by Emil Morhardt
A recent paper discussed in the previous post (Galbraith and Eggleston, 2017) claims that during the past 800,000 years when the Earth has been in a glacial condition with the occasional interglacial period (such as now), there is a strong correlation between global temperature and atmospheric CO2 levels, and that they tend to go to the same low point again and again and stay there. These authors argue that if CO2 were to go lower, so would the temperature, and that therefore something is keeping the CO2 level from going any lower then 190 ppm. One intriguing possibility they bring up comes from a paper (Pagani et al., 2009) by Mark Pagani at Yale, and his colleagues at the Carnegie Institution in Stanford and at the University of Sheffield who claim that plants stop effective photosynthesis if CO2 levels fall below 190 ppm, depriving the carbon cycle of two sources of removal of atmospheric CO2; photosynthesis, and a more subtle plant activity called biologically enhanced silicate chemical weathering. The mechanisms of these two processes are interesting.
Loss of Effective Photosynthesis—Plants use atmospheric CO2 as a food source, using photosynthesis driven by the energy in light to capture it out of the air and convert the carbon into sugars and cellulose which are organic hydrocarbon molecules made largely of carbon and hydrogen. When atmospheric CO2 levels go up, as now, they initially tend to make it easier for plants to photosynthesize, promoting plant growth in a process called CO2 fertilization. What happens when they go down? The dominant type of photosynthesis (C3) is catalyzed by an enzyme (Rubisco: ribulose bisphosphate) which also reacts with oxygen, and when it does, the photosynthesized CO2 is released back into the air (photorespiration), hence no sugars are formed, and the plants can’t grow. By 190 ppm CO2, net photosynthesis effectively stops, so plants can’t capture any more CO2.
Decreased Plant-Induced Silicate Weathering—Chemical weathering occurs when CO2 dissolves in rainwater as it falls, producing carbonic acid. When the slightly acidic rain falls on silicate rocks, it dissolves them, mineralizing the CO2 into bicarbonate ions which tend to be captured as calcium carbonate in the shells of marine organisms and removed from the atmosphere for a long time. Growing plant roots enhance this process by mechanically fracturing rocks, increasing the surface area for the acidified water to interact with, and releasing CO2 from their roots, acidifying the water even more. The mycorrhizal fungi associated with growing plant roots facilitate the process. None of this happens if plants stop growing.
So, there is a negative feedback loop here. If atmospheric CO2 falls too low it is no longer removed by plants, so it tends to stabilize at 190 ppm.
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.