**U.S. Senate Committee on Commerce, Science and Transportation**

**Hearing on "The Case for Climate Change Action"**

#### October 1, 2003

**Stephen H. Schneider**

**Professor, Department of Biological Sciences**

**Co-Director, Center for Environmental Science and Policy**

**Stanford University**

**Stanford, CA 94305**

**shs@stanford.edu**

***Introduction and Personal Perspective***. If I may indulge in a
personal note at the outset: it is a pleasure to appear again in front
of both Senators McCain and Lieberman on climate change issues, having
had that honor on several occasions since the mid-1980s with Senator
McCain and the mid-1990s with Senator Lieberman. As these hearings today
are about the "case for action" on climate change based on sound science
assessment, I will try to emphasize aspects of the science of climate
change less exhaustively covered by other witnesses, such as Dr. Tom
Wigley of the National Center for Atmospheric Research, whose testimony
on climate change science I fully associate myself with. Instead, I will
focus more on climate change impacts. The problem was well-stated by
Senator Lieberman when I commented to the Senate Environment and Publics
Works Committee, chaired by the late Senator Chaffee, in July 1997. At
that time, Senator Lieberman said:

> Changes in climate have major implications for human health, water
> resources, food supplies, infectious diseases, forests, fisheries,
> wildlife populations, urban infrastructure, and flood plain and
> coastal developments in the United States. Although uncertainties
> remain about where, when, and how much climate might change as a
> result of human activities, the changes---when they happen---may have
> severe impacts on many sectors of the U.S. economy and on the
> environment. These are serious risks that we must start considering
> (p. 15).

This statement is equally valid today and can be further supported by
substantially more scientific studies pointing out potentially serious
climate impacts. I will briefly review some of these and put them in the
context of climate change cost-benefit analyses. But first, a brief
statement about the climate change science itself.

While testifying to this Committee on May 8, 1989---when Senator McCain
was a member of the Committee---I recall a discussion about the problem
of uncertainties surrounding climate change and the question of how long
we should wait before taking action. Some debaters had asserted that
there wasn't enough direct evidence of human-induced climate change for
strong policy actions. In response to Senators from this committee on
that point, I agreed that "Most of our confidence that the future will
change is based on literally millions and millions of observations which
tell us about the heat trapping properties of gases, not based so much
on the performance of the planet this century. If we insist on waiting
for the planet to catch up to what we expect it to do, it is another 10
to 20 years to prove that beyond doubt" (p.150).

Well, it is now 14 years since I said that. I believe the work of the
Intergovernmental Panel on Climate Change (IPCC), the U.S. National
Academy of Sciences (NAS), and others has amply demonstrated that,
indeed, nature has "caught up" with our expectations of warming and in
fact added a few surprises like rapid changes in polar regions and
devastating heat-wave-induced deaths, even in modern, highly developed
countries, with the more than 15,000 mortalities occurring in France
this summer as a result of the extreme heat serving as a prime example.

***Surface warming trends are solidly grounded in observational science
and consistent with human-induced pressures***. It is
scientifically well established that the Earth\'s surface air
temperature has warmed significantly, by about 0.6 ° Celsius (C) since
1860, and that an upward trend can be clearly discerned by plotting
historical temperatures. Such a graph would show a rapid rise in
temperature at the end of the twentieth century. This is supported by
the fact that all but three of the ten warmest years on record occurred
during the 1990s. But what has been learned only in the past five years
is that this unusual warmth in recent years is not just an anomaly in
temperature records of the last 140 years, but the past 2000, as Figure
1 displays.

***Figure 1. 2,000-year reconstruction of global temperature changes in
degrees Celsius***

![](media/image1.jpeg){width="5.226388888888889in"
height="1.895138888888889in"}

The blue line represents the temperature reconstruction, with 95%
confidence band shown in yellow and the instrumental record in red.
Notice that the last several decades of the 20^th^ century exceed the
range of temperatures over the past 2000 years. (Source: Mann and Jones,
2003.)

The probability that the radical upward swing in temperature at the tail
end of the 20^th^ century is just a natural quirk of nature---as some
"contrarians" and their political supporters contend---is exceedingly
low. If, as some assert, "the sun did it", then what was the sun doing
over the previous 2 millennia? It is rather perverse to expect such
radical behavior from the sun just now, at the same time that we have
clear evidence of human-induced pressures coincident with the warming.
While the possibility (at some low probability) that natural factors are
responsible for the unusual warmth of the Earth's surface at the end of
the 20^th^ century cannot be ruled out completely, a much more likely
explanation is that the warming is the result of a mix of natural and
human-induced (anthropogenic) factors. While this alone is cause for
worry, more disquieting still are climate change projections for the
21^st^ century, especially if we assume that greenhouse gas emissions
follow a business-as-usual path.

It is for these reasons that I express my personal satisfaction for
having, over the past two decades, had the opportunity to testify to the
Senators currently leading this effort to establish a meaningful climate
change policy for the United States that will actually result in
emissions reductions. In my personal opinion, it is essential that we
get on with the job of providing (mandatory) incentives to push the
amazing industrial and intellectual capacity of our country to fashion
cost-effective solutions. I thank the Senators for having pursued this
issue over the long term.

