When the UN-organized Intergovernmental Panel on Climate Change (IPCC) presents its projections for global warming and future climate changes tomorrow, the report’s hallmark will be a far greater level of certainty and precision than what was expressed in the last IPCC report, issued in 2001. “The certainty is huge,” says Andrew Weaver of the University of Victoria in British Columbia, Canada’s top climate modeling expert and a coauthor of the new IPCC report.
To be sure, continued warming observed since 2001 is part of that certainty. But climatologists say the bigger factor is the broad accumulation of science over the past six years that has increased the precision with which climate models predict future climate change, debunked alternative hypotheses advanced by skeptics, and identified the footprint of man-made climate change in every corner of the earth.
As Weaver put it, the IPCC has not just found a smoking gun linking greenhouse gas emissions and climate change. Rather, its fourth report delivers a smoking arsenal. “There are many, many smoking guns,” he says. “It’s a battalion of smoking intercontinental ballistic missiles.”
Jerry Mahlman, a senior research associate at the U.S. National Center for Atmospheric Research (NCAR), in Boulder, CO, and a peer reviewer for the IPCC, says that while the final language is still being hammered out, the report might end up expressing 99 percent certainty that greenhouse gas emissions, primarily from burning fossil fuels, are warming Earth, up from “greater than 90 percent” confidence in the previous report. “It’s very obvious that the earth is warming up exactly as we’ve projected it to do so,” says Mahlman. One recent draft notes that IPCC’s projection in 1990 that global average temperature would rise by between 0.15 and 0.3 °C per decade through 2005 compares well with the 0.2 °C increase that actually occurred.
Today, many detailed scientific reports are detecting global warming’s fingerprints rather than simply glimpsing the outline of its footprint. The second and third IPCC assessments, issued in 1996 and 2001, respectively, built a case for man-made climate change on increased global average temperature above that expected from natural variability. Weaver says the fourth report, in contrast, will identify the signal of man-made climate change in every region of the globe and in many more variables beyond temperature, such as increases in intense tropical cyclones and forest fires.
“We’re finding the signal of climate change in more and more places,” says Stanley Solomon, a scientist with NCAR’s Earth & Sun Systems Lab. For example, last year Solomon published the first definitive identification of man-made climate change in the thermosphere, the uppermost layer of Earth’s atmosphere. The thermosphere was, paradoxically, predicted to cool and thin with increased carbon dioxide. And that is exactly what Solomon and his colleagues found. They detected the expected cooling by noting a small but statistically significant decline in the drag on satellites traveling through the thermosphere.