Climate Change 2001:
Working Group I: The Scientific Basis
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This report is the first complete assessment of the science of climate change since Working Group I (WGI) of the IPCC produced its second report Climate Change 1995: The Science of Climate Change in 1996. It enlarges upon and updates the information contained in that, and previous, reports, but primarily it assesses new information and research, produced in the last five years. The report analyses the enormous body of observations of all parts of the climate system, concluding that this body of observations now gives a collective picture of a warming world. The report catalogues the increasing concentrations of atmospheric greenhouse gases and assesses the effects of these gases and atmospheric aerosols in altering the radiation balance of the Earth-atmosphere system. The report assesses the understanding of the processes that govern the climate system and by studying how well the new generation of climate models represent these processes, assesses the suitability of the models for projecting climate change into the future. A detailed study is made of human influence on climate and whether it can be identified with any more confidence than in 1996, concluding that there is new and stronger evidence that most of the observed warming observed over the last 50 years is attributable to human activities. Projections of future climate change are presented using a wide range of scenarios of future emissions of greenhouse gases and aerosols. Both temperature and sea level are projected to continue to rise throughout the 21st century for all scenarios studied. Finally, the report looks at the gaps in information and understanding that remain and how these might be addressed.

This report on the scientific basis of climate change is the first part of Climate Change 2001, the Third Assessment Report (TAR) of the IPCC. Other companion assessment volumes have been produced by Working Group II (Impacts, Adaptation and Vulnerability) and by Working Group III (Mitigation). An important aim of the TAR is to provide objective information on which to base climate change policies that will meet the Objective of the FCCC, expressed in Article 2, of stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. To assist further in this aim, as part of the TAR a Synthesis Report is being produced that will draw from the Working Group Reports scientific and socio-economic information relevant to nine questions addressing particular policy issues raised by the FCCC objective.

This report was compiled between July 1998 and January 2001, by 122 Lead Authors. In addition, 515 Contributing Authors submitted draft text and information to the Lead Authors. The draft report was circulated for review by experts, with 420 reviewers submitting valuable suggestions for improvement. This was followed by review by governments and experts, through which several hundred more reviewers participated. All the comments received were carefully analysed and assimilated into a revised document for consideration at the session of Working Group I held in Shanghai, 17 to 20 January 2001. There the Summary for Policymakers was approved in detail and the underlying report accepted.

Strenuous efforts have also been made to maximise the ease of utility of the report. As in 1996 the report contains a Summary for Policymakers (SPM) and a Technical Summary (TS), in addition to the main chapters in the report. The SPM and the TS follow the same structure, so that more information on items of interest in the SPM can easily be found in the TS. In turn, each section of the SPM and TS has been referenced to the appropriate section of the relevant chapter by the use of Source Information, so that material in the SPM and TS can easily be followed up in further detail in the chapters. The report also contains an index at Appendix VIII, which although not comprehensive allows for a search of the report at relatively top-level broad categories. By the end of 2001 a more in-depth search will be possible on an electronic version of the report, which will be found on the web at

We wish to express our sincere appreciation to all the Co-ordinating Lead Authors, Lead Authors and Review Editors whose expertise, diligence and patience have underpinned the successful completion of this report, and to the many contributors and reviewers for their valuable and painstaking dedication and work. We are grateful to Jean Jouzel, Hervé Le Treut, Buruhani Nyenzi, Jim Salinger, John Stone and Francis Zwiers for helping to organise drafting meetings; and to Wang Caifang for helping to organise the session of Working Group I held in Shanghai, 17 to 20 January 2001.

We would also like to thank members of the Working Group I Bureau, Buruhani Nyenzi, Armando Ramirez-Rojas, John Stone, John Zillman and Fortunat Joos for their wise counsel and guidance throughout the preparation of the report.

We would particularly like to thank Dave Griggs, Maria Noguer, Paul van der Linden, Kathy Maskell, Xiaosu Dai, Cathy Johnson, Anne Murrill and David Hall in the Working Group I Technical Support Unit, with added assistance from Alison Renshaw, for their tireless and good humoured support throughout the preparation of the report. Thanks go to Christoph Ritz and Bettina Buechler of ProClim (Forum for Climate and Global Change), a programme of the Swiss Academy of Sciences in Bern for their assistance in producing the index to this report. We would also like to thank Narasimhan Sundararaman, the Secretary of IPCC, Renate Christ, Deputy Secretary, and the staff of the IPCC Secretariat, Rudie Bourgeois, Chantal Ettori and Annie Courtin who provided logistical support for government liaison and travel of experts from the developing and transitional economy countries.

Robert Watson
IPCC Chairman

John Houghton
Co-chair IPCC WGI

Ding Yihui
Co-chair IPCC WGI

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