Global climate response to idealized deforestation in CMIP6 models

Abstract

Abstract. 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 in a consistent setup, nine Earth system models (ESMs) carried out an idealized experiment in the framework of the Coupled Model Intercomparison Project, phase 6 (CMIP6). Starting from their pre-industrial state, models linearly replace 20×106 km2 of forest area in densely forested regions with grasslands over a period of 50 years followed by a stabilization period of 30 years. Most of the deforested area is in the tropics, with a secondary peak in the boreal region. The effect on global annual near-surface temperature ranges from no significant change to a cooling by 0.55 ∘C, with a multi-model mean of -0.22±0.21 ∘C. 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∘ N, with a multi-model mean of 23∘ 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 observed changes emerge first in the centre of deforestation in tropical regions and propagate edges, indicating the influence of non-local effects. The biogeochemical effect of deforestation are 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 a warming by 0.46 ± 0.22 ∘C, 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. Future analyses of the global deforestation experiments could further explore the effect on changes in seasonality of the climate response as well as large-scale circulation changes to advance our understanding and quantification of deforestation effects in the ESM frameworks.

The biogeophysical effects of idealized land cover and land management changes in Earth System Models

“…A similar but opposite pattern is also apparent in the cropland expansion experiment with CESM (Figure 5b), but appears as a warming blob with lower magnitude. The same warming blob was also found in the LUMIP deforest-glob experiments by Boysen et al (2020). A where the local effects are generally small.…”

“…This is in strong contrast to EC-EARTH, where an increase in latent heat flux causes a cooling which is offset by a stronger decrease in sensible heat flux (Figure 9c). This is most likely caused by over productive cropland in the tropics in EC-EARTH as was also found for grasslands in Boysen et al (2020). The simulated decrease in sensible heat flux in MPI-ESM reduces the heat transport away from the surface, therefore amplifying the warming, while in CESM an increase in sensible heat contributes to a cooling.…”

“…The near-surface air temperature in MPI-ESM is relatively insensitive to deforestation except north of 40°N as was also shown in (Winckler et al, 2019c). However, it should be considered that near-surface air temperature is a highly contested measure as its definition tends to vary strongly across different ESMs, especially over grid cells or grid cell fractions covered with tall vegetation (Boysen et al, 2020;Winckler et al, 2019c). Therefore, in the remainder of this study, we will focus on the response of LCLMC on surface temperature, while the maps for near-surface air temperature are added in 335 appendix D for reference.…”

“…A first set of studies attempted to use Earth System Models (ESMs) to understand the global effects of land cover change, both in idealised (Davin and de Noblet-Ducoudre, 2010;Boysen et al, 2020;Meier et al, 2021) and in more realistic setups (Pitman et al, 2009;Pongratz et al, 2010;Boisier et al, 2012;Ito et al, 2020). However, these studies only show aggregated effects of the biogeophysical processes highlighted above and no direct separation is made between effects caused by local and non-local processes.…”

CM2Mc-LPJmL v1.0: biophysical coupling of a process-based dynamic vegetation model with managed land to a general circulation model

“…However, a functioning biosphere ensures stable energy, carbon and water cycles; hence, the atmospheric composition and radiative forcing are maintained. While plants sequester carbon dioxide (CO 2 ), they also contribute to water cycling, albedo and roughness length, influencing the exchange of energy on multiple timescales (Green et al, 2017;Chapin et al, 2008; Published by Copernicus Publications on behalf of the European Geosciences Union.…”