More than 2,000 scientists from 154 countries participated in the IPCC process, which will release three more reports this year. This first report examined only the physical science of climate change. Scientists drafted as lead authors prepared chapters on subjects ranging from a historical overview of climate change science to regional projections. Governments and other reviewers then submitted more than 30,000 comments. Finally, the lead authors and diplomats gathered in Paris to review the final document word by word, changing an emphasis here ("unequivocal" triumphed over "evident") or leaving out a controversial finding there.
For example, after objections by Saudi Arabia and China, the report dropped a sentence stating that the impact of human activity on the earth's heat budget exceeds that of the sun by fivefold. "The difference is really a factor of 10," says lead author Piers Forster of the University of Leeds in England: compared with its historical output, the sun currently contributes an extra 0.12 watt of energy for each square meter of the earth's surface, whereas man-made sources trap an additional 1.6 watts per square meter.
The document's conservatism also reflects the nature of climate change science. Various models running different scenarios predict sea-level rise as little as 18 centimeters (seven inches) or as much as 59 centimeters (23 inches). None of these models, however, completely includes the potentially greater contributions to such a rise from the melting of glaciers in Greenland and Antarctica. Climate modelers do not include effects on land-based ice in these regions because they cannot reduce them to equations, such as x amount of extra heat equals y amount of melting.
Greenland's glaciers are melting and moving faster on average, but those shifts do not follow a simple, upward linear trend. For example, Kangerdlugssuaq glacier has lost mass from melting and, in its thinner form, has less weight to speed the flow of its ice toward the sea. Additionally, roughly 80 percent of its recent increase in water discharge occurred in just one year before stabilizing, according to Ian Howat of the University of Washington. As glaciologist Richard Alley of Pennsylvania State University notes: "The ice sheet is losing mass, this loss has increased over time, [and] it is not the dominant term in sea-level rise--but it matters." In fact, many variables come into play in Greenland's ice sheet. "You're trying to figure out what is going on with an immense, remote and complex beast, and it isn't easy," Alley adds.
And other important factors, such as the convection that forms thunderstorms, can only be approximated because they occur on too small a scale. "There is no way that the models are able to directly simulate these things," says climate modeler Stephen Zebiak of Columbia University. "So researchers just try to capture the net effect of the processes."
Despite these flaws, global models are increasingly credible: when fed the factors at play in climate over the past 100 years, they accurately match what has been observed to occur. Such precision gives scientists greater confidence in their ability to assign probabilities to the future. And all models agree that the world will warm at least 0.4 degree Celsius in the next 20 years.