This graphic says it all:
Abram et al.‘s Figure 5| Melt response over the past millennium. a, Schematic of Prince Gustav ice shelf history showing its presence (blue), intervals of rapid retreat (1957 and 1989; yellow) and collapse (1995; red). b,c, JRI mean temperature anomaly (green;b) and melt percentage (red;c) shown as 11-year moving averages. Thick lines are 21-year Gaussian kernel filters; dashed lines denote 1981–2000 mean. Lowest temperatures and melt occurred at AD 1410–1460, followed by progressive warming and a nonlinear melt increase. d, The occurrence of melt layers (grey lines) and a 100-year stepped average of melt frequency (purple) at Siple Dome in West Antarctica.
New research published in Nature Geoscience from Nerilie J. Abram et al. (subs. req’d) presents evidence that West Antarctic ice melt accelerated over the course of the last 1,000 years. About 400 years ago, average temperature anomalies (based off the 1981-2000 mean) increased from -1°C to -0.75°C (green curve in above graphic). You can see the interannual and interdecadal variability in this time period, which was natural. Then, starting 100 years ago, temperature anomalies rose from -0.75°C to today’s slightly positive anomaly. As a result, the melt percentage jumped to 5% at James Ross Island. That melt jump was nonlinear due to the ~0C melt threshold. As the authors state, “where summer temperatures do exceed the melting threshold, the amount of melt produced is proportional to the sum of the daily positive temperatures rather than their mean. This means that as average summer temperature increases and positive temperature days become warmer and more frequent, the amount of melt produced will exhibit an exponential increase”.
That cause-and-effect relationship is one reason why a 3°C average temperature rise carries so much more impact than a 2°C average temperature rise in polar regions. It also explains why small changes in historical temperatures allowed the ice shelves to form in the first place. The large “permanent” ice shelf collapses in recent history are the effect of rising temperatures. It should be obvious too that predicting the timing of future ice shelf collapses is difficult if not impossible.
The Wilkins Ice Shelf collapsed suddenly in 2009. This shelf is located southwest of the James Ross Island site cited above. As I wrote in the Wilkins post, six other shelves completely collapsed in contemporary times: Prince Gustav Channel, Larsen Inlet, Larsen A, Larsen B, Wordie, Muller and the Jones Ice Shelf. These ice shelves responded to the West Antarctic Ice Sheet (WAIS) warming observed in the last century or so. WAIS warming is occurring faster than almost any other location on the globe. There are areas in the Arctic and now the Antarctic that have observed +2.4°C warming from 1958 through 2009. In addition to anthropogenic near-surface temperature rise, the ocean surrounding Antarctica has warmed recently. Ice shelves are therefore being melted from above as well as below. Does the following sound familiar? “Over the past 18 years, Martinson and his colleagues have measured the physical properties of the ocean around Antarctica and came to the startling conclusion that the majority of the heat anomalies they have measured have occurred since 1960. Unfortunately, those anomalies have been growing exponentially ever since.”
Based on the above, we know that West Antarctica is warming very rapidly. We know that warming anomalies are growing exponentially. Problematically, even small temperature changes cause exponential changes in melt. Exponential change growing off of exponential change creates a highly nonlinear, and therefore very unpredictable system. What might that mean for the WAIS? It could mean that rapid effects take place in the future. In other words, ice sheet properties could change quickly. Large melt areas could start one day without very little prior signal. Additional ice sheet collapses could take place without much notice. Increasing greenhouse gas emissions will cause increasing radiative forcing, which in turn will cause increased heat storage by some climate component (primarily the ocean to date, but also the atmosphere). Current global energy imbalance guarantees decades’ worth of additional heating. That heat will eventually impact Antarctica and its massive ice sheet. Melting of global land-based ice to date increased global sea level by an average of 8 inches in the last 100 years. If the entire West Antarctic Ice Sheet melted (which would happen sooner than East Antarctica because it rests on bedrock below sea level), sea levels would rise 4.8 meters. The entire WAIS won’t melt for centuries, but sea levels would easily rise more quickly than the current 3mm/yr as annual WAIS melt increases due to increasing temperatures.
There is no catastrophe knocking on the door today, but WAIS melt will affect coastal regions this century. Total sea level rise off the east coast of the US exceeded the global average, which has already caused communities to re-examine infrastructure. Higher levees and other protective structures either have been built or are being considered by cities such as Washington, D.C., Norfolk, and New York City. Efforts to date haven’t been sufficient (see Hurricane Sandy damage along the New Jersey shore), which points to a need for more aggressive analysis of needs and implementation of new climate-based policies. Costs to these and other communities will grow as international mitigation efforts stall.
