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Is climate more susceptible to human activities than previously thought?

STANFORD -- Earth's climate may be more sensitive to the by-products of human activity than scientists have previously estimated.

That is one of the contentions Stanford University climatologist Stephen H. Schneider makes in the paper "Detecting Climatic Change Signals: Are There Any 'Fingerprints'?" that appears in the Jan. 21 issue of the journal Science.

This greater-than-accepted sensitivity stems from a possibly significant mistake that scientists have made in their attempts to estimate how much of the 0.5 to 1 degree Celsius rise in global average temperatures recorded in the last 200 years might be due to man-made sources.

Schneider points out that these efforts have assumed that in preindustrial times the atmosphere had the same basic characteristics - such as temperature profile, cloudiness and humidity - as it does today. The problem with that, Schneider said, is that conditions 200 years ago were probably quite different.

"Unfortunately, while we know it was cooler on average, we don't know exactly what these differences were," Schneider said, "but it is likely that the preindustrial atmosphere differed in ways that have led us to underestimate the climate's sensitivity."

By climatic sensitivity, the scientist means how much the global temperature varies in response to a change in the total amount of energy that drives the atmosphere. Greenhouse gases, such as carbon dioxide, increase atmospheric heating levels by trapping sunlight in much the same fashion as do the glass panes in a greenhouse. On the other hand, most airborne aerosols reduce the energy input to the atmosphere by reflecting sunlight back into space.

If the climatic sensitivity is greater than had been thought, as Schneider argues, then larger temperature variations could accompany changes in energy inputs caused by such human by-products as greenhouse gases and aerosols. Thus a 1 degree Celsius warming trend might very well mask a 2 degree Celsius greenhouse warming offset by a 1 degree aerosol cooling, he says.

The reason that such larger temperature responses have not been detected unambiguously, Schneider argues, is that the process of separating man- made from natural variations is even more difficult than most studies have assumed.

One reason for this difficulty stems from the fact that different types of human by-products act on the climate in fundamentally different ways. For example, the greenhouse effect is relatively global in nature: The gases are spread fairly evenly around the globe. Aerosols, however, tend to be distributed regionally. In addition, greenhouse warming takes place continuously, while aerosol cooling takes place only when the sun shines. So a 1 degree greenhouse warming cannot be uniformly canceled by a 1 degree aerosol cooling. In some times and places they may cancel, but in other circumstances they may actually reinforce each other.

Such variations, Schneider said, are further complicated by regional differences in the atmosphere itself. Ocean areas, for example, heat and cool much more slowly than continents. Similarly, an increase in temperatures over large bodies of water results in increases in cloud cover in the area, while similar temperature increases over desert areas do not. As a result, small changes in the global atmospheric energy balance are likely to have relatively large regional effects, causing heating in some places and cooling in others.

Much of the current effort to understand and predict climate change is done using large computer models of the atmosphere, oceans and land surfaces called coupled global circulation models. According to Schneider, these models are still decades away from incorporating enough detail to adequately account for such regional effects and so cannot provide much help right now in determining the actual regional distribution of impacts that human activities are having on the climate.

Until such a level of sophistication is reached, the best way to estimate if human activities are causing global warming is to compare the recent climatic trend with similar trends found in proxies for prehistoric climate, such as tree-ring variations or glacial moraine time series, the climatologist says. While these proxy records are not a global network of thermometers, they do go back thousands of years and suggest that events like the current, centuries-long warming trend occur perhaps once or twice a millennium. Thus, the odds are less than one in five that what the world is now experiencing is a wholly natural climate fluctuation, Schneider estimates.

"Unfortunately, while the scientists debate, the real Earth continues to perform this experiment for us," he cautions.



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