Abstract
Global atmospheric CO₂ distributions were simulated with a chemical transport model (GEOS-Chem) and compared with space-borne observations of CO₂ column density by GOSAT from April 2009 to January 2010. The GEOS-Chem model simulated 3-D global atmospheric CO₂ at 2°×2.5°horizontal resolution using global CO₂ surface sources/sinks as well as 3-D emissions from aviation and the atmospheric oxidation of other carbon species. The seasonal cycle and spatial distribution of GEOS-Chem CO₂ columns were generally comparable with GOSAT columns over each continent with a systematic positive bias of ~1.0%. Data from the World Data Center for Greenhouse Gases (WDCGG) from twelve ground stations spanning 90°S-82°N were also compared with the modeled data for the period of 2004-2009 inclusive. The ground-based data show high correlations with the GEOS-Chem simulation (0.66≤R₂≤0.99) but the model data have a negative bias of ~1.0%, which is primarily due to the model initial conditions. Together these two comparisons can be used to infer that GOSAT CO₂ retrievals underestimate CO₂ column concentration by ~2.0%, as demonstrated in recent validation work using other methods. We further estimated individual source/sink contributions to the global atmospheric CO₂ budget and trends through 7 tagged CO₂ tracers (fossil fuels, ocean exchanges, biomass burning, biofuel burning, net terrestrial exchange, shipping, aviation, and CO oxidation) over 2004-2009. The global CO₂ trend over this period (2.1 ppmv/year) has been mainly driven by fossil fuel combustion and cement production (3.2 ppmv/year), reinforcing the fact that rigorous CO₂ reductions from human activities are necessary in order to stabilize atmospheric CO₂ levels.
Highlights
Understanding the global atmospheric CO2 distribution and budget are important for achieving CO2 emission reduction targets in the 21st century
We focused on the comparison of simulated global CO2 by a 3-D global chemical transport model (GEOS-Chem) with the gases Observing Satellite (GOSAT) retrievals from April 2009 and January 2010
1 GOSAT CO2 Products GOSAT, is the first successful satellite designed to measure the concentrations of atmospheric carbon dioxide and methane with good sensitivity near the surface
Summary
Understanding the global atmospheric CO2 distribution and budget are important for achieving CO2 emission reduction targets in the 21st century. Past studies have used satellite observations of CO2 for constraining fluxes with varying degrees of success (Nassar et al, 2011; Chevallier et al, 2009, 2005), owing to the limited information on CO2 near the Earth’s surface provided by the thermal-infrared satellite measurements used. In January 2009, the Greenhouse gases Observing Satellite (GOSAT) was launched and has observed carbon dioxide (CO2) and methane (CH4) distributions since April 2009 with good observational sensitivity to these gases near the surface. 2. 1 GOSAT CO2 Products GOSAT ( known as Ibuki), is the first successful satellite designed to measure the concentrations of atmospheric carbon dioxide and methane (the two greenhouse gases making the largest contribution to climate change) with good sensitivity near the surface. The details of the products including the retrieval processes and observation results are at http://www.gosat.nies.go.jp/index_e. html
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