National Academy of Sciences Raises More Climate Questions

Report acknowledges limitations of scientific understanding

    "A thorough understanding of the uncertainties is essential to the development of good policy decisions."

    - "Climate Change Science" Report, Page 22
    "The most valuable contribution U.S. scientists can make is to continually question basic assumptions and conclusions, promote clear and careful appraisal and presentation of the uncertainties about climate change as well as those areas in which science is leading to robust conclusions, and work toward a significant improvement in the ability to project the future."

    - "Climate Change Science" Report, 6/6/01, Page 23

On June 6, 2001, an 11-member panel of the National Academy of Sciences (NAS) released "Climate Change Science: An Analysis of Some Key Questions," a report they prepared for President George W. Bush. The report confirms important points that many analysts critical of mainstream portrayals of climate change science and policy have argued for years.

In this report, NAS points out that:

  • Uncertainties in climate science throw the question of human causality of climate change into doubt;

  • Uncertainties in projecting future social trends make predictions of future climate conditions "tentative;"

  • Political influences played a significant role in shaping the "Summary for Policymakers of the United Nations' Intergovernmental Panel on Climate Change (IPCC), a key formal document in the U.N.'s three-volume Third Assessment Report on climate change; and

  • Understanding of climate change science is far from complete and is, in fact, still rudimentary in many areas.

The NAS report begins with an adamant statement that "temperatures are, in fact, rising." This is not news, however; virtually no one has argued that this is not the case. While the NAS goes on to affirm some of the technical claims from both the third Assessment Report of the U.N�s IPCC and the National Assessment Report of the United States Global Change Research Project, the NAS report has many sharply cautionary warnings scattered throughout.

This document culls key statements from the NAS report into discrete categories:

  1. Key statements on understanding of the climate system and our forecasting abilities;

  2. Key statements on human causation of observed 20th century climate changes;

  3. Key statements on research needs (the only actual recommendations given by the NAS); and

  4. Key statements on the IPCC process, scientific representation, and political influence on the "Summary for Policymakers" in the U.N.'s third Assessment Report.


1. Key Statements on Understanding of the Climate System and our Forecasting Abilities

While the NAS "generally agrees with the assessment of human-caused climate change" presented by the United Nations' IPCC, the authors of the NAS report seek to "articulate more clearly the level of confidence that can be ascribed to those assessments, and the caveats that need to be attached to them."

The following quote from the NAS report summarizes that effort quite well:

    "Because there is considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, current estimates of the magnitude of future warming should be regarded as tentative and subject to future adjustments upward or downward."

    - "Climate Change Science" Report, Page 1

The NAS report points out the many weaknesses in current understanding of climate processes:

    "Much of the difference in predictions of global warming by various climate models is attributable to the fact that each model represents these [feedback] processes in its own particular way."

    - "Climate Change Science" Report, Page 4
    "The study of the role of black carbon in the atmosphere is relatively new. As a result, it is characterized poorly as to its composition, emission source strengths, and influence on radiation."

    - "Climate Change Science" Report, Page 13
    "There is the possibility that decreasing black carbon emissions in the future could have a cooling effect that would at least partially compensate for the warming that might be caused by a decrease in sulfates."

    - "Climate Change Science" Report, Page 13
    "Climate forcing by anthropogenic aerosols is a large source of uncertainty about future climate change."

    - "Climate Change Science" Report, Page 13
    "The greatest uncertainty about the aerosol climate forcing�indeed, the largest of all the uncertainties about global climate forcings�is probably the indirect effect of aerosols on clouds."

    - "Climate Change Science" Report, Page 14
    "The great uncertainty about this indirect aerosol climate forcing presents a severe handicap both for the interpretation of past climate change and for future assessments of climate change."

    - "Climate Change Science" Report, Page 15

And while the NAS report clearly affirms the usefulness and importance of climate models, it observes that:

    "However, climate models are imperfect. Their simulation skill is limited by uncertainties in their formulation, the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature."

    - "Climate Change Science" Report, Page 15
    "Projecting future climate change first requires projecting the fossil-fuel and land-use sources of CO2 and other gases and aerosols. However, there are large uncertainties in underlying assumption about population growth, economic development, life style choices, technological change and energy alternatives, so that it is useful to examine scenarios developed from multiple perspectives in considering strategies for dealing with climate change."

    - "Climate Change Science" Report, Page 18
    "Scenarios for future greenhouse gas amounts, especially for CO2 and CH4 are a major source of uncertainty for projections of future climate. Successive IPCC assessments over the past decade each have developed a new set of scenarios with little discussion of how well observed trends match with previous scenarios."

    - "Climate Change Science" Report, Pages 18�19

Finally, in another powerfully cautionary statement, the NAS confirms that some of the proposed factors involved in climate change are so uncertain that it is unknown whether the factors will cause warming or cooling:

    "The range of model sensitivities and the challenge of projecting the sign of the precipitation changes for some regions represent a substantial limitation in assessing climate impacts."

    - "Climate Change Science" Report, Page 21


2. Key Statements on Human Causation of Observed 20th Century Climate Changes

When it comes to the all-important question of causality, the NAS report contains cautionary statements far stronger than those seen from other august scientific panels:

    "Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past twenty years. Whether it is consistent with the change that would be expected in response to human activities is dependent upon what assumptions one makes about the time history of atmospheric concentrations of the various forcing agents, particularly aerosols."

