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Earth is heading towards a climate that last existed more than three million years ago (Ma) during the 'mid-Pliocene warm period'.sup.1, when atmospheric carbon dioxide concentrations were about 400 parts per million, global sea level oscillated in response to orbital forcing.sup.2,3 and peak global-mean sea level (GMSL) may have reached about 20 metres above the present-day value.sup.4,5. For sea-level rise of this magnitude, extensive retreat or collapse of the Greenland, West Antarctic and marine-based sectors of the East Antarctic ice sheets is required. Yet the relative amplitude of sea-level variations within glacial-interglacial cycles remains poorly constrained. To address this, we calibrate a theoretical relationship between modern sediment transport by waves and water depth, and then apply the technique to grain size in a continuous 800-metre-thick Pliocene sequence of shallow-marine sediments from Whanganui Basin, New Zealand. Water-depth variations obtained in this way, after corrections for tectonic subsidence, yield cyclic relative sea-level (RSL) variations. Here we show that sea level varied on average by 13 [plus or minus] 5 metres over glacial-interglacial cycles during the middle-to-late Pliocene (about 3.3-2.5 Ma). The resulting record is independent of the global ice volume proxy.sup.3 (as derived from the deep-ocean oxygen isotope record) and sea-level cycles are in phase with 20-thousand-year (kyr) periodic changes in insolation over Antarctica, paced by eccentricity-modulated orbital precession.sup.6 between 3.3 and 2.7 Ma. Thereafter, sea-level fluctuations are paced by the 41-kyr period of cycles in Earth's axial tilt as ice sheets stabilize on Antarctica and intensify in the Northern Hemisphere.sup.3,6. Strictly, we provide the amplitude of RSL change, rather than absolute GMSL change. However, simulations of RSL change based on glacio-isostatic adjustment show that our record approximates eustatic sea level, defined here as GMSL unregistered to the centre of the Earth. Nonetheless, under conservative assumptions, our estimates limit maximum Pliocene sea-level rise to less than 25 metres and provide new constraints on polar ice-volume variability under the climate conditions predicted for this century. Sea level varied by 13 [plus or minus] 5 metres on average, but up to 25 metres, over glacial-interglacial cycles during the Pliocene epoch, due to partial collapses of Antarctic Ice Sheets., Author(s): G. R. Grant [sup.1] [sup.2] , T. R. Naish [sup.1] , G. B. Dunbar [sup.1] , P. Stocchi [sup.3] , M. A. Kominz [sup.4] , P. J. J. Kamp [...]

We present two narratives on the future of Antarctica and the Southern Ocean, from the perspective of an observer looking back from 2070. In the first scenario, greenhouse gas emissions remained unchecked, the climate continued to warm, and the policy response was ineffective; this had large ramifications in Antarctica and the Southern Ocean, with worldwide impacts. In the second scenario, ambitious action was taken to limit greenhouse gas emissions and to establish policies that reduced anthropogenic pressure on the environment, slowing the rate of change in Antarctica. Choices made in the next decade will determine what trajectory is realized.The future of Antarctica and the Southern Ocean by 2070 is described under two scenarios, one in which action is taken to limit greenhouse gas emissions, and one in which no action is taken., Author(s): S. R. Rintoul [sup.1] [sup.2] [sup.3] , S. L. Chown [sup.4] , R. M. DeConto [sup.5] , M. H. England [sup.6] , H. A. Fricker [sup.7] , V. Masson-Delmotte [...]

On page 234 of this Perspective, '50% decrease' has been corrected online to '50% increase' in the sentence 'The pH of surface waters south of 60° S decreased by 0.2 between 2017 and 2070, equivalent to a 50% increase in the concentration of hydrogen ions since the pre-industrial period.sup.1.', Author(s): S. R. Rintoul [sup.1] [sup.2] [sup.3] , S. L. Chown [sup.4] , R. M. DeConto [sup.5] , M. H. England [sup.6] , H. A. Fricker [sup.7] , V. Masson-Delmotte [...]