Impact of Idealized Deforestation in CMIP6 Models
While there is general agreement across models in their response to deforestation in terms of change in global temperatures and land carbon pools, the underlying changes in energy and carbon fluxes diverge substantially across models and geographical regions. These differences are quantified and assessed within the context of an idealized deforestation experiment that is a core experiment of the Land Use Model Intercomparison Project (LUMIP).
The multimodel assessment of the idealized deforestation experiments indicates that deforestation leads to biophysical warming in the tropics and a cooling in the boreal region, though when the biogeochemical aspects are considered, it is net warming everywhere. Strong model differences continue to be seen across the current generation of Earth System Models.
Changes in forest cover have a strong effect on climate through the alteration of surface biogeophysical and biogeochemical properties that affect energy, water, and carbon exchange with the atmosphere. To quantify biogeophysical and biogeochemical effects of deforestation, nine Earth system models (ESMs) carried out an idealized experiment as outlined in the Land Use Model Intercomparison Project of CMIP6. Starting from their pre-industrial state, models linearly replace 20 million km2 of forest area in densely forested regions with grasslands over a period of 50 years followed by a stabilization period of 30 years. The effect on global annual near-surface temperature ranges from no significant change to cooling by 0.55 K, with a multi-model mean of −0.22±0.21 K. Five models simulate a temperature increase over deforested land in the tropics and a cooling over deforested boreal land. In these models, the latitude at which the temperature response changes sign ranges from 11 to 43 degrees N. A multi-ensemble analysis reveals that the detection of near-surface temperature changes even under such a strong deforestation scenario may take decades and thus longer than current policy horizons. The biogeochemical effect of deforestation is land carbon losses of 259±80 PgC that emerge already within the first decade. Based on the transient climate response to cumulative emissions (TCRE) this would yield warming by 0.46 ± 0.22 K, suggesting a net warming effect of deforestation. Lastly, this study introduces the “forest sensitivity” (as a measure of climate or carbon change per fraction or area of deforestation), which has the potential to provide lookup tables for deforestation–climate emulators in the absence of strong non-local climate feedbacks. While there is general agreement across models in their response to deforestation in terms of change in global temperatures and land carbon pools, the underlying changes in energy and carbon fluxes diverge substantially across models and geographical regions.