TCS Daily


The Science and Politics of Climate

By Freeman J. Dyson - April 24, 2001 12:00 AM

In the nineteen-sixties the fluid dynamicist Syukuro Manabe was running global climate models on the supercomputer at the Geophysical Fluid Dynamics Laboratory in Princeton. Manabe began very early (before it became fashionable) to run models of climate with variable amounts of carbon dioxide in the atmosphere. He ran models with carbon dioxide at two and four times the present abundance, and saw in the computer output the rise in average ground temperature that is now called Global Warming. He told everybody not to believe the numbers. But the politicians in Washington believed. They wanted numbers, he gave them numbers, so they naturally believed the numbers.

It was not unreasonable for politicians to believe Manabe`s numbers. Politics and science are two very different games. In science, you are not supposed to believe the numbers until you have examined the evidence carefully. If the evidence is dubious, a good scientist will suspend judgment. In politics, you are supposed to make decisions. Politicians are accustomed to making decisions based on shaky evidence. They have to vote yes or no, and they generally do not have the luxury of suspending judgment. Manabe`s numbers were clear and simple. They said if the carbon dioxide goes up, the planet will get warmer. So it was reasonable for politicians to believe them. Belief for a politician is not the same thing as belief for a scientist.

Manabe`s numbers were unreliable because his computer models did not really simulate the physical processes going on in the atmosphere. Over and over again he said that his purpose when he ran computer models was not to predict climate but to understand it. But nobody listened. Everyone thought he was predicting climate, everyone believed his numbers.

The biosphere of the earth contains four reservoirs of carbon: the atmosphere, the ocean, the vegetation and the soil. All four reservoirs are of comparable size, so that the problem of climate is inescapably mixed up with the problems of vegetation and soil. The intertwining between the four reservoirs is so strong that it makes no sense to consider the atmosphere and ocean alone. Computer models of atmosphere and ocean, even if they can be made reliable, give at best a partial view of the problem. The large effects of vegetation and soil cannot be computed but must be observed and measured.

The way the problem is customarily presented to the public is seriously misleading. The public is led to believe that the carbon dioxide problem has a single cause and a single consequence. The single cause is fossil fuel burning, the single consequence is global warming. In reality there are multiple causes and multiple consequences. The atmospheric carbon dioxide that drives global warming is only the tail of the dog. The dog that wags the tail is the global ecology: forests, farms and swamps, as well as power-stations, factories and automobiles. And the increase of carbon dioxide in the atmosphere has other consequences that may be at least as important as global warming - increasing crop yields and growth of forests, for example. To handle the problem intelligently, we need to understand all the causes and all the consequences.

Several successful, important programs of local observation have been started in recent years. One program is measuring directly the fluxes of carbon dioxide moving between the atmosphere and the biosphere. This is done by putting instruments on towers above the local trees or other vegetation. In daytime in the summer, the vegetation is vigorously absorbing carbon dioxide. At night or in winter, the flux is going the other way, with plants giving off carbon dioxide by respiration. The soil also gives off substantial fluxes of carbon dioxide, mostly from respiration of microbes and fungi. The instruments do not distinguish between vegetation and soil. They measure the total flux leaving or entering the atmosphere.

During the last few years, instrumented sites have been built in many countries around the world. Within a few years, we will know for sure how much of the carbon released by fossil fuel burning is absorbed by forests and how much by the ocean. And the same technique can be used to monitor the carbon fluxes over agricultural croplands, wetlands and grasslands. It will give us the knowledge required, so that we can use the tools of land management intelligently to regulate the carbon in the atmosphere. Whether we manage the land wisely or mismanage it foolishly, we shall at least know what good or harm we are doing to the atmosphere.

The amount of money spent on local observations is small, but the money has been well spent. The Department of Energy is funding another successful program called Atmospheric Radiation Measurements (ARM). ARM`s activities are mainly concentrated at a single permanent site in Oklahoma, where systematic observations of radiation fluxes in the atmosphere are made with instruments on the ground and on airplanes flying at various heights. Measurements are made all the year round in a variety of weather conditions. As a result, we have a database of radiation fluxes, in a clear sky and in cloud and between clouds.

One of the most important measurements is made by two airplanes flying one above the other at different heights. Each airplane measures the fluxes of radiation coming up from below and down from above. The difference measures the local absorption of radiation by the atmosphere. The measured absorption of sunlight turns out to be substantially larger than expected. The expected absorption was derived partly from theory and partly from space-based measurements. The discrepancy is still unexplained. If it turns out that the anamolous absorption measured by ARM is real, this will mean that all the global climate models are using wrong numbers for absorption.

Check back on Wednesday for Part II of "The Science of Politics and Climate"
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