As mentioned, nature has cooperated with theory in the past few decades,
as evidenced by the record warming. In addition, it is well-established
that human activities have caused increases in radiative forcing, with
radiative forcing defined as a change in the balance between the
radiation coming into and going out of the surface-atmosphere system. In
the past few centuries, atmospheric carbon dioxide has increased by more
than 30 percent, and virtually all climatologists agree that the cause
is human activities, and the burning of fossil fuels in particular.

Despite the many well-established aspects of the science of climate
change (e.g., anthropogenic forcing of global warming), other aspects
(e.g., specific regional changes) are still being vigorously debated. In
fact, the climate change debate is characterized by deep uncertainty,
which results from factors such as lack of information, disagreement
about what is known or even knowable, linguistic imprecision,
statistical variation, measurement error, approximation, subjective
judgment, and disagreement about structural models, among others (see
Moss and Schneider, 2000). These problems are compounded by the global
scale of climate change, which produces varying impacts at local scales,
long time lags between forcing and its corresponding responses**,** very
long-term climate variability that exceeds the length of most
instrumental records, and the impossibility of before-the-fact
experimental controls or empirical observations (i.e., there is no
experimental or empirical observation set for the climate of, say, 2050
AD, meaning all our future inferences cannot be wholly "objective,"
data-based assessments --- at least not until 2050 rolls around).
Moreover, climate change is not just a scientific topic but also a
matter of public and political debate, and degrees of uncertainty may be
played up or down (and further confused, whatever the case) by
stakeholders in that debate.

***Can We Define "Dangerous" Climate Change***? Article 2 of the UN
Framework Convention on Climate Change (UNFCCC) states that: "The
ultimate objective of this Convention and any related legal instruments
that the Conference of the Parties may adopt is to achieve, in
accordance with the relevant provisions of the Convention, stabilization
of greenhouse gas concentrations in the atmosphere at a level that would
prevent dangerous anthropogenic interference with the climate system".
The Framework Convention on Climate Change further suggests that:

"Such a level should be achieved within a time frame sufficient

## to allow ecosystems to adapt naturally to climate change, 

## to ensure that food production is not threatened and 

## to enable economic development to proceed in a sustainable manner."

Thus, the term "dangerous anthropogenic interference" may be defined or
characterized in terms of the consequences (or impacts) of climate
change outcomes, which can be related to the levels and rates of change
of climate parameters. These parameters will, in turn, be determined by
the evolution of emissions and consequent atmospheric greenhouse gas
concentrations. Evaluating the consequences of climate change outcomes
to determine those that may be considered "dangerous" is a complex
undertaking, involving substantial uncertainties as well as value
judgments. In this context, the role of scientists is to assess the
literature with a view to providing information that is policy-relevant,
without being policy prescriptive.

***Climate Sensitivity and Climate Scenarios to 2100 and Beyond***. By
how much will humans and natural changes in the Earth each contribute to
future climate disturbance? The IPCC has attempted to tackle this
controversial question in its Special Report on Emission Scenarios
(SRES), which contains a range of possible future climate scenarios
based on different assumptions regarding economic growth, technological
developments, and population growth, arguably the three most critical
determinants of future climate change. Together, the fan of possible
climate scenarios and the probability distributions of possible climate
sensitivities determine what policy makers often want to know---by how
much will it warm in, say, 2100 (or any other time), depending on what
policies we choose to change emissions scenarios (e.g., Schneider,
2002).

The SRES scenarios and other climate change projections depend on
detailed modeling. The most typical way scientists codify knowledge is
by constructing models made up of the many subcomponents of the climate
system that reflect our best understanding of each subsystem. The most
comprehensive models of atmospheric conditions are three-dimensional,
time-dependent simulators known as general circulation models (GCMs).
Because of the complexity and computational costs of GCMs, simpler
models are often constructed to explore the sensitivity of outcomes to
plausible alternative assumptions (e.g., Wigley's, testimony to this
session). The system model as a whole cannot be directly tested before
the fact --- that is, before the future arrives --- but it can be
verified against historical situations that resemble what we believe
will be analogous to what will occur in the future (see "Model
Validation" below).

While modeling has become both more complex and more accurate as
computing abilities have advanced and more is understood about the
climate problem, scientists still have to deal with an enormous amount
of uncertainty, as mentioned above. In modeling, a major uncertainty is
climate sensitivity, the amount by which the global mean surface air
temperature will increase for a doubling of CO~2~ concentrations from
pre-industrial levels. Many scientists have done extensive modeling and
observational research on this subject over the past 20 years, and most
agree that climate sensitivity probably falls somewhere within the
IPCC\'s range of 1.5-4.5 ^o^C. However, that old consensus is changing,
as several recent studies (e.g., Andronova and Schlesinger, 2001;
Forrest et al, 2001) have estimated that climate sensitivity could be an
alarming 6 ^o^C or higher. (To give a sense of the magnitude, a 5-7 ^o^C
***drop*** in temperature is what separates Earth's present climate from
an ice age.) 

