Gail Whiteman and colleagues suggest that the opening up of the Arctic Ocean could bring more economic costs than benefits, owing to climatic effects resulting from a sudden release of 50 gigatonnes of methane from the area (Nature 499, 401–403; 2013). However, our literature review of the impact of sea-ice decline on Arctic greenhouse-gas exchange indicates that methane release is likely to be more gradual because of a slow rate of heat penetration into the subsea permafrost (see F. J. W. Parmentier et al. Nature Clim. Change 3, 195–202; 2013). We therefore believe that the proposed scenario is unlikely.

Although the Arctic Ocean represents a substantial source of methane, there are still many unknowns. Any research that assumes a large increase in emissions from that region should therefore include ample discussion of the uncertainties relating to this source.