Thursday, July 8, 2021

Bringing Back Forests Could Boost Summer Rainfall Across Europe by 7.6%

Converting agricultural land to forests is one way of boosting rainfall and counteracting some of the effects of climate change, according to a new study – with water availability one of the most important climate drivers alongside temperature.

Based on computer models trained on real-world data, the study authors conclude that "a realistic reforestation scenario, constrained by sustainability safeguards" would be enough to increase summertime rainfall across Europe by 7.6 percent per year.

That's based on converting 20 percent of the available land, as judged by satellite mapping techniques, with the extra rain not only falling on the forests themselves but also downwind – particularly in the summer months.

"Our results imply that forestation could trigger substantial changes in precipitation over Europe," write the researchers in their published paper.

The good news also comes with a warning – that heavier winter rainfall across the continent could become more intense with these designated areas converted to forests. It's something that needs careful consideration in the future, the team says.

While it's well established that forested areas tend to have more rainfall, the mechanisms for this have not been fully explained. Vapor-emitting tree leaves certainly contribute to air moisture as part of the photosynthesis cycle (a process known as evapotranspiration), but there's more going on.

In this study, the team hypothesizes that forests, with their increased surface roughness compared to agricultural land, also have an effect on air turbulence, causing precipitating air masses to slow down and linger in place.

Additionally, forested areas lead to warmer land surfaces during winter, and cooler ones during summer. "This could explain the seasonal cycle of the local signal we observe as warmer temperatures at the land surface destabilize the planetary boundary layer, thereby favoring the creation of precipitation," the researchers write.

Based on what we know about trees and rainfall though, adding more to the mainland in certain areas might help mitigate some of the drying summertime trends expected over Europe in the coming years, as the globe continues to get hotter.

"Probably the most threatening climate change signal that we expect in relation to precipitation, is this decrease in summer precipitation that is expected in the southern parts of Europe like the Mediterranean," environmental scientist Ronny Meier, from ETH Zurich in Switzerland, told BBC News.

"And there, according to our study, forestation would lead to an increase in precipitation. So the forestation would probably be very beneficial in terms of adapting to the adverse effects of climate change."

While the research leaves some questions unanswered, and the results are based on estimations in the modeled data, the study authors are calling for more attention to be paid to the influence that changing land use has on the climate over the long term.

Adding more trees to the environment is almost always a good idea: besides potentially increased precipitation, reforestation has also been linked to carbon capture, increased biodiversity, and better soil protection.

However, it's worth bearing in mind that no solution to the climate crisis is as effective as substantially reducing the amount of greenhouse gases we're pumping into the atmosphere - reducing emissions needs to remain our top priority.

"Adding new trees or restoring lost forests can never compensate for the greenhouse gas emissions arising from the burning of fossil fuels," climatologist Wim Thiery, from the Free University of Brussels in Belgium and who wasn't involved in the study, told BBC News. "We need to stop generating those emissions in the first place.

But cutting back on our emissions won't be enough: we will also need to actively remove carbon from the atmosphere should we wish to stay below 1.5 °C of warming.

The research has been published in Nature Geoscience.

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