***Model Validation***. In the presence of so much uncertainty, how can
modelers be more confident in their model results? How do they know that
they have taken into account all economically, ecologically, and/or
climatologically significant processes, and that they have
satisfactorily "parameterized" processes whose size scales are below
that of their models\' grid cells? The answer lies in a variety of model
validation techniques, most of which involve evaluating a model\'s
ability to reproduce --- in the case of climate models --- known
climatic conditions in response to known forcings.

Volcanic eruptions are one good form of model validation. Major volcanic
eruptions inject so much sulfuric acid haze and other dust into the
stratosphere that they exert a global cooling influence that lasts
several years. Such eruptions occur somewhat randomly, but there is
typically one every decade or so, and they constitute natural
"experiments" that can be used to test climate models. The last major
volcanic eruption, of the Philippine volcano Mt. Pinatubo in 1991, was
forecast by a number of climate modeling groups to cool the planet by
several tenths of a degree Celsius. That is indeed what happened.

***Figure 2. Predicted and observed changes in global temperature after
the 1991 eruption of Mt. Pinatubo***

![](media/image2.wmf){width="3.592361111111111in"
height="1.7805555555555554in"}

Solid curve is derived from measured air temperatures over land and
ocean surfaces. Broken curves represent climate model runs with slightly
different initial conditions. In both cases the models included the
effect of dust injected into the atmosphere by the volcanic eruptions.
(Source: Hansen et al, 1996.)

Figure 2 shows a comparison between actual global temperature variations
and those predicted by a climate model for a period of five years
following the Mt. Pinatubo eruption. Now, a drop in temperature of a few
tenths of a degree Celsius is small enough that the observed variation
just could be an unusual natural fluctuation. However, earlier
eruptions, including El Chichón in 1983 and Mt. Agung in 1963, were also
followed by a marked global cooling of several tenths of a degree
Celsius. Studying the climatic effects from a number of volcanic
eruptions shows a clear and obvious correlation between major eruptions
and subsequent global cooling. Furthermore, a very simple calculation
shows that the negative forcing produced by volcanic dusts of several
watts per square meter is consistent with the magnitude of cooling
following major volcanic eruptions. Viewed in light of these data, the
graph above suggests that climate models do a reasonably good job of
reproducing the large-scale climatic effects of volcanic eruptions over
a time scale of a few years.

Seasonality provides another natural experiment for testing climate
models. Winter weather typically averages some fifteen degrees Celsius
colder than summer weather in the Northern Hemisphere and five degrees
Celsius colder in the Southern Hemisphere. (The Southern Hemisphere
variation is lower because a much larger portion of that hemisphere is
water, whose high heat capacity moderates seasonal temperature
variations.) Climate models do an excellent job reproducing the timing
and magnitude of these seasonal temperature variations, although the
absolute temperatures they predict may be off by several degrees in some
regions of the world. However, the models are less good at reproducing
other climatic variations, especially those involving precipitation and
other aspects of the hydrological cycle. Of course, being able to
reproduce the seasonal temperature cycle alone --- since it comes full
circle in only one year --- does not guarantee that a model will
accurately describe the climate variations resulting from other driving
factors (such as increasing anthropogenic greenhouse gas concentrations)
that will likely occur over decades or centuries. On the other hand, the
fact that models do so well with seasonal variations is an assurance
that the models\' climate sensitivity is unlikely to be off by a factor
of 5 - 10, as some contrarians assert.

***Joint Probability Estimation.*** The combined effects of
uncertainties in emissions and uncertainties in climate sensitivity are
also known as a "joint probability" (i.e., sensitivity and emissions
varied jointly). How do we approach this question of the joint
probability of temperature rise to 2100 *and* crossing some "dangerous"
warming threshold, to use the language of the UNFCCC---which, by the
way, was signed by President Bush in 1992 and ratified by the Senate.
Instead of using two probability distributions, an analyst could pick a
high, medium, and low range for each factor and plot the results, as I
will demonstrate. For example, a glance at Andronova and Schlesinger's
(2001) calculations shows that the 10 percentile value for climate
sensitivity is 1.1 ^o^C for a doubling of CO~2~ (i.e., 4 W/m^2^ of
radiative forcing). 1.1 ^o^C is, of course, below the IPCC\'s lower
limit climate sensitivity value of 1.5 ^o^C. However, this merely means
that there is a 10 percent chance climate sensitivity will be 1.1 ^o^C
or less --- that is, a 90% chance climate sensitivity will be 1.1 ^o^C
or *higher*. The 50^th^ percentile result --- that is, the value that
climate sensitivity is as likely to be above as below --- is 2.0 ^o^C.
The 90^th^ percentile value for climate sensitivity from Andronova and
Schlesinger (2001) is 6.8 ^o^C, meaning there is a 90% chance climate
sensitivity is 6.8 ^o^C or less, but there is still a very uncomfortable
10% chance it is even higher than 6.8 ^o^C --- a value well above the
4.5 ^o^C figure that marks the top of the IPCC\'s range. Using these
three values to represent a high, medium, and low climate sensitivity,
we can produce three alternate projections of temperature over time,
once an emissions scenario is decided on.

