Another paper in this issue of Nature is a review by Shepherd et al. doi: 10.1038/s41586-018-0171-6 of the satellite record across Antarctica.
Somewhat surprisingly, they state that the overall rate of ice loss has "changed little" since 1992. I suspect they have a different definition of substantial change than I do.
"these studies show that the continent has contributed 7.6 ± 3.9 mm to global sea levels since 1992. Two-fifths (3.0 ± 0.6 mm) of this loss occurred during the past five years 10 . Although the rate of ice loss from the entire Antarctic ice sheet has changed little during the satellite record, speedup of glacier flow in the Amundsen Sea sector has led to accelerated losses from this region [37,38] ."
They note the evicence for ocean forced dynamic thinning:
"Although most of Antarctica has remained stable over the past 25 years, there are clear patterns of imbalance in many coastal sectors—such as the thickening of the Kamb Ice Stream and the thinning of glaciers flowing into the Amundsen Sea and at the Antarctic Peninsula. These changes reflect imbalance between ice flow and snow accumulation within the surrounding catchments. The pace of ice flow at the Kamb Ice Stream is unusually low [40] and has not altered in recent decades, but analysis of ice-penetrating radar measurements [41] shows that it stagnated over a century ago. Elsewhere, inland glacier thinning is almost exclusively coincident with contemporaneous ice speedup [21,42,43] (indicating that the thinning is dynamic in nature) and with perturbations at the marine termini of the glaciers [44] (indicating that the thinning has resulted from ocean forcing)."
On subglacial lakes:
"However, localized and episodic rises and falls of the ice sheet surface were then spotted in satellite interferometric [46] and altimetric records [47,48] , suggesting otherwise. These fluctuations, amounting to changes in height of 1–10 m over sub-decadal timescales, are interpreted to be the surface expressions of water transferring between active subglacial lake networks. More than a hundred active lakes have now been identified using this approach [49] ... Thanks to these data, we now know that in addition to periodically flushing subglacial cavities, the presence [52] of and fluctuations [27] in subglacial lake water can lubricate ice flow in parts of the continent."
Ice shelves: They note the majors (Ross, Ronne, Amery) are stable since the 1990s
"Although the major Ross, Filchner-Ronne, and Amery ice shelves have remained stable since the 1990s,"
That is something of a relief, I worry about Amery. (Not that i dont worry about the other two ...)
They continue:
"many ice shelves in West Antarctica have experienced long-term thinning over the same period. In the locations where retreat or thinning have occurred, the grounded ice inland has also been destabilized. The dominant control on this pattern is believed to be the presence (or absence) of warm ocean currents offshore [59] . Altogether, the volume of Antarctic ice shelves has declined through net overall thinning (166 ± 48 km^3/yr between 1994 and 2012; ref. 11 ) and through progressive calving-front retreat of those at the Antarctic Peninsula (210 ± 27 km^3/yr between 1994 and 2008; ref. 3 ). Combined, these losses amount to less than 1% of their volume. However, the highest ice shelf thinning rates have occurred in the Amundsen and Bellingshausen seas [12] , where five have lost between 10% and 18% of their thickness [11] owing to ocean-driven melting at their bases [67] ."
They briefly discuss the Larsen collapses also nd point out that thinning may not be a prerequisite for collapse:
" The relationship is, however, not universal; for example, although the Wilkins ice shelf collapsed in 2009, it did not thin in the preceding five years [79] ."
Wilkins was one of Mercer's canaries.
They discuss the speedup in glacier flows after shelves disappear leading to dynamic thinning:
"Over the past two decades, for example, surface lowering has spread inland across the drainage basins of the Pine Island and Thwaites glaciers at speeds of between 5 and 15 km/yr, and the majority of their catchments are now in a state of dynamical imbalance (they are thinning owing to accelerated flow)."
They point out reasons that the Amundsen region is vulnerable:
"Glaciers flowing into the Amundsen Sea sector of West Antarctica are particularly susceptible to climate forcing, owing to their geometrical configuration and the absence of any substantial ice shelf barrier [90] , and today the pace of ice sheet retreat along parts of this coastline dwarfs that during the Holocene period. The region’s ice shelves have thinned [11,12] by 3 to 6 m/yr, and its glacier grounding lines have retreated [85,86] by 10 to 35 km since 1992, which is 20 to 30 times the rate since the Last Glacial Maximum ..."
Ocean forcing is clearly implicated:
"The forcing for these events is now widely regarded to lie in the surrounding ocean, because ice drawdown has originated at and evolved from the terminus of neighbouring but distinct ice flow units [42], and because warm [67] and warming [44] water is present within the cavities beneath their peripheral ice shelves."
But the situation is complicated. Although
"it has been concluded [94] that the region is now undergoing marine ice sheet instability, with no geometrical obstacles to prevent irreversible decline."
there other clues that the process is not so straightforward, and doom might take it's own sweet time:
"However, satellite observations have revealed that retreat of the Pine Island Glacier halted around 2011 [95,96],and that ice thinning inland abated in the following years [88]. This suggests that the situation is more complicated than a consideration of the glacier geometry alone, and may involve changes in the degree of ocean forcing, as has occurred in the recent past [87,97] ."
Thy have an extended discussion of sea ice which i will not cover here.
In the summary:
"Although persistent ice shelves have fringed Antarctica for thousands of years 56 , there is now widespread evidence of changes in their extent [14]] and thickness [3,11] . Altogether, their volume has decreased by more than 300 km^3/yr since 1994 [3,11] , notably due to collapse and calving at the Antarctic Peninsula and rapid thinning of those in the Amundsen and Bellingshausen seas. These events have triggered retreat [85,86] and acceleration [21,43] of marine-terminating glaciers and ice streams around the continent, leading to the drawdown of ice from their inland catchments [39,42] . Since 1992, the grounded ice sheet has lost 1,350 ± 1,010 Gt of ice, causing a net 3.8 ± 2.8 mm contribution to global sea level rise [36] ."
The question remains:
"A key unanswered science question is how long it will take for the ice shelves that are currently thinning to reach a point whereby they are no longer providing effective buttressing for the grounded ice inland."
Indeed. Inquiring minds want to know, and every coastal community in the world needs to know.
I attach figs 1 and 2. They are very pretty.
sidd
