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1.1. Overview of the Assessment
The world community faces many risks from climate change. Clearly, it is important
to understand the nature of those risks, where natural and human systems are
likely to be most vulnerable, and what may be achieved by adaptive responses.
To understand better the potential impacts and associated dangers of global
climate change, Working Group II of the Intergovernmental Panel on Climate Change
(IPCC) offers this Third Assessment Report (TAR) on the state of knowledge concerning
the sensitivity, adaptability, and vulnerability of physical, ecological, and
social systems to climate change. Building on the Second Assessment Report (SAR),
this new report reexamines key findings of the earlier assessment and emphasizes
new information and implications on the basis of more recent studies.
Human activities�primarily burning of fossil fuels and changes in land cover�are
modifying the concentration of atmospheric constituents or properties of the
Earth�s surface that absorb or scatter radiant energy. In particular, increases
in the concentrations of greenhouse gases (GHGs) and aerosols are strongly implicated
as contributors to climatic changes observed during the 20th century and are
expected to contribute to further changes in climate in the 21st century and
beyond. These changes in atmospheric composition are likely to alter temperatures,
precipitation patterns, sea level, extreme events, and other aspects of climate
on which the natural environment and human systems depend.
One of several primary issues this report has been organized to address is
a key question before the United Nations Framework Convention on Climate Change
(UNFCCC): What are the potential impacts for societies and ecosystems of different
atmospheric concentrations of GHGs and aerosols that absorb and scatter sunlight
(United Nations, 1992)? Answering this question is a necessary step in assessing
what constitutes �dangerous anthropogenic interference in the climate system.�
This report does not make any judgments about what level of concentrations is
�dangerous� because that is not a question of science per se but a value judgment
about relative risks and tradeoffs. The task is to make the evidence about relative
risks as clear as possible. This report therefore describes what is known about
the distribution of impacts; how, why, and to what extent they differ from region
to region or place to place; and how this relates to the distribution of vulnerability
and capacity to adapt. However, it critically assesses the literature to help
inform policymakers about effects associated with different concentration levels,
so they may judge what levels of risk are acceptable. Assessment of what constitutes
dangerous interference in the climate systems will require analysis of the interactions
of climate change and social and economic conditions, which are inextricably
linked. Understanding the role of socioeconomic factors, particularly adaptive
responses and capacity, is critical.
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Figure 1-1: Regions for the IPCC Working Group
II Third Assessment Report. Note that regions in which small island
states are located include the Pacific, Indian, and Atlantic Oceans, and
the Caribbean and Mediterranean Seas. The boundary between Europe and
Asia runs along the eastern Ural Mountains, River Ural, and Caspian Sea.
For the polar regions, the Arctic consists of the area north of the Arctic
Circle, including Greenland; the Antarctic consists of the Antarctic continent,
together with the Southern Ocean south of ~58�S.
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Part of the justification for a TAR at this time is the abundance
of new evidence that has come to the attention of the expert community since publication
of the SAR. The evidence is drawn predominantly from published, peer-reviewed
scientific literature. Evidence also is drawn from published, non-peer-reviewed
literature and unpublished sources such as industry journals; reports of government
agencies, research institutions, and other organizations; proceedings of workshops;
working papers; and unpublished data sets. The quality and validity of information
from non-peer-reviewed and unpublished sources have been assessed by authors of
this report prior to inclusion of information from these sources in the report.
The procedures for the use of information from non-peer-reviewed and unpublished
sources are described in IPCC (1999a) and discussed in Skodvin (2000).
Although this report builds on previous assessments, including the SAR and
the IPCC�s Special Report on Regional Impacts of Climate Change (IPCC, 1998),
the TAR departs from them in important respects. In comparison to previous assessments,
greater attention is given to climate change adaptation; multiple pressures
on systems; links between climate change, sustainable development, and equity;
and characterization of the state-of-the-science and confidence levels associated
with key conclusions of the assessment (see Box 1-1). This
overview chapter does not attempt to provide a comprehensive summary of the
principal findings of the TAR, but it helps to illustrate basic concepts by
selectively reporting on a few key conclusions, as well as providing a more
comprehensive road map to the materials presented later in the report:
- Part I sets the stage for assessment of impacts, adaptation, and vulnerability
by discussing the context of climate change, methods for research on impacts,
and development of scenarios. These are important additions to what was emphasized
in the SAR. Consideration of the context of change (in this chapter) draws
attention to the relationship of climate change and sustainable development,
including interactions of climate variability and change with other environmental
changes and evolving demographic, social, and economic conditions that affect
driving forces of change and resources available for adaptation. Assessment
of methods (Chapter 2) and approaches for developing
and applying scenarios (Chapter 3) cover scientific
and technical aspects of research on impacts, providing a review of the science
underlying the topics covered in other chapters of the report.
