Back in 2019, a study by Zurich State University, led by Thomas Crowther, calculated that there was room for an additional 1,2 trillion trees, capable of absorbing up to two-thirds of historical emissions. This idea spurred global campaigns like the UN's 'One Trillion Trees' initiative. But reality has proven to be far more complex , and Crowther himself has since qualified his conclusions.
The risks of massive reforestation
During COP28, Crowther surprised many by stating that massive tree plantations can be harmful when used as an excuse for not reducing emissions. The scientist described it as a form of greenwashing , since companies and governments offset their carbon footprint with reforestation certificates without addressing the root causes of the problem. This practice, far from helping, diverts attention from more effective measures such as phasing out fossil fuels.
Furthermore, planting trees in unsuitable locations can disrupt existing ecosystems. Grasslands and savannas also store carbon , and converting them to forests can be counterproductive. Lack of long-term maintenance and increased local temperatures due to the albedo effect are other factors that experts identify as risks.
The case of China: the Great Green Wall
One of the most ambitious examples is China's Great Green Wall, a project initiated in 1978 to combat desertification. With nearly 78.000 billion trees planted, forest cover increased from 10% to 25%. However, a study published in 'Earth's Future' reveals that this massive reforestation is disrupting the hydrological cycle . Evapotranspiration from the new forests has shifted moisture towards the Tibetan Plateau, leaving arid regions in the northwest and the eastern monsoon zone, which together comprise 74% of the country, without water.
The paradox is clear: the trees meant to protect these lands have ended up depriving them of their most valuable resource, water. Northwest China has lost vast volumes of water , while the Tibetan Plateau has gained availability. This imbalance exacerbates the scarcity in a region that already provides only 20% of the country's water for 46% of the population, highlighting the complex way in which forests generate water and regulate the climate.
Keys to responsible reforestation
Faced with these problems, experts agree that reforestation needs to be rethought. Karen Holl, an environmental scientist, advocates for allowing ecosystems to regenerate naturally instead of imposing massive plantations. Crowther proposes prioritizing biodiversity and the direct benefit of local communities, using native species and avoiding competition with agricultural needs.
Africa's Great Green Wall, which aims to halt the Sahara's advance, is cited as a more positive example because it collaborates with local communities. In contrast, programs in China and Uganda have led to water and arable land losses due to the use of fast-growing, non-native species. The lesson is clear: environmental restoration is not simply about planting trees, and any project must study how long it takes for a forest to regenerate and its interactions with water and soil.
While reforestation can be a useful tool, it should not be considered a panacea. The priority remains reducing emissions through clean energy and energy efficiency . Only a comprehensive approach that combines emissions reduction with well-planned reforestation will be able to effectively address climate change.
