a significant 

influence with respect to the frequency and intensity of wildfires and drought in the western United 

States..In addition, we disagree with the comment that it remains uncertain that climate change can 

increase wildfire frequency and severity in the future. The TSD notes multiple lines of evidence that link 

warming temperatures and other climate change effects (i.e., longer summer season, timing of snowmelt) 

to an increased probability and risk of wildfires and wildfire frequency. As summarized in the TSD, the 

IPCC concludes that large, stand-replacing wildfires will likely increase in frequency over the next 

several decades because of climate warming (Ryan et al., 2008). The TSD specifically notes that general 

climate warming encourages wildfires by extending the summer period that dries fuels, promoting easier 

ignition and faster spread. With regard to certainty, the IPCC (Field et al., 2007) concludes with very high 

confidence that disturbances such as wildfire and insect outbreaks are increasing and are likely to 

intensify in a warmer future with drier soils and longer growing seasons. 

Climate change effects on the frequency and severity of wildfires will vary by region. Nevertheless, the 

assessment literature as summarized in the TSD concludes that, “Climate change has very likely increased 

the size and number of forest fires, insect outbreaks, and tree mortality in the interior West, the 

Southwest, and Alaska, and will continue to do so.” The TSD also includes the following conclusions 

from the IPCC and USGCRP: 

26 
Evidence indicates that since 1980, an average of about 22,000 km2

/year has burned in wildfires 

in the United States, almost twice the 1920-1980 average of about 13,000 km2

/year (Field et al., 

2007). 

Wildfires are projected to increase, especially in the Southwest (Karl et al., 2009). 

Alaska has experienced large increases in fire, with the area burned more than doubling in recent 

decades, and as in the western United States, higher air temperature is a key factor (Karl et al., 

2009). 

We reviewed the submitted articles, and our responses to each of the specific studies are as follows: 

Regarding the Schoennagle et al. (2005) study, we find that this paper is generally consistent with 

the underlying scientific literature summarized in Section 10 of the TSD, which states that there 

are multiple influencing factors (e.g., forest management practices, natural climate variations, and 

climate change) on wildfire patterns. 

Regarding the Westerling and Swetnam (2003) study, we find that this paper is generally 

consistent with the underlying scientific literature summarized in Section 10 of the TSD, which 

states that there are multiple influencing factors (e.g., forest management practices, natural 

climate variations, and climate change) on wildfire patterns and which reports that in the 

southwestern United States, fire activity is correlated with ENSO positive phases (La Niña) and 

higher Palmer Drought Severity Indices (Field et al., 2007). 

Regarding the McCabe et al. (2004) study, we note that, although the study found that the AMO 

and PDO likely have important influences on changes in drought frequency, the study also found 

that one component of the temporal pattern in drought matched well with the trends in Northern 

Hemisphere temperatures. The component that matched well with temperature trends, and which 

therefore might be an indicator of how patterns of drought might respond to continued increasing 

global average temperatures, included increasing drought in locations such as California and the 

northern Rockies, but decreasing drought in locations such as the Gulf Coast and the Pacific 

Northwest. We note this is generally consistent with the underlying scientific literature of the 

TSD as summarized in Section 10, which states that there are multiple influencing factors (e.g., 

forest management practices, natural climate variations, and climate change) on wildfire patterns. 

Therefore, we find no evidence that this study contradicts the IPCC conclusion summarized in the 

TSD that large, stand-replacing wildfires will likely increase in frequency over the next several 

decades because of climate warming (Ryan et al., 2008). 

Regarding the Riano et al. (2007) study, although the authors found no global trends in burned 

area in the last 20 years, they found that burned area “did increase for the northern hemisphere in 

the mid-latitudes and subtropical areas of North America, Africa and southwest Asia” (including 

an earlier start for the fire season). This increase was compensated by decreases in tropical 

Southeast Asia and Central America. These conclusions are generally consistent with the TSD. 

Therefore, we find no evidence that contradicts the conclusions summarized in the TSD. This 

study is also discussed in our response (6-32) below. 

