Methane and nitrous oxide are greenhouses gases with warming potentials many times higher than that of carbon dioxide. Previous work has indicated that the emission of both of the former from soils can be stimulated by increasing ambient carbon dioxide above the current atmospheric concentration, but the evidence has been piecemeal and the full picture unclear. Now a meta-analysis by Kees Jan van Groenigen et al. confirms that, overall, increasing carbon dioxide stimulates soil emissions of nitrous oxide from terrestrial ecosystems and of methane from rice paddies and natural wetlands. The authors suggest that these responses will negate at least 17% of the mitigation effect of the increase in the terrestrial carbon sink expected from a rise in carbon dioxide alone (to the second half of this century) — implying that the natural capacity of land ecosystems to slow climate change has been overestimated.