Extraordinarily rapid melting of the bottom 1000 to 1500 feet of solid ice of west Antarctica’s Smith glacier was discovered by NASA’s detailed aerial radar surveys in 2002 and 2007 of the ice shelves of the Amundsen Sea embayment (ASE). Melt volumes in the ASE, the highest in Antarctica, are already a large contributor to sea level rise. This new study shows that the ice shelves of west Antarctica may be extremely unstable and that shockingly high melting rates are possible. Because west Antarctica stores huge volumes of ice, destabilization of the ice shelves would lead to rapid global sea level rise.
West Antarctica’s ice shelves are vulnerable to collapse because west Antarctic glaciers are grounded in deep coastal channels which bottom out at thousands of feet below sea level. If circumpolar deep water, which is relatively warm and salty, intrudes into these channels the base of these glaciers will melt rapidly and the ice dams stopping the landward side of the glaciers from rapidly flowing into the sea will be broken. Global sea level studies, discussed by Jim Hansen in his recent paper www.atmos-chem-phys.net/...on superstorms and rapid sea level rise show that a sudden jump in sea level to over 20 feet above today’s sea level at the end of the last interglacial about 118,000 years ago. The collapse of the west Antarctic ice sheet (WAIS) was likely a large contributor to the jump in sea level the last time the climate was warmer than today’s climate. The scientists studying Smith glacier may be witnessing the beginning of a modern collapse of the WAIS that will parallel the collapse that took place 118,000 years ago.
The details of the study showed the complexity of the conditions and processes affecting the stability of the west Antarctic ice, but those details should not obscure the big picture that the weight of the Antarctic ice cap has pushed a large area of the Antarctic continent well below sea level. Thus, when cold Antarctic bottom water formation slows and relatively warm water rises up along the coast, the whole west Antarctic ice sheet is vulnerable to sudden collapse. Rising warm deep water appears to be have rapidly melted Smith glacier between 2002 and 2007. This process may be self-stabilizing because light melt water floats to the surface, impeding the formation of deep water. Moreover, this light melt water layer acts as an insulating blanket reducing oceanic heat loss in the cold Antarctic winter. Thus the heat content of the southern ocean may increase as the ice melts.
The details described by the glaciologists are stunning www.nature.com/… :
We find melting rates in the grounding zones of the three glaciers to be much higher than steady-state levels, removing as much as 300–490 m between 2002 and 2009 in the case of Smith Glacier (40–70 m per year). The intense unbalanced melting supports the hypothesis that a large increase in ocean heat influx into the sub-ice-shelf cavities of the ASE occurred in the mid-2000s. Averaged melting rates between 2009 and 2014 are similar or lower, which also is consistent with the slower increases in mass loss observed regionally by the end of the 2000s. Nonetheless, while Pope and Kohler glaciers stabilized, the grounding line retreat of Smith persisted.
We attribute the different evolution of Smith Glacier to the retreat of its grounding line deeper allowing warmer waters to flood its grounding zone, and increasing ocean thermal forcing due to the lowering of the in situ melting point; as well as to the exposure of the glacier bottom to ocean water as the grounding line retreated rapidly. In contrast, Pope and Kohler had retreated to shallower terrain. Such combinations of ocean conditions and topography sustaining high submarine melting can hasten mass loss from West Antarctica.
The bottom line from this study and many others is that rapid melting of the Antarctic ice sheet has started. We know from many studies of global coastlines that the last time temperatures were as warm as were 1.5 Celsius above baseline — the Paris agreement target — sea levels were 20 to 30 feet above today’s level. We now know from this study that glaciers are already melting at astonishing rates but predicting how rapidly the west Antarctic ice sheet will collapse is a major scientific challenge. Thus predicting the changing rates of sea level rise and the effects on coastlines will be controversial and political, but 20 to 30 feet (minimum) of sea level rise is pretty much inevitable. It's only a matter of time.
Although 20 to 30 feet of sea level rise will be very destructive to coastlines, efforts to slash CO2 emissions are worth doing to prevent far higher levels of sea level rise that will take place if the whole Antarctic and Greenland ice caps melt. Also, slowing down the rates of rise by limiting emissions will make mitigation efforts far easier. However, the longer we delay strong action to stop emitting carbon the greater the inevitable damage will be. Sea level rise, once set in motion, will continue for hundreds of years as the oceans and icecaps slowly move towards a new steady state. Thus the costs of delaying climate action will be extremely high over the long term.