Global greening is playing an increasing, and largely overlooked, role in the delicate balance of the Earth’s energy flow, and hence its climate.
A recent JRC-led study provides novel and robust evidence that vegetation is increasingly controlling the heat and evaporation fluxes at the Earth’s land surface, ultimately leading to an intensification of the terrestrial water cycle, and that this effect is likely to be exacerbated by climate change.
These findings are particularly relevant for climate action and policy, including the European Green Deal – the EU’s bid to lead the world on climate action.
Energy partitioning – how the sun’s energy is used on the Earth’s surface
The recently published Nature Climate Change article describes the interplay between vegetation and climate, by showing the impact of global greening on energy partitioning at the Earth's surface from 1982 to 2016.
When solar radiation reaches the Earth’s surface it is converted into heat (sensible heat flux) or used in the evaporation of water (latent heat flux). How solar energy is divided among these two processes plays a crucial role in the climate system, particularly as it affects global water and energy cycles, leading to changes to air temperature, cloud cover and precipitation.
Recent studies suggest that vegetation density may play an important role in energy partitioning by modulating water and energy exchanges between land and the atmosphere.
This is the first study to quantify how variations in vegetation structure affect this global energy partitioning.
Recent changes in the Earth’s energy balance caused by global greening
Based on an analysis of data-driven products and land surface models, the study shows that the increase in vegetation over the past thirty-five years (1982-2016) favoured the release of energy as latent over sensible heat.
Changes in leaf area index (LAI) were found to lead to a global increase in latent heat of about 3.66 W/m2 per unit of leaf area (particularly in warm and dry regions), and a reduction in sensible heat of about 3.26 W/m2 per unit of leaf area.
Globally, the study found that energy fluxes were 20-24% more sensitive to vegetation in recent years (2000–2016 compared with 1982–1999), which suggests that the impact of global greening on energy fluxes is growing.
The authors highlight that this sensitivity was very likely driven by climate variations. This increased control of vegetation appears plausibly connected to the exacerbation of water-limited conditions and the progressive increase in evaporative surface associated with the global greening.
These findings highlight the important climate control effect exerted by vegetation, particularly during extreme events such as meteorological droughts and heatwaves.
Land surface models – which are used to predict future climate trajectories – misrepresent such vegetation control, possibly due to underestimation of the biophysical response to water availability changes and poor representation of vegetation dynamics.
Croplands have the highest impact on surface energy partitioning
The impact of vegetation on the energy balance depends on both its greening (the increase in vegetation density) and the sensitivity of energy fluxes, which depend on the type of vegetation and climate conditions.
Although forests accounted for most (48%) of the global greening in the observation period, they only accounted for 21-27% of the global effect due to their lower sensitivity levels.
By contrast, natural grasses (which show lower greening rates than forests but have a larger sensitivity) contributed 32–38% to the global effect of greening on the surface energy partitioning.
Croplands, despite the fact that they only represent 25% of the land surface vegetation, contribute 41–43% to the global surface energy partitioning due to their high sensitivity and greening levels, the latter of which is possibly driven by agricultural intensification in many regions of the world.
Relevance for climate policy
The study findings are particularly relevant in the framework of the European Green Deal, since they prove how vegetation is increasingly controlling the heat and evaporation fluxes at the land surface.
An active and climate-oriented management of vegetation is therefore critical to foster land-based mitigation and adaptation strategies and policies.
Increased control of vegetation on global terrestrial energy fluxes
- Publication date
- 16 April 2020