From the EUROPEAN COMMISSION JOINT RESEARCH CENTRE and the “Dr. Roger Pielke Sr. was right” department. I suspect a whole bunch of climate models that don’t take this into consideration, and think CO2 is the dominant climate driver, are going to need to be revised.
Land use change has warmed the Earth’s surface
Natural ecosystems play a crucial role in helping combat climate change, air pollution and soil erosion. A new study by a team of researchers from the Joint Research Centre, the European Commission’s science and knowledge service, sheds light on another, less well-known aspect of how these ecosystems, and forests in particular, can protect our planet against global warming.
The research team used satellite data to analyse changes in global vegetation cover from 2000 to 2015 and link these to changes in the surface energy balance. Modifying the vegetation cover alters the surface properties – such as the amount of heat dissipated by water evaporation and the level of radiation reflected back into space – which has a knock-on effect on local surface temperature. Their analysis reveals how recent land cover changes have ultimately made the planet warmer.
“We knew that forests have a role in regulating surface temperatures and that deforestation affects the climate, but this is the first global data-driven assessment that has enabled us to systematically map the biophysical mechanisms behind these processes”, explains Gregory Duveiller, lead author of the study.
The study also looked beyond deforestation, analysing changes between different types of vegetation, from evergreen forests to savannas, shrublands, grasslands, croplands and wetlands. However, they found that the removal of tropical evergreen forest for agricultural expansion is the vegetation cover transition most responsible for local increases in surface temperature.
From a greenhouse gas perspective, the cutting of forests might only affect the global climate in the mid-to-long term. However, the scientists point out that local communities living in areas where the trees are cut will immediately be exposed to rising temperatures.
The mark of vegetation change on Earth’s surface energy balance
Changing vegetation cover alters the radiative and non-radiative properties of the surface. The result of competing biophysical processes on Earth’s surface energy balance varies spatially and seasonally, and can lead to warming or cooling depending on the specific vegetation change and background climate. Here we provide the first data-driven assessment of the potential effect on the full surface energy balance of multiple vegetation transitions at global scale. For this purpose we developed a novel methodology that is optimized to disentangle the effect of mixed vegetation cover on the surface climate. We show that perturbations in the surface energy balance generated by vegetation change from 2000 to 2015 have led to an average increase of 0.23 ± 0.03 °C in local surface temperature where those vegetation changes occurred. Vegetation transitions behind this warming effect mainly relate to agricultural expansion in the tropics, where surface brightening and consequent reduction of net radiation does not counter-balance the increase in temperature associated with reduction in transpiration. This assessment will help the evaluation of land-based climate change mitigation plans.
Our novel approach adopts the space-for-time logic to multi-scale remote sensing products to quantify the potential effect that a complete transition from one vegetation class to another would have on the individual components of the surface energy balance and on the resultant change in land surface temperature. This information is spatially and temporally explicit, enabling us to draw a comprehensive picture of the geographic and seasonal patterns of these potential changes. The resulting data set is freely available and fully described in an accompanying data description publication30. We use this global data set to quantify the total effect on the surface energy balance resulting from all vegetation changes that have occurred during the period 2000–2015, and then translate this effect into a change of 0.23 ± 0.03 °C over the concerned land.
This study makes the first global scale data-driven assessment of how different vegetation changes can influence the surface energy balance. Altogether, our results quantify these influences across different geographic regions and biomes, confirming the need to jointly assess both radiative and non-radiative processes in order to estimate the changes in surface climate induced by land cover change. In particular, this assessment shows that in ecosystems where vegetation growth is limited by water availability the climate impacts of a vegetation cover transition are dominated by changes in evapotranspiration, whereas in ecosystems where vegetation growth is limited by energy, such as boreal shrublands, the perturbation of the surface temperature is dominated by changes in the radiative and aerodynamic properties of those ecosystems. The origin of actual vegetation cover change in the recent past is divided along the same lines, with direct anthropogenic changes (such as agricultural intensification) occurring mostly where evaporation dominates, while changes within natural ecosystems have generally been confined to higher latitudes where radiative and aerodynamic effects prevail.
Our results show that vegetation cover change over the period 2000–2015 has produced on average a brighter but warmer land surface. This apparently contradictory signal is controlled by the three dominant transitions driven by agricultural expansion in mostly tropical regions (from evergreen broadleaf forests, shrublands or deciduous broadleaf forests to cropland, Fig. 5), which each lead to similar increases in albedo, and consequent reductions in absorbed radiation and turbulent energy fluxes. This perturbation of the surface energy balance ultimately produces a counter-intuitive warming of areas with higher albedo because of stronger plant-mediated constraints on evaporative cooling, in accordance with recent findings that prove the central role of non-radiative biophysical effects mediated by evapotranspiration18.
Full paper, open access: https://www.nature.com/articles/s41467-017-02810-8