One side effect of increased greenhouse gases and global warming is that vegetation is growing faster. This accelerated growth (greening) impacts the climate system, but so far its overall effects at the global level have not been fully understood.
This article explores how changes in leaf area index (LAI) – a measure of vegetation density - affect local climate, in order to quantify the biophysical impacts of the accelerated greening of the Earth. It uses satellite data of LAI, surface energy fluxes and global climate drivers for the period 1982-2011.
The authors find that changes in surface energy (heat) are closely related to changes in LAI . Increased LAI in boreal and cold temperate regions leads to enhanced absorbed radiation and surface warming, as it reduces the surface albedo (the fraction of solar energy reflected from the Earth back into space). By contrast, increased LAI in warm regions is associated with cooling due to evapotranspiration (evaporation from the soil and other surfaces and transpiration from plants). These effects are found to increase five-fold under extreme climate conditions (e.g. extreme warm and dry, and cold and wet years).
Given this connection between vegetation density and surface energy, the authors investigate the global impacts of the recent boost in greening. They found big differences in the sensitivity of temperature to changes in LAI in different parts of the world. The cold and wet climates of boreal regions, particularly in northern Canada and central Europe, showed significant LAI-related warming due to a reduction in albedo. This was offset by browning in north-eastern America and Eurasia (mainly due to forest disturbances), which had a mild cooling effect.
By contrast, dry regions of the Southern Hemisphere (South Africa, south-eastern America and Australia) show an LAI-related cooling trend, mainly due to daytime evapotranspiration.
The overall impacts of the recent greening on global temperature are limited due to the compensation of opposite local effects across different climate regions. The authors estimate an overall biophysical cooling effect related to long-term changes in LAI, which outweighs the recent estimates of climate warming driven by deforestation. They also found that greening limits the variation in daytime surface temperature, and that positive feedbacks in the land-climate system may amplify the biophysical impacts of variations in LAI on surface energy fluxes.
The findings help explain the contrasting regional climate responses in scenarios of global warming and widespread greening. In cold and humid regions, greening increases surface warming due to variations in albedo, while in warm regions greening has a cooling effect through evapotranspiration. Altogether the recent greening has therefore reduced the spatial variability of temperatures across the Earth.
Greening is amplifying air temperature trends in some biomes. This is of particular concern for cold biomes (e.g. tundra and boreal forest/taiga), where the rapid greening and sensitivity of surface temperature to LAI are contributing to the accelerated warming of ecosystems that are particularly vulnerable to climate change.
The relationships between vegetation cover and surface energy fluxes described in the article may serve as a benchmark for global climate models, as future changes in environmental conditions could alter the dominant mechanisms observed in today's climate. Given that climate change is expected to intensify worldwide, it is likely that greening and climate change will be even more connected and have even greater impacts than they do now. How these biophysical feedbacks will develop will impact the future climate of the Earth.
The authors propose that the biophysical impacts of dynamics of global vegetation on local climate, in particular under extreme weather conditions, should be accounted for in the design of local mitigation and adaptation strategies.
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Satellites reveal contrasting responses of regional climate to the widespread greening of Earth