Earth from space at night
James Hansen et al.
Atmospheric Chemistry and Physics
March 22, 2016

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,  butnthe record is too short to confirm the nature of the response.nThe  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 (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the  timescale  for  paleo-global  climate,  ice  sheet,  and  sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale 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 C 6–9 m 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 2C global warming above the preindustrial level could 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) non-linearly growing sea level rise, reaching several meters over a  timescale  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.