Global Warming: New Scenarios from the Intergovernmental Panel on Climate Change

Abstract

The Intergovernmental Panel on Climate Change (IPCC) was set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme to review the state of knowledge about human‐induced climate change and assess possible responses. Most of its activities are conducted by three working groups, concerned respectively with scientific aspects of the climate system, with the vulnerability of human and natural systems to climate change, and with options for mitigating that change. The major IPCC reports have been highly detailed statements of scientific consensus on changes in the climate system, issued at roughly five‐year intervals. These reflect the input of some hundreds of scientists, with drafts scrutinized by expert reviewers, revised to attain consensus, and eventually approved (or “accepted”) by the full Panel. The first such assessment, published in 1990, was influential in formulating the Framework Convention on Climate Change adopted at the 1992 Rio conference. The second assessment report (SAR), Climate Change 1995, produced the widely cited estimate that global warming would raise average temperatures by 1°–3.5°C by 2100, with a “best estimate” of 2°C, and produce a sea‐level rise of 0.13 – 0.94 meters. That report took the further step of explicitly linking the warming to anthropogenic (human‐caused) emissions of greenhouse gases. Its cautious conclusion: “The balance of evidence suggests a discernible human influence on global climate.” Climate Change 2001: The Scientific Basis is Working Group I's contribution to the third IPCC assessment. The document was finalized at the Group's meeting in Shanghai in January 2001. A brief (18‐page) “Summary for Policymakers” was also released at this meeting, distilling the findings of the full report and putting them in more accessible language. Two sections of this summary document, presenting the Group's projections of atmospheric temperature trends and sea‐level rises, are reproduced below.The procedure followed was to assemble hypothetical alternative combinations of future greenhouse gas emissions in the form of emission scenarios, which were fed into large‐scale climate models to produce estimates of future temperature and sea‐level trends. For the third assessment report the scenarios used were set out in the IPCC Working Group Ill's Special Report on Emission Scenarios (March 2000), and are referred to below as the “SRES scenarios.” There are 35 of them in all. They fall into six groups, detailed in the text box, from each of which an illustrative case is plotted in the charts. (The shaded areas in the charts are envelopes spanning the 35 scenarios. Some additional details shown in the original charts have been omitted here for clarity.) The SAR scenarios are referred to as IS92.The major difference from the second assessment is in the projected temperature increase, which is now put at 1.4°–5.8°C (or in Fahrenheit degrees, 2.5°–10.4°). The projected sea‐level rise is slightly smaller, at 0.09 – 0.88 meters. There is also a strengthening of the statement on anthropogenic causes, which now reads: “There is new and stronger evidence that most of the [atmospheric] warming observed over the last 50 years is attributable to human activities.” The report notes that even with stabilization of greenhouse gas emissions, there may be continuing climate effects beyond the twenty‐first century. One such effect is the “weakening of ocean thermohaline circulation “—the ocean currents that, for example, transport heat into high northern‐hemisphere latitudes and moderate the coastal climates of those regions.The summary report is available online at the IPCC's website, www.ipcc.ch. The complete third assessment report, covering also the conclusions of Working Groups II and III (particularly on the social and economic costs of forecast climate change), will be released shortly.