Diagnosing Carbon-Climate Feedbacks in the Contemporary Carbon Cycle

The amount of carbon being taken up by the biosphere is clearly increasing; however, the variance in the global carbon cycle also appears to be increasing. This increasing variance may be indicative of C sink instability, or it may simply be noise in the global C cycle. Therefore identifying the mechanisms driving this variance is critical for predicting future behavior of the global C cycle. Here we combine satellite observations of terrestrial productivity and atmospheric observations of CO2 and its isotopic composition as diagnostics to gain new insight into the changing behavior of the global C cycle. We use these observations to partition net C uptake into photosynthetic gains and respiratory losses. Based on this approach, we conclude that much of the observed variance in the global C cycle appears to be due to the respiratory response of the terrestrial tropics and that much of the recently observed increase in net C uptake is due to diminished respiratory losses during the well documented warming hiatus. Lastly, we use isotopic analyses of CO2 in this respiratory flux to infer the biosphere's response to changing climate. From these analyses we conclude that water vapor feedbacks present a first-order control on the biosphere's capacity to continue taking up atmospheric CO2 as Earth's atmosphere continues to warm.