We know ice ages occur because a feedback system removes much of the carbon dioxide from the atmosphere, but where does all that carbon go? The question becomes rather important if we are ever to reverse the modern pattern of ever-increasing atmospheric carbon, and a new paper presents evidence we've been overlooking some of the most important processes.
Although land plants can remove a lot of carbon dioxide in the short term, some of which stays captured in the soil, and atmospheric carbon can react with minerals on much longer timescales, the biggest sink for atmospheric carbon is the oceans. Yet we have a poor understanding of how oceans remove carbon dioxide as ice ages set in.
During ice ages, the concentration of carbon dioxide in the atmosphere falls by around 90 parts per million (ppm), less than we are likely to need to remove when we finally get our fossil fuel addiction under control, but it is a pretty good start.
According to Professor Samar Khatiwala of Oxford University, the contributions from lower ocean temperatures have often been ignored entirely, and the estimates that have been made range from 16 to 30ppm. In Science Advances, Khatiwala argues this is a major error. Carbon dioxide is more soluble in cold water, and the cooling of the oceans during an ice age reduces carbon dioxide in the atmosphere by 44ppm, Khatiwala calculates. Most of this is through direct effects, with a small portion being the result of changes in biological production.
Khatiwala thinks iron fertilization is another neglected route. Large areas of the oceans are biologically unproductive because they lack the iron needed for photosynthetic algae to bloom. Increased dust storms during glacial periods dump more iron into the oceans. All this extra life removes 26ppm carbon dioxide from the atmosphere, Khatiwala claims.
An expansion in sea ice affects carbon storage in multiple ways, but these can be both positive and negative, and circumstances determine which ends up larger.
Previous explanations for oceanic carbon removal have relied on the deep ocean becoming more isolated and stratified, trapping more carbon at a depth where it can't get exchanged back to the atmosphere.
If Khatiwala is right, it's a mix of good and bad news. Cooling the oceans at a time when the world is warming up is clearly not practical. Indeed the extra heat may release even more oceanic carbon dioxide than current models allow, creating a vicious circle.
On the other hand, consideration has been given to fertilizing unproductive ocean regions with iron to draw down carbon dioxide. While the practicality and merits of this remain debated, Khatiwala's work suggests that if it proves realistic it could make a substantial contribution to removing excess atmospheric carbon.