ETH Zürich researchers find reforestation can help increase rain

Researchers at ETH Zürich have published a new study that's based on observational data from around Europe. Climate researchers have shown for the first time that forests lead to an increase in precipitation. The study claims that if agricultural land were reforested, the precipitation in Europe could increase by more than seven percent.

Climate researchers have known for a long time that forests impact regional climates. Numerous studies show that they usually lower the surface temperatures of the land in summer, helping to adapt to the effects of global warming locally. It has been less clear how forests and the reforestation of agricultural land could impact precipitation locally and regionally.

Scientists on the project looked at precipitation data for over 5800 measuring stations belonging to different measurement networks. The analysis focused on five regions of Europe because of the availability of measured data in the areas. Regions in the study included in and around Great Britain, Germany, the Netherlands, Sweden, and Finland. Pairs of measuring stations were chosen in those regions, with one station in a forested area and another on agricultural land.

The difference between forest cover on the land was at least 20 percent, and the stations had to be placed at a similar elevation and not more than 84 kilometers apart. For the second phase of the study, the team looked at station data using statistical modeling to explain the amount of precipitation and isolate the effect of for station while ruling out other factors that might impact precipitation.

According to the first author of the study, Ronnie Meier, while there were outliers, the data showed a clear trend. In forested areas, precipitation is considerably higher than in agricultural areas. Project team members also found that differences in precipitation were more pronounced in the winter than in summer. Meier's hypothesis that the forest's surface roughness is probably more important than previously thought and is a key factor for increased precipitation.