Regarding the Kitzberger et al. (2007) study, we conclude this is generally consistent with the 

underlying scientific literature summarized in Section 10 of the TSD, which states that there are 

multiple influencing factors (e.g., forest management practices, natural climate variations, and 

changes in climate variables) on wildfire patterns. 

Regarding the Zhang et al. (2008) study, we conclude it does not include any information about 

fire activity or drought in the western United States and therefore was not considered here. 

For these reasons, we conclude that the TSD’s summary of the relationship between projected climate 

change and wildfire patterns is consistent with and accurately reflects the conclusions of the body of 

scientific literature. 

27 
Comment (6-32): 

Two commenters (3283.1, 5846) indicate forest management practices in the past have contributed to the 

prevalence of recent wildfires, not climate change, and that risk would be reduced if land managers such 

as the USDA Forest Service were to implement better practices in the future. 

Response (6-32): 

Section 10(b) of the TSD summarizes the findings of the assessment literature with respect to the impact 

of climate change on wildfires. This discussion presents findings regarding observed changes to United 

States wildfires as well as information on projected trends. We note that the TSD includes multiple lines 

of evidence that climate change will affect wildfire risk and frequency; see the preceding response (6-31) 

for more information. We disagree with the commenters’ assertion that anthropogenic climate change is 

not a significant factor that will influence the frequency and intensity of wildfires and drought in the 

western United States. 

Consideration of forest management practices and concerns regarding the practices implemented by the 

USDA Forest Service are not germane to this action. See the Findings, Section III.C, “Adaptation and 

Mitigation,” for our response to comments on the treatment of adaptation and mitigation in the Findings. 

Comment (6-33): 

At least one commenter (11459) indicates that current wildfires in California are receiving a lot of media 

attention, and at least one reporter has attributed its intensity to global warming. The commenter notes 

that anyone familiar with southern California wildfires are aware that the Santa Ana winds are one of the 

biggest factors in the ability of firefighters to control these wildfires. The commenter further states that in 

the 1930s, about 38 million acres were burned in the United States per year, and an average of 25 million 

acres per year were burned in the 1920s, and new fires were allowed to burn out because most of the 

forested areas had no buildings or people. In the 1990s, the average acreage burned was about 5 million 

per year. In the cool 1960s, the acreage burned also averaged about 5 million per year. According to the 

commenter, the big change has been the fuel buildup in these forested areas. The commenter cites Riano 

et al. (2007), which examined global satellite data for the period of July 1981 through December 2000. 

The commenter notes the study showed an increase in burned area in the western United States, but a 

decrease globally. 

Response (6-33):

The commenter appears to dispute that wildfire activity, particularly in California, can be attributed to 

global warming. See Volume 3 for our responses to comments regarding the attribution of observed 

changes from climate including extreme events. We do not disagree with the commenter’s statement that 

wildfire activity in southern California is correlated with climate systems such as the Santa Ana winds or 

El Niño and La Niña conditions. However, we note the commenter does not provide specific references 

for the information on acres burned in the United States, and we disagree that observed climate change 

has played no role in past wildfire activity. The scientific assessment literature as reflected in the TSD is 

clear that, with warming temperatures, both the frequency of large wildfires and the length of the fire 

season have increased substantially in recent decades in North America and will continue in the future. 

See our response (6-31) above for further response to this issue, including the Riano et al. (2007) study. 

Comment (6-34): 

28 
Commenters (3347.1, 3447.3, 3747.1) assert that the information presented in the TSD regarding 

uncertainties in models used to represent disturbances such as wildfires do not support the 

Administrator’s endangerment finding. The commenters state that because the TSD’s discussion of 

disturbances such as wildfires relies on models with significant uncertainties, this information is not 

robust enough to support the Administrator’s endangerment finding. One commenter additionally argued 

that EPA must provide estimates of nationwide net changes in wildfire frequency, scope, and intensity 

before the Agency could assess the risks to public health and welfare. 

