Abstract
The impact of land use on the global carbon cycle and climate is assessed. The Bern carbon cycle-climate model was used with land use maps from HYDE3.0 for 1700 to 2000 A.D. and from post-SRES scenarios for this century. Cropland and pasture expansion each cause about half of the simulated net carbon emissions of 188 Gt C over the industrial period and 1.1 Gt C yr-1 in the 1990s, implying a residual terrestrial sink of 113 Gt C and of 1.8 Gt C yr-1, respectively. Direct CO2 emissions due to land conversion as simulated in book-keeping models dominate carbon fluxes due to land use in the past. They are, however, mitigated by 25% through the feedback of increased atmospheric CO2 stimulating uptake. CO2 stimulated sinks are largely lost when natural lands are converted. Past land use change has eliminated potential future carbon sinks equivalent to emissions of 80–150 Gt C over this century. They represent a commitment of past land use change, which accounts for 70% of the future land use flux in the scenarios considered. Pre-industrial land use emissions are estimated to 45 Gt C at most, implying a maximum change in Holocene atmospheric CO2 of 3 ppm. This is not compatible with the hypothesis that early anthropogenic CO2 emissions prevented a new glacial period.
Highlights
Past and current land use and land use changes (LULUC) contribute to the ongoing anthropogenic climate change (Forster et al, 2007)
While land use change has many socio-economic and climatic consequences, here, we are interested in the impact of LULUC on the global carbon cycle and atmospheric CO2 and CO2 related climatic changes over the industrial period and the future
The goals of this study are: (i) to estimate carbon emissions from LULUC over the industrial period and the past decades in the Bern Carbon Cycle-Climate (BernCC) model framework, thereby contributing to the ongoing assessment of the magnitudes und uncertainties of LULUC induced carbon fluxes; (ii) to make an appraisal of the potential impact of pre-industrial LULUC on atmospheric CO2 and climate; (iii) to project atmospheric CO2 and climate in three new scenarios for 21st century land use and emissions of CO2 and other anthropogenic forcing agents; (iv) to quantify the different environmental feedbacks and interactions of LULUC with past and future atmospheric CO2 employing a range of factorial model experiments and (v) to assess the impact of past LULUC on future atmosphere-land carbon fluxes, atmospheric CO2, and climate
Summary
Past and current land use and land use changes (LULUC) contribute to the ongoing anthropogenic climate change (Forster et al, 2007). While land use change has many socio-economic and climatic consequences, here, we are interested in the impact of LULUC on the global carbon cycle and atmospheric CO2 and CO2 related climatic changes over the industrial period and the future. Uncertainties in the quantitative understanding of the impact of LULUC on the global carbon cycle lead to uncertainties in projections of atmospheric CO2 and climate, and affect the formulation of emission mitigation strategies. Carbon fluxes due to LULUC constitute the least well quantified flux in the global carbon budget (Pacala et al, 2001; Prentice et al, 2001; Goodale et al, 2002; Houghton et al, 2004; Denman et al, 2007)
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