Turning Barren Lands into Forests Won’t Cool Planet

Planting forests in vast semi-arid areas will not moderate global warming and may even worsen it, according to researchers at the Technion – Israel Institute of Technology and the Weizmann Institute of Science in Israel in a study that contradicts some long-held assumptions.

The researchers attribute their findings partly to the fact that forested areas retain more heat than barren land, which is more reflective of solar radiation.

Based on their extensive study of the climate change mitigation potential of the world’s semi-arid areas over a total of 448 million hectares, the researchers also developed a global-scale smart map that shows how their findings can be applied anywhere at a resolution of up to a few kilometers.

In a paper published today in Science, the researchers presented a study of the potential climatic impact on global warming of large-scale afforestation. Their findings show that while semi-arid areas offer the largest potential for afforestation, even extensive afforestation is not an effective climate change mitigation solution.

These unexpected findings are attributed partly to the often overlooked “albedo effect,” which warms the Earth’s surface, according to the research led by Professor Yohay Carmel and his then-PhD student (now Dr) Shani Rohatyn from the Technion’s Faculty of Civil and Environmental Engineering, along with Professor Dan Yakir and Dr. Eyal Rotenberg from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science.

Prof. Yohay Carmel and Dr Shani Rohatyn

“When we began the research, we expected to show that extensive planting of forests in semi-arid areas would significantly slow down climate warming,” said Prof. Carmel. “But our study disproved this accepted hypothesis. It is disappointing indeed. Yet, this is how science works – it discovers the truth regardless of what we want to discover. There is also an important lesson here – planting forests, no matter how extensive, will not save us from climate change. Instead, we should focus on reducing emissions.”

The study shows that even if trees are planted in every possible location, their carbon absorption by year 2100 would offset only about 1% of all carbon emissions from the burning of fossil fuels. This is because areas darkened due to forestation absorb more solar radiation than exposed areas like deserts and glaciers (the so called “albedo effect”). Therefore afforestation, while absorbing large quantities of carbon, also typically increases the heating of the previously exposed surface.

The albedo effect was already known in the context of climate change, but this is the first study to produce a worldwide mapping of the phenomenon and to calculate the site-specific balance between the two contrasting effects of forestation – the cooling effect of carbon sequestration and the warming effect of change in albedo. The resulting maps give not only a global perspective of afforestation’s potential for climate change mitigation, but also inform the intelligent planning of forestry.

Dr. Shani Rohatyn, who built the model, points out that smart afforestation – that is, planting forests only in climatically beneficial places based on the current research – is expected to double the emissions offset by afforestation and to increase it even more significantly at the local and regional levels, as the researchers showed in an accompanying article.

“The climatic consequences of planting forests depend on many factors, including the reflection of local radiation from the ground, precipitation and trees’ ability to fix carbon,” said Prof. Yakir. “The good news of our research lies in the tools we developed that make it possible to predict where afforestation can indeed have a positive effect. We hope planners will take these findings into account and use them for optimal planning of planting trees.”

Prof. Dan Yakir

About half of the world’s afforestation potential is located in semi-arid areas, and large-scale plantings are already underway presently in China, Saudi Arabia, the Sahel in western and north-central Africa, and beyond. These projects are expected to transform about 5 million square kilometers of barren land into forests. The current research shows that without proper planning, these projects are liable to create undesirable climatic results, so it is important to conduct site-specific planning of planting in semi-arid areas.

“Afforestation is a process that has many advantages, including local cooling, prevention of soil erosion and more,” concluded Prof. Carmel. “However, uninformed afforestation may destroy rare species adapted to live in the open desert and thus harm biological diversity and, as mentioned, also harm the greater goal of minimizing climate warming. That is why it is so important to take into account all the considerations before starting the wholesale afforestation of large areas.”

The research was supported by the Technion and the Weizmann Institute of Science, the Stephen and Nancy Grand Water Research Institute at the Technion, Israel’s National Science Foundation, the Minerva Foundation, the Yotam Project, and the Sustainability and Energy Research Initiative (SAERI) at the Weizmann Institute of Science.

Net climatic effects of dryland afforestation, including both the carbon cooling and albedo warming effects. An interactive map of the results can be found here: https://tinyurl.com/mrt4ycha.

To read the full article in Science, click here