 

Similarly, a number of commenters (3347.3, 3427.1, 3440.1, 5846) argue that the TSD does not provide 

evidence to substantiate the statement from the Proposed Findings that climate change will cause more 

wildfires; thus, increasing wildfire frequency cannot be used as a justification for the endangerment 

finding. One commenter provides the following references as evidence that wildfire frequency is not 

increasing: Bergeron and Archambault (1993) and Bergeron et al. (2004). According to another 

commenter (3394.1), Higuera et al. (2009) find that past climate change has not been directly linked to 

increases in wildfires, that a variety of other factors are instead more closely related to increases in 

wildfire events, and that climate interactions with other factors can override any influence of climate 

change on wildfire events. Another commenter (3347.1) states that the TSD does not mention the benefit 

of forest fires for managed forest ecosystems. 

Response (6-34): 

The purpose of the TSD is to summarize the findings of the assessment literature, with appropriate 

uncertainty information. In this case, Section 10(b) summarizes the science on drought and wildfire risk. 

The TSD summary cites the IPCC (Field et al., 2007) “regarding a number of observed changes to United 

States wildfire size and frequency, often associated these changes with changes in average temperatures.” 

These findings include: 

Since 1980, an average of about 22,000 km2

/year (8,500 mi2/year) has burned in wildfires, almost 

twice the 1920-1980 average of about 13,000 km2

/year (5,020 mi2/year). 

The forested area burned in the western United States from 1987-2003 is 6.7 times the area 

burned from 1970-1986. 

Human vulnerability to wildfires has increased, with a rising population in the wildland-urban 

interface. 

In the last three decades, the wildfire season in the western United States has increased by 78 

days, and burn durations of fires greater than 1,000 hectares (ha) (2,470 acres) have increased 

from 7.5 to 37.1 days, in response to a spring/summer warming of 1.6°F (0.87°C). 

Earlier spring snowmelt has led to longer growing seasons and drought, especially at higher 

elevations, where the increase in wildfire activity has been greatest. 

In the southwestern United States, fire activity is correlated with ENSO positive phases (La Niña) 

and higher Palmer Drought Severity Indices. El Niño events tend to bring wetter conditions to the 

southwest, enhancing the production of fine fuels and, La Niña events tend to bring drier 

conditions. Major fire years tend to follow the switching from El Niño to La Niña conditions due 

to buildup of material during wet years followed by desiccation during a dry year, whereas small 

fires are strongly associated directly with previous year’s drought. Other modes of atmospheric 

and oceanic variability are known to impact temperature and precipitation (Gutowski et al., 2008) 

and hence wildfire patterns and activity. 

Increased temperature in the future will likely extend fire seasons throughout the western United 

States, with more fires occurring earlier and later than is currently typical, and will increase the 

total area burned in some regions. 

29 
As noted by the commenter, the TSD also clearly acknowledges the state of the science with respect to 

wildfire modeling. It states, “Though fires and extreme events are not well represented in models, current 

climate modeling studies suggest that increased temperatures and longer growing seasons will elevate fire 

risk in connection with increased aridity.” The finding that fires and extreme events are not well 

represented in models does not preclude the consideration of how wildfires are changing now and are 

expected to continue changing due to climate change. We note the impacts and risks to forestry are 

assessed from more than just projections of extreme events related to future climate. Importantly, the TSD 

summarizes several studies that are based on observed changes, including the statement that “climate 

change has very likely increased the size and number of forest fires.” 

We have reviewed the submitted studies and note that the IPCC (Easterling et al., 2007) cites the 

Bergeron et al. (2004) study; therefore, it has been reviewed and considered by the assessment literature 

on which the TSD primarily relies. Regarding the Bergeron and Archambault (1993) study, we note this 

study finds decreased fire frequency over a 300-year period after the end of the “Little Ice Age” (1688-

1988). In addition, both of these studies focus on a specific fire regime in a boreal forest of Canada and 

thus, are not representative of the overall risks and impacts to U.S. forestry from climate change. 

