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  Geotimes - January 2008 - Comment
A COMMENT ON ...

Climate Change: Teaching a Moving Target
William Ruddiman

Climate change has emerged as a major issue only in the last few decades, with important new studies appearing nearly every week. Given the fast pace of discoveries in this important area, how are teachers — let alone textbook writers — supposed to keep up?

Consider the changes during the last two decades. In 1988, NASA climate scientist James Hansen testified before Congress that global warming was occurring and that future warming would be large. At that point, surface stations had only been showing a warming since 1977, after a cooler interval that had persisted since the early 1940s. Many climate scientists (myself included) thought that Hansen was probably right, but also felt that going public with this conclusion might have been a bit premature. In 1989, an appendix in a pioneering academic-style textbook (Paleoclimatology by T.J. Crowley and G.R. North) noted that rising carbon dioxide levels were on a path likely to match those estimated for the Cretaceous era of approximately 100 million years ago, a time known to have been much warmer than now.

Fast-forward to 2000, by which time global temperatures reconstructed from surface stations had warmed for another 12 years, interrupted by a brief cooling after the 1991 Mount Pinatubo eruption. Published late in 2000, the first edition of my textbook, Earth’s Climate, favored the view that global warming was under way and likely to be large in size in the future, but it also acknowledged uncertainties noted in the 2001 Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). For example, temperatures reconstructed from satellite measurements had shown no net warming since the late 1970s, contrary to the warming trend from surface stations. In addition, several well-respected scientists argued that changes in incoming solar radiation explained a substantial part of the warming during the 20th century.

Fast forward again to late 2007, by which time surface stations had recorded another seven years of very warm temperatures, and summer sea ice had shrunk to limits far below anything measured during the 30-year era of satellite observations. Also, the earlier satellite estimates of temperature that hadn’t detected a warming trend had been found to be erroneous because of incomplete adjustments to the initial measurements, and the most respected advocates of solar forcing had by then acknowledged that changes in solar irradiance were considerably smaller than previously thought. In the almost two decades since Hansen’s initial testimony, the cumulative increase in scientific understanding has been profound. The second (2008) edition of my textbook makes a much more forceful case that the modern warming is primarily anthropogenic, as did the 2007 IPCC report.

Textbook writers should describe the science as they see it, but not venture beyond the data into policy advocacy. Personally, I feel comfortable in concluding that the large changes we have begun to impose on Earth’s climate carry considerable risk, but I know I do not have the expertise needed to predict changes in the complex mixture of social, economic, technological and environmental factors that will affect future human well-being. Each classroom instructor has the liberty to express opinions on such issues, including the choices of other media they use in their courses. These resources also provide a way to keep up with new discoveries between editions of the textbooks.

Meanwhile, the science moves on. Some scientists are claiming that we are close to a “tipping point,” when the Greenland ice sheet will begin an irreversible disintegration, eventually raising sea level by as much as six meters and threatening the world’s enormous coastal populations. At this point, this alarming-sounding conclusion is far from majority opinion among climate scientists. But we should probably take no comfort from the fact that James Hansen is again the most prominent voice sounding the warning. I wonder what textbooks a decade from now will say about this issue.


Ruddiman is a professor emeritus of environmental sciences at the University of Virginia and author of Earth’s Climate. He presented these conclusions at the annual meeting of the Geological Society of America in October.

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