The Intergovernmental Panel on Climate Change (IPCC) made no bones about where it stands on global warming in its fourth report, released early today in Paris. "Warming of the climate system is unequivocal," its report stated, adding that most of the warming is "very likely" due to human activity. If people keep spewing greenhouse gases into the atmosphere, they will "very likely" change climate in this century much more than they did in the 20th century, the report concludes.
The IPCC hasn't rushed to judgment on climate change. It took 600 authors from 40 countries 6 years to produce hundreds of pages, which in turn were scanned by 600 reviewers. Then the wording--but not the science--of the 21-page "Summary for Policy Makers" got worked over by 300 delegates from 113 governments this week in Paris. The bottom line is that "there's an irrefutable consensus that [global warming] is real," says geoscientist Michael Oppenheimer of Princeton University in New Jersey. And "there's an irrefutable consensus that it will get worse" if greenhouse emissions are not reined in.
The IPCC's heightened confidence flows from several developments of the past few years. More observations of climate--from satellites to tree rings--have been analyzed. More computer models have grown more realistic and been run multiple times. And the natural world has continued to behave as if it is warming under a strengthening greenhouse. So IPCC upgraded its 2001 statement that "most of the observed warming ... is likely to have been due to" rising greenhouse gases to the warming being "very likely" human-caused.
As for the future, the panel concluded that the climate system is likely to be moderately to strongly sensitive to rising greenhouse gases. The particulars of that sensitivity will depend on how fast greenhouse gases are released, but according to the models, a middle-of-the-road emissions scenario would lead to 2°C to 4°C of warming by the end of the century; the world has warmed 0.6°C in the past century.
The only constant would be change: High latitudes would warm more than low latitudes, but low latitudes would dry out more. Summertime Arctic sea ice would continue shrinking, heat waves and droughts would continue to become more frequent, and melting ice sheets would continue to raise sea level. IPCC's report on the social and economic impacts of such climate change is due out in early April. Another report on what might be done to avoid or adapt to such changes is scheduled for early May.