POLAR ICE AND SEA LEVEL: INSIGHTS AND UNCERTAINTIES

Warming melts ice including the polar ice sheets, as shown by physical understanding, paleoclimatic data, and ongoing trends. The increase in accumulation rate with warming from the rise in saturation vapor pressure is almost always outweighed by the increase in melting over large areas. Melting of ice from above in regions such as coastal Greenland is important, and model results show clearly that the response time for loss of Greenland’s ice decreases with increasing warmth, potentially to as little as a few centuries. Melting is potentially more rapid for ice ending in the ocean, with thinning or removal of ice shelves unbuttressing non-floating ice and contributing to sea-level rise, sometimes quite rapidly. The large ice shelves exist in waters that are about as cold as any large volume of the ocean, so any warming or change in circulation is unfavorable to their long-term existence. Paleoclimatic data show that sustained forcings similar to or even smaller than that to which we are now committed have produced meters-higher sea levels; those data allow but do not require that substantial sea-level rise from ice volume similar to today occurred notably more rapidly than recent rates. Ice-sheet shrinkage of ~0.6 mm/yr over the two decades leading up to the IPCC Fifth Assessment Report was a little under 0.001% of ice-sheet sea-level equivalent per year, equivalent to a professor losing about 1/3 of one potato chip per year in a diet. Physical understanding and modeling show the potential for much faster rise than this. The most-likely projections of sea-level rise from the IPCC are toward the low end of the full range of possibilities, with at least a slight chance of “much faster” rise not offset by a similar chance of “much slower”. Costs probably rise more rapidly than linearly with sea-level rise, so each meter of rise costs more than the previous meter. If so, and if we have already committed to ~3.3 m of rise from West Antarctica, then the value of preventing other sea-level rise is greatly increased, as is the value of research results that can inform wise policies on the other potential sources of sea-level rise.