There is a considerable difference between the statement of the IPCC (‘high risk of extinction’) and that from the expert survey participants (‘median expert judgement of 28% decline in total population’). They are not directly comparable, however, partly because the sea-ice scenarios on which they are based are different. The IPCC statement assumes that global warming of 2·8 °C causes a 62% loss of summer sea-ice extent (a multi-model mean, with individual simulations within the range 40–100%; Table 4·1, IPCC 2007a), relative to pre-industrial conditions. No time frame was given over which the sea-ice retreat would take place. In the study reported here, experts were asked for projections based on a 47% loss of summer sea-ice extent, relative to 1961–1990 conditions, by the 2050s. These were diagnosed from model simulations under the A1B scenario, under which the multi-model ensemble global temperature rise from pre-industrial to 2050 was 1·9 °C (Table II.4, IPCC 2001). The observed temperature rise of 0·4 °C from pre-industrial to 1961–1990 leaves a projected future rise of 1·5 °C from 1961–1990 to 2050. If ice retreat is linearly related to global temperature change (probably a poor assumption), then the 47% loss of summer ice relative to 1961–1990 would be equivalent to a 60% (= 47% × 1·9/1·5) loss relative to pre-industrial. This suggests that the scenario used in this expert survey is, in fact, very similar to that used as the basis for the IPCC statement, and thus, the reason for the differences must lie elsewhere. In particular, we urge caution in interpreting the extrapolated statement from the expert survey; it is based on the median of the experts’ mean values and thus does not demonstrate the full range of expert projections, and there is considerable uncertainty and regional variation.