Climate change 2001: The scientific basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change David J. Griggs and Maria Noguer Met Office Hadley Centre, Bracknell Background The earth’s climate system has demonstrably changed since the pre-industrial era, with some of these changes attributable to human activ- ities. The consequences of climate change pose a serious challenge to policy-makers. Hence they need an objective source of information about climate change, its impacts and possible response options. Recognising this, the World Meteorological Organization (WMO) and the United Nations Environmental Programme jointly established the Intergovernmental Panel on Climate Change (IPCC) in 1988. The terms of reference of the IPCC include: (i) to assess available scientific and socio- economic information on climate change and its impacts, and the options for mitigating climate change and adapting to it; and (ii) to provide, on request, scientific/tech- nical/socio-economic advice to the Conference of the Parties to the United Nations Framework Conven- tion on Climate Change (UNFCCC). The IPCC is organised into three Working Groups and a task force on national green- house inventories. Working Group I (WGI) addresses the scientific aspects of the climate system and climate change, Working Group II (WGII) assesses the impacts of and adaptation to climate change, and Working Group III (WGIII) addresses the options for the mitigation of climate change. From 1990, the IPCC has produced three major Assessment Reports (1990, 1996, and 2001), and Special Reports, Technical Papers, methodologies and other products that have been widely used by policy- makers, scientists and other experts. More information about IPCC can be found on the IPCC website (http://www.ipcc.ch). Components of the Third Assessment Report The WGI report (IPCC 2001) on the scientific basis of climate change is the first part of Cli- mate change 2001, the Third Assessment Report (TAR) of the IPCC. Other companion assessment volumes have been produced by WGII (Impact, adaptation and vulnerability) and by WGIII (Mitigation). As well as the underlying chap- ters, each volume contains a ``Summary for Policymakers’’ and a ``Technical Summary’’ to maximise the ease of utility of the report. In addition, a Synthesis Report has been pro- duced, drawing from all IPCC reports scientific and socio-economic information relevant to nine questions addressing particular policy issues raised by the UNFCCC. Production of the WGI contribution to the TAR The WGI TAR 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, and 21 review editors oversaw the production of the report. The draft report was circulated for review to experts, with 420 reviewers sub- mitting 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 assim- ilated into a revised document for considera- tion at the session of the IPCC WGI held in Shanghai, from 17 to 20 January 2001, with delegates from 99 countries and 50 scientists representing the lead authors. There the ``Summary for Policymakers’’, drafted by a team of 59 authors, was approved word by word, and the underlying report accepted. 267 Weather Vol. 57 August 2002