5 August 2018

Semi-arid forests could reduce global warming

Although new trees are cropping up in many places around the globe in an effort to fight climate change, it has become apparent that not all types of forests are created equal.

A new study piloted by Weizmann Institute scientists suggests that planting trees that are common in semi-arid forests in two previously neglected areas could significantly counteract human-induced global warming. The research, published earlier this year in Nature’s Scientific Reports, showed that such trees could absorb an amount of carbon equal to 10% of the world’s uptake and help cool the Earth within around six years.

Through years of measurements in Israel’s Yatir Forest, Professor Dan Yakir, from the Department of Earth and Planetary Sciences, and senior staff scientist Dr Eyal Rotenberg previously found that semi-arid pine forest forests are surprisingly good carbon sinks—or reservoirs for carbon storage. They have been operating the Yatir Station, located at the edge of the Negev Desert, for the past 18 years. The Yatir Forest mainly includes Aleppo pine trees—also known as Jerusalem pines—which thrive in the Mediterranean climate.

In the group’s latest study, Dr Gil Yosef, who recently completed his PhD under Professor Yakir’s supervision, sought out to explore the question of what could be the effects if a forest similar to Yatir would be planted over much larger scales. He focused on two areas with similarly low rainfall as the Yatir semi-arid forest—the Sahel zone bordering the Sahara and a large semi-arid tract of Northern Australia.

Dr Yosef and Professor Yakir used a global climate model designed by the University of Miami’s Professor Roni Avissar and Dr Robert Walko to simulate an imaginary forest filled with such trees. The University of Miami model granted the scientists a telescopic view so that they could investigate differences in geographic areas of a few kilometers and then zoom out to see larger, continental-scale patterns. In their simulations, they used real weather data from a 15-year time span, which added further nuances like rainfall amounts to their simulations.

Their results showed that not only could the planting produce local cooling, but the effects could also extend more broadly to the regional atmospheric circulation—significantly increasing rain over these areas. The scientists estimated that afforestation in these areas alone could create a carbon sink equivalent to around 10% of the global biospheric carbon uptake, thereby helping cool the planet within about six years.

“Our model simulations showed that planting the appropriate forest, such as Yatir, at a large scale in the Sahel and Australia modifies the local climate in a way that could sustain the forest and even expand it,” Professor Yakir said.

The main reason for this phenomenon, said Dr Yosef and Professor Yakir, is that trees in semi-arid forests are typically excellent “water miners” and have deep roots. The trees reach down into the Earth to pull up more water and evaporate it through their leaves, making the surface measurably cooler. They found that the trees, acting together on a sufficiently large scale, could promote cloud formation due to this ability to increase “evapotranspiration”—intensifying moisture in the atmosphere above them and cooling the surface.

“In turn, modification of the surface temperatures trigger a chain of surface and atmospheric events that ultimately further enhances both precipitation and carbon uptake,” Professor Yakir explained.

“Trees are not only able to adapt to the climate they inhabit, but they are also able to shape that climate as well,” he said.

While afforestation in the Sahel and Northern Australia could theoretically give rise to an enormous carbon sink, accomplishing such a task is probably easier said than done, Professor Yakir acknowledged.

“But if this is also done in other places, the effects will add up. There is no ‘silver bullet’ in dealing with climate change. We need everything at our disposal,” he said.

Professor Dan Yakir is supported by the Sussman Family Center for the Study of Environmental Sciences, the Cathy Wills and Robert Lewis Program in Environmental Science, and Dana and Yossie Hollander.

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