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

Abstract

Urbanization can cause changes in carbon fluxes, which, in turn, impacts atmospheric carbon dioxide (CO 2 ) 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 ~16% urbanized in 1973; however, by 2002, the region was ~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 ~150 g in [1978][1979][1980] 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 CO 2 fertilization, urban expansion may not decrease the region-wide NEP.KEY WORDS: Net ecosystem production · NEP · Net ecosystem exchange · NEE · Urban emissions · Carbon · Carbon dioxide · CO 2 Resale or republication not permitted without written consent of the publisherClim Res 30: [201][202][203][204][205][206][207][208][209][210][211][212][213] 2006 concentrations is through changes in net ecosystem production (NEP). NEP can be defined as follows: NEP = GPP -(R a + R h )where GPP is gross primary production, R a is autotrophic respiration (i.e. respiration by vegetation) and R h is heterotrophic respiration (e.g. soil microbial respiration). NEP is measured using biometric methods (i.e. ecological-inventory technique; Barford et al. 2001, Curtis et al. 2002. NEP is similar to net primary production (NPP), with the major disparity being that R h is not used in the calculation of NPP (i.e. NPP = GPP -R a ). NEP is equivalent to net ecosystem exchange (NEE), with NEE having a negative value if NEP has a positive value. NEE is measured using the eddy-covariance method, which employs towermounted instruments to measure trace gas flux densities between the biosphere and the atmosphere (Baldocchi et al. 1996). The longitudinal dimensions of flux footprints for eddy-covariance towers range from 100 m to several km (Schmid 1994), thus flux values are applicable only at the local scale. Potential impacts of urbanization on NEPThere are multiple ways that urbanization can decrease NEP. The pr...

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