Governments around the world have just received one of the most important scientific reports ever written. It provides the starkest assessment yet of how the earth's climate is responding to rising levels of greenhouse gases in the atmosphere and creating risks for billions of people from extreme weather events and rising sea levels.
A confidential draft of the new report on the causes and consequences of global warming was sent to governments to review on June 7, ahead of the publication of the final version this autumn. Compiled for the Intergovernmental Panel on Climate Change by 255 scientific experts from universities and research institutes in 38 countries, the report (which later leaked) provides an up-to-date overview of the findings of thousands of recent peer-reviewed research papers.
Most important, the latest IPCC report _ part of its fifth comprehensive assessment in its 25-year history _ includes an analysis of new computer projections of how global warming might develop by the end of the century. The initial results show that, at current rates of emissions of carbon dioxide and other greenhouse gases, the average global temperature could be at least three degrees centigrade higher by the end of this century than it was before the onset of the Industrial Revolution and widespread burning of fossil fuels.
At a United Nations summit in 2010, governments agreed that emissions should be reduced sharply in order to limit global warming to two degrees centigrade by the end of this century. So the new IPCC report is likely to increase pressure on world leaders ahead of another UN meeting in 2015 to conclude a new international treaty on climate change, including legally binding emissions cuts.
Among the major issues that have been covered by recent research _ and that are addressed in the IPCC report _ are current trends in greenhouse-gas concentrations and global temperatures. The scientific literature indicates that today the level of atmospheric carbon dioxide is about 40% higher than the pre-industrial level. It is at its highest since the Pliocene Epoch about three million years ago, when the planet was 2-3 degrees centigrade warmer, the polar ice caps were much smaller, and the global sea level was about 20 metres higher.
Meanwhile, the global surface temperature has already risen about 0.8 degrees centigrade. Although the rate of increase has been lower over the past 15 years than it was before, nearly all climate scientists believe that the slowdown is temporary, and that warming will accelerate again in the near future.
Governments will negotiate a summary of the new IPCC report line by line at a special meeting in Stockholm at the end of September, with the main report to be published shortly thereafter. Two more major reports _ focusing on the challenges of adapting to the effects of climate change and how to mitigate the worst potential consequences through emissions cuts _ will follow next year. Together with a synthesis of the main conclusions, they will complete the fifth assessment.
The IPCC, established by the World Meteorological Organisation and the UN Environment Programme, has provided policymakers with authoritative information concerning the state of knowledge about climate change since 1988. Its last comprehensive assessment, in 2007, concluded that global warming over the previous 50 years had been "unequivocal", and that there was a 90% chance that most of it was caused by human activity, such as the burning of fossil fuels.
But the IPCC also attracted controversy when it admitted in 2010 that a volume on the consequences of climate change erroneously suggested that, at current rates of melting, all of the glaciers in the Himalayas would disappear by 2035, rather than within a few centuries.
This small but significant mistake prompted an invitation from the IPCC to the world's leading national science academies to review its procedures.
As a result of the academies' recommendations, the IPCC tightened its review methods, created a new process for correcting possible mistakes in future reports, and introduced a more explicit policy for dealing with potential conflicts of interest among authors.
Opponents of the IPCC have unsuccessfully attempted to undermine the new report by selectively leaking earlier versions. They have quoted sections out of context in order to create a misleading impression of its contents, falsely claiming that research shows that cosmic rays from outer space are responsible for global warming.
Nevertheless, governments and the public can be confident that the report will be the most reliable scientific assessment of climate change that has ever been produced. Most critically, it will allow people to read for themselves the authoritative verdict of the world's scientific community on the evidence for climate change. Citizens can then judge the effectiveness of efforts to reduce greenhouse-gas emissions _ and decide whether their governments are doing enough to manage the risks posed by climate change. 2013 PROJECT SYNDICATE
Bob Ward is policy director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics and Political Science.