- Part II assesses recent advances in experimental work, observations, and
modeling that contribute to the current state of knowledge of baseline trends,
vulnerabilities, and adaptation options in six sectors or resource areas.
This section of the report integrates material that had been covered in 18
chapters in the SAR, focusing to a greater extent on cross-sectoral issues.
For example, Chapter 5, Ecosystems and their Goods and
Services, integrates what had been separate chapters on forests, rangelands,
deserts, mountain regions, wetlands, agriculture, food security, and other
systems; it also adds assessment of climate change impacts on wildlife�an
issue not covered in the SAR. Integrating these issues into a single chapter
provides an opportunity for improved assessment of interactions across these
systems, the effects of change on landscapes, and the distribution of land
use and cover. Information on the impacts of natural variability and the potential
for nonlinear interactions also is included in the chapters in Part II.
- Chapters in Part III build on key findings of the IPCC�s Special Report
on Regional Impacts of Climate Change. Each chapter and subchapter in this
part of the report explores what has been learned regarding the context of
change, sensitivity, adaptation, and vulnerability of key sectors. A chapter
is devoted to each of eight regions of the world: Africa, Asia, Australia
and New Zealand, Europe, Latin America, North America, polar regions, and
small island states (see Figure 1-1). These regions are
chosen to correspond to continents or�in the case of polar regions and small
island states�to bring together in one chapter areas that share important
attributes related to climate change vulnerability. This regionalization of
the world is convenient for organizing the report, but it must be recognized
that there is a high degree of heterogeneity within each of these regions
in terms of climate, ecosystems, culture, and social, economic, and political
systems. Consequently, climate change impacts, adaptive capacity, and vulnerability
vary markedly within each of these regions. These chapters provide an opportunity
to place vulnerability and adaptation in the context of multiple stresses
and regional resources for adaptation. This is an extremely important development
because it calls attention to the issues that regional and local decisionmakers
in the private and public sectors will be facing in each of the regions.
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Box 1-1 Uncertainties and Confidence Scale
The many conclusions presented in this report are subject to varying
degrees of uncertainty. The degree of uncertainty attached to conclusions
in this report are assessed and reported in two different ways. One is
to assess and report a confidence level for a conclusion, using a Bayesian
probability framework. (Bayesian assessments of probability distributions
would lead to the following interpretation of probability statements:
The probability of an event is the degree of belief that exists among
lead authors and reviewers that the event will occur, given observations,
modeling results, and theory currently available.) The second is to assess
and report the quality or level of scientific understanding that supports
a conclusion.
The 5-point confidence scale below is used to assign confidence levels
to selected conclusions. The confidence levels are stated as Bayesian
probabilities, meaning that they represent the degree of belief among
the authors of the report in the validity of a conclusion, based on their
collective expert judgment of all observational evidence, modeling results,
and theory currently available to them.
5-Point Quantitative Scale for Confidence Levels
| 95% or greater |
Very High Confidence |
| 67�95% |
High Confidence |
| 33�67% |
Medium Confidence |
5�33%
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Low Confidence |
| 5% or less |
Very Low Confidence |
For some conclusions, the 5-point quantitative scale is
not appropriate as a characterization of associated uncertainty. In these
instances, authors qualitatively evaluate the level of scientific understanding
in support of a conclusion, based on the amount of supporting evidence
and the level of agreement among experts about the interpretation of the
evidence. The matrix below has been used to characterize the level of
scientific understanding.
Key to Qualitative �State of Knowledge� Descriptors
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Well-Established: Models incorporate known processes; observations
are consistent with models; or multiple lines of evidence support
the finding.
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Established but Incomplete: Models incorporate most known processes,
although some parameterizations may not be well tested; observations
are somewhat consistent but incomplete; current empirical estimates
are well founded, but the possibility of changes in governing processes
over time is considerable; or only one or a few lines of evidence
support the finding.
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Competing Explanations: Different model representations account for
different aspects of observations or evidence or incorporate different
aspects of key processes, leading to competing explanations.