In Schneider (2003), the three climate sensitivities just explained were
combined with two SRES storylines: A1FI, the very high emissions, fossil
fuel-intensive scenario; and A1T, the high technological innovation
scenario, in which development and deployment of advanced technologies
dramatically reduces the long-term emissions. This comparison pair
almost brackets the high and low ends of the 6 SRES representative
scenarios' range of cumulative emissions to 2100, and since both are for
the "A1 world," the only major difference between them is the technology
component. This component should be viewed as a "policy lever" that
could be activated through the implementation of policies to encourage
decarbonization, for example---like the bill before this committee.
Therefore, studying how different the evolution of projected climate is
to 2100 for the two different scenarios is a very instructive exercise
and can help in exploring the different likelihoods of crossing
"dangerous" warming thresholds.

***Figure 3. Three climate sensitivities and two scenarios (source:
Schneider, 2003)***

![](media/image3.jpeg){width="3.5180555555555557in"
height="3.8958333333333335in"}

As noted in Figure 3 above, the three climate sensitivities --- 10^th^,
50^th^ and 90^th^ percentiles --- designated by Andronova and
Schlesinger (2001) are combined with the radiative forcings for the A1FI
and A1T scenarios laid out in the SRES. The dashed horizontal lines in
both graphs represent the 3.5 ^o^C cut-off --- a very conservative
number picked by me as the threshold value for "dangerous" climate
change --- and the blue shaded area marks the extent to which each
temperature change scenario exceeds that 3.5 ^o^C threshold. As shown,
these scenarios produce similar projections of warming for the first
several decades of the 21^st^ century, but diverge considerably---
especially in the high-sensitivity 90^th^ percentile case --- after
mid-century. The 50^th^ and 90^th^ percentile A1FI cases both exceed the
threshold of 3.5 ^o^C warming before 2100, and the area shaded in blue
is much more dramatic in the fossil-intensive scenario than the
technological innovation scenario. In fact, at 2100, when the A1T curves
are stabilizing, the A1FI curves are still upwardly sloped --- implying
even greater warming in the 22^nd^ century. In order to fully assess
"dangerous" climate change potential, simulations that cover well over
100 years will be necessary, since it is widely considered that warming
above a few degrees Celsius is likely to be much more harmful than
changes below a few degrees (see Figure 4 below).

***How Long is a "Long View"?*** The most striking features of both
scenarios in Figure 3 are the top (red) lines, which rise very steeply
above the two lines below them. That is because of the peculiar shape of
the probability density function for climate sensitivity in Andronova
and Schlesinger (2001). \[For those concerned with the technical
details, that is because the probability density function has a long
tail on the right-hand side, representing the possibility that aerosols
have been holding back not-yet-realized warming and the rise in
temperature could be much higher than currently expected.\] Also
striking is that both the 10^th^ and 50^th^ percentile results for both
the A1FI and A1T scenarios don't differ much in 2050, but then diverge
considerably by 2100. This has led some to declare (erroneously, in my
view) that there is very little difference in climate change across
scenarios or even among different climate models with different
sensitivities. This is clearly wrong, for although both A1FI and A1T
have emissions, and thus CO~2~ concentration, projections that are not
very different for the first several decades of the 21^st^ century, they
diverge after 2050, as does the temperature response. For the 90^th^
percentile results, both the A1FI and the A1T temperature projections
exceed the "dangerous" threshold of 3.5 ^o^C at roughly the same time
(around 2040), but the A1FI warming not only goes on to outstrip the A1T
warming, but is still steeply sloped at 2100, implying warming beyond 13
^o^C in the 22^nd^ century, which would undoubtedly leave a dramatic
legacy of environmental damage for distant posterity and great
ecological stress for nature. 

Figure 3 shows, via a small number of curves (6 in all), the probability
of temperature changes over time for three climate sensitivity
probabilities, but it does not give probabilities for the emissions
scenarios themselves; only two are used to "bracket uncertainty," and
thus no joint probability can be gleaned from this exercise. This is the
next step that needs to be taken by the research community. An MIT
integrated assessment group (Webster et al, 2003) has already attempted
to fashion a probability distribution for future climate using a series
of different models and expert judgments. Like other assessments, their
work also suggests a wide range of possibilities, with some representing
quite "dangerous" potential outcomes. That approach, I predict, will be
the wave of the future in such analyses, but given the heavy
model-dependence of any such results, individual "answers" will remain
controversial and assumption-bound for a considerable time to come.

The likelihood of threshold-crossing is quite sensitive to the
particular selection of scenarios and climate sensitivities used.
However, in these bracketing studies, the probability of crossing
"dangerous" thresholds of climate change is typically around ten
percent---a risk society will have to weigh against the costs of climate
mitigation activities. As will be discussed shortly, that is a high risk
indeed.

If conventional economic discounting were applied, some present-day
"rationalists" might argue that the present value of damages postponed
for a century or so is virtually nil. But what if our behavior were to
trigger irreversible changes in sea levels and ocean currents or the
extinction of species (on generational time scales)? Is it fair to
future generations for us to leave them the simultaneous legacy of more
wealth and severe ecosystem damage? That is the dilemma thoughtful
analysts of the climate policy debate have to ponder, since the next few
generations' behaviors will precondition to a considerable extent the
long-term evolution of the climate and the planetary ecosystems. 

