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      Trajectories of the Earth System in the Anthropocene

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          We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System—biosphere, climate, and societies—and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.

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          Most cited references 89

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          Tipping elements in the Earth's climate system.

          The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.
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            Geology of mankind.

             P J Crutzen (2002)
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              Contribution of Antarctica to past and future sea-level rise.

              Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics-including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs-that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.

                Author and article information

                Proc Natl Acad Sci U S A
                Proc. Natl. Acad. Sci. U.S.A
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                14 August 2018
                6 August 2018
                6 August 2018
                : 115
                : 33
                : 8252-8259
                aStockholm Resilience Centre, Stockholm University , 10691 Stockholm, Sweden;
                bFenner School of Environment and Society, The Australian National University , Canberra, ACT 2601, Australia;
                cCenter for Macroecology, Evolution, and Climate, University of Copenhagen, Natural History Museum of Denmark , 2100 Copenhagen, Denmark;
                dEarth System Science Group, College of Life and Environmental Sciences, University of Exeter , EX4 4QE Exeter, United Kingdom;
                eThe Beijer Institute of Ecological Economics, The Royal Swedish Academy of Science , SE-10405 Stockholm, Sweden;
                fSchool of Geography and Development, The University of Arizona , Tucson, AZ 85721;
                gScott Polar Research Institute, Cambridge University , CB2 1ER Cambridge, United Kingdom;
                hJasper Ridge Biological Preserve, Stanford University , Stanford, CA 94305;
                iEarth and Life Institute, Université catholique de Louvain , 1348 Louvain-la-Neuve, Belgium;
                jBelgian National Fund of Scientific Research , 1000 Brussels, Belgium;
                kResearch Domain Earth System Analysis, Potsdam Institute for Climate Impact Research , 14473 Potsdam, Germany;
                lDepartment of Environmental Sciences, Wageningen University & Research , 6700AA Wageningen, The Netherlands;
                mDepartment of Physics and Astronomy, University of Potsdam , 14469 Potsdam, Germany
                Author notes
                1To whom correspondence may be addressed. Email: will.steffen@ or john@ .

                Edited by William C. Clark, Harvard University, Cambridge, MA, and approved July 6, 2018 (received for review June 19, 2018)

                Author contributions: W.S., J.R., K.R., T.M.L., C.F., D.L., C.P.S., A.D.B., S.E.C., M.C., J.F.D., I.F., S.J.L., M.S., R.W., and H.J.S. wrote the paper.

                Copyright © 2018 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                Page count
                Pages: 8
                Physical Sciences
                Earth, Atmospheric, and Planetary Sciences
                Social Sciences
                Sustainability Science


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