Victor Brovkin, Stephen Sitch, Werner von Bloh, Martin Claussen, Eva Bauer and Wolfgang Cramer

Global Change Biology, 10, 1253-1266 (2004)

Potsdam Institute for Climate Impact Research (PIK), Telegrafenberg, P.O. Box 60 12 03, 14412 Potsdam, Germany.

Abstract

We assess the role of changing natural (volcanic, aerosol, insolation) and anthropogenic (CO₂ 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 CO₂ and delta¹³CO₂ 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 CO₂ emissions.

Land cover changes led to estimated increases in atmospheric CO₂ of between 22 and 43 ppmv. Over the entire period 1800-2000, simulated delta¹³CO₂ 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 CO₂. 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 CO₂ 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 CO₂ increase during the last 150 years.

Keywords: atmospheric CO₂ concentration, atmospheric delta¹³CO₂, biogeophysical effects, biosphere atmosphere interaction, earth system modelling, historical land cover changes, interactive carbon cycle