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Reforestation holds promise for Europe’s increasingly drier summers

The Rietholzbach catchment is a hilly pre-alpine basin in north eastern Switzerland
  • A new study in Nature Geoscience suggests that if all land suitable for reforestation was forested in Europe, average summer rainfall would increase by 7.6%, partially ameliorating drier summers predicted as a result of climate change.
  • While the study is based on all the potentially reforestable land in Europe after accounting for food security and biodiversity, the amount of land people are willing and able to reforest is likely to be lower in practice.
  • As a statistical model, the study helps scientists and policymakers understand the relationship between forests and precipitation and highlight the benefits beyond carbon sequestration.

Plant more trees! This message has been one of the cornerstones of the European response to worsening climate change. A new study published in Nature Geoscience suggests that if Europe planted trees across all the land suitable for reforestation, it might not only sequester carbon, but also partially ameliorate the increasingly dry European summers predicted by climate change models. In all, mass reforestation could increase summer rainfall by an average of 7.6%.

“When planned carefully, reforestation could result in additional benefits in regions where it is implemented,” said Ronny Meier, a researcher at the Swiss Federal Institute of Technology, Zurich (ETH Zürich), and lead author on the study.

Scientists have long identified a link between forests and rainfall, with scientists in the 1850s expressing alarm at reduced rainfall following deforestation. The exact mechanisms of this relationship are, however, still not fully understood. Meier and his colleagues used data from 3,481 rain-gauge stations across Europe to build a statistical model linking forest cover and rainfall levels. The research team then used the model to predict how much rainfall might change if forest cover increased.

To try and model a realistic forestation scenario, the team used the Global Reforestation Potential Map produced by a previous study exploring natural solutions to climate change.

The Global Reforestation Map shows areas of potential reforestation whilst also accounting for food security and biodiversity (green = areas with realistic reforestation potential). The reforestation potential varies across Europe. Map by ‘Global Reforestation Potential Map’ Griscom et al (2017) (CC BY-SA)

“[The map] finds the locations that used to have forest, but currently lack forest cover and then excludes croplands (we need to eat!), native grasslands to protect biodiversity, and locations relevant for other natural climate solutions,” said Susan Cook-Patton, a senior forest scientist at The Nature Conservancy, who was not involved in either study.

The remaining area is viewed by scientists as the maximum possible reforestable area while still protecting food security and biodiversity. In practice, the area people are willing and can afford to reforest is likely to be smaller, Cook-Patton said.

The map finds that 14.4% of Europe’s land area is suitable for reforestation, with particular concentrations of suitable area across the U.K., Portugal, western and southern France, Italy, and Eastern Europe. In 2015, 38% of Europe’s land area was under forest cover, and although forest area has been consistently increasing for the last 20 years, foresting the entire reforestable area would be a massive undertaking. For context, the recent European Commission pledge to plant 3 billion trees by 2030 to tackle climate change would increase tree cover by around 3 million hectares (7.4 million acres), an area the size of Belgium, or just 0.7% of Europe’s land area.

See related: How to pick a tree-planting project? Mongabay launches transparency tool to help you decide

The increased ‘surface roughness’ of forests creating turbulence above trees and slowing the movement of clouds is one way in which scientists believe forests affect rainfall. Image courtesy of Jim Tan.

The complexity of climates

As anyone knows who has ever planned for a sunny day based on the weather forecast, only to be soaked to the skin, predicting the weather is a tricky business. Climate change prediction models tend to agree on both the magnitude and rate of warming temperatures across Europe, but the impact on rainfall is less clearly understood. There is some agreement among climate models, however, that precipitation will generally increase in Northern Europe and decrease in Southern Europe, and that rainfall will decrease in the summer months and increase in winter.

“Rainfall is a very [complex] parameter,” said Daniela Jacob, director of the Climate Service Center Germany. “It is challenging to find robust signals in modeling and observational studies.”

One such challenge for Meier and his team was interpreting the two-way interaction between forests and rainfall. The higher surface “roughness” of forests compared to “smoother” agricultural land creates more turbulence in the air column above trees. This can slow the movement of heavy clouds, leading to rain downwind of forests. Also, forests often evaporate more water than agricultural land, which could lead to more rain nearby. However, forests are more likely to grow in areas with higher rainfall due to other reasons, such as topography. The team had to be careful not to wholly attribute precipitation to forests, when in fact the forests might be there because of precipitation generated in other ways.

While increased summer rainfall could be very beneficial, Meier warns there could also be adverse effects. For example, given that scientists have tied higher temperatures to an increased likelihood of heavier showers, more precipitation could increase the risk of flash flooding. Increased rainfall in one place could also lead to reduced water availability and therefore reduced precipitation in another area.

“In addition to this study based on statistical methods, the physical processes behind the findings have to be further investigated,” said Jacob, who was not involved in the recent study.

The Rietholzbach catchment is a hilly pre-alpine basin in north eastern Switzerland used by the Swiss Federal Institute of Technology for hydrological process studies. Image courtesy of Ronny Meier.

Physical models, as opposed to statistical models, that account for the complex interaction of meteorological factors are particularly important for scientists in predicting how the weather changes under different conditions, such as the rapidly rising temperatures of climate change. Meier’s study adds to the growing body of knowledge that can then feed into and improve the accuracy of climate predictions, particularly highlighting the potential benefits of forestation beyond carbon sequestration.

While all are very positive about the benefits of planting more trees, there is one point that Meier, Jacob and Cook-Patton are all keen to point out.

“Forestation alone cannot mitigate all the greenhouse gas emissions produced by mankind currently,” Meier said. “To tackle this problem, we need rapidly to minimize our greenhouse gas emissions in the first place.”

Additional listening from Mongabay’s podcast: “Reforestation done right, from Haiti to Honduras and Ho Chi Minh City,” listen here:

Citations:

Meier, R., Schwaab, J., Seneviratne, S. I., Sprenger, M., Lewis, E., & Davin, E. L. (2021). Empirical estimate of forestation-induced precipitation changes in Europe. Nature Geoscience, 14(7), 473-478. doi:10.1038/s41561-021-00773-6

Bennett, B. M., & Barton, G. A. (2018). The enduring link between forest cover and rainfall: A historical perspective on science and policy discussions. Forest Ecosystems5(1). doi:10.1186/s40663-017-0124-9

Griscom, B. W., Adams, J., Ellis, P. W., Houghton, R. A., Lomax, G., Miteva, D. A., … Fargione, J. (2017). Natural climate solutions. Proceedings of the National Academy of Sciences114(44), 11645-11650. doi:10.1073/pnas.1710465114