Thursday, August 7, 2014

Physics Shows Instability in Antarctic Ice Shelf

Fig.1: Locations affected in West Antarctica and Wilkes Basin (from Physics Today)  Physics of melting and onset of shelf instability shown below:

One of the constant points of issue in the nonsensical climate change debate (which shouldn't exist at all) is the extent of melting and instability in the Antarctic. As I noted in previous blogs, the take by the denialists is usually off because they don't grasp the processes at work. See e.g.

Now, new research shows that West Antarctica's unstoppable ice shelf collapse has likely begun. (Physics Today, July, p. 10). First, a team led by Eric Rignot at UC-Irvine and NASA have documented accelerated glacial retreat in the red-shaded region of Fig. 1.. Most importantly, the researchers point out the lack of any geological features (e.g. bedrock 'bumps')  that might re-stabilize the ice. (One computer model shows a full scale collapse is likely to occur over the next several centuries, if not sooner.)

Meanwhile, a team led by Matthias Mengel and Anders Levermann at the Potsdam Institute for Climate Impact Research in Germany have shown that the larger, thicker ice sheet of East Antarctica may not be as stable as previously thought. They found that the region shaded in yellow (Fig. 1) is held in place by only a small volume of coastal ice that eventually could melt. Such loss would have catastrophic consequences for global sea level rise.

The fragility of the West Antarctic ice shelf can be traced to several factors which physics can shed light upon. These are illustrated in Fig. 2. First, as shown, the ice rests on a bed that lies below sea level. Second, note that the bed slopes backward, actually falling deeper below sea level farther inland. It is this confluence of conditions that gives rise to marine ice-sheet instability since currents of warmer water eat away at the ice from below. When the 'grounding line' (see Fig. 2) gets pushed back by warmer-than-usual water then the rate of melt discharge increases and the glacier retreats further.

Rignot and colleagues used 20 years of interferometric synthetic aperture radar (Insar) data to track the retreat of grounding lines in 4 West Antarctic glaciers, including the two shown in Fig. 1. Their data showed that for these two areas the grounding lines had retreated between 10 and 35 km over the 20 year time and the rates are now speeding up.

What if there was a total collapse of both ice sheets - for Western and Eastern Antarctica? How large would the effects be, respectively? In the first case, there is enough ice mass to raise sea levels by about 3m or 9.9 feet. In the second case, there is enough ice to raise sea levels by more than 50 m or 165 feet. Thus, in such an event most of the planet's lower lying regions would be under water, including Florida. While you wouldn't have the extreme phase depicted in 'Waterworld' you would have a world with a lot more land claimed by water - perhaps 40 percent more. The Earth's image from space would appear radically different from what we see now.

How precise are the models being used? As with most climate change modeling, or modeling any complex systems, there's always a tradeoff between the scope (or scale) and accuracy. If I increase my model's scope, say using data beyond the resolution level of instruments to detect solar granulation, then I lose accuracy in the process. 

For example, very early in his Ph.D. dissertation (p. 17), Jason Lisle concedes an inability to properly resolve granules via his MDI (Michelson Doppler Imaging) data, though he does make an appeal to local correlation tracking (LCT) as a kind of savior since it allows motions to be deduced from the images even when individual moving elements aren't well resolved. The question that arises, of course, is whether the product of such LCT manipulation is real, or to put it another way, an objective physical feature that is not associated with instrumental effects, distortions or errors.

Exacerbating this suspicion is Lisle's own admission (ibid.) that LCT "suffers from a number of artifacts".   Thus, the question emerged of whether Lisle really had enough signal to ascertain a "persistent N-S alignment" in solar supergranules,  taken as exhibiting a fundamental polarity preference.

In the case of the Antarctic glacier modeling, the researchers were faced with a choice of representing a glacier's flow along a single line, or simulating a whole ice sheet using a coarse-grained model. In the end, they used something in between, a "regional scale model that covered an entire basin but not the full ice sheet" according to one of the researchers. Of course, it definitely helps if one can use a construct or entity pertinent to some "midway" position that actually exists!

In the end, the constellation of models shows the Antarctic ice sheets, both for the Eastern and Western sectors, are under severe and constant stress from climate change-global warming. The portents and predictions are far from sanguine but meanwhile too many human politicos fiddle while the glaciers and ice sheets melt.

Sadly, it is our future generations that will have to pay the price.

See also:

No comments: