The amount of Earth’s green cover (measured as Leaf Area Index[6]) has increased globally since 1980, especially in northern latitudes, where growing seasons have lengthened. This is due mainly to increasing CO2 concentration, but also to warmer temperatures and changing precipitation patterns, nitrogen deposition, and land-use change (such as afforestation in China). Higher ambient CO2 can stimulate photosynthesis and reduce water loss, but the extent of the CO2 fertilization depends on the availability of other key nutrients (nitrogen and phosphorus) and water. Warmer temperatures due to climate change have increased greening in northern latitudes by extending the growing season, but diminished greening in the tropics, where temperatures were already optimal.
Greater global green cover has observable feedbacks on climate and the carbon cycle. In addition to offsetting 28% of anthropogenic emissions since 1980, vegetation affects hydrological cycles and air-surface temperatures. Since the 1980s, increased global evapotranspiration (ET) is mainly attributable to increased global greening. Higher transpiration rates from vegetation can reduce or enhance groundwater storage locally, depending on how the atmospheric moisture generated through evapotranspiration is recycled into rain and where that rain falls. In the world’s great rainforests, vegetation preserves groundwater.
The enhanced precipitation over transpiring regions is particularly evident in moist forests like the Amazon or Congo, which are ‘closed’ atmospheric systems where 80% of the rainfall originates from upwind ET. Such an efficient atmospheric water recycling mitigates water loss from the soil, sustains inland vegetation and maintains mesic[7] and humid ecosystems [Piao 2019: 9].
Vegetation affects land-surface temperature by way of ET (cooling effect) and albedo (warming or cooling effect, depending on how dark or light the surface is). While the relative strength of ET versus albedo varies by latitude, the net global effect of increasing vegetation cover is one of cooling the land surface.
[6] Leaf Area Index (LAI) is the quantity of leaf area per unit ground surface area; it’s a way to quantify the thickness of a vegetation canopy.
[7] Mesic refers to moderate moisture levels.