THE Cretaceous period was a time of global warmth(1-4). MidCretaceous equatorial temperatures were similar to today’s(5), but the equator-to-pole temperature gradient is the subject of some controversy(5-7). Although it is unlikely that the poles were ice-frees(8-10), fossil evidence(3-5,11,12) indicates that near-polar temperatures were much higher than they are today. Little is known, moreover, about oceanic poleward heat transport, and this makes it hard to model the Cretaceous climate or to evaluate the extent to which it provides an analogue for a ’greenhouse’ world warmed by increased atmospheric CO2 alone. Here we use relationships between leaf physiognomy (such as shape and size) and modern climate to determine Cretaceous climate conditions in the Arctic region from fossil leaves. We find that the Arctic Ocean was relatively warm, remaining above 0 degrees C even during the winter months. This implies that there was significant poleward heat transport during all seasons.