One of the biggest uncertainties about global climate change is how much of the planet's carbon dioxide is, so to speak, going down the sink. While carbon dioxide is going up into the atmosphere, large amounts are absorbed by such things as oceans and forests, areas that are known as "carbon sinks."

The better identification of all the carbon sinks - and figuring out whether their ability to absorb carbon dioxide will increase or decrease in the future - is pivotal in the effort to predict the impacts of future climate changes.

Even as the amount of carbon dioxide in the atmosphere has grown, there still isn't as much up there as scientists say there should be. In 1996, for instance, more than a fifth of the 8.1 billion tons of carbon dioxide gas pumped into the atmosphere was absorbed by sinks other than those that have already been identified. In this, areas like the Bay, and the neighboring ocean, could be important players.

"The ocean is a tremendous reservoir of dissolved CO2; it soaks up CO2 like mad," said Hugh Ducklow, of the Virginia Institute of Marine Science. "If the ocean didn't have this tremendous absorptive capacity for CO2, there would be a lot more of it in the atmosphere, and things would probably be a lot warmer right now. But we don't understand all the mechanisms, or have a good predictive understanding, of what the ultimate capacity of the ocean is for CO2."

In general, the ocean stores carbon because CO2 dissolves in seawater and because sunlight and nutrients spur algae growth which absorbs CO2, changing it into organic matter. That organic matter is consumed by other animals which ultimately die and sink to the bottom where they decompose. "In effect, biology creates kind of a pump that's moving CO2 out of the surface water, pumping it down, and redissolving it as CO2 again," Ducklow said. "Once you get that CO2 dissolved in the deep water, it is down thousands of meters deep."

At that point, Ducklow said, it is tied up in slow ocean circulation patterns and it can take centuries for the carbon dioxide to return to the surface and re-enter the atmosphere. But as the Earth warms, no one knows how the function of that biological pump will change.

It's possible, Ducklow said, that increased temperatures would speed up biological processes, allowing more CO2 to be absorbed - sort of like a "self-correction mechanism" for the global carbon cycle.

But, he said, heat also tends to inhibit the ability of ocean waters to mix. That would slow down the delivery of nutrients from deep waters to the surface which, in turn, would serve to cut off biological activity.

"Those are two interesting opposing effects," he said. "The heat alone might intensify biological activity, but the change to the physical circulation of the ocean may slow it down. And one can make other arguments that these same changes in physical circulation patterns could also intensify biological activity. The real interesting point is, we don't even know what direction some of these changes are going to go. It's complete speculation."

Just as the ocean stores carbon, the same could be true of estuaries, Ducklow said. If so, estuaries could be an important sink because - like the Chesapeake - they tend to be nutrient-enriched. Working against that hypothesis, he said, is that estuaries are shallow, so they don't offer the potential for long-term storage unless the carbon becomes buried in sediment.

"It's a controversial area," Ducklow said. But the role of oceans is probably more important - even though they are not as rich in nutrients as estuaries - simply because they are so large.

Whatever the case, Ducklow said there is concern among some scientists that a little bit of warming could go a long way toward disrupting normal ocean processes. And because of a "lag time" of about 50 years from the time CO2 is emitted to the time its full impact is felt in the atmosphere, it is possible that those few degrees of warming could, in effect, already be locked in.

"There is so much CO2 in the atmosphere right now that even if we were able to stabilize things tomorrow, there could still be large effects," he said. "Even in a very short amount of time, I would say in 25 to 50 years, some aspects of ocean physics and chemistry could be really fundamentally altered by a few degrees of global warming."