Dam reservoirs may emit far less carbon than previous estimates suggested

Greenhouse gas emissions from dammed reservoirs may be substantially smaller than widely reported, according to new research published in Science Advances. The study suggests that dammed reservoirs may contribute only 18.6% of the global emissions projected by earlier approaches, after accounting for the carbon that the same water bodies would have released even without a dam in place.

Hydroelectricity has grown in popularity worldwide, and so too has the number of dams constructed to generate it. These structures trap organic carbon in their reservoirs, where it can be converted and released as methane and carbon dioxide. “High carbon emissions from reservoirs have sparked controversy over dams and even raised questions about the green credentials of hydropower,” Wenqing Shi and colleagues write.

The team simulated both dam and non-dam emissions from the same 5,346 dammed reservoirs across the globe. By comparing the two scenarios for each individual reservoir, rather than treating reservoir emissions as a standalone total, the researchers found that estimates of dams’ greenhouse gas contributions have been too large.

The inaccuracy, the authors argue, arises because conventional techniques have not considered baseline emissions from dammed reservoirs under no-dam conditions. Rivers and natural water bodies emit carbon regardless of whether they are impounded, and that pre-existing flux has often been attributed entirely to the dam itself.

When this baseline was incorporated, the analysis established that damming leads to an average increase in carbon emissions of 21.5% over the no-dam condition. That increment represents only 18.6% of the emissions previously estimated for these reservoirs, a gap the researchers attribute to the overlooked baseline rather than to any change in the underlying measurements.

The study is a modeling exercise covering thousands of reservoirs worldwide, and its conclusions depend on the assumptions built into the simulations of dammed and undammed conditions. The authors are explicit about one important limitation: the work does not consider downstream estuarine carbon emissions, which fall outside the scope of the current analysis.

“Future dam planning and environmental impact assessments should evaluate carbon consequences at the scale of the integrated river-estuary continuum,” the team writes. “This holistic perspective will facilitate the development of low-carbon hydropower systems and provide a scientific basis for balancing renewable energy expansion with climate mitigation goals.”

The findings arrive amid ongoing debate about the climate accounting of hydropower, which is often treated as a low-carbon source of electricity but has faced scrutiny over the methane and carbon dioxide released from impounded waters. By separating the emissions a dam adds from the emissions that would occur anyway, the new framework offers a different benchmark for judging how carbon-intensive individual reservoirs really are.

The research, “Dams not to blame for all reservoir carbon emissions from retained organic carbon,” was published in Science Advances on 28 October 2026.

Subject of Research: Greenhouse gas emissions from dammed reservoirs

Article Title: Carbon emissions from dam reservoirs may have been overestimated

Article References: Carbon emissions from dam reservoirs may have been overestimated. Original publication (Bibliographic details unavailable.)

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Keywords: dams, reservoirs, greenhouse gas emissions, methane, carbon dioxide, hydropower, carbon accounting, Science Advances, river-estuary continuum, climate mitigation, Carbon, emissions