The only thing for sure is concentrations of atmospheric carbon dioxide have fluctuated naturally by about 80 to 100 parts per million between glacial and interglacial periods during Earth's recent geological history.

"The overall conclusion is that CO2 has changed in the past, and at glacial periods it was roughly 30 to 35 percent lower than it was in pre-industrial times," Karen Kohfeld, assistant professor of earth and environmental science at Queens College of the City University of New York, told UPI's Climate.

"This has occurred consistently over glacial periods, at least for the last 800,000 years," Kohfeld said. "Every 100,000 years, we have these time periods where CO2 drops. It drops for natural reasons."

The question is: what are those so-called natural reasons? The explanation has eluded researchers for at least the past 20 years, Kohfeld said.

One hypothesis suggests a chain of events that begins whenever iron content increases in the oceans.

Increased iron fertilizes plankton growth, which enhances the water's ability to absorb CO2, pulling it out of the atmosphere and thereby leading to colder temperatures.

Some years ago, the late John Martin, an oceanographer with the Woods Hole Oceanographic Institution, dramatized this hypothesis by saying, "Give me half a tanker of iron, and I will give you an ice age."

Scott Doney, an associate scientist at Woods Hole, told Climate of the attempts to explain the CO2 variability.

"I jokingly call this the Holy Grail of chemical oceanography," he said. "No one has found a single hypothesis to explain the CO2 drawdown."

Still, the biological activity connection carries some credence. Plankton and other ocean plants, just like land plants, ingest carbon from the atmosphere, then die and gradually drift to the bottom of the ocean where they - and the carbon - become trapped in sediment.

The process removes CO2 from the atmosphere "at least on the time scale of ocean circulation," Kohfeld said. "If marine biology were much more active in the past during these glacial periods, maybe that's what caused CO2 to be lower."

The problem is that scientists using atmospheric computer models have attempted to draw down CO2 during glacial periods by changing the temperature of sea ice, but their calculations have not reproduced the numbers observed from the paleoclimate data.

Kohfeld's work, which was published in the April 1 issue of the journal Science, concluded that ocean biology can explain only about 40 percent of ocean carbon uptake during glaciation.

Doney said Kohfeld's work presents a couple of scientifically interesting aspects.

One has to do with her attempt to draw inferences from "a large number of ice cores. Lots of people have looked at individual cores (but Kohfeld is) trying to look at a whole collection of cores with the same method ... and rather than looking at only the height of the last glacial maximum, (she is) also looking at the period 80,000 to 100,000 years ago, when something had already happened to cause the (CO2) drawdown."

Based on other data, it appears CO2 levels actually trailed temperatures during interglacial periods, rather than leading them, according to paleoclimatologist Casper Amman of the National Center for Atmospheric Research in Boulder.

"We find that the change of CO2 in particular can explain about half of the climate change between glacial maximum and the Holocene (at the end of the last Ice Age)," Amman told Climate.

Unlike the current climate situation, where CO2 buildup in the atmosphere is forcing changes in the climate, in the past the situation was probably reversed - CO2 concentrations were determined by temperature changes.

One possibility for the decline in CO2 beyond the uptake from biological processes are physical ones.

When ocean temperatures are colder, the water can absorb CO2 more efficiently, Amman said. "Even without any biology, the colder the ocean gets, the more it dissolves CO2."

Scientists think that during the glacial period the thermohaline circulation -- a global conveyor belt of ocean current that sinks warm water down and brings cold water to the surface - somehow shut down.

It probably did so in some other periods as well, such as the Younger Dryas, which occurred roughly 12,000 years ago.

Amman said recent thinking on this subject is that during the glacial maximums, the thermohaline might have been quite vigorous, though the locations of the cold-water upwellings may have changed.

This would continue to bring fresh water to the surface to absorb CO2, while the water saturated with CO2 would be circulated to the deeper ocean.

Kohfeld's paper, he said, which argues that less than half of the CO2 change can be linked to biological activity, may encourage more research into the issue of physical mechanisms for CO2 uptake. Doney agreed.

"In terms of this science question, where we're headed is that there won't be one single mechanism that led to the entire drawdown of atmospheric CO2," Doney said.

"A series of different mechanisms occurred at different times. It may have been changes in ocean physics 120,000 years ago, then dust and iron fertilization that led to a further drawdown."

In terms of relevance to current climate issues, Doney said: "There has been a lot of discussion of whether the CO2 problem will solve itself - where there is some self-regulating mechanism.

The resounding answer that's coming out of the research is that there is not any self-regulating systems that are going to clean things up over the next 200 years or so."

Even if there was some attempt to manipulate ocean CO2 absorption - such as by dumping iron into the Southern Ocean, for example - "this paper would call into question the effectiveness of that action," Doney said.

"This is another piece of evidence that says maybe the role of iron in the carbon cycle isn't as effective at storing carbon as we might hope."

Regarding Martin's quote about using a half-tanker of iron to jump-start an ice age, he said, "You might be able to draw down CO2, but John couldn't give you a new ice age."

Climate is a weekly series examining the potential human impact on global climate change, by veteran environmental reporter Dan Whipple. E-mail: [email protected]

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