IPCC Third Assessment Report


The IPCC Third Assessment Report, Climate Change 2001, is an assessment of available scientific and socio-economic information on climate change by the IPCC. Statements of the IPCC or information from the TAR were often used as a reference showing a scientific consensus on the subject of global warming. The Third Assessment Report was completed in 2001 and consists of four reports, three of them from its Working Groups: Working Group I: The Scientific Basis; Working Group II: Impacts, Adaptation and Vulnerability; Working Group III: Mitigation; Synthesis Report. A number of the TAR's conclusions are given quantitative estimates of how probable it is that they are correct, e.g., greater than 66% probability of being correct. These are "Bayesian" probabilities, which are based on an expert assessment of all the available evidence.
"Robust findings" of the Synthesis Report include:
  • "Observations show Earth's surface is warming. Globally, 1990s very likely warmest decade in instrumental record". Atmospheric concentrations of anthropogenic greenhouse gases have increased substantially.
  • Since the mid-20th century, most of the observed warming is "likely" due to human activities.
  • Projections based on the Special Report on Emissions Scenarios suggest warming over the 21st century at a more rapid rate than that experienced for at least the last 10,000 years.
  • "Projected climate change will have beneficial and adverse effects on both environmental and socioeconomic systems, but the larger the changes and the rate of change in climate, the more the adverse effects predominate."
  • "Ecosystems and species are vulnerable to climate change and other stresses and some will be irreversibly damaged or lost."
  • "Greenhouse gas emission reduction actions would lessen the pressures on natural and human systems from climate change."
  • "Adaptation has the potential to reduce adverse effects of climate change and can often produce immediate ancillary benefits, but will not prevent all damages." An example of adaptation to climate change is building levees in response to sea level rise.

    Working groups

The IPCC was established in 1988 by the United Nations Environment Programme and the UN's World Meteorological Organization "... to assess scientific, technical and socio-economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation." The IPCC is organized as three working groups and a task force :
  • WGI: Scientific aspects of climate.
  • WGII: Vulnerability, consequences, and options.
  • WGIII: Limitation and mitigation options.
  • Task Force: National Greenhouse Gas Inventories Programme
WG I covers the same areas as the Second Assessment Report of 1995, but WG II & III cover slightly different areas in the TAR.

Conclusions

Working Group I

The key conclusions of Working Group I were:
  1. An increasing body of observations gives a collective picture of a warming world and other changes in the climate system
  2. Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate
  3. Confidence in the ability of models to project future climate has increased
  4. There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities
  5. Human influences will continue to change atmospheric composition throughout the 21st century
  6. Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.
The TAR estimate for the climate sensitivity is 1.5 to 4.5 °C; and the average surface temperature is projected to increase by 1.4 to 5.8 Celsius degrees over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 metres over the same period. The wide range in projections is based upon several different scenarios that assume different levels of future CO2 emissions.

Synthesis Report

The TAR Synthesis Report includes a summary of the TAR's main findings and uncertainties. "Robust findings" of the TAR include:
  • Observed warming of the Earth's surface, attribution of observed warming to human activities, projected increases in future global mean temperature, rising sea levels, and increased frequency of heat waves.
  • Future warming will have both beneficial and adverse effects, but for higher levels of warming, adverse effects will predominate.
  • Developing countries and poor persons are most vulnerable to climate change.
"Key uncertainties" in the TAR include:
  • Estimated climate forcings of natural climatic factors and anthropogenic aerosols, future changes in greenhouse gas emissions, and the role of climate feedbacks, which may amplify or reduce the magnitude of future climate change;
  • Assigning probabilities to projections of changes in sea level and temperature, as well as uncertainties related to regional projections of climate change.

    List of greenhouse gases

The following tables are adapted from Chapter 6 of the Working Group 1 report.

