Forests in different regions have different effects on cloud cover

A recent study shows that cloud cover is increased over most temperate and boreal forests in comparison to non-forest landscapes. However, over the Amazon, Central Africa, and the Southeast US, cloud cover is reduced over forests. The study was led by Ru Xu and Yan Li, at Beijing Normal University, and involved collaboration with Dominick Spracklen, at the University of Leeds, who helped to interpret the study’s results. This research is available with free online access via Nature Communications.

Large-scale and global models have previously linked the presence of forests to increased cloud cover and rainfall, however relatively few studies have considered the links between forests and clouds at smaller scales. Those that have done so suggest small-scale deforestation could actually increase cloud cover. These smaller scales may be particularly important for understanding how more local changes in forest ranges could affect climates within different regions.

The team set out to uncover the ways that forests affect cloud cover around the world, using high resolution imagery from multiple satellites to collect their data.

They found that differences in cloud cover between regions were determined by the exchange of energy and moisture between the surface and atmosphere. In the tropics, forests provide a cooling effect on the landscape but, further from the equator, this effect is reduced and forests can be warmer than the surrounding landscape when closer to the poles. This study found that clouds were more likely to be reduced over tropical forests with a cooling effect and increased over boreal forests with a warming effect.

Ongoing forest cover loss has led to cloud increase over forest loss hotspots in the Amazon (+0.78%), Indonesia (+1.19%), and Southeast US (+ 0.09%), but cloud reduction in East Siberia (-0.20%) from 2002-2018.

In the tropics, forests are cooler than the surrounding landscape. This cool air causes less convection over forests than nearby grassland, resulting in less clouds over forests.
In temperate and boreal forests a different mechanism is more important. Emission of water from forests in a process called transpiration increases humidity and enhances cloud formation over forests.

Visualisation of transpiration as described in the main text

As well as the effects of forests, the study showed that cloud cover can also be influenced by the land’s elevation. However, elevation was only able to explain 30% of cloud variation whereas forest cover explained 41% of cloud increase and 22% of cloud reduction. 7% of cloud cover could be explained by the cumulative effects of a range of other factors.

This research is available with free online access via Nature Communications.