The Effects of Anthropogenic Land Cover Change on Global and Regional Climate in the Preindustrial Holocene: A Review

Abstract:

The recent development of anthropogenic land cover change (ALCC) scenarios that cover all or part of the preindustrial Holocene (11,700 BP to ~AD 1850) has led to a number of modelling studies on the impacts of land cover change on climate, using both GCMs and regional climate models. Because most ALCC scenarios arrive at similar estimates of anthropogenic deforestation by the late preindustrial, most models agree that the net biogeophysical effect of ALCC by AD 1850 is regional cooling at mid- to high-latitudes and warming and drying over the tropics and subtropics. In particular, tropical deforestation appears to lead to local amplification of externally forced drought cycles, e.g., from ENSO. The spatial extent of these climate changes varies between models because the choice of ALCC scenario leads to large differences in the initial forcing. Those model studies that considered biogeochemical feedbacks show that the importance of preindustrial CO₂ emissions ranges from being insignificant to larger than the global biogeophysical feedback, depending on assumptions made about potential natural atmospheric CO₂ at the beginning of the Industrial Revolution. While the net magnitude of deforestation is similar among ALCC scenarios at AD 1850, the timing of deforestation varies widely, which, in addition to affecting the inferred importance of biogeochemical feedbacks, leads to large differences in the estimated importance of ALCC on climate earlier in the Holocene. For example, modelling experiments performed on Europe and the Mediterranean representing conditions at the peak of the Roman Empire or in Mesoamerica for the Classic Maya period show large differences in the estimated importance of the biogeophysical feedback to regional climate depending on the ALCC scenario used. The wide variety of results gained so far from ALCC and climate modelling experiments shows that the question of “how much did humans influence the state of the Earth System before the Industrial Revolution?” is far from being resolved. Future improvements to ALCC scenarios that improve thematic resolution to go beyond simple deforestation are essential, for example to include locally important types of historical land use such as irrigation and forest pasture, and Earth System models should move towards coupling between ALCC and climate.