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Edited by Susan E. Trumbore, Max Planck Institute for Biogeochemistry, Jena, Germany, and approved March 22, 2016 (received for review October 3, 2015)
Significance
Carbon uptake by terrestrial ecosystems mitigates the impact of anthropogenic fossil fuel emissions on atmospheric CO2 concentrations, but the strength of this carbon sink is highly sensitive to large-scale extreme climate events. In 2012, the United States experienced the most severe drought since the Dust Bowl period, along with the warmest spring on record. Here, we quantify the impact of this climate anomaly on the carbon cycle. Our results show that warming-induced earlier vegetation activity increased spring carbon uptake, and thus compensated for reduced carbon uptake during the summer drought in 2012. This compensation, however, came at the cost of soil moisture depletion from increased spring evapotranspiration that likely enhanced summer heating through land-atmosphere coupling.
Abstract
The global terrestrial carbon sink offsets one-third of the world’s fossil fuel emissions, but the strength of this sink is highly sensitive to large-scale extreme events. In 2012, the contiguous United States experienced exceptionally warm temperatures and the most severe drought since the Dust Bowl era of the 1930s, resulting in substantial economic damage. It is crucial to understand the dynamics of such events because warmer temperatures and a higher prevalence of drought are projected in a changing climate. Here, we combine an extensive network of direct ecosystem flux measurements with satellite remote sensing and atmospheric inverse modeling to quantify the impact of the warmer spring and summer drought on biosphere-atmosphere carbon and water exchange in 2012. We consistently find 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 Eastern Temperate Forests 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 results also suggest that the positive carbon cycle effect of warm spring enhances water limitations and can increase summer heating through biosphere–atmosphere feedbacks.
- seasonal climate anomalies
- carbon uptake
- ecosystem fluxes
- biosphere–atmosphere feedbacks
- eddy covariance
Footnotes
- ↵1To whom correspondence should be addressed. Email: sewolf{at}ethz.ch.
↵2Present address: Earth and Environmental Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
Author contributions: S.W., T.F.K., J.B.F., D.D.B., A.R.D., and A.D.R. designed research; S.W. performed research; S.W. analyzed data; S.W., T.F.K., J.B.F., D.D.B., A.R.D., A.D.R., R.L.S., B.E.L., M.E.L., N.A.B., W.P., and I.T.v.d.L.-L. wrote the paper; S.W. compiled the flux datasets; T.F.K. performed the flux data gap-filling and partitioning; J.B.F. compiled and processed the Moderate Resolution Imaging Spectroradiometer (MODIS) and Coupled Model Intercomparison Project Phase 5 (CMIP5) data; A.R.D. performed the CarbonTracker analysis (CT2013B); and W.P. and I.T.v.d.L.-L. performed the CarbonTracker analysis (CTE2014 and CTE2015).
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The eddy-covariance data are available in the AmeriFlux data archive at the Carbon Dioxide Information Analysis Center at the Oak Ridge National Laboratory (cdiac.ornl.gov/ftp/ameriflux/data).
See Commentary on page 5768.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1519620113/-/DCSupplemental.
Warm spring reduced impact of summer drought
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