Regarding the Higuera et al. (2009) study, we note that the authors examine past relationships between 

fire dynamics and climate change over millennium time scales (the time scales over which humaninduced climate change is generally most relevant span the near term over the next few decades and the 

longer term out to the end of the 21st century). Furthermore, we note the important contribution of the 

Higuera et al. study to our understanding of climate change effects on wildfires, but we do not conclude 

from the Higuera et al. study that our understanding that climate change increases the risk of wildfires, as 

stated in the assessment literature, should be immediately and fundamentally altered. Higuera et al. note 

that, “Although the response of fire regimes to climate change is complex and will vary regionally, there 

is general agreement that area burned across arctic and boreal regions will increase over the next century 

as climate change lengthens the fire season, decreases effective moisture, and increases ignition rates . 

These predictions are based primarily on short-term fire–climate relationships established in recent 

decades, but paleoecological studies also suggest that changes in relative moisture have influenced fire 

regimes throughout the Holocene.” 

The TSD focuses on evaluating the impacts from changes in climate on the forestry sector; therefore, we 

do not evaluate the benefits of managed forest fires as related to managed forest ecosystems. See the 

Findings, Section III.C, “Adaptation and Mitigation,” for our response to comments on the treatment of 

adaptation and mitigation in the Findings. Finally, please see Section IV.B, of the Findings for a 

discussion of how the Administrator weighed the evidence related to the impacts of climate change on 

forests, and Sections II.A and IV.B of the Findings for our response to comments regarding the treatment 

of uncertainty in the Administrator’s endangerment determination. 

Comment (6-35):

A commenter (3347.1) indicates that the TSD attributes to one of the IPCC chapters the implicit claim 

that climate change will increase the size and severity of wildfires in the United States. However, the 

commenter asserts that Easterling et al. (2007) make no such statement. The commenter states that EPA 

asserts climate change will cause more wildfires in the Summary section of the Proposed Findings, but 

that this claim is not supported anywhere else in the notice. 

Response (6-35):

We have reviewed the TSD and Findings and disagree with the commenter’s characterization of the 

Proposed Findings and TSD with regard to citing the IPCC’s finding that climate change will increase the 

size and severity of wildfires in the United States. The summary section of the Proposed Findings stated, 

30 
“The effects of climate change observed to date and projected to occur in the future—including but not 

limited to the increased likelihood of more frequent and intense heat waves, more wildfires, degraded air 

quality, more heavy downpours and flooding, increased drought, greater sea level rise, more intense 

storms, harm to water resources, harm to agriculture, and harm to wildlife and ecosystems—are effects on 

public health and welfare within the meaning of the Clean Air Act.” See Section IV.B.2 of the Final 

Findings for a discussion of forestry-related risks and impacts and how the Administrator weighed these 

issues in the endangerment finding to public welfare. 

The TSD summarizes findings from both IPCC and CCSP with regard to the incidence of future wildfires. 

The CCSP indicates that “climate change has very likely increased the size and number of forest fires, 

insect outbreaks, and tree mortality in the interior west, the Southwest, and Alaska, and will continue to 

do so (Ryan et al., 2008).” In addition, the IPCC (Field et al., 2007) concludes, “Disturbances such as 

wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future with drier soils 

and longer growing seasons (very high confidence).” Field et al. (2007) and Ryan et al. (2008) were 

primarily used to summarize this issue in the TSD since they focus on North America and the United 

States in particular. The commenter did not provide literature to support the conclusion that wildfires are 

not increasing. To be clear, the statement by Easterling et al. (2007) in Section 10(b) of the TSD on 

wildfires and drought risk indicates, “Some research identifies the possibility of a 10% increase in the 

seasonal severity of fire hazard over much of the United States under climate change.” For these reasons, 

we conclude the TSD and the Findings accurately reflect and are consistent with the IPCC and other 

supporting scientific assessment literature. 

