New research finds that reforesting Europe would increase rainfall, fighting a drying trend

Good News Notes:

““Plant more trees” is often the first idea that comes to mind when we think about how to prevent further climate change or at least adapt to its impacts. There are good reasons for this.

Multiple studies have shown that as well as trees being a fantastic way to store carbon dioxide, they offer other benefits, such as a cooling effect in cities, the ability to reduce flood risk and boost biodiversity, among other things.

Our new study in Nature Geoscience shows that trees could also affect rainfall patterns.

We used measurements of rainfall across Europe to investigate what effect forests have on rainfall totals. We know that forests mostly increase local and downwind rainfall in the summer and winter, but the magnitude of this effect varies across regions and seasons.

To identify a realistic reforestation strategy we used the global reforestation potential map. In the area we looked at in our research (most of Europe), 14.4% of the land surface was considered suitable for reforestation, an area larger than France.

Much of Europe could be reforested (green = areas with realistic reforestation potential) (‘Global Reforestation Potential Map’ Griscom et al (2017)CC BY-SA)

We then compared the effect of turning all that land into forest to the precipitation changes in a future scenario in which the world faces intermediate levels of climate change, based on current predictions. While the climate scenario projects wetter winters and drier summers, the inclusion of reforestation could enhance European summertime rainfall by an average of 7.6%, potentially offsetting some of the drying that climate change is projected to cause. However, we also found reforestation may exacerbate the increase in winter rainfall.

In the UK and Ireland for example, where around 37% of the land area has the potential for reforestation, we estimate that reforestation on this scale would increase precipitation by an average of 0.74 mm/day (24%) in winter and 0.48 mm/day (19%) during summer.

Several factors potentially contribute to this. Forests typically have a higher surface “roughness” than agricultural land. This creates more turbulence over the trees and slows the movement of heavy clouds causing them to rain over and downwind of the forests. The same is true of urban areas too – increased surface roughness from buildings can amplify the precipitation over cities and downwind of cities. And forests typically evaporate more water than agricultural land, particularly during the summer season, which likely means more rain….”

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