Description: The Palaeocene/Eocene thermal maximum, 55 million years ago, was a brief period of widespread, extreme climatic warming (Zachos et al., 2003; Kennett and Stott, 1991, doi:10.1038/353225a0; Tripati and Elderfield, 2005, doi:10.1126/science.1109202), that was associated with massive atmospheric greenhouse gas input (Dickens et al., 1995, doi:10.1029/95PA02087). Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition (Backman et al., 2006, doi:10.2204/iodp.proc.302.2006). We show that sea surface temperatures near the North Pole increased from 18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations (Shellito et al., 2003, doi:10.1016/S0031-0182(02)00718-6), but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms -perhaps polar stratospheric clouds (Sloan and Pollard, 1998, doi:10.1029/98GL02492) or hurricane-induced ocean mixing (Emanuel et al., 2004, doi:10.1175/1520-0469(2004)061〈0843:ECOTCI〉2.0.CO;2)- to amplify early Palaeogene polar temperatures.

Description: Quasi-periodic variation in sea-surface temperature, precipitation, and sea-level pressure in the equatorial Pacific known as the El Niño - Southern Oscillation (ENSO) is an important mode of interannual variability in global climate. A collapse of the tropical Pacific onto a state resembling a so-called 'permanent El Niño', with a preferentially warmed eastern equatorial Pacific, flatter thermocline, and reduced interannual variability, in a warmer world is predicted by prevailing ENSO theory. If correct, future warming will be accompanied by a shift toward persistent conditions resembling El Niño years today, with major implications for global hydrological cycles and consequent impacts on socioeconomic and ecological systems. However, much uncertainty remains about how interannual variability will be affected. Here, we present multi-annual records of climate derived from growth increment widths in fossil bivalves and co-occurring driftwood from the Antarctic peninsula that demonstrate significant variability in the quasi-biennial and 3-6 year bands consistent with ENSO, despite early Eocene (~50 Mya) greenhouse conditions with global average temperature -10 degrees higher than today. A coupled climate model suggests an ENSO signal and teleconnections to this region during the Eocene, much like today. The presence of ENSO variation during this markedly warmer interval argues for the persistence of robust interannual variability in our future greenhouse world.