-
Speculative: Conceptually plausible ideas that haven�t received much
attention in the literature or that are laced with difficult to reduce
uncertaint
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Areas of important new findings include detection of impacts of observed climatic
changes on environmental systems, transient scenarios, vulnerability to changes
in climate variability, and vulnerability to strongly nonlinear, complex, and
discontinuous responses to climate change. Another distinction from previous
assessments is the recognition in the TAR that the many complexities of analysis
logically lead to a focus on ranges of outcomes and characterizations, using
subjective probabilities of events, rather than primary emphasis on �best guesses,�
point estimates, single �optimum,� or aggregate conclusions. Finally, the TAR
has circulated cross-cutting �guidance papers� to all three working groups to
try to achieve more consistency in dealing with four areas: development, sustainability,
and equity; uncertainties; costing methodologies; and decision analytic frameworks
(see Box 1-2).
The IPCC�s charge to Working Group II for the TAR implies that consideration
of the impacts of climate change in the SAR is insufficient per se as a basis
for decisionmaking. In general, the SAR was able to address the implications
of climate change only for single economic sectors or environmental components.
With this in mind, Chapters 4�19
consider the various decision analysis framework tools to improve upon the responses
to the impacts of climate change provided in the SAR. In the current exercise,
not only are possible implications of climate change for the various economic
sectors or environmental components assessed, options to alleviate identified
impacts are investigated. In addition, direct and indirect costs of adaptation
options are explored, and an extensive assessment of direct and indirect benefits
is provided. Where monetary values can be assigned, CBA is employed to determine
the optimal value of adaptation measures, including sensitivity analysis to
critical parameters, and CEA is adopted to identify the least-cost solution
to targeted mitigation objectives.
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Box 1-2. Cross-Cutting Issues Guidance Papers
Four cross-cutting guidance papers (Pachauri et al., 2000) were available
to all Lead Authors of all three IPCC working groups. Many of the concepts
in these papers were previously unfamiliar to a large number of the Lead
Authors. Significant efforts were made to incorporate the uncertainties
guidance and at least consider the guidance in other papers by each working
group. Future assessments will be increasingly able to benefit from the
suggestions and frameworks in these four cross-cutting guidance papers.
Development, Sustainability, and Equity (DSE) (Munasinghe, 2000)
DSE is relevant to Working Group II with respect to three questions:
How do development paths and equity conditions influence vulnerability
to climate change and the capacity to adapt to or cope with climate change?
How might climate change impacts and adaptation responses affect prospects
for attaining sustainable development and equity goals? What types of
policies are capable of reducing climate change vulnerability and promoting
sustainable development and equity objectives?
DSE is a response to principles contained in UNFCCC Article 3.4 (to promote
sustainable development). Article 3.2 takes into account the special needs
and circumstances of developing countries. Article 4 deals, for example,
with the responsibility of developed nations, competing priorities for
developing nations, and �common, but differentiated responsibilities,�
and Article 2 says to avoid �dangerous� interference with the climate
system (United Nations, 1992). DSE is closely tied to sustainable development
with respect to three underlying dimensions: economic, social, and environmental.
Development has been characterized as �qualitative improvement� (Ishida,
1998), including economic growth and social dimensions. Sustainability
of a system refers to its durability or its capacity to withstand and
recover from disturbances (WCED, 1987)�in other words, its resilience.
Equity refers to procedural as well as distributional issues. Procedural
issues relate to how decisions are made (e.g., internal equity and governance
structures within nations could have significant effects on adaptive capacity).
Distributional equity, on the other hand, relates to how the costs of
impacts, mitigation, and adaptation are shared. Equity considerations
are important in addressing global climate change for several reasons,
including moral and ethical concerns; facilitating cooperation because
equitable decisions carry greater legitimacy; the social dimension of
sustainable development; and the UNFCCC itself, which considers equity
as one of its basis principles (in Article 3.1).
Climate change could undermine social welfare, equity, and the sustainability
of future development. In particular, it is generally believed that developing
countries and disadvantaged groups within all countries are more vulnerable
to the impacts of climate change (e.g., Chapter 18)
as a result of limited resources and low adaptive capacity.
Uncertainty (Moss and Schneider, 2000)
Anticipating the imperfect nature of available information, UNFCCC Article
3.3 provides guidance to the effect that �where there are threats of serious
or irreversible damage, lack of full scientific certainty should not be
used as a reason for postponing measures to anticipate, prevent, or minimize
the causes of climate change and mitigate its adverse effects� (United
Nations, 1992). The uncertainties guidance paper develops a unified approach
for assessing, characterizing, and reporting uncertainties in the TAR.