***Climate Impacts***. Let us consider some of the effects that might
occur in the next century if the SRES emissions *do* occur. We can use
models to calculate the climatic consequences of those scenarios
unfolding, which then allow us to estimate potential impacts of climate
changes, and in turn, the benefits of avoiding some of those potential
damages through mitigation and/or other measures.

Table 1 shows the IPCC's summary of a number of such projected impacts.
These effects have been consolidated into five major reasons for concern
and represented graphically, as shown in Figure 4.

***Table 1--- Projected effects of global warming during the 21^st^
Century (adapted from IPCC 2001b, table SPM-1)***

+-----------------------+------------+---------------------------------+
| Projected Effect      | P          | Examples of Projected Impacts   |
|                       | robability | with high confidence of         |
|                       | estimate   | occurrence (67 -- 95%           |
|                       |            | probability) in at least some   |
|                       |            | areas                           |
+-----------------------+------------+---------------------------------+
| Higher maximum        | Very       | Increased deaths and serious    |
| temperatures, more    | likely     | illness in older age groups and |
| hot days and heat     | (90-99%)   | urban poor                      |
| waves over nearly all |            |                                 |
| land areas            |            | Increased heat stress in        |
|                       |            | livestock and wildlife          |
|                       |            |                                 |
|                       |            | Shift in tourist destinations   |
|                       |            |                                 |
|                       |            | Increased risk of damage to a   |
|                       |            | number of crops                 |
|                       |            |                                 |
|                       |            | Increased electric cooling      |
|                       |            | demand and reduced energy       |
|                       |            | supply reliability              |
+-----------------------+------------+---------------------------------+
| Higher minimum        | Very       | Decreased cold-related human    |
| temperatures, fewer   | likely     | morbidity and mortality         |
| cold days, frost days | (90-99%)   |                                 |
| and cold waves over   |            | Decreased risk of damage to a   |
| nearly all land areas |            | number of crops, and increased  |
|                       |            | risk to others                  |
|                       |            |                                 |
|                       |            | Extended range and activity of  |
|                       |            | some pest and disease vectors   |
|                       |            |                                 |
|                       |            | Reduced heating energy demand   |
+-----------------------+------------+---------------------------------+
| More intense          | Very       | Increased flood, landslide,     |
| precipitation events  | likely     | avalanche, and mudslide damage  |
|                       | (90-99%)   |                                 |
|                       | over many  | Increased soil erosion          |
|                       | areas      |                                 |
|                       |            | Increased flood runoff          |
|                       |            | increasing recharge of some     |
|                       |            | floodplain aquifers             |
|                       |            |                                 |
|                       |            | Increased pressure on           |
|                       |            | government and private flood    |
|                       |            | insurance systems and disaster  |
|                       |            | relief                          |
+-----------------------+------------+---------------------------------+
| Increased summer      | Likely\    | Decreased crop yields           |
| drying over most      | (67-90%)   |                                 |
| mid-latitude          |            | Increased damage to building    |
| continental interiors |            | foundations caused by ground    |
| and associated risk   |            | shrinkage                       |
| of drought            |            |                                 |
|                       |            | Decreased water resource        |
|                       |            | quantity and quality            |
|                       |            |                                 |
|                       |            | Increased risk of forest fire   |
+-----------------------+------------+---------------------------------+
| Increase in tropical  | Likely\    | Increased risks to human life,  |
| cyclone peak wind     | (67-90%)   | risk of infectious disease      |
| intensities, mean and | over some  | epidemics and many other risks  |
| peak precipitation    | areas      |                                 |
| intensities           |            | Increased coastal erosion and   |
|                       |            | damage to coastal buildings and |
|                       |            | infrastructure                  |
|                       |            |                                 |
|                       |            | Increased damage to coastal     |
|                       |            | ecosystems such as coral reefs  |
|                       |            | and mangroves                   |
+-----------------------+------------+---------------------------------+
| Intensified droughts  | Likely\    | Decreased agricultural and      |
| and floods associated | (67-90%)   | rangeland productivity in       |
| with El Niño events   |            | drought- and flood-prone        |
| in many different     |            | regions                         |
| regions               |            |                                 |
|                       |            | Decreased hydro-power potential |
|                       |            | in drought-prone regions        |
+-----------------------+------------+---------------------------------+
| Increased Asian       | Likely\    | Increase in flood and drought   |
| summer monsoon        | (67-90%)   | magnitude and damages in        |
| precipitation         |            | temperate and tropical Asia     |
| variability           |            |                                 |
+-----------------------+------------+---------------------------------+
| Increased intensity   | Uncertain  | Increased risks to human life   |
| of mid-latitude       | (current   | and health                      |
| storms                | models     |                                 |
|                       | disagree)  | Increased property and          |
|                       |            | infrastructure losses           |
|                       |            |                                 |
|                       |            | Increased damage to coastal     |
|                       |            | ecosystems                      |
+-----------------------+------------+---------------------------------+

***Figure 4 --- Reasons for concern about climate change impacts
(source: IPCC Working Group 2 Third Assessment Report, figure SPM-2)***

