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POLICYFORUM
CLIMATE CHANGE
The Limits of Consensus
The establishment of consensus by the IPCC is
no longer as critical to governments as a full
exploration of uncertainty.
he Intergovernmental Panel on Climate Change (IPCC) has just delivered its Fourth Assessment Report
(AR4) since 1990. The IPCC was a bold
innovation when it was established, and
its accomplishments are singular (1, 2). It
was the conclusion in the IPCC First
Assessment Report that the world is likely
to see “a rate of increase of global mean
temperature during the next century … that
is greater than seen over the past 10,000
years” (3) that proved influential in catalyzing the negotiation of the United Nations
Framework Convention on Climate Change.
The conclusions of the Second Assessment
with regard to the human influence on
climate (4) marked a paradigm shift in the
policy debate that contributed to the negotiation of the Kyoto Protocol. IPCC conclusions from the Third, and now the Fourth,
assessments have further solidified consensus behind the role of humans in changing
the earth’s climate.
The emphasis on consensus in IPCC
reports, however, has put the spotlight on
expected outcomes, which then become
anchored via numerical estimates in the
minds of policy-makers. With the general
credibility of the science of climate change
established, it is now equally important that
policy-makers understand the more extreme
possibilities that consensus may exclude or
downplay (5).
For example, the Working Group I
(WGI) “Summary for Policymakers” (SPM)
of AR4 anticipates a rise in sea level of
between 18 and 59 cm by the year 2100 (6),
a “model-based range” composed largely of
thermal expansion of oceans, melting of
nonpolar glaciers, and the gradual response
of ice sheets. The range does not include the
CREDIT : NATIONAL SNOW AND ICE DATA CENTER, BOULDER, CO
T
potential for increasing contributions from
rapid dynamic processes in the Greenland
and West Antarctic ice sheets (WAIS),
which have already had a significant effect
on sea level over the past 15 years and could
eventually raise sea level by many meters.
Lacking such processes, models cannot
fully explain observations of recent sealevel rise, and accordingly, projections
based on such models may seriously understate potential future increases. Although
the AR4 SPM recognizes the possibility of a
1Woodrow
Wilson School of Public and International
Affairs, Princeton University, Princeton, NJ, USA. 2Department of Geosciences, Princeton University, Princeton, NJ,
USA. 3International Institute for Applied Systems Analysis,
Laxenburg, Austria. 4Watson Institute for International
Studies, Brown University, Providence, RI, USA. 5MIT Joint
Program on the Science and Policy of Global Change,
Massachusetts Institute of Technology, Cambridge, MA,
USA. 6Organization for Economic Cooperation and
Development (OECD), Paris, France.
*Author for correspondence. E-mail: [email protected]
The views expressed in this paper are those of the authors
and not necessarily those of the institutions they are
affiliated with.
Not captured by ice-sheet models. (Top) The Larsen
B ice shelf along the Antarctic Peninsula on 31
January 2002. (Bottom) A large section has disintegrated, 5 March 2002. Glaciers behind the collapsed
section of the ice shelf subsequently accelerated their
discharge into the ocean, apparently because of the
loss of buttressing by the ice shelf. Neither rapid collapse nor buttressing are captured by ice-sheet models, and both could substantially affect the rate of
future sea-level rise as larger ice shelves to the south
in West Antarctica warm (26).
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VOL 317
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larger ice-sheet contribution, its main
quantitative results indicate the opposite:
Uncertainty in sea-level rise is smaller, and
its upper bound is lower, for the 21st century than was indicated in the Third Assessment Report (7). On the related question of sea-level rise beyond the 21st century, whereas the Third Assessment’s SPM
provided a numerical estimate of a potential contribution from WAIS, the AR4 WGI
SPM doesn’t mention WAIS at all. This
omission presumably reflects a lack of consensus arising from the inadequacy of icesheet models for WAIS made so apparent
by recent observations.
Nevertheless, alternatives to model-based
approaches, such as empirical analysis and
expert elicitation, were available for exploring uncertainty in 21st-century (8) and longterm sea-level rise (9), respectively. Such
information certainly would have been
useful to policy-makers, particularly for
WAIS, which contains enough ice to raise
sea level by about 5 m.
