Those who know a little about photosynthesis may wonder how it may interact with the severity or speed of anthropogenic climate change. Plants and organic systems take in carbon to produce energy and output oxygen back into the atmosphere, so it seems logical that, if atmospheric carbon increases, plants will take in some of this carbon and therefore reduce the net carbon left in the air.
This is the common view currently held, supported by Earth system models that show that atmospheric carbon increases do indeed increase the uptake by the terrestrial biosphere. But it’s actually uncertain what this increased uptake will then do to carbon concentrations, as there are additional mechanisms at play – most importantly, organic matter decomposing and re-releasing the carbon back to the atmosphere. This is an important area to investigate further because it’s difficult for experiments to detect the amount of organic carbon in soil, so climate models have difficulty incorporating its effect into their predictions.
A new meta-analysis in Science gives a first answer to this question, reviewing evidence for effects of increased CO2 concentration on soil carbon storage. Contrary to assumption, increased CO2 in the atmosphere increases the decomposition of CO2 in the soil, leading to greater emission back into the air. Therefore, hopes that increased photosynthetic activity might limit CO2 additions to the atmosphere appear unfounded.
There is a simple and more complicated answer to this. The simple answer is what scientists generally already understand – using first-order kinetic models, researchers have found that as you increase the amount of CO2 in the atmosphere, the amount of organic matter in the soil increases and therefore the amount of CO2 re-emitted back to the atmosphere due to decomposition increases (I’ll call this the proportionality rule, since it’s a simple association between the amount of CO2 in the soil being related to the amount released back to the air). But these authors have now found evidence that increased CO2 in the atmosphere leads to CO2 emission increase from the soil above and beyond that predicted by just the increase in amount from the proportionality principle (16.5% higher, to be exact). This is the more complex observation to understand because it indicates that it’s not just the amount of CO2 in the soil – somehow, increased CO2 concentrations in the air increases the basic rate of decomposition in the soil!
How can this happen? It’s due to an effect scientists call priming, in which carbon deposits in the soil, previously thought protected from microbes that decompose them, are now vulnerable. However, this has always been thought to be capricious and short-term process that shouldn’t be seen on such a widespread, consistent scale as seen by this meta-analysis. These results indicate priming is a very general process that should apply to most organic carbon deposits in the soil, although studies specifically looking at this effect should now be done to confirm this interpretation of the results.
This finding provides another important parameter to include in climate models of atmospheric-Earth interactions to better predict future climate change. The inclusion of this increased decomposition rate due to CO2 atmospheric concentrations will likely lead to predictions of expedited climate change, as CO2 will stay in the atmosphere longer than previously expected.
van Groenigen, K., Qi, X., Osenberg, C., Luo, Y., & Hungate, B. (2014). Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage Science, 344 (6183), 508-509 DOI: 10.1126/science.1249534