Are We Subduing Our Allies in Fight Against Climate Change?

New studies suggest that plants and oceans may absorb less carbon dioxide because of temperature changes.

Lush forests and windswept oceans are two of humanity's most important natural allies in the battle against global warming. But until recently, scientists didn't know how climate change itself might weaken these allies, which absorb heat-trapping gases being pumped into the atmosphere.

Now, for the first time, scientists are getting a fuller picture of the effects warmer temperatures may have on the ability of plants and the ocean to store carbon dioxide (CO2), the "greenhouse" gas of most concern.

Researchers have found that rising temperatures have the potential to significantly reduce the amount of CO2 oceans can absorb. And although plants will continue to scrub rising amounts of CO2 from the atmosphere into the next century, they start to lose that ability as they become saturated with CO2 and face other stresses.

These studies, detailed in a recent issue of the journal Nature, are "an important step forward" in the effort to better forecast the impact of human-induced climate change, says climate expert David Schimel. And while they are far from the final word on global warming, the studies could lead to better estimates on the effects of climate change - estimates that may determine how much countries reduce their reliance on coal, petroleum, and natural gas, which give off CO2 as they burn.

Even small differences in the planet's CO2 budget from a climate standpoint "are enormously important economically," says Dr. Schimel, who divides his time between the National Center for Atmospheric Research in Boulder, Colo., and the Max Planck Institute in Germany.

In 1995, the Intergovernmental Panel on Climate Change (IPCC) produced a set of global-warming estimates that became the basis for CO2 emission targets set last year in Kyoto, Japan. But the IPCC assumed that rising temperatures had little or no effect on the terrestrial and oceanic carbon cycles.

In the latest simulation of the ocean's carbon cycle, two teams of scientists in Princeton, N.J., found that as temperatures rose, the ocean surface acted like a can of warm soda going flat - its ability to dissolve and retain atmospheric CO2 diminished. Moreover, warming led to more rainfall at higher latitudes, increasing the amount of fresh water entering the ocean - which slows the rate at which the ocean can take up CO2.

According to new estimates based on temperature and mixing alone, between 1765 and 1990, the oceans took up 309 billion tons less CO2 than the IPCC estimated. Between 1990 and 2065, the oceans are projected to take up 1 trillion tons less CO2 than the IPCC estimates.

As for plants, another team of researchers from the University of Sheffield in Sheffield, England, found that between 1861 and 2070, soil and vegetation will have taken up 309 billion tons of CO2. But that uptake flattens after 2070. At that point, terrestrial plants can't deal with more CO2, either because they lack the additional nutrients they would need, because of temperature stress, or because they have reached maximum growth.

Yet the researchers say that their newest models, created by using powerful supercomputers to simulate the interplay between warmer temperatures and the carbon cycle, contain some large uncertainties.

For example, the oceans' ability to take up CO2 depends on how aquatic life responds, notes Jorge Sarmiento, the lead researcher running the ocean simulation. And taking living organisms into account, he says, can be difficult. "When fresh water slows ocean circulation, you don't know how the biology will respond," he says. When he adds this so-called biological pump to the ocean simulation, the oceans will absorb an additional 760 billion tons of CO2 between 1990 and 2065.

In plant simulations, Schimel adds, the model fails to account for changes in the relative abundance of nutrients or for human-induced changes, such as deforestation.

Such uncertainties point to new directions for research to bring the simulations closer to what actually occurs. "The big effects we saw in the ocean from the biology were a real surprise," Dr. Sarmiento says. "Now we need to go out and see how the biology responds in more detail."