Though small water bodies cover just 6% of the global inland water surface area, they are responsible for 15% of carbon dioxide emissions and 28% of methane emissions from inland waters—a disproportionate impact driven largely by agricultural nutrient inputs and land-use intensity, according to a new study.

The research, led by the Research Center for Eco-Environmental Sciences (RCEES) of the Chinese Academy of Sciences (CAS) in collaboration with Peking University, the University of Science and Technology of China, Quaid-i-Azam University in Pakistan, and other partners, was recently published in Proceedings of the National Academy of Sciences.

In this study, small water bodies are defined as inland waters smaller than 1 km². They are widespread across landscapes but have long been overlooked in global carbon-cycle assessments. Past estimates of greenhouse gas emissions from inland waters have largely concentrated on large lakes and rivers, often relying on data collected from these larger systems and extrapolating to smaller ones.

However, small water bodies differ substantially from their larger counterparts. They typically have higher perimeter-to-area ratios, shallower depths, and a greater proportion of oxygen-depleted sediments. These features can create favorable conditions for intense carbon processing and methane production, making them potential emission hotspots on a per-area basis.

Despite their significance, the contribution of small water bodies has remained highly uncertain. A major reason is the lack of high-resolution environmental data that captures local human pressures—such as fertilizer application, aquaculture activities, nutrient runoff, and land-use intensity. These localized pressures can strongly modify how small water bodies respond to climate warming.

To bridge this gap, the research team collected field observations from 470 small water bodies worldwide and used this dataset to train machine-learning models. The models were then applied to a global database containing 3.28 million small water bodies, enabling the researchers to quantify emissions and assess the extent to which human activities amplify climate-driven greenhouse gas output.

The findings confirm that while small water bodies account for only 6% of the global inland water surface area, they contribute 15% of CO₂ and 28% of methane emissions. Their area-specific methane flux was found to be 148% higher than that of larger water bodies. Importantly, the study also revealed that methane ebullition—a pathway often excluded from global assessments—accounted for 56% of total methane emissions from these small systems.

Further analysis pinpointed agricultural nutrient inputs and land-use intensity as key drivers of this amplification effect. Methane fluxes from small water bodies in agricultural catchments were five times higher than those from forested systems. Fertilizer application was also significantly and positively associated with methane ebullition fluxes.

Using partial least squares path modeling, the researchers demonstrated that land-use intensity indirectly increased CO₂ emissions through nutrient enrichment. Fertilizer application affected methane emissions both directly and indirectly, with nutrient enrichment serving as a critical pathway linking human activities to enhanced methane release.

The team also projected future emissions under different socioeconomic and climate pathways. Under the fossil-fuel-driven scenario SSP5-8.5, CO₂ and methane emissions from small water bodies are projected to rise by 30% and 14%, respectively, by 2100. In contrast, under the sustainable-development pathway SSP1-2.6, reducing nutrient loads could limit these increases to 12% and 4%, respectively.

These results suggest that greenhouse gas emissions from small water bodies are not only climate-sensitive but also socioeconomically manageable. They underscore the need to more fully incorporate small water bodies into global carbon budgets and climate mitigation strategies.

The study emphasizes the value of integrated land-water-climate management. Reducing nutrient inputs from agriculture, improving fertilizer management, and strengthening watershed-scale ecological governance could help mitigate emissions from small water bodies while advancing broader goals for climate action and water-quality protection.

Prof. ZHUANG Xuliang from the RCEES is the first author and a corresponding author of the paper. Prof. OUYANG Zhiyun and Associate Prof. JIANG Cancan are co-corresponding authors.