Comment (6-36): 

Some commenters (3136.1, 3596.2) dispute the conclusions of CCSP (and as cited in the TSD) from 

Westerling et al. (2006) regarding climate change and wildfires in the western United States. According 

to the commenters, Westerling et al. claim that increased wildfire frequency is caused by a trend toward 

earlier spring snowmelt. The commenters submit Figure 1b from Westerling et al. (2006) and claim it 

does not show a trend toward earlier spring snowmelt. Commenters also state that there is no correlation 

between global surface temperature and the Palmer Drought Severity Index in the western United States. 

In addition, they cite a study by Cook et al. (2004) that reconstructed western U.S. drought history back to 

800 A.D. and found that compared to past megadroughts, the current drought does not stand out as an 

extreme event. The commenter concludes that the CCSP is therefore not a reliable source on the historical 

relationship between global temperature, drought, and western fires and should not be used as a source for 

the endangerment finding. 

Response (6-36): 

After careful review of the literature cited, we note that the commenter’s interpretation of the Westerling 

et al. (2006) study from IPCC (Field et al., 2007) regarding the relationship between global temperature, 

drought, and wildfires in the western United States is not supported. Westerling et al. (2006) correlate 

wildfire with legitimate trends toward earlier spring snowmelt. Regarding drought, Westerling et al. do 

not correlate recent trends in wildfires with drought trends, contrary to the implication of the commenters. 

Westerling et al. (2006) show that large wildfire activity increased suddenly and markedly in the mid1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. They 

note the greatest increases occurred in mid-elevation, Northern Rockiesforests, where land-use histories 

have relatively little effect on fire risks and are strongly associated with increased spring and summer 

temperatures and an earlier spring snowmelt. 

We find that the commenter’s conclusion that there is no trend toward earlier snowmelt is 

unsubstantiated. They indicate that Figure 1b in Westerling et al. (2006) shows no trend in snowmelt, but 

31 
provide no analysis to support that statement. Furthermore, it is well-established that there have been 

trends toward earlier spring snowmelt in the West. In Section 4(j) of the TSD, we describe the CCSP 

(Lettenmaier et al., 2008) conclusion that finds where shifts to earlier snowmelt in the West. This 

conclusion is supported, in part, by the Stewart et al. (2005) study which finds: “Widespread and 

regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place 

across most of western North America, affecting an area that is much larger than previously recognized.” 

Westerling et al. (2006) do not attempt to correlate drought with wildfires, but rather temperature. See 

Volume 2 for our responses to comments on observed drought trends, including a response to the Cook et 

al. (2004) study. 

For these reasons we conclude that the TSD’s summary of the science is accurate and reflects the current 

scientific literature. We disagree with the commenter’s interpretation of the Westerling et al. study and 

conclude the relationship among spring snow melt timing, longer fires seasons, and drought, as described 

by the scientific assessment reports (IPCC and CCSP), is accurately reflected in the TSD. 

Comment (6-37): 

A commenter (3136.1) indicates that the TSD states “In the south-western U.S., fire activity is correlated 

....with higher Palmer Drought Severity Indices.” The commenter indicates that higher values of the 

Palmer Drought Severity Index mean wetter conditions, so both the TSD (Section 10b) and the IPCC 

(Field et al., 2007) finding it relies on are seemingly inconsistent with this by claiming that wetter 

conditions yield more fires, even though they are predicting drier conditions in the future. The commenter 

references a finding from McKenzie et al. (2004) that large fire years are associated with current-year 

drought but wetter-than-average conditions in the five previous years in the Southwest. The commenter 

states that, in contrast, the TSD predicts increasing frequency of drought in the Southwest caused by 

lower rainfall and higher temperatures, which would argue for fewer large fire years. The commenter 

concludes that the TSD’s discussion of drought and fire is internally inconsistent and does not provide 

evidence that supports endangerment. 

Response (6-37): 

We have carefully reviewed the TSD and the underlying literature upon which these statements were 

derived. The statements in the TSD the commenter believes to be inconsistent are related to overall 

precipitation trends and fire activity in the southwestern United States. According to studies in the 

assessment literature, forest systems in the southwestern United States experience warm and wet 

conditions (higher Palmer Drought Severity Index) influenced by the El Niño ENSO events, followed by 

La Niña events that are associated with dry conditions (Westerling et al., 2003; McKenzie et al., 2004). 