The most important contribution of the uncertainties guidance paper is
the construction of confidence schemes and qualitative terms to describe
the state of science, which are reproduced in Box 1-1. The goal is to
promote consistency in evaluating the judgments of scientific experts,
to facilitate communication of these judgments to nonspecialists, and
to provide peer-reviewed guidance for policymakers. Thus, a great deal
of importance is attached to assessing the scientific merit of information
in the literature and �explicitly distinguishing and communicating which
findings are well understood, which are somewhat understood, and which
are speculative� (Moss and Schneider, 1997). Section
2.6 discusses in more detail the differences between the well-calibrated
ranges in the literature and the much larger, full range of uncertainty
as well as the cascade of uncertainty that occurs when ranges in climate
scenarios are cascaded with uncertainties in each successive step of assessment.
In Chapters 4�19, selected
findings are assigned levels of confidence, using either the scale for
assessing confidence level quantitatively or the matrix for assessing
the state of knowledge qualitatively.
Especially in the regional chapters, uncertainty about future climate
is the dominant cause of uncertainty about the character and magnitude
of impacts. In such cases, confidence estimates are evaluated conditionally
on a specific climate change scenarios to avoid �cascades� (see Figure
2-2) in which confidence in the occurrence of an event does not include
compounded uncertainties in each factor that contributes to the final
outcome. Instead, the assessment evaluates each step in the cascade separately�what
is called a �traceable account��and is particularly appropriate for any
aggregate conclusions.
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The concluding section of this report (Chapters 18 and
19) examines global issues and offers a synthesis. A significant
addition to previous assessments is Chapter 18, which
is devoted to assessment of opportunities for and barriers to adaptation. This
chapter considers determinants of adaptive capacity; lessons from adaptation
to present-day climate variability and extremes; the potential effectiveness
of adaptation measures; and global-, national-, and local-scale options for
strengthening adaptive capacity, especially for vulnerable populations, countries,
or zones. Chapter 19, a synthesis, also is new to the
TAR. It draws on the analyses of other chapters, synthesizing information that
is important for interpretation of Article 2 of the UNFCCC and key provisions
of international agreements to address climate change. Potential global impacts
of different stabilization levels of atmospheric concentrations of GHGs are
assessed. Chapter 19 assesses vulnerability within the
framework of sustainable development and equity, acknowledging common but differentiated
responsibilities.
The issue of what constitutes sustainable development was advanced in 1987
by the World Commission on Environment and Development (the so-called Brundtland
Commission; WCED, 1987). The commission defined sustainable development as �development
that meets the needs of the present without compromising the ability of future
generations to meet their own needs� and notes that �even the narrow notion
of physical sustainability implies a concern for social equity between generations,
a concern that must logically be extended to equity within each generation.�
The goal of sustainable development is a stable human environmental system in
which available resources are sufficient to meet the needs of society in perpetuity.
Questions have been asked about whether �needs,� as conceived in the Brundtland
Commission report, should be limited to environment-dependent basic necessities
of food, clothing, shelter, and health or should include more qualitative aspects
such as comfort, convenience, or other �quality of life� measures. There is
no consensus in the literature regarding what constitutes the limits of �needs�
in this context.
Because available studies have not employed a common set of climate scenarios
and methods and because of uncertainties regarding the sensitivities and adaptability
of natural and human-dominated systems, assessment of regional vulnerabilities
is necessarily qualitative. Whenever possible, quantitative estimates of the
impacts of climate change are cited in this report. Field or experimental data
often provide quantitative underpinnings for specific circumstances, but rarely
are complex systems sufficiently described by the limited number of cases in
which a large quantity of �hard data� is available. Thus, most quantitative
estimates are still dependent on the specific assumptions employed regarding
future changes in climate, as well as the particular methods and models applied
in the analyses. On the other hand, issues in which there is a great deal of
relevant field or lab data are likely to carry higher confidence in any such
quantitative estimate. To interpret these estimates, it is important to bear
in mind uncertainties regarding the character, magnitude, and rates of future
climate change that will affect society�s degree of exposure. Of comparable
importance are uncertainties associated with future states of the human condition�for
example, the extent and quality of economic development throughout the world
and the evolution of traditions and institutions in societies�that will affect
profoundly the capacity for coping and adaptation, hence level of vulnerability.
These uncertainties impose limitations on the ability of the research community
to project the impacts of climate change, particularly at regional and smaller
scales.
This introductory chapter is organized to address a series of questions: What
is potentially at stake as a result of changes in climate (Section
1.2)? How has society responded to the risks and potential opportunities
(Section 1.3)? How are impacts, adaptation, and vulnerability
assessed in the report (Section 1.4)? How do the complexities
of analysis affect the assessment (Section 1.5)? How can
this assessment be used to address policy-relevant questions (Section
1.6)?
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