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. Continue reading →