Renowned climate modeler James Hansen, whose summer 1988 congressional testimony helped kick off the climate change debate, now says climate models don't work. Writing in the Proceedings of the National Academy of Sciences, Hansen states flatly, "The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate changes." Climate modeling and forecasting needs to be rethought, he concludes, arguing, "The summary implication is a paradigm change for long-term climate projections: uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue."
Through his position at the Goddard Institute of Space Science, Hansen is one of the lead climate modelers for the National Aeronautics and Space Administration. Ten years ago he said his model predicted that doubling atmospheric CO2 levels would drastically increase global temperature. In response, Congress ramped up the Climate Change Research Program which, after spending over $10 billion, has revealed deep uncertainties in climate forecasting. Hansen first noted some of these publicly almost year ago (ED, Feb. 14). Then he admitted his own model was wrong in July (ED, July 27). Now he says that modeling per se is inaccurate.
The culprit is an unpredictable mix of countervailing forces that drive long-term climate. As Hansen puts it, "Anthropogenic greenhouse gases (GHGs), which are well measured, cause a strong positive (warming) forcing. But other, poorly measured, anthropogenic forcings, especially changes of atmospheric aerosols, clouds, and land-use patterns, cause a negative forcing that tends to offset greenhouse warming. One consequence of this partial balance is that the natural forcing due to solar irradiance changes may play a larger role in long-term climate change than inferred from GHGs alone."
Hansen's article may deal a stunning blow to the scientific foundations of climate policy, smashing whatever perceived consensus there may have been on the effects of increasing atmospheric carbon dioxide levels. "There never really was a consensus," says Joel Bucher, climate policy analyst for Citizens for a Sound Economy, "but Hansen has made it official. The King of Modelers has courageously recanted."
"Hansen has always been an independent thinker," said S. Fred Singer, a long-time critic of the science of climate change advocates. "He doesn't always move with the pack, and he was among the first to highlight the chaotic nature" of the science involved.
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