Setting aside or minimizing the importance of key structural uncertainties in
underlying processes is a frequent outcome
of the drive for consensus (5, 10). For example, ranges of projected warming and atmospheric composition in AR4 include an
amplifying effect of interactions between
climate and the carbon cycle. However, the
estimated uncertainty in this effect is based
largely on models that omit a number of
poorly understood processes (11), such
as feedbacks on carbon contained in permafrost; changes in marine ecosystem structure; and responses to land-use history,
nutrient limitation, and air-pollution effects.
These models also share similar assumptions about the temperature sensitivity of
carbon fluxes from soils based on experimental results that cannot be reliably scaled
to the ecosystem level (12). A fuller accounting of uncertainty would be more
appropriate.
Similarly, the narrowing of uncertainty
(relative to previous assessments) associated
with potential changes in the meridional
overturning circulation relies on agreement
across models, but the structural uncertainty
in all the models means that less may
be known than suggested by the numerical
estimates (13).
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Michael Oppenheimer,1,2* Brian C. O’Neill,3,4 Mort Webster,5 Shardul Agrawala1,6
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POLICYFORUM
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Avoiding Premature Consensus
The IPCC has made progress over four
assessment cycles in its treatment of uncertainties. However, this progress is limited and
uneven across its Working Groups. Several
additional modifications to the current practice could reduce the risk of ignoring or
underemphasizing critical uncertainties.
First, given the anchoring that inevitably
occurs around numerical values, the basis
for quantitative uncertainty estimates provided must be broadened to give observational, paleoclimatic, or theoretical evidence of poorly understood phenomena
comparable weight with evidence from
numerical modeling. In areas in which
modeling evidence is sparse or lacking,
IPCC sometimes provides no uncertainty
estimate at all. In other areas, models are
used that have quantitatively similar structures, leading to artificially high confidence
in projections (e.g., in the sea-level, oceancirculation, and carbon-cycle examples
above). One possible improvement would
be for the IPCC to fully include judgments
from expert elicitations (23), as Working
Group II has sometimes done. Beyond this,
increased transparency, including a thorough narrative report on the range of views
expressed by panel members, emphasizing
areas of disagreement that arose during the
assessment, would provide a more robust
evaluation of risk (24). It would be critical
to include this information not only in the
chapters, but in the summaries for policymakers as well.
Second, IPCC should revise its procedure
for expert review to guard against overconfidence. External reviewers should ferret out
differences between chapters or author subgroups, and a special team of authors could be
instructed to examine the treatment of unlikely
but plausible processes, perhaps in a separate
chapter. Integration of risk assessment across
Working Groups in advance of drafting of the
Synthesis Report would highlight internal discussions and disagreements. At the end of an
assessment cycle, a small external team of
ombudsmen should review key problematic
issues (of a scientific nature) that may have
emerged from the report and should recommend modifications of approaches for handling these areas in subsequent reports.
Third, IPCC could also formalize a
process of continuous review of its structure
and procedures. A useful example is provided by the history of IPCC emissions scenario development, which included a series
of reviews for production of the SA90, IS92,
and SRES scenarios (25).
Fourth, and perhaps most important,
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national governments now need to confront a
more fundamental question of how often they
need comprehensive assessments of climate
change. Addressing the special risks entailed in
particular aspects of the climate system, like
the ice sheets or carbon cycle, might be better
approached by increasing the number of concise, highly focused special reports that can be
completed relatively quickly by smaller groups,
perhaps even by competing teams of experts.
At this juncture, full assessments emphasizing
consensus, which are a major drain on participants and a deflection from research, may not
be needed more than once per decade.
References and Notes
1. S. Agrawala, Clim. Change 39, 621 (1998).
2. N. Oreskes, Science 306, 1686 (2004).
3. IPCC, Summary for policymakers, in Scientific Assessment
of Climate Change: Report of Working Group I (IPCC
Secretariat, Geneva, 1990).
4. IPCC, Summary for policymakers, in Climate Change
1995: The Science of Climate Change (IPCC Secretariat,
Geneva, 1995).
5. A. G. Patt, Risk Decis. Policy 4, 1 (1999).
6. IPCC, Summary for policymakers, in Climate Change
2007: The Physical Science Basis: Contribution of
Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change, S. Solomon
et al., Eds. (Cambridge Univ. Press, New York, 2007).
7. IPCC, Summary for policymakers, in Climate Change
2001: The Physical Science Basis, J. T. Houghton et al.,
Eds. (Cambridge Univ. Press, Cambridge, 2001).
