How robust are modeled non-local temperature effects of historical land use changes really?

Historically, large areas across the globe have been affected by deforestation or irrigation expansion. The replacement of forests with agricultural land and increased water availability in irrigated croplands altered the land’s surface properties, leading to influences of biogeophysical changes on near-surface temperature. From limited observations and mostly idealized simulations, we know that sufficiently large alterations of land surface properties can theoretically lead to systematic temperature and precipitation changes outside and even far from the altered areas. Not only the advection of temperature anomalies, but also changes in circulation and ocean feedbacks have been shown to be potential drivers of such non-local responses in single and multi-model studies.

We tested the robustness of non-local temperature signals to internal variability in the fully coupled Community Earth System Model 2 (CESM2) simulations of the historical period (1850 – 2014) with all forcings vs. all-but-land-use-change forcings. Doing so, we first found seemingly robust non-local temperature effects of land use change on the global and regional scale. But when accounting for the sampling of internal variability in the model using a large initial condition ensemble, the global scale signal was found to be indistinguishable from noise. Only regionally in some hotspots, we found robust and historically important non-local temperature signals. Through increasingly rigorous analysis, we reached a partly negative and unexpected but important finding, which may have implications for future assessments of comparably weak or spatially heterogeneous forcings to the Earth system.