Warm Spring Reduced Carbon Cycle Impact of the 2012 US Summer Drought

Carbon uptake by terrestrial ecosystems mitigates the impact of anthropogenic fossil fuel emissions but the strength of this carbon sink is highly sensitive to large-scale extreme climate events. In 2012, the US experienced the warmest spring on record and the most severe drought since the 1930s Dust Bowl, resulting in substantial damage for agricultural production. It is crucial to understand the dynamics of such events as warmer temperatures and a higher prevalence of drought are projected in a changing climate. We combined an extensive network of direct ecosystem flux measurements from AmeriFlux with satellite remote sensing and atmospheric inverse modelling to quantify the impact of the warmer spring and summer drought on biosphere-atmosphere carbon and water exchange in 2012. We found that earlier vegetation activity increased spring carbon uptake and compensated for the reduced uptake during the summer drought, which mitigated the impact on net annual carbon uptake. The early phenological development in the Northeast played a major role for the continental-scale carbon balance in 2012. The warm spring also depleted soil water resources earlier and thus exacerbated water limitations during summer. Our results show that the detrimental effects of severe summer drought on ecosystem carbon storage can be mitigated by warming-induced increases in spring carbon uptake. However, the positive carbon cycle effect of warm spring enhances water limitations and can increase summer heating through biosphere-atmosphere feedbacks.