This cycling results in buildup of fuel material during the wet years, followed by desiccation during a dry 

year as conditions follow the ENSO patterns. For this reason, major fire years tend to follow the transition 

from El Niño to La Niña conditions. Although the southwestern United States is expected to become drier 

in the future (Karl et al., 2009), this does not imply that these processes will not continue. We note that it 

is possible for a region to experience a long-term circulation pattern that produces drought, and to have 

short-term changes in this long-term pattern that result in short-term wet spells. Likewise, it is possible 

for a long-term wet circulation pattern to be interrupted by short-term weather spells that result in shortterm drought. McKenzie et al. (2004) find that climate variability is a dominant factor affecting large 

wildfires in the western United States; however, they also note that it is unclear how these indices (e.g., 

Pacific Decadal Oscillation-PDO, ENSO) will respond to climate warming. In addition, the study finds 

that, “Increased temperature in the future will likely extend the fire seasons throughout the western U.S., 

with more fires occurring earlier and later than is currently typical, and will increase the total are burned 

in some regions.” This is consistent with the IPCC (Field et al., 2007) and the summary in the TSD. 

32 
For these reasons, we disagree with the commenter that these statements from the TSD are internally 

inconsistent. See the Findings, Section IV.B, “The Air Pollution Is Reasonably Anticipated to Endanger 

Both Public Health and Welfare,” for our response to comments on how the Administrator weighed the 

scientific evidence underlying her endangerment determination in general, and with regard to the forestry 

sector in particular. 

Comment (6-38): 

Commenters (3136.1 and 3596.2) state that the IPCC (Easterling et al., 2007) and EPA ignored the work 

of Nemani et al. (2003), which shows an enhancement of global vegetation in the last two decades. This 

study ascribes a 6% increase in global net primary productivity from 1982-1999 to the direct effect of 

warming temperatures and increased CO2 concentrations. The commenters argue that the TSD speaks 

equivocally about the effects of climate change despite the availability of reliable data sensed from 

satellites.

Response (6-38): 

We have reviewed Nemani et al. (2003) and note that this study is acknowledged and cited by the IPCC in 

the context of ecosystem properties, goods, and services in Fischlin et al. (2007). The Nemani et al. 

reference submitted by at least one commenter is consistent with the TSD statement that elevated CO2

concentrations and moderate temperature increases over the near term can result in enhanced vegetative 

growth. The findings of Nemani et al. (2003) are broadly consistent with IPCC projections of an increase 

in carbon uptake or storage and an increase in net primary productivity (particularly in the tropics) in the 

early part of this century (Fischlin et al., 2007). However, we note that this reference considers the period 

from 1982 to 1999, whereas the TSD summarizes information on this issue over a longer time period and 

discusses future projections, thus exploring the important interplay of elevated CO2 with projected 

temperature increases, and other relevant factors (e.g., changes in precipitation and tropospheric ozone). 

The summary of the scientific assessment literature in the TSD explains that there is regional variability 

in the responses of forest systems to climate change and thus, productivity gains in one area can occur 

simultaneously with productivity losses in other areas. Furthermore, there are a number of climate change 

effects that can act to offset forest productivity gains. Discussion of these and related issues of the varying 

impacts of climate change can be found in earlier responses to comments in this section. 

We disagree with the commenter that the TSD is equivocal with respect to climate impacts. The 

discussion in Section 10 is clear that, while climate change may have positive impacts on the productivity 

of forest systems, changes in disturbance patterns are expected to have a substantial impact on overall 

gains or losses. As summarized in the TSD, the IPCC concludes with very high confidence that 

disturbances like wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future 

with drier soils and longer growing seasons, and that continuing increases in disturbances are likely to 

adversely impact forestry by limiting carbon storage, facilitating invasive species, and disrupting 

ecosystem services. 

