Beschreibung: 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 (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 +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 2 °C 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) nonlinearly 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.

Beschreibung: We document massive deposition of carbonate sands along the south shore of Bermuda that were emplaced during one or two great storms during the last interglacial. As determined by their stratigraphic position and geochronological data, these deposits formed during marine isotope substage (MIS) 5c ca. 100 ka ago. Within a leeward set of eolian beds, evidence of a living landscape was preserved that includes delicate footprints of a shorebird (Scolopacidae, Catoptrophorus) preserved in frothy dune foreset beds. In the same stratigraphic unit, outlines of a standing forest of palm trees (Sabal bermudana), some evidently with fronds in place, were molded in the fine carbonate dune sand. All available evidence points to an MIS 5c sea level positioned several meters below the present datum, which would require great intensity of storms to transport such voluminous deposits well above present sea level. Waves and storm currents transported loose sediments from the shallow shelf onto the shore, where hurricane winds piled up sand sufficiently deep to bury established forests of 8- to 10-m-tall trees. Evidence of such powerful storms preserved in the rock record is a measure of the intensity of past hurricanes, and a possible bellwether of future storm events. Entombment of the trees involved rapid burial and cementation creating external molds in limestone, a process that is important in the development of vertical hydrological conduits commonly observed in eolianites.