Impacts of urbanization on land-atmosphere carbon exchange within a metropolitan area in the USA

Urbanization can cause changes in carbon fluxes, which, in turn, impacts atmospheric carbon dioxide (CO2) concentrations and possibly global surface temperatures. Using the Atlanta, Georgia, region as a case study, this paper explores the impact of urban expansion from 1973 to 2002 on land-atmosphere carbon exchange. The major objectives were to estimate net ecosystem production (NEP) values for multiple land-cover classes and to link urbanization-induced changes in land-cover to changes in NEP and overall carbon fluxes. The principal data were daily climatic data, year-specific land-cover data, annual net ecosystem exchange (NEE) values, and annual anthropogenic carbon emissions estimates. The principal methods were testing for climatic trends, determining the composition of the land-cover classes, estimating annual NEP values for the land-cover classes, and estimating the overall carbon exchange. The major findings: (1) there were no significant trends for any of the climatic variables; (2) the region was only similar to 16% urbanized in 1973; however, by 2002, the region was similar to 38% urbanized; (3) the NEP in 1978-1980 of 443 g C m(-2) yr(-1) may have continued until 1996-1998, despite the substantial loss of forest land; and (4) net carbon emissions increased from similar to 150 g in 1978-1980 to similar to 940 g C m-2 yr-1 in 1996-1998. Therefore, urban expansion greatly increased the carbon emissions of the Atlanta region; however, it is possible that, through increasing the growing-season length as well as increasing nitrogen and CO2 fertilization, urban expansion may not decrease the region-wide NEP.