2 ° C Global Warming is Dangerous

We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth’s energy imbalance and heat flux into most of the global ocean’s surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10-40 year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (5002000 year) time scale of deep ocean ventilation affects the time scale for natural CO2 change and thus the time scale for paleo global climate, ice sheet, and sea level changes, but this paleo millennial time scale should not be misinterpreted as the time scale for ice sheet response to a rapid large human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6-9 meters with evidence of extreme storms while Earth was less than 1°C warmer than today. Ice melt cooling of the North Atlantic and Southern Oceans, increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2°C global warming above the preindustrial level would be dangerous. Continued high fossil fuel emissions this century are predicted to yield: (1) cooling of the Southern Ocean, especially in the Western Hemisphere, (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss, (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region, (4) increasingly powerful storms, and (5) nonlinearly growing sea 1 Climate Science, Awareness and Solutions, Columbia University Earth Institute, New York, NY 10115, USA 2 Department of Environmental Studies, University of North Carolina at Wilmington, North Carolina 28403, USA 3 Trinnovium LLC, New York, NY 10025, USA 4 NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA 5 Institut Pierre Simon Laplace, Laboratoire des Sciences du Climat et de l’Environnement (CEA-CNRS-UVSQ), Gif-sur-Yvette, France 6 Key Lab of Aerosol Chemistry & Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China 7 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91109, USA 8 Department of Earth System Science, University of California, Irvine, California, 92697, USA 9 Program for the Study of Developed Shorelines, Western Carolina University, Cullowhee, NC 28723, USA 10 Department of Geological Sciences, East Carolina University, Greenville, NC 27858, USA 11 GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, Kiel 24148, Germany 12 Mediterranean Institut of Oceanography, University of Toulon, La Garde, France 13 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, 10027, USA 2 level rise, reaching several meters over a time scale of 50-150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.