The new evidence suggests (1) it has been losing ice for more than two decades, and (2) there is significant change in the pace of change. During 2012-2016, the Pine Island Glacier continues to shrink, and that, in turn, has slowed the retreat of its neighbor, the Ross Sea Glacier, as well as much of the surrounding ice flow. To make matters worse, a strong, continuing storm system in the Ross Sea has stalled out the rest of the Ross Sea’s flow, resulting in an absence of any measurable ice gain in the rest of the system.
There are some interesting trends, though, that do come as no surprise. They are:
The thinning of Pine Island Glacier during 2012-2016 is more complex than what is being reported in the headlines. A change in the nature of ice flow is causing glaciers to lose ice at a faster pace than previously thought. Gulf of Rovira, a large, thinning glacier that borders Pine Island Glacier, continues to change its flow patterns, but these variations are not significantly impacting the speed at which Pine Island Glacier thinning is accelerating. The most dramatic change occurred in 2012 – it became almost entirely unstable, and moved in an almost random fashion. This allowed the Pine Island Glacier to accelerate its retreat in the short term. There were also some signs it had begun to gain ice, but if things continued at their current pace, there was little doubt it would be the next big victim of the long-term trend toward thinning, and ice-loss elsewhere. Such speedier retreat also meant that in 2012 Pine Island Glacier went from being one of the fastest retreaters on the planet to the slowest retreater in the last 50 years.
The biggest change during this period was the loss of ice at Pine Island. During 2012 and 2013, roughly 80% of the ice on Pine Island went away. In 2014, that figure was cut in half, and by 2016 it had fallen to less than 10%. This is a really slow rate of change, and represents the slowest loss of ice on the planet. During the past five decades, the rate of loss of ice on Pine Island has been about the same as average – in fact, if we use historical melt rates as a proxy, this would be the second slowest rate of loss in the entire twenty-first century. Ice loss has been more extreme on the westside of Pine Island Glacier this year than in the rest of the system. In 2014, for example, Pine Island Glacier retreated by an average of about 80 kilometers a day, an average loss that is almost twice as fast as in years with other major ice loss events like the 2001, 2004, and 2012 storms, which reduced flows by 30 kilometers a day. In contrast, in this year’s Pine Island event the flow over the island was reduced by about half because the storm system stalled at the base of the glacier.
This year’s retreat is also a bit of a surprise. After earlier and more rapid retreats during past years, the Pine Island Glacier began this year with a slower rate of decline than we had anticipated, largely because it lost ice in its interior. Some of this lost ice made it into its interior, where, with increasing ice pressure, it was exposed to greater warmth, and that added heat made it melt even faster, causing the ice to thicken in the rest of the system. As the Pine Island Glacier became more warm, the runoff rate in the system decreased, and a more gradual increase in the flow rate across the glacier occurred, although a brief, sharp recovery is expected during the summer. All of this helps explain why the Pine Island Glacier continues to be shedding more and more ice to the west.
The Pine Island Glacier is one of the most productive glaciers in Antarctica, and when it thins, it lowers the volume of that glacier and leads to a reduction in glacier volume through calving, which has led to the dramatic change we have seen. The Pine Island Glacier retreats as it melts, and these changes are not random. In particular, thinning should bring about more changes in the flow, flow velocity, and so on. Although other smaller, thinning glaciers have been changing their flow patterns and their velocities all this time, even the most rapid retreat is a gradual change overall.
The new evidence also helps explain the massive changes being seen in the Ross Sea, and these are real and not just a trend or coincidence. To see the actual changes being made in the region, you have to look at the radar imagery from the MHA project. It shows one of the major changes to be occurring in a series of images from 2014 to 2016, as the Ross Sea ice retreat rate increased rapidly:
As you can see, the Ross Sea ice shelf in 2013 got so thick it effectively covered