Reforestation may help mitigate droughts

Climate researchers have known for a long time that forests affect regional climates. Numerous studies show that forests usually lower the surface temperature of the land in summer, helping to adapt to the effects of global warming locally. However, it is less clear how forests and the reforestation of agricultural land affect precipitation locally and regionally. The research group lead by Sonia Seneviratne, Professor of Land-Climate Dynamics at ETH Zurich, explored this question for the first time with observational data (instead of model data) for Europe.

The researchers looked at precipitation data from over 5,800 measuring stations belonging to different measuring networks. Their analysis focused on five European regions because of the availability of measured data in those areas: regions in and around Great Britain, Germany, the Netherlands, Sweden and Finland. The researchers selected pairs of measuring stations in these regions - one station in a forested area and the other located on agricultural land. The difference in forest cover was at least 20 percent. Further criteria included that the stations are placed at a similar elevation and are not located more than 84 kilometres apart. During a second phase, the team adjusted the paired station data using a statistical model to explain the amount of precipitation. This was to isolate the effect of forestation and rule out other factors that might affect the levels of precipitation.

Big effects especially in winter

"Despite outliers, our data shows a clear trend," explains Ronny Meier, first author of the study recently published in Nature Geoscience and postdoc in the research group of Land-Climate Dynamics. "In forested areas, precipitation is on average considerably higher than in agricultural areas." While the researchers had expected this outcome, they were still surprised that the local effects were much more pronounced in winter than in summer. In fact, the climate researchers were expecting that the evaporation of water that occurs particularly in summer and is promoted by forests, would lead to more precipitation.

Meier's hypothesis: "The surface roughness of forests is probably more important than previously thought and is a key factor for increased precipitation." As a result, forests hold up air masses longer and induce more turbulence, which favours precipitation. Furthermore, forests are warmer in winter than their surroundings. This also promotes precipitation. In addition to the pronounced local effects in winter, the researchers also observed strong non-local effects further away from forested areas in summer. In winter, these non-local effects were mostly observed in coastal areas, whereas they were much weaker in continental and northern Europe.

"Based on these findings, we estimated how much the amount of precipitation in Europe could be influenced by additional reforestation," explains Edouard Davin, a member of the team and climate researcher at ETH Zurich. They based their analysis on the Global Reforestation Potential Map, which shows how much land is potentially available for reforestation projects, excluding arable land, human settlements and Scandinavia, where reforestation might lead to an undesirable local temperature rise.

The results: reforestation of 14.4 percent of the total area included in the study, which corresponds to an area slightly larger than France, would increase average precipitation by as much as 7.6 percent. The amount of precipitation for just over a quarter of Europe's landmass would even increase by more than 10 percent. "The geographical distribution of the additional precipitation is highly variable," explains Meier. "And so far we are unable to say whether reforestation would lead to more frequent or more intense precipitation."

A remedy for droughts?

The findings are not only relevant for researchers, but also for political decisions. Reforestation is currently one of the most discussed measures for reducing CO₂ emissions. The study suggests that such efforts could have additional benefits. This is because climate models have shown that heat waves and droughts in summer will increase in Europe, accompanied by a decrease in precipitation, with the exception of Scandinavia. Governments could counteract this trend through additional reforestation efforts and compensate for the decline in precipitation.

At the same time, Meier warns against too high hopes: "A forest does not grow overnight; it takes 20-30 years." Nor is it clear yet whether young forests, such as those planted ten years ago, have already increased the precipitation locally. Furthermore, he thinks it is important to place the findings in the larger context of local residents' situations and needs. "The increased evaporation caused by adding forests in one location might draw water away from streams and rivers needed for agricultural irrigation elsewhere."

And what about extreme weather events like heavy precipitation, which are becoming more frequent due to global warming just like droughts? Is it possible that such events might even be exacerbated by more precipitation? "One model study for Europe has shown that reforestation tends to counteract extreme precipitation," says Meier. "One can therefore not directly infer from average precipitation to extreme events." There is still a lot of uncertainty about changes in precipitation due to reforestation, however, and the current study is only based on spatial and not on temporal comparisons of precipitation. Meier and his colleagues argue that the interplay of land use and water availability deserves more attention in the climate debate.