State of the Planet

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Does temperature control atmospheric carbon dioxide concentrations?

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.

Compilation of CO2 records and EPICA Dome C temperature anomaly over the past 800,000 years. Reprinted by permission from Macmillan Publishers LTD: Lüthi et al., High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature, 453 (7193): 379-382.

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.

This misconception is widespread, appearing in blogs and documents spanning the full spectrum of views about global warming, for example herehere and here.

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.

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Brock Ervin
Brock Ervin
6 years ago

To State of the Planet/Bob Anderson: I’ve got a question. You state early in the article that the cooler waters would only account for about a third of the reduced atmospheric CO2. The rest of the article focuses solely on other processes that could reduce atmospheric CO2, some of which appear to be dependent on cooler oceans/climates for those processes to occur. At the end of the article, you explain that ocean temperature played a minor role in reducing atmospheric CO2.

First question: Is a third of the cooling really minor? Or are you saying that cooler water temperatures don’t even explain 1/3 of the CO2 decrease as mentioned earlier on, and that the other processes are that much more significant?

Second: As you stated, sometimes these processes work together and sometimes separately, but wouldn’t at least a few of these be reinforcing as the processes continue (i.e. increased CO2 intake by proliferating organisms and their sediment lock-in CO2, thereby cooling the planet and spurring production of sea ice, which then stifles physical mixing), all of which reduce atmospheric CO2 and therefore the earth’s temperatures?

Third: Where do the Milankovitch Cycles fit in to all of this? Are these cycles still considered to be the dominating cause, or at least the impetus, of the ice ages?

Thanks very much. I know this article is from 2010, so I hope this message doesn’t go into a black hole.

Bob Anderson
Bob Anderson
6 years ago

This is a response to Brock Ervin from Bob Anderson, entered as a comment since I am no longer affiliated with the blog and do not have access to submit a formal response.

To respond to your first point, the additional absorption of CO2 by the ocean during the last ice age was not negligible, as you say, but I think most experts would agree that 1/3 of the total reduction is an upper limit. The actual amount could be substantially less, perhaps 15%. Recent work by Jeff Severinghaus and his group at the Scripps Institution of Oceanography indicates that the average temperature of the ocean during the last ice age was about 2°C less than in the preindustrial modern ocean. If that is the case, then the lowering of atmospheric CO2 by ocean cooling during the last ice age must have been substantially less than 1/3 of the total depletion of CO2 from the atmosphere. I would consider this to be a minor contribution, but not negligible.

It is possible, or even likely, that ocean cooling induced positive feedbacks that lowered CO2 further, for example by reducing the physical mixing that brings the CO2 that accumulates in the deep sea due to respiration back to the surface ocean where it can be vented to the atmosphere. But this is a feedback, not a direct response to cooling due to the greater solubility of gases as water cools, as explained in my original posting.

In response to your last question, yes, Milankovitch cycles are still thought to be the initiator of the main ice-age cycles that have characterized the Pleistocene, covering roughly the past 2 million years. But there must be other factors involved, as well, factors that are often referred to as feedbacks. For example, a feedback is required to explain why the ice ages in the Southern Hemisphere are largely (although not precisely) in phase with the ice ages in the Northern Hemisphere. Were Milankovitch forcing the only factor involved, that is, were the summer insolation at high latitudes that melts ice sheets the only factor, then one would expect the ice ages in the Northern and Southern Hemispheres to be out of phase, contrary to observations. However, because CO2 is fairly well mixed in the atmosphere, the rise and fall of atmospheric CO2 levels in response to Milankovitch forcing provides the feedback required to explain the paradox of concurrent ice ages in both hemispheres through the greenhouse properties of CO2.