Comment (6-39): 

A commenter (3136.1) disagrees with the TSD statement that ecosystems in the northeast and southeast 

United States are projected to become carbon sources by the end of the 21st century. The commenter 

argues that this statement implies a very static use of forests, and that because they are largely managed 

ecosystems, efficient logging for wood products and housing will maintain forests as a carbon sink. 

Similarly, a commenter (11166) indicates that without forest management, older trees will be retained on 

both private and public forest land, which produces less oxygen and then produces more carbon dioxide 

33 
when they die or burn in forest fires. The commenter asks how the EPA will measure and regulate the 

CO2 released by the death of older trees, and how much this regulation would cost the economy. The 

commenter indicates support for active forest management, forest protection, and forest fire control by 

professional foresters. 

Response (6-39): 

The statement referenced by the commenter, “By the end of the 21st century, ecosystems in the northeast 

and southeast United States are projected to become carbon sources, while the western United States 

remains a carbon sink,” comes from the IPCC (Field et al., 2007), which cites a study by Bachelet et al. 

(2004). The Bachelet et al. (2004) study provides model projections of the carbon storage potential for 

various regions in the United States based on ecosystem type and other influencing climatic factors. The 

study primarily addresses the effects of changes in climate variables on the total ecosystem carbon pool 

and does not directly incorporate forest management practices into the analysis. Regarding the notion that 

effective management practices can prevent forests from becoming carbon sources, we did not separately 

assess different management scenarios over time. See the Findings, Section III.C, “Adaptation and 

Mitigation,” for our response to comments on the treatment of adaptation and mitigation in the Findings. 

The economic effects of potential future regulation and forest management activities are not within the 

scope of determining whether there is endangerment from elevated levels of GHG concentrations. See the 

Finding, Section III.C, “Adaptation and Mitigation,” for our response to comments on the treatment of 

adaptation and mitigation in the Findings. 

 

Comment (6-40): 

A commenter (3283.1) references the TSD statement the Arctic/Sub-Arctic tree line has advanced 6 miles 

northward in the last 50 years, which is comparable to earlier Holocene changes. The commenter states 

that the Holocene period is representative of “natural” climate change conditions, and notes that birch 

forests existed on Greenland during the Medieval Warm Period, indicating that it was natural for tree 

ranges to extend further north than they do today. The commenter concludes that effects of changing 

forest composition are neutral, or even potentially beneficial, because they have happened before, and that 

any potential detrimental effects can be mitigated through adaptation and technology. 

Response (6-40): 

Section 10(c) of the TSD summarizes the findings of the assessment literature that climate change and 

associated changes in disturbance regimes will cause shifts in the distributions of tree species and alter 

forest species composition, which in turn can alter the frequencies, intensities, and impacts of 

disturbances such as fire, insect outbreaks, and disease. Regarding the effects of climate change on the 

Arctic tree line in particular, the TSD summarizes the conclusions of the Arctic Climate Impact 

Assessment (ACIA, 2004) that vegetation zones are projected to continue to migrate northward, with 

forests encroaching on tundra and tundra encroaching on polar deserts. We disagree with the commenter’s 

notion that because northward migration of the treeline happened to some extent in the past, future shifts 

in the distributions of tree species will have only beneficial effects. The commenter provides no evidence 

to support this statement. 

See the Finding, Section III.C, “Adaptation and Mitigation,” for our response to comments on the 

treatment of adaptation and mitigation in the Findings.

Comment (6-41): 

34 
A commenter (3394.1) indicates that projections of invasive species impacts provided in the TSD should 

be reexamined in light of recent studies indicating that climate change will have considerably less of an 

impact in this regard (Sax and Gaines, 2008; Bradley et al., 2009). 

Response (6-41): 

See Volume 7, Section 7.3 of the Response to Comments document for our response to comments on the 

Sax and Gaines (2008) and Bradley et al. (2008) studies and invasive species impacts to ecosystems and 

wildlife. 

Comment (6-42):

A commenter (3394.1) indicates that the TSD distorts the relevant science in this area by suggesting that 

ambient CO2 levels could result in “noticeable die-off” due to root anoxia but fails to explain that this 

effect will not occur as a result of any projected CO2 concentrations likely to occur. The commenter notes 

that, in fact, it has been observed only where CO2 makes up 20% to 95% of soil gas, such as is found in 

areas exposed to volcanic activity. Thus, the commenter indicates this information is irrelevant and 

misleading. 

