Comment▼ The IPCC Fourth Assessment
W
ill history show that February 2, 2007, marked the beginning of the end of the fossil-fuel age? That’s when the scientific basis of the Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report (AR4) was unveiled in Paris. It also may be the day when we first took seriously the threat of human-induced global warming, the gravest environmental problem of our time. Climate Change 2007: The Physical Science Basis AR4 from Working Group 1 is a consensus report written by ∼150 authors from 100 countries and vetted by 600 reviewers. It states, with 90% certainty, that human activities, especially the burning of fossil fuels, have caused global warming during the past 50 years. Predicting a warming for the 21st century of 2.0–4.5 C, it narrows the range of 1.4–5.8 C from the Third Assessment Report. The tone of the assessment has become more strident (and certain) with each report: “. . . the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger humaninduced greenhouse warming.” —IPCC First Assessment Report, 1990 “The balance of evidence suggests a discernible human influence on global climate.” —IPCC Second Assessment Report, 1995 “There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities.” —IPCC Third Assessment Report, 2001 Despite the fact that these reports are consensus documents, the reality is still lost on many Americans. The Bush Administration has done an amazingly effective job of creating confusion where little existed. And the press has contributed to the fiasco by “fair and balanced” reporting, that is, by always finding an opposing quotation despite the lack of support for its research content or the questionable credentials of the interviewee. AR4 tracks 20 climate models from groups all over the world. Unanimous agreement exists among model results that the 21st century will be significantly warmer. Modelers disagree only on exactly how much warmer it will be. Understanding climate sensitivity to increasing CO2 is still crucial, and the uncertainties are narrowing. Like a vigilant lawyer, the report lays out multiple lines of evidence as to how we know that humans are causing global warming: satellite corroboration of land surface warming, parallel ocean warming and commensurate sea-level rise, stratospheric cooling, increasing nighttime minimum temperatures, and melting ice shelves and glaciers. © 2007 American Chemical Society
Why is AR4 so important? For the first time, it seeks to define “dangerous climate interference”. Earth’s vulnerabilities include the potential disintegration of Greenland and Antarctic ice sheets, increased storm severity, rapidly rising sea levels, and shutdown of the thermohaline circulation in the North Atlantic. Most researchers estimate that those effects initiate at 2 C of total warming. Because we have already experienced 0.6 C of warming and we likely have already loaded an additional ~1.0 C of warming into the sluggish climate system, we don’t have much leeway before anthropogenic interference becomes dangerous. That is why nations must begin to reduce emissions within the next decade or so. Drastic measures are needed—an ~80% cut in emissions to stabilize the atmospheric concentration of CO2 at 550 ppmv by 2100 (a doubling from preindustrial times). The Kyoto Protocol was designed to achieve a reduction of a few percent, and it will fall short of that target. Eighty percent is a formidable goal, a much greater challenge than sending a man to the moon or rebuilding Europe and Japan after World War II. It is the environmental challenge defining our century and future generations. Actually, I’m optimistic that (finally) we are beginning to accept and respond to the challenge. Bob Dylan sang that you don’t need a weatherman to know which way the wind blows and, for the first time, you can feel the winds shifting. Companies in the EU, North America, Japan, and even China are announcing their own emission reduction programs; states are passing legislation to serve as incubators for greenhouse-gas mitigation; and people understand that emitting CO2 has consequences and that there’s a price to pay. The much-debated Stern report from the U.K., The Economics of Climate Change, states that price: 5–20% of gross world product (GWP) if we fail to act, compared with 1% of GWP if we respond now. I believe even the laggard U.S. will enact a cap-andtrade program within the next year. In case you missed the nuance, the release of the IPCC report coincided with Groundhog Day in the U.S., the day when that awkward furry animal wakes from hibernation to predict what the weather will be in the future. Let’s hope the U.S. can make the right choice.
Jerald L. Schnoor Editor
[email protected] MARCH 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 1503
