Studies May Alter Insights Into Global Warming

By Curt Suplee
Washington Post Staff Writer
Monday, March 15, 1999; Page A07

Two new studies of the Earth's ancient atmosphere may alter the way scientists understand the relationship between airborne carbon dioxide and climate change--and hence the dynamics of future "greenhouse" global warming.

In one paper, published in the March 11 issue of the journal Nature, researchers said they found that during the past 11,000 years--the period known as the Holocene epoch that began around the end of the last ice age and extends to the present--levels of carbon dioxide, a potent greenhouse gas, did not remain constant until the onset of the industrial revolution, as many had long supposed.

Instead, although average global temperatures stayed relatively stable, carbon dioxide levels fluctuated considerably during the Holocene, according to a team from the Scripps Institution of Oceanography and the University of Bern, Switzerland. "The system was never in equilibrium because the carbon dioxide levels never stabilized," said Martin Wahlen of Scripps, part of the University of California at San Diego.

Presumably this occurred because of still-unexplained changes in the amount of carbon dioxide taken up by oceans and vegetation, especially short-term variations of 10 percent or more in the quantity absorbed by plants. "This suggests that the terrestrial biosphere may also exhibit changes in the future," said Thomas F. Stocker of the University of Bern. What they might be, however, is uncertain.

In the other study, reported in the March 12 issue of the journal Science, Scripps investigators addressed one of the most vexing "chicken-and-egg" questions in climate research. Namely, when the Earth shifts from glacial to warm periods (as it does every 100,000 years or so), which comes first: an increase in atmospheric carbon dioxide levels, or an increase in global temperature? Contrary to what many believe, the team concluded that the temperature rise comes first, followed by a carbon dioxide boost 400 to 1,000 years later.

That's what the researchers found at glacial-interglacial transitions from 240,000, 140,000 and 13,000 years ago. That sequence of events appears to contradict the fundamental logic of simple greenhouse warming theories, which argue that increases in heat-trapping gases will be followed by higher surface temperatures.

The analysis also points to vegetation as a major source of the carbon spike. "Previously it was thought to have originated primarily in the ocean biosphere," said Julie Palais of the National Science Foundation, which supported both studies. "But this work suggests that the terrestrial biosphere played a significant role. . . . There are clearly implications for the future, considering the rate at which the rain forests in South America are currently being destroyed."

Both research projects used evidence from hundreds of ice specimens, or "cores," taken far below the surface of Antarctica. Each layer contains tiny bubbles of air snared in the ice when it formed from snow. Investigators placed the samples in vacuum chambers, released the traces of air, and analyzed them for carbon dioxide content and an isotope of carbon that indicates where the gas was last.

Carbon, a ubiquitous element in the Earth's surface and atmosphere, exists in three isotopes. Photosynthesis, the process whereby plants use sunlight to turn carbon dioxide into organic matter, favors the uptake of the lightest, carbon-12 atom, leaving the atmosphere with a relative plenitude of carbon-13, the next heaviest isotope. When plants die and rot, they release that carbon-12 back into the atmosphere. So the ratio between the two isotopes at any time indicates how much carbon dioxide is being absorbed by plants.

That quantity may be critical to future climate changes. In general, civilization releases about 6 billion tons of carbon (in the form of carbon dioxide) into the atmosphere every year. But only 3 billion tons stay there; the rest is absorbed into what are called carbon "sinks." Half dissolves into the ocean; the remainder ostensibly is taken up by vegetation. Consequently, the performance of these sinks is an important variable in how much carbon dioxide is left in the air to trap heat and possibly raise global temperatures.

The Scripps-Bern authors writing in Nature found that at the beginning of the Holocene, the atmosphere contained about 268 parts per million by volume of carbon dioxide, up from 180 to 200 ppmv in the depths of the last ice age about 18,000 years ago. By the late 1700s, it had risen to 285 ppmv. (Since then, the concentration has climbed to 364 ppmv and is still growing. That is, it rose by the same amount--80 ppmv--in the past 200 years that it had from the coldest part of the previous ice age to the late 1700s.)

In accordance with orthodox notions, "one commonly referred to the 'preindustrial CO2 concentration of 280 ppmv,' " as if it were constant, Stocker said. But now "this has to be revised," he noted.

As the world warmed its way out of the last ice age, carbon dioxide levels first dipped to 260 ppmv about 8,200 years ago, probably because receding glaciers made way for the increasing vegetation that took up a lot of gas. But then the carbon dioxide content began to creep back up as ocean temperatures rose (decreasing the amount of dissolved gas oceans could hold) and land masses cooled and dried out (decreasing the carbon-trapping activity of photosynthesis).

"The direct relevance of this finding," said Jean Lynch-Stieglitz of Lamont-Doherty Earth Observatory at Columbia University, "is that we can expect that as climate warms, the terrestrial biosphere will probably be capable of holding more carbon than it can today."

But uncertainty is high. Over the past few thousand years, there have been dramatically abnormal climate events. But "none of those involved a rate of CO2 change like that occurring now," said Lamont-Doherty's Gerard Bond, and that "is a measure of how serious the problem might be."

As for the carbon dioxide lag-time findings, "the crux of the issue" for nonscientists is that "if the observation that increases in temperature lead increases in CO2 in the natural system, then industrialization is forcing the natural system backwards, and our natural analogs deduced from past behavior might not be good models for predicting what's going to happen in the future," said Joan J. Fitzpatrick, technical director of the U.S. Geological Survey's National Ice Core Laboratory in Denver. "That's a sobering thought."

Indeed, despite the sizable margin of possible error in the analysis, "greenhouse skeptics will probably jump on this paper as 'proof' " that there is no necessary causal relation between carbon dioxide levels and temperatures, said Anthony J. Broccoli of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J. But in fact, he said, the new findings are completely consistent with a "positive CO2-temperature feedback" system in which changes in one prompt changes in the other.


Copyright 1999 The Washington Post Company