Global climate response to idealized deforestation in CMIP6 models

Lena Boysen ,P Lawrence ,Matthias Rocher ,Philippe Peylin ,David M Lawrence ,Vivek K Arora ,Yanwu Zhang ,Wim Thiery ,Spencer Liddicoat ,Deborah Lawrence ,Lars Nieradzik ,Julia Pongratz ,Marysa M Laguë ,Nicolas Vuichard ,Victor Brovkin ,Tomohiro Hajima ,Peter Anthoni ,Christine Delire ,Roland Séférian ,Ming Lo

doi:10.5194/bg-17-5615-2020

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.

Highlights

Forests cover about 32 × 106 km2, or about a quarter of the ice-free land surface (Hansen et al, 2010)
Since dynamic vegetation was still switched on outside the deforested areas in UKESM, we considered forest cover changes only until year 50 to exclude forest changes afterwards that originate from outside the study area
MIROC does not simulate specific forest cover and provided a separate deforestation map based on prescribed land cover changes replacing primary with secondary vegetation; regrowth of forest could not be suppressed

Summary

Introduction

Forests cover about 32 × 106 km, or about a quarter of the ice-free land surface (Hansen et al, 2010). A dominant driver of climate change effects is deforestation, as forest replacement with crops and pastures has a strong influence on land surface albedo (reflectivity) and transpiration, and it leads to carbon losses to the atmosphere. While the biogeochemical effects of deforestation, associated with release of carbon to the atmosphere, always lead to a warming at the global scale, biogeophysical effects, associated with changes in energy fluxes, differ in direction and magnitude between tropical and boreal regions (Pongratz et al, 2010). Boreal deforestation generally cools the climate due to increased land surface albedo during the snow season (Bonan, 2008), especially in the spring, when the snow-masking effect of forests strongly affects the net radiation at the surface (Brovkin et al, 2006). Climate consequences of temperate deforestation are intermediate, with possible cooling in spring but warming in summer (Betts, 2000)

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