A record of temperature and atmospheric CO2 from Antarctic ice cores shows the tight relationship between the two over the past 800,000 years (see figure). It is frequently asserted that changes in Earth’s temperature or, more specifically, the temperature of the ocean, caused atmospheric CO2 concentrations to vary over that time period.
The underlying principle is that during ice ages the cold ocean absorbs more gases, thereby lowering atmospheric CO2 concentrations. This explanation is appealing. It is based on the well-known fact that cold water holds more dissolved gases than warm water. Unfortunately, the statements are misleading. Ocean cooling was a factor, but it was not the main cause of lower CO2 levels during ice ages.
Of course, cooling did cause the ocean to absorb more CO2 during the ice ages. All other things held equal, ocean cooling could have accounted for about 1/3 of the observed lowering of atmospheric CO2. However, other factors changed as well. For example, the oceans became saltier as water was locked up on land in great ice sheets, and saltier water holds less gas than fresh water. This offset a large part of the temperature effect.
Furthermore, the growing ice sheets reduced the amount of carbon contained in the terrestrial biosphere. Altogether, these factors offset nearly all of the CO2 lowering caused by cooling the ocean. These features of the global carbon cycle have been known for many years, and were summarized nicely in reviews by Danny Sigman and by David Archer published in 2000.
Something else caused CO2 levels to drop during ice ages, and then rise rapidly by 80 ppm or more as ice ages ended. These other factors involve changes in the physics, chemistry and biology of the ocean, all of which affect the amount of CO2 stored in the ocean and, conversely, the level of CO2 in the atmosphere.
In last week’s issue of Nature (1 July, 2010), Danny Sigman and coworkers published a new review describing the complex interplay of processes by which the ocean governs the CO2 content of the atmosphere. As noted in their review, processes that regulate CO2 levels almost certainly occur in the ocean around Antarctica.
Three conditions most likely to lower CO2 levels during ice ages are:
1) Reducing the rate of physical mixing that stirs deep water up to the surface. Centuries of respiration produce high levels of CO2 in the deep sea, and much of this CO2 is released to the atmosphere when deep waters are brought up to the surface. Other factors held constant, reducing the mixing of deep water to the ocean surface will lower the CO2 concentration of the atmosphere.
2) Stimulating the growth of algae that consume CO2 in surface waters and later transfer that carbon to the deep sea when they die and their remains sink toward the sea floor. Other factors held constant, if more carbon is transferred to the deep ocean by organisms, the CO2 content of the atmosphere is lowered.
3) Increasing sea ice cover across the ocean around Antarctica, as sea ice inhibits the release of CO2 brought to the surface by physical mixing. It also inhibits the growth of algae that consume CO2. Other factors held constant, increasing ice cover will lower the CO2 content of the atmosphere.
Of course, the processes identified above do not operate in isolation of one another. Sometimes they act synergistically and sometimes antagonistically. Much remains to be learned about these processes, and how they differed during ice ages compared to conditions that we observe today. The Sigman et al. review paper goes into more detail about the conditions during ice ages that allowed the oceans to absorb more CO2.
One thing is certain: varying the amount of gases absorbed by the ocean due to changes in its average temperature played a minor role in regulating the observed changes in the CO2 content of the atmosphere. The leading actors involved physical, chemical and biological processes in the ocean.