title: North American terrestrial CO2uptake largely offset by CH4and N2O emissions: toward a full accounting of the greenhouse gas budget.
abstract: The terrestrial ecosystems of North America have been identified as a sink of atmospheric CO2though there is no consensus on the magnitude. However, the emissions of non-CO2greenhouse gases (CH4and N2O) may offset or even overturn the climate cooling effect induced by the CO2sink. Using a coupled biogeochemical model, in this study, we have estimated the combined global warming potentials (GWP) of CO2, CH4and N2O fluxes in North American terrestrial ecosystems and quantified the relative contributions of environmental factors to the GWP changes during 1979-2010. The uncertainty range for contemporary global warming potential has been quantified by synthesizing the existing estimates from inventory, forward modeling, and inverse modeling approaches. Our "best estimate" of net GWP for CO2, CH4and N2O fluxes was -0.50 ± 0.27 Pg CO2eq/year (1 Pg = 1015g) in North American terrestrial ecosystems during 2001-2010. The emissions of CH4and N2O from terrestrial ecosystems had offset about two thirds (73 %±14 %) of the land CO2sink in the North American continent, showing large differences across the three countries, with offset ratios of 57 % ± 8 % in US, 83 % ± 17 % in Canada and 329 % ± 119 % in Mexico. Climate change and elevated tropospheric ozone concentration have contributed the most to GWP increase, while elevated atmospheric CO2concentration have contributed the most to GWP reduction. Extreme drought events over certain periods could result in a positive GWP. By integrating the existing estimates, we have found a wide range of uncertainty for the combined GWP. From both climate change science and policy perspectives, it is necessary to integrate ground and satellite observations with models for a more accurate accounting of these three greenhouse gases in North America.
authors.affiliation: International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849 USA.
authors.forename: Hanqin
authors.initials: H
authors.lastname: Tian
authors.affiliation: International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849 USA.
authors.forename: Guangsheng
authors.initials: G
authors.lastname: Chen
authors.affiliation: International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849 USA.
authors.forename: Chaoqun
authors.initials: C
authors.lastname: Lu
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Xu
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Hayes
authors.affiliation: International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849 USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Ren
authors.affiliation: International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849 USA.
authors.forename: Shufen
authors.initials: S
authors.lastname: Pan
authors.affiliation: School of Earth Sciences and Environmental Sustainability, North Arizona University, Flagstaff, AZ 86011 USA.
authors.forename: Deborah N
authors.initials: DN
authors.lastname: Huntzinger
authors.affiliation: Department of Earth and Planetary Science, Harvard University, 29 Oxford St., Cambridge, MA 02138 USA.
authors.forename: Steven C
authors.initials: SC
authors.lastname: Wofsy