The Keeper and Breaker of the Global Carbon Cycle Since the Industrial Revolution

Lianhong Gu, ORNL, lianhong-gu@ornl.gov
Tony King, ORNL, kingaw@ornl.gov (Presenting)
Mac Post, ORNL, postwmiii@ornl.gov

There have been growing concerns on the potential weakening of terrestrial carbon sink due to droughts, heat waves, wild fires, spring temperature fluctuations, autumn warming and deforestation and of oceanic sink due to the reduced carbonate buffering capacity and climate change. Corroborating these concerns is a detectable increase in the airborne fraction since 1960. To gain insight into the future status of the global carbon sinks, we integrated broad observational datasets and multiple model predictions to construct the past sink trajectories and identify processes controlling them. Here we show that the global carbon sink strength (GCSS, the annual sum of net oceanic and terrestrial carbon balance) has been increasing since 1900 up to the present (2006), after a relatively stable period of small global source since the industrial revolution (~ 1750). A tight linear relationship exists between GCSS and atmospheric carbon dioxide (CO2) for the whole 256-year period. However, the partition of this global pattern into land and oceanic contributions with two independent approaches consistently reveals that the land carbon sink has long saturated with respect to rising atmospheric CO2 and the saturation has been masked by a strong oceanic response in the global signal. The land carbon sink saturation apparently is due to the neutralization of the CO2 fertilization effect by warming which has moved the land surface temperature regime from a positive relationship with land carbon uptake into a negative one. These findings imply that continuous increase in atmospheric CO2 will no longer enhance the land carbon sink and the oceans have already become the only reservoir to cushion the rapidly growing fossil fuel emissions for atmospheric CO2.