![](media/image4.jpeg){width="4.688888888888889in"
height="3.332638888888889in"}

In Figure 4 above, the left part of the figure displays the observed
temperature increase up to 1990 and the range of projected increases
after 1990, as estimated by IPCC Working Group I (IPCC, 2001a). The
right panel displays conceptualizations of five reasons for concern
regarding climate change risks evolving through 2100. White indicates
neutral or small negative or positive impacts or risks, yellow indicates
negative impacts for some systems, and red means negative impacts or
risks that are more widespread and/or greater in magnitude. This figure
shows that the most potentially dangerous impacts (the red colors on the
figure) typically occur after a few degrees Celsius of warming --- thus,
my use of 3.5 ^o^C as a tentative "threshold" for serious climate
damages in Figure 3 is very conservative. (The European Union has
suggested the "dangerous" threshold is about 2 ^o^C.) The risks of
adverse impacts from climate change increase with the magnitude of
climate change.

Despite uncertainties surrounding emissions scenarios and climate
sensitivity, the IPCC has projected that, if its latest estimate that
the Earth\'s atmosphere will warm somewhere between 1.4 and 5.8 ^o^C by
2100 is correct, likely effects will include: more frequent heat waves
(and less frequent cold spells); more intense storms (hurricanes,
tropical cyclones, etc.) and a surge in weather-related damage;
increased intensity of floods and droughts; warmer surface temperatures,
especially at higher latitudes; more rapid spread of disease; loss of
farming productivity and/or movement of farming to other regions, most
at higher latitudes; species extinction and loss of biodiversity; and
rising sea levels, which could inundate coastal areas and small island
nations (see Table 1).

The threat of rising sea levels has been studied in great detail. It is
thought that warmer atmospheric temperatures would lead to warming of
ocean water (and corresponding volumetric expansion) until the heat was
well-distributed throughout the oceans --- a mixing time known to be on
the order of 1,000 years. Instead of only up to a meter of sea level
rise over the next century or two from thermal expansion of warmed ocean
waters--- and perhaps a meter or two more over the five or so centuries
after that --- significant global warming would likely trigger nonlinear
events like a deglaciation of major ice sheets near the poles. That
would cause many additional meters of rising seas for many millennia,
and once started, might not be reversible on the time scale of thousands
of years.

It is important that scientists continue to develop stronger models and
probe the issue of climate sensitivity, as improvements in the science
will lead to improvements in our understanding of the potential impacts
of various levels of temperature change.

***In What Units Can We Measure Climate Damage?*** Schneider,
Kuntz-Duriseti, and Azar (2000) have argued that the best way to
estimate the full extent of the climate change-induced damages described
above is by examining not just monetarily-quantifiable (\"market\")
damage, but the \"five numeraires\": monetary loss (market category),
loss of life, quality of life (including coercion to migrate, conflict
over resources, cultural diversity, loss of cultural heritage sites,
etc.), species and/or biodiversity loss, and distribution/equity.
Assessing climate impacts in all these categories should ensure a
fairer, more accurate assessment of the actual costs of global warming.

The last numeraire, the issue of equity in climate change, is, and will
likely continue to be, contentious. Climate change inequality will
likely come in two forms. First, it will produce inequity in effects.
Some countries or sectors within countries will benefit from a certain
degree of warming, whereas others will be harmed by it. The developed
countries, who are responsible for most of the greenhouse gases emitted
into the atmosphere thus far may not be affected as much as the
developing countries for two reasons: first, there is usually higher
adaptive capacity in richer, cooler countries than in poorer, warmer
ones. Second, developing countries that have not yet experienced the
economic fruits of an industrial revolution and want their chance to
emit and industrialize fear that policies to restrict emissions will
deny them their "fair share" of the atmospheric commons to use---quite
literally---as a waste dump. One strategy to solve this problem is
"technology leapfrogging," the transfer or development of cleaner
technologies to developing countries on a much-accelerated time schedule
(relative to the developments that have emerged over a century in
now-rich countries).

Moreover, as there are disparities in countries' abilities to pay for
global warming-related problems, once again, the developing countries
will be affected more yet have less of an ability to pay than the rich
nations. While I agree it is essential to deal with climate policy at
home---and thus personally applaud this bill before the committee
today---we will have to join with other countries to fashion joint
solutions in the near future if we are to make progress on the climate
change problem.

***Nature Is Already Responding***. Another numeraire mentioned above
was the loss of biodiversity. Very recent studies (e.g., Root et al,
2003; Parmesan and Yohe, 2003) have shown that nature is already
responding to climate trends of the past several decades. Figure 5
(below), for example, shows the activities of many plants and animals
--- such as the flowering of trees and the migrating of birds in the
spring --- have been occurring earlier due to observed climate trends.
That warmer weather would make flowers bloom earlier is hardly
surprising, but that "only" 0.6 ^o^C of warming to date has already
caused a statistically significant "discernible impact" on plants and
animals is surprising. Moreover, it is sobering to consider what major
movements --- and extinctions --- would likely take place in plant and
animal communities if the climate changes by several degrees or more.

