An Urgent Plea on Global Warming

Judging from presentations at a meeting convened by the American Association for the Advancement of Science (AAAS) on June 15, it would seem that scientists are becoming more and more alarmed about the consequences of climate change.

Ten leading climate-change scientists spoke at the meeting. Rising levels of greenhouse gases are already affecting the world's weather, sea level, and ecological systems, they explained. Unless strong, near-term actions are taken to curb emissions, the changes are likely to be far greater--even devastating--by the end of the century.

The meeting, organized by Donald Kennedy, editor-in-chief of Science, and Albert H. Teich, director of science and policy programs for AAAS, laid out in clear, stark language and images how the changes under way will lead by 2100 to conditions unlike anything Earth has experienced for millions of years. "We are in the middle of a large, uncontrolled experiment on the only planet we have," Kennedy said.

Speakers described the basic concepts underlying climate-change science. The atmospheric concentration of CO₂, now at 380 ppm, is 30% higher than the preindustrial level and expected to rise to somewhere between 800 and 1,000 ppm by 2100 unless large cuts are made in emissions, they said. Over the same period, the average global temperature is expected to increase 2.5 to 10.4 &°F (1.4 to 5.8 °C). There is absolutely no question that Earth will warm as CO₂ concentrations increase, they said.

Daniel P. Schrag, professor of geochemistry at Harvard University, pointed out that the current CO₂ level is higher than it has been at any time in the past 430,000 years. By century's end, the level will likely be higher than at any time since the Eocene--36 million to 55 million years ago. At that time, "palm trees lived in Wyoming, crocodiles lived in the Arctic, Antarctica was a pine forest, and sea level was at least 300 feet higher than today," he said. "The future may have some big surprises in store for us in terms of melting of polar ice sheets and global sea level rise," he warned.

The global sea level rose 10 to 20 cm over the past century, and in this century it may rise as much as 88 cm, according to Michael Oppenheimer, professor of geosciences and international affairs at Princeton University. Greenland is already rapidly losing ice around the periphery, and many glaciers along the coasts are shrinking, he said. If the CO₂ buildup continues unabated and local temperatures in Greenland increase by 3 °C, the huge ice sheet covering the island would very likely melt completely, as it seems to have melted during the previous interglacial period. Complete melting would raise sea level about 7 meters. According to models, total melting would take 1,000 years or more, but factors not accounted for in the models may accelerate it, he noted.

In addition, Oppenheimer said, the West Antarctic Ice Sheet may eventually collapse if warming on that part of the continent exceeds 4 to 8 °C. This collapse would raise sea level about 5 meters. Over the past decade, floating ice shelves along the Antarctic Peninsula "have undergone spectacularly rapid disintegration," he said. This deterioration does not itself contribute to sea level rise, but allows the grounded glaciers behind the ice shelves to move more rapidly into the sea, raising sea level, he explained.

A mere 1-meter sea level rise "would cause a huge loss of land in Bangladesh," Oppenheimer pointed out, and would also inundate a large part of South Florida. "If we have a sea level rise of 1 meter per century, there is no way to protect South Florida and Bangladesh, though it might be possible to protect New York with seawalls," he observed.

The speakers emphasized that climate rarely behaves in a linear fashion. Large, abrupt changes have taken place repeatedly in the past, said Richard B. Alley, professor of geosciences at Pennsylvania State University, University Park. Temperature increases as great as 18 °F (10 °C) have occurred in a decade in some locales, often caused by sudden shifts in oceanic circulation. Abrupt shifts "seem to happen when some part of the Earth system is forced across a threshold, triggering change to a new and persistent state," he said.

The scientists urged policymakers not to use uncertainties and gaps in knowledge as an excuse for inaction. They called for a major shift to cleaner fuel technologies and for other investments that will reduce the rapid buildup in greenhouse gases. "The time it takes to develop and transfer technologies is such that investments need to be made very soon," said David S. Battisti, professor of atmospheric sciences at the University of Washington, Seattle.

Given the dire consequences--spelled out so graphically at AAAS--that we as a society may encounter in just a few generations if CO₂ emissions are unconstrained, it would seem prudent to buy some insurance by investing in clean technologies. I agree with the U.K.'s chief science adviser, David A. King, who wrote, "In my view, climate change is the most severe problem that we are facing today--more serious even than the threat of terrorism" [Science, 303, 176 (2004)].