How long will the Earth’s CO2 disposal system keep up with humans?

1 August 2012 Justin Gillis The earth is performing an enormous disposal service for the human race. About half of the carbon we are dumping into the atmosphere does not stay there and is instead taken up by the oceans…

1 August 2012

Justin Gillis

The earth is performing an enormous disposal service for the human race. About half of the carbon we are dumping into the atmosphere does not stay there and is instead taken up by the oceans and land. Were this not the case, scientists say, the earth would probably be warming far more rapidly.

One of the biggest questions in climate science is: How long will that disposal service last?

Remarkably, the earth’s ability to keep socking away carbon has for decades kept up with human activity, with the proportion that disappears from the atmosphere remaining close to 50 percent even as our emissions soared. Computer analyses of the climate have long predicted that the uptake would become less efficient sometime in this century. If that happened, the level of carbon dioxide in the air would begin rising faster, trapping more of the sun’s heat.

Two new scientific papers shed some light on this issue. One of them is reassuring, at least in the short run, while the other offers new reasons to worry about the long-term stability of the “carbon sink,” as scientists call it.

The good news first: A new paper suggests that the earth’s ability to take up carbon has not yet begun to weaken.

The study, published online Wednesday by the journal Nature and led by Ashley Ballantyne of the University of Colorado, Boulder, is the latest installment in a debate that has been going on for several years.

Some research, focused on regional carbon sinks on land (such as forests) or in the ocean, had suggested the beginnings of a decline. Earlier this week, for instance, this paper found a decline in the sink in the Western forests of North America as a result of a drought from 2000 to 2004. These regional findings led some scientists to suggest that the carbon sink might be weakening on a global scale, but that claim was countered by papers that found no significant drop.

The new paper is an attempt at a definitive take on this question. Basically it is a meticulous statistical analysis of global carbon dioxide measurements that have been going on for decades, including the renowned monitoring effort on the slopes of the Mauna Loa volcano in Hawaii. Taking rigorous account of all possible sources of error, Dr. Ballantyne and his colleagues found no convincing evidence that the global carbon sink was weakening.

They warned, however, that this finding should not lead to complacency about the risks of unchecked carbon dioxide emissions. “It’s not a question of whether or not natural sinks will slow their uptake of carbon, but when,” Dr. Ballantyne was quoted as saying in a news release.

Still, assuming that his paper holds up to critical scrutiny, it is unquestionably good news that the shift has not yet occurred.

A second paper, published earlier in the week by the journal Nature Geoscience, provides insight into how the disposal service in the ocean is actually working. The surprising finding is that a handful of relatively concentrated spots in the Southern Ocean account for a high proportion, roughly 20 percent, of the entire oceanic carbon uptake.

The reason is that while carbon dioxide can easily dissolve out of the air into ocean water, it tends to stay in a surface layer that does not mix well with the colder, denser water below. It can even escape that surface layer to re-enter the atmosphere. The scientists found that certain combinations of winds and currents are required to overcome the barrier and pump carbon dioxide into the deep ocean, where much of it stays locked away for thousands of years.

The paper, led by Jean-Baptiste Sallée of the British Antarctic Survey, found “hot spots” in the Southern Ocean where this process is operating most efficiently. The two most important are in the Drake Passage, between Antarctica and South America, and in a region of the ocean due south of western Australia. The scientists used measurements from the new Argo network of floating robots to make the most complete analysis yet of the role of currents.

The obvious concern the paper raises is that climate change could disturb the existing pattern of winds and currents and shut down the hot spots, making the entire ocean less efficient as an absorber of carbon dioxide.

In principle, of course, things could go the other way too, with climate change perhaps creating more such hot spots and increasing the efficiency of the disposal service. But we know from the geological record that past jumps in the earth’s temperature have tended to raise the amount of carbon dioxide in the air, which then reinforced the warming trend. So that’s a pretty good reason to think that things will unfold the same way as a result of human-caused warming.

“The good news is that today, nature is helping us out,” said James W. C. White, a University of Colorado researcher and a co-author on the Nature paper, in a news release. “The bad news is that none of us think nature is going to keep helping us out indefinitely.”