***Figure 5. Frequency of species and groups of species with a
temperature-related trait changing by number of days in 10 years for
data gathered primarily since 1960***

![](media/image5.png){width="4.872916666666667in"
height="2.986111111111111in"}

The arrow indicates the mean and the "x" indicates no data were
tabulated for species showing no clear trait changes. This is a highly
statistically significant result demonstrating that there has been a
discernible impact of recent climate trends on plants and animals. Their
vital activities that are linked to temperature are occurring earlier,
in concert with global warming trends. (Source: Root et al, 2003.)

Another clear climate impact is the retreat of mountain glaciers. This
problem goes beyond just the disruption of scenic beauty as glaciers in
places like Glacier National Park continue to disappear; it can be
damaging to societies that are flooded during the glacier-melting stage
and will later suffer from lack of water as their current supplies
disappear with the glaciers. Figure 6 shows the dramatic disappearance
of Mt. Kilimanjaro's glaciers, which have decreased in size by 80-90%
relative to 100 years ago.

***Figure 6. What will happen to the snows of Kilimanjaro?***

![](media/image6.jpeg){width="4.25in" height="2.7493055555555554in"}

The extent of ice cover on Mt. Kilimanjaro decreased by 81% between 1912
and 2000. Disappearing paleoclimate archives such as this are a priority
target of the Global Paleoclimate Observing System currently being
proposed by the Past Global Changes (PAGES) scientists.

***Climate Surprises?*** The IPCC and others have stated that
\"dangerous\" climate change, including surprises, is more likely to
occur with more than a few degrees Celsius of additional warming.
Surprises, better defined as "imaginable abrupt events", could include
deglaciation and/or the alteration of ocean currents, the most
widely-used example of the latter being a slowdown of the Thermohaline
Circulation, or THC, system in the North Atlantic Ocean. Ecosystems,
especially those already stressed by land use pressures, are
particularly vulnerable to rapid climate changes.

Estimating climate damages that are expected to occur gradually and
their effects is simple relative to forecasting \"surprise\" events and
their consequences. But rather than being ignored as unlikely, surprises
and other irreversibilities like plant and animal extinctions should be
treated like other climate change consequences by scientists performing
risk assessments, where ***risk*** is defined as ***probability x
consequence***. While the possible consequences of climate change have
been discussed thoroughly, they are often not accompanied by
probabilities. The probability component of the risk equation will
entail subjective judgment on the part of scientists, but this is far
preferable to overlooking the risk equation entirely.

Policymakers will be better able to determine what is \"dangerous\" and
formulate effective legislation to avoid such dangers if probabilities
appear alongside scientists\' projected consequences. These
probabilities and consequences will vary regionally. In general,
temperature rises are projected to be greatest in the subpolar regions,
and to affect the polar winter more dramatically than the summer. 
Hotter, poorer nations (i.e., developing nations near the equator) are
expected to suffer more dramatic effects from climate change than their
developed neighbors in the North. This is partly due to the lower
expected adaptive capacities of future societies in developing nations
(when compared with their developed country counterparts), which depend
on their resource bases, infrastructures, and technological
capabilities. This implies that damages may be asymmetrically felt
across the developed/developing country divide. The scenario in which
climate change brings longer growing seasons to the rich northern
countries and more intense droughts and floods to the poor tropical
nations is clearly a situation ripe for increasing international
tensions and could cause developing nations to feel increasing
resentment towards the most-polluting nations in the twenty-first
century. That scenario has clear security implications for the United
States.

Regardless of the different levels of vulnerability and adaptive
capacity that future societies are expected to have and the need for
regional-level assessments that that implies, all people, governments,
and countries should realize that \"we\'re in this together.\" In all
regions, people\'s actions today will have long-term consequences. Even
if humanity completely abandons fossil fuel emissions in the 22^nd^
century, elevated CO~2~ concentrations are projected to remain for a
millennium or more. The surface climate will continue to warm from this
greenhouse gas elevation, with a transient response of centuries before
an equilibrium warmer climate is established. How large that equilibrium
temperature increase is depends on both the final stabilization level of
the CO~2~ and the climate sensitivity.

***Implications for Climate Policy Choices.*** In the face of such
uncertainty, potential danger, and long-term effects of present actions,
how should climate change policy be confronted? As discussed previously,
climate change, like many other complex socio-technical issues, is
riddled with "deep uncertainties" in both probabilities and
consequences. They are not resolved today and may not be resolved to a
high degree of confidence before we have to make decisions regarding how
to deal with their implications. With imperfect, sometimes ambiguous,
information on both the full range of climate change consequences and
their associated probabilities, decision-makers must decide whether to
adopt a \"wait and see\" policy approach or follow the \"precautionary
principle\" and hedge against potentially dangerous changes in the
global climate system. Since policymakers operate on limited budgets,
they must determine how much to invest in climate protection versus
other worthy improvement projects --- like new nature reserves, clean
water infrastructure and other health improvement, and better education.

 

Ultimately, the decision on whether or not to take action on climate
change entails a value judgment on the part of the policymaker regarding
what constitutes \"dangerous\" climate change, ideally aided by complete
risk assessments provided by scientists. Cost-benefit analyses (CBAs)
are also useful in deciding the ifs and whats of climate change policy,
but uncertainties and the need for multiple metrics (e.g., the "five
numeraires") make this exercise difficult as well, especially when
attempting to estimate the costs of surprise and other catastrophic
events.

