The UN Intergovernmental Panel on Climate Change whirrs into action this week with a significant assessment of extreme events and disasters. The final report is due on Nov. 18. A draft summary for policymakers obtained by Bloomberg shows the caution and rigor with which scientists approach attributing observed trends to man-made climate change.
The panel says it's “virtually certain” that warm daily temperature extremes will increase in this century. It’s "likely" that human influences have led to a warming of extreme daily minimum and maximum temperatures across the globe, and that instances of heavy rainfall will increase. The report finds the average maximum wind speed of hurricanes is likely to increase, though storm frequency is likely to drop or remain the same.
“Likely" or "virtually certain" imply precision in science that's generally absent from everyday speech. So when they say "virtually certain," they're using a definition of 99 to 100 percent probability. "Very likely" is 90 to 100 percent, and "likely" is 66 to 100 percent.
The values assigned in this week's report are meant to inform and constrain public policy discussions, and will be fodder for delegates to chew on at the UN's annual climate treaty talks which start Nov. 28 in Durban, South Africa. NASA climatologist and IPCC contributor Gavin Schmidt explains how nonscientists might interpret these assessments: “If it is likely to rain, will you take an umbrella with you? If people answer yes, then that is your answer -- responses to likely events are sensible. If they answer 'no,' then there isn't much point in continuing the conversation.”
Climate scientists distinguish between the “likelihood” of certain events occurring and their “levels of confidence” in those assessments. Granular findings may carry a lower level of confidence. For example, the authors of the new study had “low confidence” data on increased tropical cyclone activity. Storm observation technology has improved in the past few decades, making it difficult to separate reporting improvements from actual increases in frequency. They said with “high” confidence that economic damages from natural disasters will increase without new defenses, such as flood barriers.
The study's low-confidence findings illustrate how hard it is to statistically link climate pollution with floods, storms and droughts that have always occurred and may be getting worse. Some regions of the world will be more affected by climate change than others. Some billion-dollar disasters result more from the human proclivity to build dense cities in flood-prone areas than the increasing frequency of such accidents.
The IPCC has stated in its last three major assessment reports that “unequivocal attribution” to the causes of climate change is impossible because it would require a controlled experiment. There’s only one Earth. Instead, climate scientists have shown that trends are consistent with predicted change from pollution, and inconsistent with alternative explanations, such as solar variability or increased volcanic activity.
The UN climate treaty talks later this month may result in a reduction in the scope of the only global climate protection deal out there, the Kyoto Protocol. Russia, Canada and Japan say they won’t take on new commitments to cut greenhouse gases after 2012 unless the U.S., China and other large holdout nations join in. The question is whether reports like the IPCC’s will spur negotiators to close the so-called gigaton gap, the UN’s phrase for the disparity between the current course of action and the global warming limit of 2 degrees Celsius (3.6 degrees Fahrenheit), after which life on earth is virtually certain to become more difficult.