Abstract
The terrestrial ecosystems of North America have been identified as a sink of atmospheric CO2 though there is no consensus on the magnitude. However, the emissions of non-CO2 greenhouse gases (CH4 and N2O) may offset or even overturn the climate cooling effect induced by the CO2 sink. Using a coupled biogeochemical model, in this study, we have estimated the combined global warming potentials (GWP) of CO2, CH4 and 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, CH4 and N2O fluxes was −0.50 ± 0.27 Pg CO2 eq/year (1 Pg = 1015 g) in North American terrestrial ecosystems during 2001–2010. The emissions of CH4 and N2O from terrestrial ecosystems had offset about two thirds (73 %±14 %) of the land CO2 sink 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 CO2 concentration 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10584-014-1072-9) contains supplementary material, which is available to authorized users.
Highlights
Radiative forcing of climate is increasing at unprecedented rates in Earth’s atmosphere, largely due to rapid increases in the atmospheric concentrations of greenhouse gases (GHGs) such as CO2, CH4, and N2O (Forster et al 2007)
Since close linkages exist among CO2, CH4 and N2O fluxes and one gas flux altered by environmental forces would affect the other two, a systems approach incorporating all three GHGs would be needed to provide an accurate estimate on global warming potential (GWP) (Lu and Tian 2013)
Based on results and data from the North American Carbon Program (NACP) Regional Interim Synthesis (Huntzinger et al 2012; Hayes et al 2012), Non-CO2 GHG regional interim synthesis (Tian et al 2012b) and model simulations with the Dynamic Land Ecosystem Model (DLEM), this study intends to: 1) estimate the overall GWP of CO2, CH4, and N2O fluxes in the terrestrial ecosystems of North America; 2) quantify the relative contributions of individual environmental factors to GWP changes during recent 32 years (1979– 2010); and 3) identify gaps and uncertainties in existing estimates of the GHG balances for improving climate prediction and guiding climate change policy-making in North America
Summary
Radiative forcing of climate is increasing at unprecedented rates in Earth’s atmosphere, largely due to rapid increases in the atmospheric concentrations of greenhouse gases (GHGs) such as CO2, CH4, and N2O (Forster et al 2007). The fluxes of CO2, CH4, and N2O could be greatly influenced by multiple environmental changes, such as climate, nitrogen deposition, CO2 fertilization, land use, and land management practices (e.g., nitrogen fertilizer uses and irrigation) Roles of these driving forces on North American CO2 balance are extensively studied based on the long-term experiments and observations (e.g., FLUXNET sites), remote sensing data, and modeling synthesis activities (Xiao et al 2011; Hayes et al 2012; Huntzinger et al 2012), but only a few studies have investigated their roles on the fluxes of CH4 and N2O (e.g., Tian et al 2010, 2012b; Xu et al 2010, 2012). Based on results and data from the North American Carbon Program (NACP) Regional Interim Synthesis (Huntzinger et al 2012; Hayes et al 2012), Non-CO2 GHG regional interim synthesis (Tian et al 2012b) and model simulations with the Dynamic Land Ecosystem Model (DLEM), this study intends to: 1) estimate the overall GWP of CO2, CH4, and N2O fluxes in the terrestrial ecosystems of North America; 2) quantify the relative contributions of individual environmental factors to GWP changes during recent 32 years (1979– 2010); and 3) identify gaps and uncertainties in existing estimates of the GHG balances for improving climate prediction and guiding climate change policy-making in North America
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