To mitigate climate change, afforestation worldwide has the potential to sequester up to 5 billion metric tons of carbon dioxide annually, according to scientific research led by a team from the Sun Yat-sen University in Guangzhou, Guangdong province.

Qin Zhangcai, a professor from the School of Atmospheric Sciences, Sun Yat-sen University, and his team published their findings, titled "Land Availability and Policy Commitments Limit Global Climate Mitigation from Forestation", in the international academic journal Science on Aug 29. The research quantified for the first time the carbon sequestration capacity of underground soil in the global forest restoration process.

To "cool down" the planet, the world is in urgent need of reducing carbon dioxide emissions and enhancing the carbon sequestration capacity of ecosystems. As natural "carbon absorbers", forests serve as indispensable "green air conditioners" for climate regulation, and afforestation is widely regarded as one of the most cost-effective nature-based climate solutions.

The research innovatively integrates three key factors — ecology, climate and policy — to redefine the potential of afforestation to mitigate climate change and provide "navigation" for future global afforestation.

After years of collecting scarce global soil data, Qin's team has successfully developed a machine learning model capable of quantifying soil carbon changes resulting from afforestation. Leveraging international cooperation and integrating existing vegetation carbon estimates, the team has also created an "all-round detector" that can monitor carbon changes in both vegetation and soil.

The team also took into account international environmental and socio-economic constraints related to afforestation, such as biodiversity, water resources, surface warming and national commitments. From the two dimensions of sustainable land "supply" and national policy "demand", the team has redeveloped a "navigation map" for future forest restoration.

"However, when land sustainability and countries' willingness are taken into comprehensive consideration, the actual emission reduction potential of afforestation worldwide can decrease to 1.5 billion metric tons per year," warned Qin, adding that climate risks can stem from "empty talk", or "theoretical plans that lack implementation".

According to Qin, there are huge discrepancies regarding how much carbon dioxide future afforestation can absorb, with the low estimate being less than 300 million tons and the high estimate exceeding three billion tons per year.

Qin said the controversy mainly centers on two key challenges.

While the carbon sequestration capacity of vegetation is relatively clear, changes in soil carbon remain difficult to detect accurately, he said.