Response (6-42):

We disagree with the commenter that the effects of elevated concentrations of CO2 on vegetation are 

irrelevant and misleading, and upon review of the TSD language, we find that it places this statement in 

the appropriate context. The TSD provides a summary of the phototoxicity of CO2 to vegetation from 

volcanic outgasing in the context of the direct effects of elevated GHG concentrations in Section 3. This 

provides evidence from the scientific literature regarding the biological response of plants from exposure 

to excessive CO2 levels. Furthermore, the sentence in the TSD indicates that “no projections show CO2 

concentrations approaching these phytotoxic levels.” 

Comment (6-43): 

A commenter (3394.1) states that EPA does not provide adequate detail on the potential competitive 

disadvantage of the United States compared to other regions in the international forest products market. 

Another commenter (3347.1) states that the TSD does not discuss how, with more potential forest 

inventory to harvest, the costs of wood and paper products to consumers are likely to decrease, as are the 

returns to owners of timberland. 

Response (6-43): 

In response to comments, EPA revised the TSD to remove the explicit discussion of the economic effects 

on the United States forest products industry in the context of broader global changes to timber markets, 

due to very limited supporting information from the underlying assessment reports. However, the 

introduction to Section 10 of the TSD does summarize one conclusion from the assessment literature 

regarding how global timber production may be affected by climate change: “Globally, the IPCC 

(Easterling et al., 2007) concludes that modeling studies predict increased global timber production but 

that regional production will exhibit large variability. However, Easterling et al. 2007 also notes CO2

enrichment effects may be overestimated in models.” In addition, Section 10(a) of the TSD states, “For 

the projected temperature increases over the next few decades, most studies support the conclusion that a 

modest warming of a few degrees Celsius will lead to greater tree growth in the United States. 

Simulations with yield models show that climate change can increase global timber production through 

location changes of forests and higher growth rates, especially when positive effects of elevated CO2

concentration are taken into consideration (Easterling et al., 2007).” See Volume 11 of this Response to 

35 
Comments document for our response to comments on the implications of the Findings for economic and 

related concerns. 

Comment (6-44):

Commenters (e.g., 2791, 10298) state their support for the Findings, noting the potential for increased 

stress from heat, drought, insects, and disease on plant and tree populations. Others (2599, 10081) from 

the western United States voice their support for the Findings and describe their experiences with hot 

summers and serious wildfires. Other commenters (e.g., 3421, 4748, 6894) state their support for the 

Findings and express concern about the effects of current and future extreme weather events on the 

forestry industry, including heat waves, droughts, floods, wildfires, and hurricanes. One commenter 

(5844) describes how projected climate impacts, particularly the increased risk of wildfire on forestry and 

forest biodiversity will affect him personally given his enjoyment of hiking, camping, and communing 

with nature in the forests of the Pacific Northwest. 

A commenter (3501) states his support for the Findings, indicating that the western United States and 

Canada are already seeing widespread changes in the natural landscape due to climate change. Hotter 

temperatures are causing more frequent and persistent drought in the West, which contribute to forest 

fires and pine beetle infestations. A weather-related pine beetle infestation has decimated millions of 

acres of forest in the western United States Western US and Canada. At the current rate of destruction, 

80% of the forests of British Columbia will have been destroyed within five years and the rest of the West 

will lose 50% of its forests by mid-century. The forest fire season in the West is now 78 days longer than 

25 years ago and it is well recognized that our forest fires have become more frequent, more intense and 

more destructive. 

Response (6-44):

We reviewed the comments provided and note they are generally consistent with the discussion of climate 

impacts on forestry in the TSD, although commenters do not provide specific references to support their 

claims. As summarized in the TSD and in our responses to previous comments in this volume, 

disturbances like wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future 

with drier soils and longer growing seasons. 

36 
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