The urgency of assessing the greenhouse gas budgets of hydroelectric reservoirs in China

China, already the largest generator of hydroelectricity, plans to accelerate dam construction. This has led to warnings that increased emissions of greenhouse gases, particularly methane, from Chinese reservoirs could constitute a 'global warming time-bomb'. A review of evidence on emissions from the Three Gorges Reservoir — the world's largest — indicates that such fears are probably misplaced. Already the largest generator of hydroelectricity, China is accelerating dam construction to increase the share of hydroelectricity in its primary energy mix to reduce greenhouse gas emissions. Here, we review the evidence on emissions of GHGs, particularly methane, from the Three Gorges Reservoir, and argue that although the hydroelectric reservoirs may release large amounts of methane, they contribute significantly to greenhouse gas reduction by substitution of thermal power generation in China. Nonetheless, more systematic monitoring and modelling studies on greenhouse gas emissions from representative reservoirs are necessary to better understand the climate impact of hydropower development in China.