Planting forests can increase rainfall and help fight drought in Europe, study shows

Climate change is expected to bring with it drier summers. But according to a study published in the July edition of the scientific journal Nature Geoscience, converting agricultural land into forests in Europe could boost summer rainfall by an average of 7.6 percent, potentially offsetting some of the effects of global warming.

The authors of the study found that in total 14.4 percent of the European land surface – larger than the area of France – is suitable for reforestation, “with concentrations over the British Isles, western and southern France, Portugal, Italy and eastern Europe”.

The researchers used an observation-based continent-scale statistical model to show that converting agricultural land in Europe triggers substantial changes in rainfall across the continent, not only locally, but also downwind from the forests.

To determine which lands are suitable, the researchers used a “global reforestation potential map”, which took into consideration sustainability safeguards, only including areas where reforestation would not threaten food and fibre security.

For example, “reforestation can only happen over grazing lands and not over crop lands, under the assumption that in the future, people will consume less animal products and we will therefore have grazing lands available,” Ronny Meier, the study’s lead author, of the ETH Zurich, explained to FRANCE 24.

Where the wind blows

Reforestation requires careful planning, not only because trees take decades to grow, but also because planting forests in one region can affect the precipitation in another region, depending on a variety of factors.

In the southern half of France, 28 percent of the land area has reforestation potential, with a possible precipitation increase of 0.54 mm/day (24 percent) in winter and 0.23mm/day (13 percent) in summer, Meier said Friday.

“It’s important to note, however, that these numbers do not represent the effect from reforestation in these regions alone. For example, the precipitation increase in northern France might be influenced by reforestation in southern France or the UK,” he said.

One cause of the increase in precipitation is that forests evaporate more water than agricultural land, particularly in the summer, likely causing rainfall in the direction in which the wind blows.

Another important factor, according to the paper, is the rough surface of forests. “Forests typically have a higher surface roughness than agricultural land, inducing more turbulence and slowing movement of precipitating air masses”, or heavy clouds, causing them to rain locally over the forests. (This increased surface roughness exists also in urban areas with high buildings and has contributed to a rise in rainfall over and near cities.)

The surface roughness factor, however, might also contribute to a drop in precipitation in certain regions downwind from forests in winter, Meier said: “It could have the effect that moist-air masses or frontal systems that come from the ocean, from the west, are slowed down by forests and cannot propagate very well into the interior of the continent.”

The report’s authors warn, too, that massive reforestation “could also have potentially adverse effects by further intensifying climate change-induced precipitation increases in the Atlantic region”.

Barking up the right trees

Another consideration in reforestation is the species of the trees planted, Meier says. “It’s important to keep in mind whether the forests will be resilient to the impacts of climate change. For example, in the Mediterranean area, temperatures might easily rise by another two to three degrees Celsius. Can the forest still exist sustainably, or will it be threatened and maybe die in case of a massive heatwave?”

The study might offer incentive for policy makers to pursue reforestation.

“As scientists, our task is to provide information so that policymakers can take well-informed decisions,” Meier said.

“In this research, we focused more on the benefits of reforestation. But our study can also be seen as a bit of a warning to try and preserve the forests we have, because the effects could also go in the other direction, and once we start losing forests – and precipitation decreases – that will lead to the loss of even more forests.”