Expanding the forest cover in Europe could enhance rainfall and partly counteract future drying trends expected with climate change, according to a new study. Researchers found that converting agricultural land to forest would increase summer rains by 7.6% on average, based on observations of existing patterns. 

Many countries around the world have started planting more trees in an effort to tackle rising global temperatures. The Bonn Challenge, signed and backed by 48 nations, pledged to restore 350 million hectares of forest by 2030. China has already spent $100 billion on trees in the last decade alone, with 22% of the country now covered in forest. The idea is that a large-scale reforestation effort could influence the local climate and rainfall patterns by altering the transfer of water, energy, and momentum between the land surface and the atmosphere, while also providing shade and numerous other environmental services. Nevertheless, the extent to which reforestation may affect rainfall is largely unknown. With this in mind, a group of researchers decided to explore the issue further. 

They considered the impact of converting rain-fed agricultural land to forests across Europe. They used an observation-based statistical model to assess how changes to forests would impact rainfall across the continent. They found that converting agricultural land to forests triggers substantial changes, tending to increase rainfall in most areas. 

More trees, more rain

Researchers analyzed the effects a 20% increase in forest spread uniformly across Europe would have — a change they described as a realistic figure. They found that it would boost local rainfall, especially in winter and with greater impacts in coastal regions. Planting trees would also cause impacts downwind of the reforested areas, as the rainfall in these locations was increased especially in the summer months. 

Taking the two impacts together, the reforestation campaign could increase summertime rainfall in Europe by an average of 7.6%, partly offsetting drying trends that are expected with climate change, the researchers found. But on the other hand, the expansion of forests would worsen climate-induced intensification of rainfall in winter.

“Probably the most threatening climate change signal that we expect in relation to percipitation, is this decrease in summer precipitation that is expected in the southern parts of Europe like the Mediterranean,” Ronny Meier, lead author and researcher at ETH Zurich, told BBC News. 

The conclusions get even more complex, as the authors note that increased rainfall could also have negative impacts, boosting rainfall patterns that have already been affected by climate change — especially in the Atlantic region. The reasons aren’t fully clear yet, but the team points out that the cloudy air that produces the rain tends to stay longer over forested areas. 

“A forest is a much rougher surface than agricultural land. So, it induces more turbulence at the land-atmosphere interface, and also, the forest exerts more drag on to the atmosphere than agricultural land,” said Meier. “This higher turbulence over the forests is probably the main reasons for the fact that we find more precipitation in regions with more forests.”

Overall, the impact of trees on rainfall is expected to be positive, but it can have complex ramifications.

Although humans have massively expanded their reach across the planet, there is still enough room to accommodate 0.9 billion hectares of forest, according to a study from 2019. Currently, there are an estimated three trillion trees sucking CO2 from the atmosphere all around the globe, with enough room to plant between 1 to 1.5 trillion more trees — a possible growth of up to 50%. However, it’s important to note that trees alone won’t save us from climate change, and the only thing that is guaranteed to help us is reducing our greenhouse gas emissions.

The study was published in the journal Nature.