by Emil Morhardt
Atmospheric CO2 levels are always lower during glacial periods than during interglacials like the one we are in now. During the last glacial maximum 20,000 years ago, for example, they were at 190 parts per million (ppm), whereas during the most recent 10,000 years, almost up to the present, they have been about 280 ppm. [We have now succeeded in raising them to over 400 ppm and still counting, but that’s a different story.] Eric Galbraith and S. Eggleston (Galbraith and Eggleston, 2017) argue that as far as we know, atmospheric CO2 levels have never gone below the typical glacial levels of 190 ppm, even in extended snowball earth conditions. Why not? Well, a carefully-reasoned 2009 paper they cite (Pagani et al., 2009) suggests that even in the mostly warm conditions of the last 24 million years, when CO2 levels fell below 190 ppm, terrestrial plants stopped effectively photosynthesizing, thus they not only stopped removing CO2 from the atmosphere directly, but they also stopped the active root growth which increases the acidity of soils and enhances chemical silicate weathering from the rocks which removes CO2 from the soil, and ultimately from the atmosphere. Galbraith and Eggleston argue that the same thing has been happening during the glacial periods of the last 800,000 years, and extend the argument to the photosynthesis of oceanic phytoplankton. To wit, when CO2 levels get below 190 ppm, CO2 removal from the atmosphere by photosynthesis and chemical weathering is sharply reduced, so they decline no further.
This paper is entirely speculative, following on the heels of many similar ones, but systematically discounts their speculations.
Is there an instability threshold such that when it gets cold enough things start to warm up and CO2 levels follow? Probably not, because the coldest periods tend to be about the same temperature, with similar levels of CO2, but of widely different durations. The authors think, instead, that something during the cold states prevented further cooling and kept the CO2 levels where they were.
Maybe the freezing point of seawater is the limiting factor; ice doesn’t sink, so once surface seawater freezes at –2C, it can no longer transfer heat to the deep ocean by sinking; global temperatures stop decreasing and so do atmospheric CO2 levels. Their arguments against this idea are that deep ocean temperatures have little effect on the atmosphere since they interact with it in only a few locations, and in any case, global climate models don’t work this way; if atmospheric CO2 is arbitrarily lowered in them, global temperatures continue to decline.
Maybe the lower the temperature, the less effective atmospheric CO2 is as a greenhouse gas, but the global climate models predict exactly the opposite.
Maybe the growth of Southern Ocean sea ice when CO2 levels are low interferes physically with the transfer of CO2 between the ocean and the atmosphere, effectively stabilizing atmospheric CO2 levels. If such a mechanism works, however, why would it not have prevented the snowball earth episodes 600 million years ago?
The idea that remains standing, in the eyes of Galbraith and Eggleston, is their photosynthetic one; when CO2 levels fall below 190 ppm, plants and phytoplankton stop removing CO2 from the atmosphere, and consequently, global temperatures stop falling as well and remain stable until something causes the world to warm up again. This something is the astronomical Milankovitch cycles but that, too, is another story.
Galbraith, E., Eggleston, S., 2017. A lower limit to atmospheric CO2 concentrations over the past 800,000 years. Nature Geoscience 10, 295-298.
Pagani, M., Caldeira, K., Berner, R., Beerling, D.J., 2009. The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years. Nature 460, 85.