Climate mitigation from vegetation biophysical feedbacks during the past three decades

The surface air temperature response to vegetation changes has been studied for the extreme case of land-cover change1–5; yet, it has never been quantified for the slow but persistent increase in leaf area index (LAI) observed over the past 30 years (Earth greening)6,7. Here we isolate the fingerprint of increasing LAI on surface air temperatureusinga coupled land– atmosphere global climate model prescribed with satellite LAI observations. We find that the global greening has slowed down the rise in global land-surface air temperature by 0.09 ± 0.02 C since 1982. This net cooling e ect is the sum of cooling from increased evapotranspiration (70%), changed atmospheric circulation (44%), decreased shortwave transmissivity (21%), and warming from increased longwave air emissivity (−29%) and decreased albedo (−6%). The global cooling originated from the regions where LAI has increased, including boreal Eurasia, Europe, India, northwest Amazonia, and the Sahel. Increasing LAI did not, however, significantly change surface air temperature in eastern North America and East Asia, where the e ects of large-scale atmospheric circulation changes mask local vegetation feedbacks. Overall, the sum of biophysical feedbacks related to the greening of the Earthmitigated 12%of global land-surfacewarming for the past 30 years. Satellite data show unequivocally that the land surface has been greening for the past 30 years, and that leaf area index (LAI) has increased by 8% globally6,7. This satellite-observed greening of the Earth is supported by increased biomass from long-term forest inventories8, model simulations6,9 and observed enhancement of seasonal exchange of CO2 (ref. 10). The increased photosynthetic removal of CO2 from the atmosphere and consequent carbon sequestration potentially driven by this greening impose a negative forcing on the climate system10,11. Yet this negative forcing from the mitigation of atmospheric CO2 growth could be enhanced or diminished by various biophysical feedbacks1–5,12–15. The biophysical feedbacks are largely controlled by LAI, a variable that regulates the amount of absorbed solar radiation by modifying albedo and the magnitude of evapotranspiration through canopy resistance16. The opposing effects of increased evapotranspiration (cooling) and decreased albedo (warming) for a given increase of LAI (refs 11,15), and further feedbacks through changes in cloud cover17, atmospheric circulation and water recycling18, make unravelling the fingerprint of the greening of the Earth on climate amajor challenge in climate research. Biophysical feedbacks are reasonably well studied for the vegetation variations associated with land-use/land-cover change1–5. Previous studies demonstrated that tropical afforestation attenuates warming locally through increasing evapotranspiration, whereas boreal afforestation exacerbates warming through decreasing albedo14. In particular, the Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) has evaluated the radiative forcing of land-surface changes only from the perspective of surface albedo (−0.15± 0.10Wm−2; ref. 1). However, biophysical feedbacks of the observed widespread greening of the Earth during the past three decades6,7 have not been examined to date. Here we quantify the response of landsurface air temperature to this greening during the past 30 years using a coupled land–atmosphere global climate model (GCM). To separate the effects of vegetation biophysical feedbacks from internal variability of the coupled climate system, we performed four simulations using the IPSLCM GCM (ref. 19) prescribed with different observed LAI and ocean sea-surface temperature (SST) distributions (see Supplementary Table 1 and Methods). In experiment (1) the differences between simulations, LAIobs_OCNobs and LAIclim_OCNobs, isolate the fingerprint of observed LAI changes (referred to as 1LAI) on climate (see Methods for details). Figure 1a shows the 1LAI-induced globalmean change in land-surface 2-m air temperature (Ta), derived from the difference between the two simulations. In response to the greening, Ta significantly decreased at a rate of −0.030± 0.006 C per decade (p< 0.001) along with the increase of LAI (0.04± 0.01m2 m−2 per decade, p< 0.001). This trend is very robust in the case of excluding the influence of significant