Role of land cover changes for atmospheric CO 2 increase and climate change during the last 150 years

Abstract

We assess the role of changing natural (volcanic, aerosol, insolation) and anthropogenic (CO 2 emissions, land cover) forcings on the global climate system over the last 150 years using an earth system model of intermediate complexity, CLIMBER‐2. We apply several datasets of historical land‐use reconstructions: the cropland dataset by Ramankutty & Foley (1999) (R&F), the HYDE land cover dataset of Klein Goldewijk (2001) , and the land‐use emissions data from Houghton & Hackler (2002) . Comparison between the simulated and observed temporal evolution of atmospheric CO 2 and δ 13 CO 2 are used to evaluate these datasets. To check model uncertainty, CLIMBER‐2 was coupled to the more complex Lund–Potsdam–Jena (LPJ) dynamic global vegetation model. In simulation with R&F dataset, biogeophysical mechanisms due to land cover changes tend to decrease global air temperature by 0.26°C, while biogeochemical mechanisms act to warm the climate by 0.18°C. The net effect on climate is negligible on a global scale, but pronounced over the land in the temperate and high northern latitudes where a cooling due to an increase in land surface albedo offsets the warming due to land‐use CO 2 emissions. Land cover changes led to estimated increases in atmospheric CO 2 of between 22 and 43 ppmv. Over the entire period 1800–2000, simulated δ 13 CO 2 with HYDE compares most favourably with ice core during 1850–1950 and Cape Grim data, indicating preference of earlier land clearance in HYDE over R&F. In relative terms, land cover forcing corresponds to 25–49% of the observed growth in atmospheric CO 2 . This contribution declined from 36–60% during 1850–1960 to 4–35% during 1960–2000. CLIMBER‐2‐LPJ simulates the land cover contribution to atmospheric CO 2 growth to decrease from 68% during 1900–1960 to 12% in the 1980s. Overall, our simulations show a decline in the relative role of land cover changes for atmospheric CO 2 increase during the last 150 years.

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