Changes to the Earth’s energy budget due to global forestation and deforestation affect remote climate via adjusted atmosphere and ocean circulation

Recent controversies about the climatic consequences of forestation and deforestation have centered on the carbon storage potential of forests and the local or global thermodynamic impacts due to biogeophysical effects. So far, not much attention has been given to the changes that biogeophysical effects of forestation and deforestation impose on the atmospheric and ocean circulation and consequently on remote weather and climate.  Here we discuss how the changes in the Earth's energy balance following global-scale forestation and deforestation alter the global atmospheric circulation patterns and even have profound effects on the ocean circulation. We perform multicentury coupled climate model simulations in which preindustrial vegetation cover is either completely forested or deforested and carbon dioxide mixing ratio is kept constant. Forestation leads to global warming of +0.5 K, which is most pronounced over northern extratropical land. Consequently, the meridional heat transport in the Northern Hemisphere decreases in the forestation simulation. The reduction mainly occurs in the ocean as a result of a weakened Atlantic meridional overturning circulation (AMOC). Extratropical land-warming results further in weaker and poleward shifted weather systems, which, via momentum feedback to the mean flow leads to an attenuation and poleward displacement of the extratropical jet stream. Deforestation leads to global cooling of -1.6 K, a stronger AMOC and extratropical jet stream, a southward shift of the intertropical convergence zone and a stronger Hadley cell in boreal winter, and a weaker Hadley cell in boreal summer. In many aspects, deforestation causes the reverse patterns compared to forestation but with larger amplitudes. These larger amplitudes are mostly related to a strong snow-ice-albedo feedback in high latitudes. Both land surface changes substantially affect regional precipitation, temperature, and surface wind patterns across the globe. The design process of large-scale forestation projects thus needs to take into account global circulation adjustments and their influence on remote climate.