As we have seen from the previous blog entry, the buffering of the oceans works more efficiently the warmer the Earth is. In other words, the warmer the oceans, the quicker carbon dioxide is returned to the atmosphere.
An interesting facet of the ice age cycles is that ice age cycles are just as much about where carbon dioxide is stored as where water and ice are stored. During ice ages, a considerable fraction of the Earth's surface water is stored in ice sheets on the continents, and the oceans fall. The amount of water on the surface of the Earth doesn't change, but where the water is changes.
The same thing happens with carbon dioxide.
During an ice age, the amount of CO2 in the atmosphere falls by about 100 ppm from the 280-300 ppm in the atmosphere during interglacials to 180-200 ppm during the depths of ice ages. But where does the CO2 go?
The answer is that it goes into the sea. The oceans absorb it, and become slightly more acid. There is a cycle that slowly changes the PH of the oceans by ~0.03 The PH of the oceans is not just controlled by the acidic and alkaline compounds dissolved within it, but also by activity factors that mitigate (or increase) ionic concentrations. The chemistry for activity factors in the oceans is very complicated and I will not be going into it here, but the net effect is to slightly mitigate, or lessen, the swings in ionic concentrations in the oceans as the amount of CO2 increases or decreases. At least in the natural history of our recent ice ages. I will discuss it briefly at the end of this entry
Some of the results of this carbon cycle are counterintuitive. During ice ages, the atmosphere has less carbon dioxide, but the oceans are more acid. How can this be?
The answer is that the amount of carbon dioxide in the oceanic/atmospheric system remains broadly the same. At least the carbon does. During ice ages, more carbon remains in methane, which is trapped in methane hydrates in cold continental shelves. The total amount of carbon dioxide in the oceans and atmosphere does fall slightly, with more methane. But the amounts of carbon remain the same.
Another factor is that colder waters can hold more dissolved oxygen, and colder waters can support more life, if other trace minerals needed are present. Evidence does suggest that oceanic biological productivity was slightly greater during ice ages, and this may also have trapped some more gigatons of carbon.
But the main thing is that carbon dioxide accumulated in the oceans.
As I said, the PH of the oceans falling as CO2 concentrations fall in the atmosphere sounds counterintuitive, but it does make sense. The amount of carbon in the oceanic/atmospheric system remains broadly constant. If there is less in the air, there is more in the sea.
Carbon dioxide was not trapped on land in vegetation. Not only were millions of square miles covered under ice sheets, but today's temperate zones were much colder and drier. Forests are the main way life stores carbon on land, and there was much less forest area during the ice ages. There was increased land areas during ice ages as continental shelves were exposed (the area of ice-free land was about the same during ice ages as today) But it was mostly dry tundra or grasslands, and even in the tropics forests shrank and became patchy in restricted areas or river valleys.
*note* Tundra can trap large amounts of carbon. If it is wet tundra. We all know about the thawing tundra bogs bubbling with methane as they thaw. But dry tundra is different. It's just frozen without much biological productivity and not much buried biological matter to become peat infused with methane. The climate of the Earth was much drier during ice ages---so dry that in many places cold enough to form ice sheets, such as Siberia, they did not form. Dust deposits, or loess, show that at best much of North America, Europe, South America and the remaining temperate parts of Asia were at best semi-arid and most of these areas were true deserts.
Much of the interior of North America resembled the Gobi Desert. The Sand Hills of Nebraska were giant sand dunes. When Native Americans first became numerous ~15,000 years ago, the climate was already becoming milder and wetter, supporting more vegetation and life. And I am skipping over the raging question of when mankind arrived in the Americas, but there is not evidence of widespread populations more than 15,000 years ago.
As the implications of Revelle's worked seeped through the geologic, oceanographic, biologic, and climatologic branches of science during the 1960s and 1970s, there was some thought that there might be a global carbon cycle, driven perhaps by a long cycle in volcanic activity over tens of thousands of years, that injected and withdrew carbon from the oceanic/atmospheric system. But it was never a widespread belief, and no evidence has been found for it.
The work on the Earth's carbon budget has had two main implications for climate science and global warming, one well grounded in fact, and the other more speculative.
The first one is that yes, warming out of the ice ages does come before carbon dioxide begins to rise in significant quantities. And it does make sense and doesn't invalidate carbon dioxide as the major driver in climate change.
The reason is this. The Milanković cycles determine how much solar energy falls in the polar and temperate zones, and the tropics as well. The Milanković cycles trigger a small warming, which then increases the buffering of CO2 by the oceans. The oceans, as they warm slightly, return CO2 more quickly to the atmosphere. This increases the warming, which then increases the rate CO2 is returned to the atmosphere, and it triggers an accelerating feedback. As the climate warms and becomes wetter, tropical forests and wetlands increase, increasing wetlands and methane emissions. More warming. Methane hydrates in marginal areas become unstable and release their methane. More warming. Swamps and wetlands increase in temperate and polar zones and emit more methane--more warming. The methane is quickly oxidized to CO2 and water, but the CO2 is still a warming gas, as we know.
As ice sheets shrink, the Earth's albedo decreases and the Earth retains more heat. More warming. Less known is the fact that forests are quite dark, while deserts are reflective. Compare the Amazon rain forest to the Sahara Desert in satellite pictures. Or the Siberian taiga to the rocky wastes of northern Canada's islands.
