Modeling the effects of two different land cover change data sets on the carbon stocks of plants and soils in concert with CO2 and climate change

[1] A geographically explicit terrestrial carbon cycle component of the Integrated Science Assessment Model (ISAM) is used to examine the response of plant and soil carbon stocks to historical changes in cropland land cover, atmospheric CO2, and climate. The ISAM model is forced with two different land cover change data sets for cropland: one spatially resolved set based on cropland statistics (Ramankutty and Foley, 1998, 1999) and another regionally specific set based on deforestation rates (Houghton and Hackler, 1999, 2001; Houghton, 1999, 2000, 2003). To our knowledge, this is the first attempt to incorporate Houghton’s regionally specific land cover change data into a spatially resolved terrestrial model. Our model results indicate that globally aggregated land use emissions are not sensitive to the spatially explicit location of the natural vegetation converted for croplands within a region. The ISAM estimated land use emissions based on Houghton’s data were substantially higher during the 1980s than those based on Ramankutty and Foley’s data. Although our results are consistent with previous model results, they do not support the ideas that the differences between the two land use emissions for cropland changes can either be related to modeling framework or global land use practices. This study suggests that differences between the two sets of land use fluxes are primarily due to the differences in the rates of changes in land area amount for croplands. The ISAM model estimates a larger contribution to net CO2 uptake from CO2 fertilization ( 2.0 GtC/yr), and a smaller contribution from biospheric CO2 release due to the climate effect (0.7 GtC/yr) during the 1980s. The negative CO2 fertilization feedback is most pronounced in the tropics and midlatitudes, whereas the positive temperature effect on CO2 uptake is most pronounced in the high-latitude regions of the Northern Hemisphere. The ISAM estimated land use emissions due to land cover changes for croplands and pasturelands during the 1980s vary between 1.60 and 2.06 GtC/yr. Most importantly, Intergovernmental Panel on Climate Change estimates based on the CO2 and O2 data indicate that terrestrial ecosystems become a sink for atmospheric CO2 in the 1980s ( 0.2 ± 0.7 GtC/yr), whereas they remain a source in simulations based on the ISAM (0.63 ± 0.20 GtC/yr). Our results leave open the possibility that the discrepancy in the magnitudes of the modeled and data-based net terrestrial uptakes for the 1980s decade reflect weaknesses in the terrestrial biosphere model and/or uncertainties in the land cover, O2 data, or data-based estimates.