We examine the abatement costs for water and climate pollutants and their respective policies while accounting for cobenefits. We construct private and social marginal cost curves for reducing greenhouse gas emissions and nutrient runoff in Finnish agriculture. We find that the social marginal costs of reducing emissions that reflect the cobenefits are lower than the private costs. Accounting for greenhouse gas cobenefits from nutrient load reduction or water cobenefits from climate emissions reduction creates a gap between privately and socially optimal reduction levels. This gap varies depending on the valuation of cobenefits. The cost-efficient reduction of the focus pollutant is increased when cobenefits from the other pollutant are accounted for. For policies, this implies a higher cap or tax on the focus pollutant. We decompose the optimal tax rate to a basic tax on the focus pollutant and on an additional tax component depending on the level of cobenefits.