Gases relevant to [radiative forcing] only

Gases relevant to radiative forcing and [ozone depletion]

Projections

Projections are used in the TAR as a guide to the possible future effects of climate change, e.g., changes in global mean temperature and sea level. In the TAR, the word "projection" is favoured over "prediction". This is because many future changes related to climate are highly uncertain. For example, climate change projections are affected by highly uncertain changes in future GHG emissions.
The TAR projects impacts according to possible future changes in global mean temperature. Other projections are based on scenarios that the IPCC has developed. In 2000, the IPCC published 40 different scenarios which contain estimates of future changes in anthropogenic emissions of greenhouse gases and aerosols. The SRES scenarios project a wide range of possible changes in future social and economic development, and projected climate change impacts vary according to the scenario considered. The IPCC has not assigned probabilities to the 40 SRES scenarios. Some authors have argued that some SRES scenarios are more likely to occur than others.

Scientific opinion

The IPCC is backed by the scientific community. For example, a joint statement of support was issued in May 2001 by the science academies of Australia, Belgium, Brazil, Canada, the Caribbean, China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden and the UK. It states: "We recognise the IPCC as the world's most reliable source of information on climate change and its causes, and we endorse its method of achieving consensus."
In 2001, the executive branch of the US federal government asked the US National Research Council to produce an assessment of climate change science. Part of the assessment by US NRC looks at the report produced by Working Group I in the TAR. Working Group I's contribution to the TAR assesses the physical scientific aspects of the climate system and climate change. US NRC generally agrees with findings of the WG I report, for example, US NRC state that " IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue."
US NRC emphasise the need for governments to have a good understanding of uncertainties in climate change science. The example cited by US NRC is the uncertainty over future changes in GHG emissions, which may be less or more than that projected by the TAR. US NRC also state:
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.

Reception

Endorsements

In 2001, 16 national science academies issued a joint statement on climate change.
The joint statement was made by the Australian Academy of Science, the Royal Flemish Academy of Belgium for Science and the Arts, the Brazilian Academy of Sciences, the Royal Society of Canada, the Caribbean Academy of Sciences, the Chinese Academy of Sciences, the French Academy of Sciences, the German Academy of Natural Scientists Leopoldina, the Indian National Science Academy, the Indonesian Academy of Sciences, the Royal Irish Academy, Accademia Nazionale dei Lincei, the Academy of Sciences Malaysia, the Academy Council of the Royal Society of New Zealand, the Royal Swedish Academy of Sciences, and the Royal Society.
The statement, also published as an editorial in the journal Science, stated "we support the conclusion that it is at least 90% certain that temperatures will continue to rise, with average global surface temperature projected to increase by between 1.4 and 5.8 °C above 1990 levels by 2100".
The TAR has also been endorsed by the Canadian Foundation for Climate and Atmospheric Sciences, Canadian Meteorological and Oceanographic Society, and European Geosciences Union.
In 2001, the US National Research Council produced a report that assessed Working Group I's contribution to the TAR. US NRC "generally agrees" with the WGI assessment, and describes the full WGI report as an "admirable summary of research activities in climate science".
IPCC author Richard Lindzen has made a number of criticisms of the TAR. Among his criticisms, Lindzen has stated that the WGI Summary for Policymakers does not faithfully summarize the full WGI report. For example, Lindzen states that the SPM understates the uncertainty associated with climate models. John Houghton, who was a co-chair of TAR WGI, has responded to Lindzen's criticisms of the SPM. Houghton has stressed that the SPM is agreed upon by delegates from many of the world's governments, and that any changes to the SPM must be supported by scientific evidence.
IPCC author Kevin Trenberth has also commented on the WGI SPM. Trenberth has stated that during the drafting of the WGI SPM, some government delegations attempted to "blunt, and perhaps obfuscate, the messages in the report". However, Trenberth concludes that the SPM is a "reasonably balanced summary".
US NRC concluded that the WGI SPM and Technical Summary are "consistent" with the full WGI report. US NRC stated:
... the full report is adequately summarized in the Technical Summary. The full WGI report and its Technical Summary are not specifically directed at policy. 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. Written responses from U.S. coordinating and lead scientific authors to the committee indicate, however, that no changes were made without the consent of the convening lead authors and most changes that did occur lacked significant impact.