In a seminal paper last year [ok, some sarcasm there], Yi et. al argued (at the end) that extreme events can cause grassland ecosystems to absorb less carbon.  I know – that was my hypothesis and contribution to the paper, which was borne from a class project and furthered by an AGU poster. In the latest issue if Nature, Reichstein et al. (2013) have written about Climate Extremes and the Carbon Cycle. While I’ve not had a chance to digest it fully, I have some thoughts.Apparently, we were on to something in 2011.  Coumou and Rahmstorf (2012) looked at weather records of the last decade to determine if extreme events are indeed increasing (which they may be, depending on the type of event).  Reichstein make the same argument that we made in 2011, that extreme events (in our case drought, which apparently play a large part of Reichstein’s argument) cause a reduction in global carbon uptake.  There are, of course, competing hypotheses – specifically, that increased CO2 will enhance plant growth and carbon uptake through CO2 fertilization (as well as increased temperatures and longer growing seasons). (Funny enough, my advisor has several papers about CO2 fertilization!).  However, CO2 fertilization often has a limit – increased CO2 generally increases the water-use efficiency of plants by decreasing the amount of water lost for the same amount of CO2 absorbed. This is effective for plants where water (or water-use efficiency) is a limiting factor on growth.  However, in many places, it is not, and other nutrients are limiting factors (see Cox et al 2013 for a recent overview).  The Reichstein et al. paper argues that extreme events can offset these gains.

I do wonder if the method that we used might be applicable to this. Of course, the difficulty is the short time-span of the CO2-flux records in determining change over time. It may be something worth looking in to. It’s also interesting to note (from the Cox et al. 2013 paper) that the tropics are a major research focus in terms of carbon cycling, and also where my main research is focused (hello, ENSO).  It also dovetails very nicely into ENSO research – ENSO is a MAJOR source of extreme weather (floods and droughts, both of which affect primary productivity) and changes in ENSO as a result of climate change will have a major impact on the tropical carbon cycle.