8. S. Rahmstorf, Science 315, 368 (2007).
9. D. G. Vaughan, J. R. Spouge, Clim. Change 52, 65 (2002).
10. M. Oppenheimer, B. C. O’Neill, M. Webster, paper presented at the Conference on Learning and Climate
Change, International Institute for Applied Systems
Analysis, Laxenburg, Austria, 10 April 2006.
11. P. Friedlingstein et al., J. Clim. 19, 3337 (2006).
12. J. M. Melilo et al., Science 298, 2173 (2002).
13. K. Zickfeld et al., Clim. Change 82, 235 (2007).
14. L. H. Goulder, S. H. Schneider, Resour. Energy Econ. 21,
211 (1999).
15. E. Baker, Resour. Energy Econ. 27, 19 (2005).
16. R. Gerlagh, B. van der Zwaan, Resour. Energy Econ. 25,
35 (2003).
17. M. Webster et al., Atmos. Environ. 36, 3659 (2002).
18. E. Parson, Protecting the Ozone Layer: Science and
Strategy (Oxford Univ. Press, New York, 2003).
19. World Meteorological Organization, Report of the
International Ozone Trends Panel—1988 (Report 18,
Global Ozone Research and Monitoring Project, World
Meteorological Organization, Geneva, 1988).
20. World Meteorological Organization, Scientific Assessment
of Ozone Depletion—1991 (Report 25, Global Ozone
Research and Monitoring Project, World Meteorological
Organization, Geneva, 1991).
21. World Meteorological Organization, Atmospheric Ozone
1985 (Report 16, Global Ozone Research and Monitoring
Project, World Meteorological Organization, Geneva, 1986).
22. F. S. Rowland, Am. Sci. 77, 36 (1989).
23. G. Morgan, M. Henrion, Uncertainty: A Guide to Dealing
with Uncertainty in Quantitative Risk and Policy Analysis
(Cambridge Univ. Press, Cambridge, 1990).
24. A. Patt, Glob. Environ. Change 17, 37 (2007).
25. The SA90 scenarios were published in the IPCC First
Assessment Report. The IS92 scenarios were published in
the 1992 Supplementary Report to the IPCC Assessment.
26. T. Scambos, J. Bohlander, B. Raup, compilers, Images of
Antarctic ice shelves [2001, updated 2002], National
Snow and Ice Data Center, Boulder, CO.
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Like models of physical processes, conclusions drawn on the basis of socioeconomic
models may also be subject to premature consensus. Estimates of the costs of mitigating
emissions come primarily from models that
omit endogenous technical change, a poorly
understood process. This omission could
cause a significant bias, not only in mitigation costs, but also in the stringency of nearterm mitigation that may be justified for a
given damage function or stabilization target (14–16). Similarly, the conventional use
of the range of emissions described by the
IPCC Special Report on Emissions Scenarios
(SRES) marker scenarios as a key determinant of uncertainty in projecting climate
change, sea-level rise, impacts, and mitigation costs may be misguided. The SRES scenarios were intended to be representative of
scenarios available in the literature at the
time they were produced, with no explicit
goal of spanning the full range of uncertainty. The SRES assessment made no
attempt to judge whether emissions pathways outside the range it covers could plausibly occur. In fact, pathways outside that
range were known at the time, and more
have been developed since the publication
of SRES (17).
To be sure, the underlying IPCC chapters
do detail the limitations and uncertainties
associated with such conclusions. But the
caveats are often cryptic or lost entirely in the
highly influential SPMs. This inevitably leads
to an anchoring by both policy-makers and
scientists around any numerical estimates
that are reported in these summaries.
Ignoring the implications of structural
uncertainty in models of key aspects of the
climate system is reminiscent of the way
assessments treated the uncertainty in ozone
photochemical models. Projections of ozone
depletion were made from 1974 onward
based on improved understanding of gasphase chemistry (18). Knowledge of stratospheric chemistry was then transformed by
the report in 1985 of large, seasonal Antarctic
depletion (the “ozone hole”); the validation in
1987 of its origin in halogen photochemistry;
and subsequent identification of depletion at
the mid-latitudes and in the Arctic (19, 20).
Various heterogeneous chemical reactions,
discounted by most researchers years before
and absent from nearly all model simulations
(21), were shown to be the missing photochemical processes required to explain
observed depletion. Their potential implications were of concern to some scientists (22),
but this structural uncertainty was generally
downplayed in assessments until the ozone
hole was reported.