  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) 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.
