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.