title: Separating the influence of temperature, drought, and fire on interannual variability in atmospheric CO2.
abstract: The response of the carbon cycle in prognostic Earth system models (ESMs) contributes significant uncertainty to projections of global climate change. Quantifying contributions of known drivers of interannual variability in the growth rate of atmospheric carbon dioxide (CO2) is important for improving the representation of terrestrial ecosystem processes in these ESMs. Several recent studies have identified the temperature dependence of tropical net ecosystem exchange (NEE) as a primary driver of this variability by analyzing a single, globally averaged time series of CO2anomalies. Here we examined how the temporal evolution of CO2in different latitude bands may be used to separate contributions from temperature stress, drought stress, and fire emissions to CO2variability. We developed atmospheric CO2patterns from each of these mechanisms during 1997-2011 using an atmospheric transport model. NEE responses to temperature, NEE responses to drought, and fire emissions all contributed significantly to CO2variability in each latitude band, suggesting that no single mechanism was the dominant driver. We found that the sum of drought and fire contributions to CO2variability exceeded direct NEE responses to temperature in both the Northern and Southern Hemispheres. Additional sensitivity tests revealed that these contributions are masked by temporal and spatial smoothing of CO2observations. Accounting for fires, the sensitivity of tropical NEE to temperature stress decreased by 25% to 2.9 ± 0.4 Pg C yr-1K-1. These results underscore the need for accurate attribution of the drivers of CO2variability prior to using contemporary observations to constrain long-term ESM responses.
authors.affiliation: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan Ann Arbor, Michigan, USA.
authors.forename: Gretchen
authors.initials: G
authors.lastname: Keppel-Aleks
authors.affiliation: Department of Earth and Environmental Science, University of Michigan Ann Arbor, Michigan, USA.
authors.forename: Aaron S
authors.initials: AS
authors.lastname: Wolf
authors.affiliation: Department of Earth System Science, University of California Irvine, California, USA.
authors.forename: Mingquan
authors.initials: M
authors.lastname: Mu
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution Woods Hole, Massachusetts, USA.
authors.forename: Scott C
authors.initials: SC
authors.lastname: Doney
authors.affiliation: Biospheric Sciences Laboratory, NASA Goddard Space Flight Center Greenbelt, Maryland, USA.
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Morton
authors.affiliation: Nicholas School of the Environment, Duke University Durham, North Carolina, USA.
authors.forename: Prasad S
authors.initials: PS
authors.lastname: Kasibhatla
authors.affiliation: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder Boulder, Colorado, USA ; NOAA Earth System Research Laboratory, Global Monitoring Division Boulder, Colorado, USA.
authors.forename: John B
authors.initials: JB
authors.lastname: Miller
authors.affiliation: NOAA Earth System Research Laboratory, Global Monitoring Division Boulder, Colorado, USA.
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Dlugokencky
authors.affiliation: Department of Earth System Science, University of California Irvine, California, USA.
authors.forename: James T
authors.initials: JT
authors.lastname: Randerson
keywords: atmospheric CO2; carbon cycle; climate variability; drought; fire; terrestrial ecosystems