Any policies that are implemented should encourage, and possibly even go
so far as to subsidize, technological change. Encouraging technological
change through energy policies, in particular, is of critical importance
when addressing climate change. As Figure 3 shows, alternate
energy-technology scenarios could dramatically lower the risk of
"dangerous" climate change.

***Is It Really Too Expensive To Mitigate Global Warming***? Christian
Azar and I (Azar and Schneider, 2002) developed a simple economy model
and estimated the present value (discounted to 1990, expressed in 1990
USD, and assuming a discount rate of five percent per year) of the costs
to stabilize atmospheric CO~2~ at 350 parts per million (ppm), 450 ppm,
and 550 ppm to be 18 trillion USD, 5 trillion USD, and 2 trillion USD,
respectively. Obviously, 18 trillion USD is a huge cost; the output of
the entire global economy in 1990 amounted to about 20 trillion USD.
Seen from this perspective, these estimates of the costs of abatement
tend to create the impression that we would, as critics suggest, have to
make draconian cuts in our material standards of living in order to
reduce emissions and achieve the desired levels of CO~2~ concentration.
These same critics view the cost estimates as unaffordable and
politically impossible.

However, viewed from another perspective, an entirely different analysis
emerges. In the absence of emission abatement and without factoring in
any damages from climate change, GDP is assumed to grow by a factor of
ten or so over the next 100 years, which is a typical convention used in
long-run modeling efforts. (The plausibility of these growth
expectations is not debated here, but the following analysis will show
how GDP is expected to grow with and without climate stabilization
policies.) If the 350 ppm target were pursued, the costs associated with
it would only amount to a delay of two to three years in achieving this
aforementioned tenfold increase in global GDP. Thus, meeting a stringent
350 ppm CO~2~ stabilization target would imply that global incomes would
be ten times larger than today by April 2102 rather than 2100 (the date
the tenfold increase would occur for the no-abatement-policies
scenario). This trivial delay in achieving phenomenal GDP growth is
replicated even in more pessimistic economic models. These models may be
very conservative, given that most do not consider the ancillary
environmental benefits of emission abatement (see Figure 7 below).

***Figure 7. Global income trajectories under business-as-usual (BAU)
and in the case of stabilizing the atmosphere at 350 ppm, 450 ppm, and
550 ppm***

![](media/image7.jpeg){width="4.688888888888889in"
height="2.613888888888889in"}

Observe that we have assumed rather pessimistic estimates of the cost of
atmospheric stabilization (average costs to the economy assumed here are
\$200/ ton Carbon (tC) for 550 ppm target, \$300/tC for 450 ppm, and
\$400/tC for 350 ppm) and that the environmental benefits in terms of
climate change and reduction of local air pollution of meeting various
stabilization targets have not been included. (Source: Azar & Schneider,
2002.)

Representing the costs of stringent climate stabilization as a few short
years of delay in achieving monumental increases in wealth should have a
strong impact on how policymakers, industry leaders, and the general
public perceive the climate policy debate. Similar results can be
presented for the Kyoto Protocol: the drop in GDP below \"baseline\"
levels that would occur if the Kyoto Protocol were implemented ranges
between 0.05% and 1%, depending on the region considered and the model
used (see IPCC Working Group III, chapter 8, IPCC 2001c, p. 537-538).
The drops in the growth rates for OECD countries over the next ten years
would likely fall in the range of 0.005-0.1 percent per year below
baseline scenario projections under the Kyoto Protocol. (It should be
kept in mind that the uncertainties about baseline GDP growth
projections are typically much larger than the presented cost-related
deviations.) Returning to the analysis Azar and I did, assuming a growth
rate of two percent per year in the absence of carbon abatement
policies, implementation of the Kyoto Protocol would imply that the OECD
countries would get 20 percent richer (on an annual basis) by June 2010
rather than January 2010, assuming the high-cost abatement estimate.

Similar statements could well be made about the costs associated with
this bill that is before the Committee. Although I have not analyzed it
myself, I strongly suspect that the loss of GDP from the costs incurred
as a result of implementing this measure would be such a small fraction
of typically-projected US GDP growth rates that only months of delay in
growth would occur, nowhere enough to prevent large increases in
personal income from occurring. Thus, this bill is likely to be an
inexpensive "insurance premium" to slow down global warming and lower
the likelihood of "dangerous" climate impacts.

Whether the costs mentioned are big or small is, of course, a value
judgment, but in any case, it is difficult to reconcile the long-term
climate benefits of a short-term delay in GDP growth with the strident
rhetoric of contrarians like Lindsey (2001) who states in a speech to a
colloquim on Science and Technology Policy (organized by the American
Association for the Advancement of Science, or AAAS) that "the Kyoto
Protocol could damage our collective prosperity and, in so doing,
actually put our long-term environmental health at risk" (p.5). Others
have made similar statements about this bill, and they have been refuted
by careful economic analyses (Pizer and Kopp, 2003; Paltsev et al,
2003). Clearly, such balanced quantitative economic assessments, rather
than pessimistic and often politically-motivated exaggerations should
guide the evaluations of making bills like this one the laws of the
land.

I thank the Committee for asking for my views on this important piece of
legislation.

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