From the UNIVERSITAT AUTONOMA DE BARCELONA and the “department of settled science” comes another global climate thermostat story. Just last week, we were told the Earth had no thermostat. Gotta love it when climate science is so robustly definite, eh?

Ice age thermostat prevented extreme climate cooling

Atmospheric CO2 concentrations swung over a range of 100 ppm (parts per million, by volume) during the ice ages. The exact processes behind this variation have been difficult to pinpoint, but it is known that changes in the storage of carbon by photosynthetic organisms played an important role.

“When we took a close look at measurements from ice cores, we noticed that atmospheric CO2 concentrations hovered close to 190 ppm during much of the past 800,000 years, but very rarely fell any lower,” said Sarah Eggleston, a researcher at the Institut of Environmental Science and Technology (ICTA-UAB) and co-author of the study. “This was surprising, because it suggests that these very low CO2 concentrations were quite stable. What’s more, we know that CO2 was often very high in the distant geological past, but we have no evidence that CO2 concentrations were ever lower than 190 ppm.”

“We know that, over hundreds of thousands of years, CO2 is regulated by slowly reacting with exposed rocks” explained Eric Galbraith, lead author of the study and an ICREA professor at ICTA-UAB. “But this would be too slow to explain the stability during periods of only a few thousand years, as we see in the ice cores. So it must have been some other mechanism that kicked in at very low CO2.”

The authors suggest that it was most likely the biosphere that maintained habitable temperatures, since at very low CO2 levels, plants and phytoplankton struggle to photosynthesize. Slower growth of these organisms would have meant less carbon in the soils and deep ocean leaving more in the atmosphere, and preventing CO2 concentrations from falling further. This might have prevented extreme cooling that would have led to Earth freezing over as a ‘snowball’.

However, the study did not reveal a corresponding regulation during the warm portions of the ice age cycles, suggesting that the Earth does not have a similar mechanism to prevent rapid warming.

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A lower limit to atmospheric CO 2 concentrations over the past 800,000 years

Global temperatures and atmospheric CO 2 concentrations varied widely over the glacial cycles of the past 800,000 years. But despite this variability, Antarctic ice cores have shown that CO 2 concentrations were very similar during all the coldest points of these cycles. Remarkably, the recurring minimum CO 2 concentrations (190 ± 7 ppm) fall on the lower bound of any known in Earth history. Here we show that although the volume of terrestrial ice sheets was normally distributed over the past 800,000 years, as might be expected from the approximately normal distribution of the orbital forcing that drove the glacial cycles, Antarctic temperatures have a strong cold mode, whereas CO 2 concentrations have both a cold mode and a central mode. Although multiple explanations are possible, the recurring CO 2 minima and pronounced cold modes are consistent with a strong negative feedback to decreasing CO 2 that resisted further cooling on timescales shorter than 10,000 years. We suggest that one possible negative feedback is CO 2 -limitation of photosynthesis, either directly or via CO 2 -limitation of N 2 fixation, which could have inhibited further lowering of CO 2 by reducing carbon storage.

Full paper: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2914.html (paywalled)

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