The effects of past land-cover changes on climate are disputed1,2,3. Previous modelling studies have generally concluded that the biogeophysical effects of historical deforestation led to an annual mean cooling in the northern mid-latitudes3,4, in line with the albedo-induced negative radiative forcing from land-cover changes since pre-industrial time reported in the most recent Intergovernmental Panel on Climate Change report5. However, further observational and modelling studies have highlighted strong seasonal and diurnal contrasts in the temperature response to deforestation6,7,8,9,10. Here, we show that historical deforestation has led to a substantial local warming of hot days over the northern mid-latitudes—a finding that contrasts with most previous model results11,12. Based on observation-constrained state-of-the-art climate-model experiments, we estimate that moderate reductions in tree cover in these regions have contributed at least one-third of the local present-day warming of the hottest day of the year since pre-industrial time, and were responsible for most of this warming before 1980. These results emphasize that land-cover changes need to be considered when studying past and future changes in heat extremes, and highlight a potentially overlooked co-benefit of forest-based carbon mitigation through local biogeophysical mechanisms.
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We acknowledge partial support from the European Union through the projects FP7 EMBRACE (grant agreement 282672), H2020 CRESCENDO (grant agreement 641816), FP7 EUCLEIA (grant agreement 607085) and ERC DROUGHT-HEAT (contract 617518), as well as from the German Research Foundation's Emmy Noether Program. We also acknowledge the World Climate Research Programme’s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank members of the climate modelling groups who took part in this project for producing and making available their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We also thank C. Jones, V. Avora, I. Bethke and D. Lawrence for providing additional data from CMIP5 simulations, and we are very grateful to U. Beyerle for management of the CMIP5 database at ETH. Finally, we thank X. Lee and colleagues for making the observational data available.
Supplementary Material S1, Supplementary Material S2 (Table S1), Supplementary Material S3 (Table S2), Supplementary Figures S1 and S2, and Supplementary References