The Earth's "lung disease" is getting worse! Deforestation is spreading to the tropics

Author: Zhang Shuanghu

"Tropical forests are equivalent to the 'lungs of the earth.' In recent years, people believe that global deforestation is gradually decreasing, but this is an illusion. Our research shows that the earth's 'lung disease' is still getting worse." Zeng Zhenzhong, associate professor at the School of Environmental Science and Engineering of Southern University of Science and Technology, told China Science Daily.

On March 1, Nature-Sustainability published the research results of Zeng Zhenzhong's team, Zheng Chunmiao's team, chair professor of the School of Environmental Science and Engineering at the Southern University of Science and Technology, and Chen Ji's team, professor of the University of Hong Kong. This study used multi-source remote sensing data for the first time to explore the interannual variation characteristics and driving factors of tropical forest carbon storage loss, and found that tropical forest carbon storage loss has doubled since the 21st century.

Rare studies on interannual dynamics

"Tropical rainforests play an extremely important role in the global carbon cycle," Zeng Zhenzhong explained. "For the same area of ​​forest, tropical rainforests have a much greater carbon sequestration capacity than temperate or boreal forests."

Tropical forests have an important impact on the global terrestrial carbon cycle and the regulation of atmospheric carbon dioxide concentration. They are not only huge carbon reservoirs (storing about 250PgC of biomass carbon; 1PgC=1015gC, or 1 billion tons of carbon), but can also fix large amounts of carbon dioxide through photosynthesis.

In recent years, extreme climate events and human activities have not only weakened the carbon sequestration capacity of tropical forests, but also caused a large amount of carbon storage loss. Existing studies have mostly focused on the magnitude of forest carbon storage loss, and few studies have reported on the interannual dynamics of forest carbon storage loss.

"The terrestrial carbon source/sink is the part with the highest uncertainty in the global carbon cycle. Accurately assessing the interannual changes in forest carbon stock loss can provide a more accurate basis for estimating the global carbon cycle."

Feng Yu, the first author of the paper and a doctoral student jointly trained by Southern University of Science and Technology and the University of Hong Kong, told China Science Daily, "Based on high-resolution remote sensing observation products and combined with stratified random sampling methods, we calculated the biomass carbon and soil organic carbon losses caused by forest loss in tropical regions from 2001 to 2019. We found for the first time that the loss of tropical forest carbon stocks increased from 0.97±0.16PgC per year in the early 21st century to 1.99±0.13PgC per year."

Feng Yu explained that human activities emit more than 10PgC of carbon into the atmosphere each year, of which more than 30% is absorbed by terrestrial ecosystems. The main force of terrestrial ecosystem carbon sequestration is tropical forests, "but the latest data shows that people's destruction of tropical forests has increased instead of decreased. From 2000 to now, the rate of deforestation has doubled."

Make data more accurate

In 2016, Zeng Zhenzhong was doing postdoctoral research at Princeton University in the United States. His mentor, Professor Eric F. Wood, happened to have a research project on tropical deforestation in northern Thailand.

At the beginning of the project, Zeng Zhenzhong went to Nan Province in northern Thailand for research. As soon as he arrived in Nan Province, he was "shocked" by the scene in front of him.

"Standing on the ridge and looking down, we can see that all over the mountain, the places that the original data showed were forests were actually planted with corn." Zeng Zhenzhong added that in many places "the terrain is so steep that people dare not squat there," but even so, large tracts of forest have still turned into fields.

"This study has two highlights," Zeng said. "We found that the rate of tropical forest deforestation has doubled, first because we used more accurate satellite data and second because we used a more accurate computational model."

Feng Yu added that previous studies in this area mostly used satellite data with a resolution of hundreds of meters. This study used satellite data with a resolution of 30 meters, which can more accurately depict small-scale deforestation and farmers' "encroachment" on forests.

In addition, the team also used high-resolution remote sensing data from the American commercial company Planet satellite, with a resolution accurate to 3 to 5 meters, and conducted a "visual interpretation" of land use after deforestation.

After analyzing 1TB of data, the researchers found that the original calculation model had errors. The team spent more than ten days manually correcting satellite images and tables, and combined with stratified random sampling methods, they finally obtained "more accurate data."

Deforestation is “going uphill”

From 2016 to 2019, Zeng Zhenzhong conducted field surveys in tropical forest areas every year. He and his partners from the United States, France, Thailand, the United Kingdom and other countries traveled to tropical rainforest areas in Africa, South America and Southeast Asia.

"We believe that countries around the world have been increasing their efforts to protect forests in recent years, and many research data show that the rate of deforestation is slowing down or basically remaining the same, but this is not actually the case," Zeng Zhenzhong said. "The current hotspot of global deforestation has shifted from mid-latitudes to tropical regions, and some new characteristics have emerged. First, deforestation is moving higher up, and second, deforestation is more sporadic and hidden."

The study believes that in tropical forest areas, the main cause of deforestation is agricultural expansion. In places with lower altitudes, where monitoring and management are less difficult, deforestation has eased, but small-scale deforestation is gradually spreading to the mountains. In addition, the characteristics of agricultural production in tropical forests are "rotational agriculture" (deforestation is abandoned for a few years, gradually forming forests naturally, and then destroyed again), which increases the difficulty of monitoring tropical vegetation dynamics using traditional remote sensing methods.

The study further found that 82% of forest carbon stock loss was caused by agricultural expansion, among which commodity-driven large-scale agricultural expansion was the main cause of forest carbon stock loss in South America and Southeast Asia, while in Africa, the main cause was small-scale rotational agriculture.

"Among them, about 70% of the forest destruction caused by agricultural expansion remained in agricultural use in 2020, proving the long-term and persistent impact of agriculture on tropical forest destruction," said Feng Yu.

The 2014 New York Declaration on Forests proposed to halve the global deforestation rate by 2020.

"Our research shows that humanity has not yet fulfilled this commitment, which also highlights the importance and urgency of the Glasgow Leaders' Declaration on Forests and Land Use at the United Nations Climate Conference to be held in Glasgow in 2021," said Zeng Zhenzhong. "There is only one Earth, and carbon emissions are the same in both tropical and temperate zones. We urgently need to strengthen our research on tropical forests."

Related paper information:

https://doi.org/10.1038/s41893-022-00854-3

China Science Daily (2022-03-02 Page 1, original title: "The Axe of Deforestation Reaches the Tropics")

Editor | Zhao Lu

Typesetting | Zhihai