Transboundary spillovers may degrade environmental quality if countries free ride. This paper examines the extent of such degradation in water quality in international rivers. Using data from river monitoring stations in the UN's Global Emissions Monitoring System (GEMS), it compares pollution levels in international and domestic rivers. The results suggest that free riding may substantially increase pollution in international rivers, but the estimates are sensitive to the inclusion of country effects.

Public policies for pollution control, including climate change policies, sometimes allow polluters in one sector subject to an emissions cap to offset excessive emissions with pollution abatement in another sector. The government may find it more costly to verify offset claims than to verify compliance with emissions caps. Concerns about such enforcement difficulties may lead regulators to restrict the use of offsets. We demonstrate that allowing offsets may increase pollution abatement and reduce illegal pollution, even if the government has a fixed enforcement budget. We explore circumstances that may make allowing pollution offsets an attractive option when enforcement is costly.