TCS Daily


Rainy Daze

By Willie Soon - February 8, 2002 12:00 AM

Mark Twain once said, "It's not the things you don't know that fool you. It's the things you do know that ain't so." Nowhere is that sagebrush aphorism more appropriate than in the realm of climate science.

Consider a couple of recent studies that have just been published in the journal Nature claiming that extreme rainfall and great floods are coming our way over the next 100 years if we keep emitting carbon dioxide to the air. The studies come from two groups of highly regarded researchers from both sides of the Atlantic: the European Centre for Medium-Range Weather Forecasts/Rossby Centre; and the U.S. Geological Survey/Geophysical Fluid Dynamics Laboratory at Princeton.

"The frequency of severe floods in large river basins has increased during the twentieth century... with only a small likelihood that these changes are due to natural climate variability," remarked Reiner Schnur of the Max Plank Institute for Meteorology at Hamburg, the expert reviewer of these Nature papers.

But how alarmed should we be by the predictions for "increasing risk of great floods" over the next 100 years in these studies? Not much.

For example, the prediction of future flooding made by the American group was based on a scenario in which the future atmosphere contains four times the current level of carbon dioxide concentration -- an extremely unrealistic likelihood. (Interestingly, the prediction for flooding was not uniform across the globe: in the case of the Ohio River, the quadrupled CO2 scenario actually predicted a decrease in river discharge, hence a reduced risk of flooding.)

Beyond that, there are more serious doubts hidden in the fine print of these reports. In the closing paragraph of their paper, the U.S. authors cautioned that "our detection of an increase in great-flood frequency and its attribution to... climate change [from man-made carbon dioxide and sulfate aerosols] are tentative... The forced signal and unforced variability in the model contains errors of unknown magnitude."

That caution needs to be reemphasized. Why? Because two members of this same group of Princeton researchers, in a study of water cycles in the Mississippi River Basin less than one year ago, concluded that "the precipitation trend is readily explained mainly as part of an unusually large internal fluctuation in the climate system. This is consistent with findings that regional water-flux changes associated with 'greenhouse warming' should not be generally detectable at this time, due to a low signal/noise ratio." These researchers also confirmed that "for the Mississippi River basin [which roughly cover an area of three million squared-kilometers] during 1949-1997, the linear trend over time in near-surface air temperature was negative, differing significantly from the regional warming in climate model."

And the U.S. experts admitted still further limitations to their simulations. "Absent from the model are forces such as solar variability, volcanic activity, land-cover change, and water-resource development, and potential biospheric feedbacks such as CO2-induced stomatal closure and water-stressed-induced root extension."

But what about the future increase in extreme rainfalls as reported by the European group? The new European effort was based on simulations from 19 (as opposed to one) global general circulation climate models. This group also introduced a new technical method -- though the method is rather computer resource-intensive -- to better account for the forecasted extreme climate events. So their numbers should be pretty good.

Moreover, the European researchers are more conservative in that their projections are for future atmosphere with double the present level of carbon dioxide. They found that with twice the amount of carbon dioxide in the air, the risk from extreme rainfall events will increase by a factor of five to seven over much of Northern Europe and over the catchment basins of the Brahmaputra, Ganges and Meghna rivers.

But can such a calculation that focused only on the effect of increasing atmospheric carbon dioxide ever be meaningful and useful for policy making? Not really. Reiner Schnur even admitted as much, saying "[s]imilar model integrations will be needed for a more complete picture of the factors contributing to climate change, such as other greenhouse gases [methane, nitrous oxide, CFCs] and sulphate aerosols, including any uncertainties attached to these agents."

In other words, once again climate modeling efforts are grasping for more experimentation and readjustment instead of providing us reliable figures for the number of sandbagged rivers and submerged streets we're likely to encounter in the next 100 years. More and more, President Bush's call to study the climate science further before embarking on massive energy reduction schemes looks like a prudent political, economic and, scientific move.
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