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Research Article

Effect of increasing CO2 on the terrestrial carbon cycle

David Schimel, View ORCID ProfileBritton B. Stephens, and Joshua B. Fisher
PNAS first published December 29, 2014; https://doi.org/10.1073/pnas.1407302112
David Schimel
aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91011; and
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  • For correspondence: dschimel@jpl.nasa.gov
Britton B. Stephens
bEarth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO 80301
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  • ORCID record for Britton B. Stephens
Joshua B. Fisher
aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91011; and
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  1. Edited* by Gregory P. Asner, Carnegie Institution for Science, Stanford, CA, and approved November 19, 2014 (received for review April 21, 2014)

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Significance

Feedbacks from terrestrial ecosystems to atmospheric CO2 concentrations contribute the second-largest uncertainty to projections of future climate. These feedbacks, acting over huge regions and long periods of time, are extraordinarily difficult to observe and quantify directly. We evaluated in situ, atmospheric, and simulation estimates of the effect of CO2 on carbon storage, subject to mass balance constraints. Multiple lines of evidence suggest significant tropical uptake for CO2, approximately balancing net deforestation and confirming a substantial negative global feedback to atmospheric CO2 and climate. This reconciles two approaches that have previously produced contradictory results. We provide a consistent explanation of the impacts of CO2 on terrestrial carbon across the 12 orders of magnitude between plant stomata and the global carbon cycle.

Abstract

Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO2 concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO2 effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO2 that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO2. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO2 concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO2 concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.

  • climate feedback
  • carbon budget
  • tropics
  • atmospheric transport

Footnotes

  • ↵1To whom correspondence should be addressed. Email: dschimel{at}jpl.nasa.gov.
  • Author contributions: D.S. and B.B.S. designed research; D.S., B.B.S., and J.B.F. performed research; D.S., B.B.S., and J.B.F. analyzed data; and D.S., B.B.S., and J.B.F. wrote the paper.

  • The authors declare no conflict of interest.

  • ↵*This Direct Submission article had a prearranged editor.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1407302112/-/DCSupplemental.

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Increasing CO2 and the carbon cycle
David Schimel, Britton B. Stephens, Joshua B. Fisher
Proceedings of the National Academy of Sciences Dec 2014, 201407302; DOI: 10.1073/pnas.1407302112

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Increasing CO2 and the carbon cycle
David Schimel, Britton B. Stephens, Joshua B. Fisher
Proceedings of the National Academy of Sciences Dec 2014, 201407302; DOI: 10.1073/pnas.1407302112
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