Stephen M. Kosslyn

on the environment, including changes to land, atmosphere and oceans, but it was also the time when human activity became globalised. This led to a whole range of new global problems, from the destruction of atmospheric ozone through to accumulation of marine litter, climate change, ocean acidifi cation, and more. These global changes would already ensure that geologists of the future would fi nd it relatively easy to spot the middle of the 20th century in any sediment layers. “One thing that has struck me is the way that a number of the activities and Earth process trends graphed by Will Steffen and his colleagues translate effectively into geology,” remarks Zalasiewicz, “the carbon emissions are refl ected in changes in carbon isotopes in plankton shells, the nitrogen shift is seen in the chemistry of lake layers, the biospheric changes will be refl ected in future fossils, and so on. It’s quite a neat interplay of action and petrifaction.” As a bonus feature, however, humanity left a foolproof geological signal that can be pinned down to a precise date, namely the fi rst testing of an atomic bomb at Alamogordo, New Mexico, on July 16, 1945. The radionuclides from this nuclear explosion — and the ~500 others that followed until 1963 — spread around the world and can be easily identifi ed in sediments and ice cores. Choosing the fi rst test as the beginning, rather than the peak of the testing craze in the 1950s, the authors argue, has the advantage of clarity, as all anthropogenic radionuclides found would automatically be counted as within the thus-defi ned Anthropocene. This suggestion, the authors emphasise, is not just a contribution to the ongoing consideration regarding formal approval of the Anthropocene as an epoch. It could be used even if it remained just an informal concept. The working group aims to make its recommendation at the International Geological Congress in 2016. If it recommends the new epoch, a subcommission of the ICS will then take over and decide whether to submit a defi nitive proposal to the ICS for its epoch-making decision. Whether or not the Anthropocene becomes a new epoch, the fact that we have already kicked the Earth system so hard that that it appears to have left the stable equilibrium of the Holocene should spur humanity into action. Considering the extreme climate swings and mass extinctions of the geological record, the Holocene was a surprisingly cosy epoch uniquely suitable for a nearly hairless ape species to colonise the whole world. Now that we are essentially driving the changes at the planetary level, we should do all we can to stay in the Holocene.