    - "Climate Change Science" Report, Page 3
    "Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time history of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established."

    - "Climate Change Science" Report, Page 17
    "The fact that the magnitude of the observed warming is large in comparison to natural variability as simulated in climate models is suggestive of such a linkage, but it does not constitute proof of one because the model simulations could be deficient in natural variability on the decadal to century time scale."

    - "Climate Change Science" Report, Page 17


3. Key Statements on Research Needs

While the NAS report does not make outright recommendations, it does point out research needs and encourages additional research. This itself points to weaknesses in the underlying scientific understanding of climate change.

    "Reducing the wide range of uncertainty inherent in current model predictions of global climate change will require major advances in understanding and modeling of both (1) the factors that determine atmospheric concentrations of greenhouse gases and aerosols, and (2) the so-called �feedbacks� that determine the sensitivity of the climate system to a prescribed increase in greenhouse gases. Specifically, this will involve reducing uncertainty regarding: (a) future usage of fossil fuels, (b) future emissions of methane, (c) the fraction of fossil fuel carbon that will remain in the atmosphere and provide radiative forcing versus exchange with the oceans or net exchange with the land biosphere, (d) the feedbacks in the climate system that determine both the magnitude of the change and the rate of energy uptake by the oceans, which together determine the magnitude and time history of the temperature increases for a given radiative forcing, (e) the details of the regional and local climate change consequent to an overall level of global climate change, (f) the nature and causes of the natural variability of climate and its interactions with forced changes, and (g) the direct and indirect effects of the changing distributions of aerosol."

    - "Climate Change Science" Report, Page 23


4. Key Statements on the IPCC Process, Scientific Representation, and Political Influence on the "Summary for Policymakers" in the U.N.'s Third Assessment Report

Perhaps the most fascinating element of the NAS report is its inquiry into the limitations of the IPCC process, and its questioning whether the IPCC "Summary for Policymakers" the most widely quoted element of all the IPCC publications faithfully represents the underlying technical reports.

While the NAS finds the underlying technical reports of the IPCC on the science of climate change (a.k.a. the "Working Group 1" section of the Third Assessment Report) to be rigorous and representative of mainstream scientific thought, it raised many concerns about the influence of political forces on the IPCC's overall reporting process and on key documents such as its "Summary for Policymakers" in the Third Assessment Report.

The NAS also confirms a practice that many critics of past IPCC reports have questioned: that of retroactively altering the technical studies to support the statements given in the "Summary for Policymakers." While "most" of these changes were acceptable to the IPCC chapter authors, the NAS suggests that "some scientists may find fault with some of the technical details, especially if they appear to underestimate uncertainty" (page 23).

Additional points raised by the NAS report include the following:

    "The 'Summary for Policymakers' reflects less emphasis on communicating the basis for uncertainty, and a stronger emphasis on areas of major concern associated with human-induced climate change. This change in emphasis appears to be the result of a summary process in which scientists work with policy makers on the document."

    - "Climate Change Science" Report, Page 5
    "Changes to the 'Summary for Policymakers' are only approved by 'a fraction of the lead and contributing authors,' not the full body of authors of the WG1 report."

    - "Climate Change Science" Report, Page 5
    "After analysis, the committee finds that the conclusions presented in the 'Summary for Policymakers' and the 'Technical Summary' are consistent with the main body of the report. There are, however, differences. The primary differences reflect the manner in which uncertainties are communicated in the 'Summary for Policymakers.' The 'Summary for Policymakers' frequently uses terms (e.g., likely, very likely, unlikely) that convey levels of uncertainty; however, the text less frequently includes either their basis or caveats."

    - "Climate Change Science" Report, Page 22
    "Confidence limits and probabilistic information, with their basis, should always be considered as an integral part of the information that climate scientists provide to policy- and decision-makers. Without them, the IPCC 'Summary for Policymakers' could give an impression that the science of global warming is 'settled' even though many uncertainties still remain."

    - "Climate Change Science" Report, Page 22
    "In addition, the preparation of the 'Summary for Policymakers' involves both scientists and governmental representatives. Governmental representatives are more likely to be tied to specific government postures with regard to treaties, emission controls, and other policy instruments."

    - "Climate Change Science" Report, Page 23
    "Without an understanding of the sources and degree of uncertainty, decision-makers could fail to define the best ways to deal with the serious issue of global warming."

    - "Climate Change Science" Report, Page 23

The newly released "Climate Change Science: An Analysis of Some Key Questions" report of the National Academy of Sciences is a noteworthy contribution to the ongoing debate over climate policy. While understanding that the Earth�s average temperature has increased recently, and affirming the mainstream scientific view that some of this warming is attributable to human action, the NAS report also acknowledges the current limitations of scientific understanding, and the dangers of mischaracterizing those limitations by exaggerated reporting that downplays uncertainty.

Dr. Kenneth Green is senior fellow at Reason Foundation and Chief Scientist at Frasier Institute.


Relevant Reason Publications

Plain English Guide 3: Exploring the Science of Climate Change

E-brief 105: Mopping Up After a Leak: Setting the Record Straight on the "New" Findings of the Intergovernmental Panel on Climate Change (IPCC)

Evaluating the Kyoto Approach to Climate Change (.pdf)

A Baker's Dozen: 13 Questions People Ask about Climate Change





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