All these effects produce enough warming to bring the Earth out of an ice age. But the key is that temperatures rise first from the Milanković cycle.
This is a point that deniers try to exploit. When they do so, you can be sure that they are either ignorant of climate processes or being deliberately dishonest. Usually they are being dishonest.
Deniers argue that because the temperature began to rise before CO2 began to rise in the atmosphere that means that CO2 is not a greenhouse gas. Or that it doesn't trigger warming. Or some such thing. No. CO2 rises as a feedback to a slight temperature increase, and then vastly increases the temperature rise far beyond what Milanković cycles can do. And the albedo effects from reductions in ice and snow cover and changes in vegetation increase temperatures still further! The Milanković cycle increases temperatures a few tenths of a degree and the feedbacks from CO2 and decreasing albedo trigger the far greater temperature rises.
Milanković cycles work because the Earth is finely balanced between different climatic states, ice ages and interglacials. Milanković cycles determine how much solar radiation reaches the polar and adjacent temperate zones. Aside from changes in the eccentricity of the orbit of the Earth, Milanković cycles do not change the total amount of solar radiation reaching the earth. When changes in the axial tilt increase solar radiation in polar zones, they decrease solar radiation in tropical zones. The total amount of solar radiation reaching the Earth remains the same. Aside from the changes in the eccentricity of the orbit of the Earth around the sun, Milanković cycles wouldn't change the temperature of the Earth at all if positive feedbacks didn't come into play when more solar radiation reaches polar and adjacent temperate zones.
The fact that the Earth's climate warms so much from subtle changes in solar radiation in polar zones shows that the positive feedbacks are very strong. And that is why what we are doing to the atmosphere is so dangerous.
This brings into play climate sensitivity. We know that Milanković cycles, aside from the minor orbital eccentricity effect, can't change the total solar radiation the Earth receives. Jule Gregory Charney (1917-1981) crafted the first definitive report on climate sensitivity in 1979. You can read it here.
Climate sensitivity compares how much a given increase in a greenhouse gas, carbon dioxide in this care, to what actually happened in the climatic record. Although Charney's report is from 1979, it is definitive. The basic physics of radiation absorption by CO2 have been well understood for decades (As I have said in previous entries, it was believed until the 1940s that CO2 in the atmosphere was saturated as far as infrared radiation absorption is concerned. In other words, that adding more CO2 would not make a difference because it already absorbed all the infrared radiation it could. I will be discussing how that was proved wrong soon in an upcoming blog entry.)
From the Charney report we know that climate sensitivity greatly increases temperature swings from changes in atmospheric carbon dioxide alone would do. And has done. The question humanity faces is how powerful these positive feedbacks will be in a warming world.
Revelle's research, and other research by scientists later, is disquieting on several fronts. Back then there was 50 times as much dissolved CO2 in the oceans as in the atmosphere. It is now about 40 times as much, as atmospheric CO2 has increased so quickly. We have added nearly 3 trillion tons of carbon dioxide to the atmospheric/oceanic system. Despite the swings in oceanic chemistry between ice ages and interglacials, the oceans are already far more acid (or less alkaline) than in the ice ages. As temperatures rise, the efficiency of carbon dioxide return to the atmosphere increases. The oceans hold hundreds of trillions of tons of carbon dioxide. Could the large increases in temperature in store turn the oceans into net carbon dioxide emitters?
We don't know.
The warmer water becomes, the less gas it can dissolve. That runs counter to what we know in daily life, because we know about how solids behave in water. Cold water doesn't dissolve much sugar, and dissolves it much more slowly than hot water.
With gases it is different. Molecules vibrate faster and travel faster as temperatures warm. In fact the motion of molecules defines temperature in our daily lives. Solids dissolve more readily in liquids as the temperature rises because the break off from the surface of solids and are incorporated into the liquid.
For gases, the greater the temperature, the more rapidly the gas molecules travel, and the more easily they can escape the liquid. That is why the warmer water is, the less dissolved gas they can hold. That is why CO2 buffering becomes more efficient as temperatures rise. Rising temperatures will also decrease the amount of oxygen dissolved in water, with impacts on biological productivity. We don't know how oceanic life will adapt to warmer, more acidic conditions. Photosynthetic algae do remove a lot of CO2 and convert it to oxygen. Algae decompose on the surface when they die, and the carbon within them remains part of the open carbon system. But animals feeding on them can sink to the ocean floor when they die, as well as diatom shells. Could the warming and acidification of the oceans decrease biological productivity enough to significantly reduce the amount of carbon that settles to the sea floors?
We don't know.
The chemical processes of the Revelle Effect are well known, and assuming no major changes in biological activity, the impact of the Revelle effect is quantifiable. All measurements agree that CO2 buffering and return to the atmosphere increases by 6%-8% for every 1° C. That is a closer agreement than many in science.
But the possibility of some sort of biological threshold being reached--a cliff--where biological productivity decreases enough to increase the Revelle effect much more than expected is not something we can safely ignore.
Here are some links to the chemistry of oceanic carbon dioxide buffering:
From the IPCC
Some of the major chemical reactions in oceanic carbon dioxide buffering from Columbia University.
And a more detailed paper on the chemical reactions of oceanic carbon dioxide buffering by Chuixiang Yi, Peng Gong, Ming Xu and Ye Qi.