Antarctica is pretty much covered with glaciers. Glaciers are dynamic entities that, unless they are in full melt, tend to grow near their thickest parts (that’s why those are the thickest parts) and mush outwards towards the edges, where the liminal areas either melt (usually seasonally) in situ or drop off into the sea.
Antarctic’s glaciers are surrounded by a number of floating ice shelves. The ice shelves are really the distal reaches of the moving glaciers floating over the ocean. This is one of the places, probably the place at present, where melting accelerated by human caused greenhouse gas pollution occurs. The ice shelves are fixed in place along their margins (they typically cover linear fjord like valleys) and at a grounding point underneath the shelf some distance form the ice margin but under sea level.
The collapse or disintegration of an ice shelf is thought to lead to the more rapid movement of the corresponding glacial mass towards the sea, and increased melting. This is the big problem right now with estimating the rate of glacial melting in the Antarctic. This is not a steady and regular process, as rapid disintegration of an ice shelf is possible. Most likely, Antarctic glacial melting over the coming decades will involve occasional catastrophic of an ice shelf followed by more rapid glacial melting at that point.
The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using eighteen years of continuous satellite radar altimeter observations we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 km3 per year for 1994-2003 to rapid loss of 310 ± 74 km3 per year for 2003-2012. West Antarctic losses increased by 70% in the last decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades.
This is one of many reasons that even the most extreme of the IPCC estimates of ice loss (generally) and its contribution to sea level rise have to be seen as a lower limit. This is a substantial change, and it is very recent. It isn’t just that the ice sheets have gotten thinner, but also, that the rate of melting at these margins is increasing.
Caption to figure: Fig. 1 Eighteen years of change in thickness and volume of Antarctic ice shelves.
Rates of thickness change (m/decade) are color-coded from -25 (thinning) to +10 (thickening). Circles represent percentage of thickness lost (red) or gained (blue) in 18 years. Only significant values at the 95% confidence level are plotted (see Table S1). Lower left corner shows time series and polynomial fit of average volume change (km3) from 1994 to 2012 for the West (in red) and East (in blue) Antarctic ice shelves. Black curve is polynomial fit for All Antarctic ice shelves. We divided Antarctica into eight regions (Fig. 3), which are labeled and delimited by line segments in black. Ice-shelf perimeters are shown as a thin black line. The central circle demarcates the area not surveyed by the satellites (south of 81.5°S). Original data were interpolated for mapping purposes (see Table S1 for percentage area surveyed of each ice shelf). Background is the Landsat Image Mosaic of Antarctica (LIMA).
A paper came out in today’s Nature about glacial melting and its contribution to sea level rise. This paper does not present new research, but rather summarizes and evaluates the last several years of research on modeling and measuring contiental glaciers and their dynamics.
From the Abstract:
Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain
There are many difficulties with measuring and understanding the dynamics of melting of large continental glaciers, the large ice sheets that cover Antarctica and Greenland. As ice melts from these glaciers, they grow lighter and this allows the underlying bedrock to rise up, and conversely, if snow is added to the surface this increases the amount of depression of the underlying bedrock. For this reason you can’t just measure the surface of the ice to estimate how much has been added or removed. When ice melts on the surface, some of it travels down into the glacier and some comes right off the surface. The ice that goes into the glacier may cause deeper ice to melt, or it may provide lubrication to the base of moving streams of ice. As a glacier loses mass at the edge through calving of ice bergs, and the margin retreats away from the sea, the degree of calving, which is an ice-ocean interaction effect probably decreases. Large masses of ice are “grounded” at the outer margin on a “grounding line” beyond which is floating glacier (not sea ice, but large masses of ice undercut by the sea). The grounding line can move towards the sea or away from it, and the dynamics of this movement are complex and difficult to model or measure. Many of the Antarctic grounding lines occur on surfaces that slope downwards in the inland direction, which makes the dynamic a bit more complicated to measure.
Major changes that have improved estimates include adding dimensions to some of the models, such as considering both vertical and horizontal forces along grounding lines. Also, newer models use a finer resolution. However, the increase in resolution is thought to be insufficient; current models are not calculated at fine enough resolution to include numerous smaller ice streams that are narrower than the sampling density of the models.
It appears that the range of uncertainty of ice-melting models has improved significantly over the years so greater confidence in their predictions may be warranted. The best estimates of future contribution to sea level rise of melting glaciers is still highly variable, however.
The current estimates of contributions to sea level rise in mm per year from various studies are between 0.59 and 0.82 from the major ice sheets, between 0.71 and 1.4 for ice caps and glaciers, about 1.1 for thermal expansion, and a negligible but positive amount from changes in terrestrial water storage. These modeled amounts sum to 1.66 mm per year or 3.11 mm per year depending on the set of sources that are used. The observed change in sea level rise over the period from 1993=2008 is 3.22, so there is good agreement though the models are a bit light.
But there is a problem. Several areas of uncertainty exist in the models that are currently in use, and my impression is that these areas of uncertainty could be associated with dramatic errors in sea level rise estimate. The dynamics of grounding line changes, the role of lubrication at the base of glaciers (which can cause ice streams to speed up on their way to the sea) and the effects of warm currents shifting their position in Antarctic to cause more melt at the boundaries are among those factors that are least known and that have the highest uncertainty. Also, the seaward edge of continental glaciers are not only held in place by their grounding line on the continent, but also by more distal parts of the floating segment of the glaciers being pinned on prominence. As far as I know the effects of pinning being disrupted or lost are not included in any of the models. Also, I’m pretty sure that the effects of sea level rise on grounding and pinning have not been adequately addressed.
That these issues may be a problem is empirically suggested. The paleo-record shows that continental ice melting and associated sea level rise may occur in fits and starts, with steady melting punctuated by brief periods of extreme melting. The current models don’t seem to predict this sort of event, though these events probably happen.
Hanna, E., Navarro, F., Pattyn, F., Domingues, C., Fettweis, X., Ivins, E., Nicholls, R., Ritz, C., Smith, B., Tulaczyk, S., Whitehouse, P., & Zwally, H. (2013). Ice-sheet mass balance and climate change Nature, 498 (7452), 51–59 DOI: 10.1038/nature12238
…I have spent a great deal of time reading blogposts and comments on skeptical sites on the Internet, and one important fact has become readily apparent: that many in our community aren’t aware of one of the most important things a skeptic should know.
Iâ€™ve seen opinions stated without any factual substantiation. Iâ€™ve seen self-styled â€œexpertsâ€ make derisive comments about others’ lack of knowledge about a topic, only to find out that it was they who were, in fact, ill-informed. Why? Because too many of us don’t know what we don’t know.