Mike Muller argues that dams can produce clean, cheap energy while shrinking or eliminating methane-emitting wetlands downstream (Nature 566, 315–317; 2019). This hypothesis is untested and should not shape decisions that could affect large regions for centuries.

Setting aside the societal and ecological impacts of shrinking natural wetlands, we find Muller’s argument problematic from the perspective of greenhouse-gas budgeting. Floodplain wetlands typically hold large stores of carbon. Disrupting the hydrology that caused these accumulations over millennia could lead to the sustained release of carbon as CO 2 , potentially counteracting any reduction in methane emissions.

Furthermore, emissions from hydropower are site-specific, depending on reservoir and catchment characteristics, among other factors (L. Scherer and S. Pfister PLoS ONE 11, e0161947; 2016). Although some hydropower reservoirs have low net life-cycle emissions, others generate large amounts of greenhouse gases.

Evaluating the pros and cons of dams will therefore require holistic, evidence-based accounting of their impact on greenhouse-gas emissions, ecology and society — on a case-by-case basis.