The Science of Melting Ice Sheets: New review in Nature

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

ResearchBlogging.orgThere 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.

These numbers are small, but they are larger than previous estimates and observations. Still, compared to the potential sea level rise when one considers that the ice in the continental glaciers equals several meters of ocean water, near future sea level rise may be expected to be relatively low if these models are correct and account for everything. Over a century of time, this amounts to about 300 mm, or one foot, of sea level rise. If, however, oceans are warming more than the air at present and a few more episodes of that occur over the next century, this may be considered a minimal estimate. One foot does not sound like a lot of sea level rise, but it is enough to remove extant barrier beaches. Also, flood tides would not be increased by one foot, but rather, more exponentially. This is how a sea level rise of about this order of magnitude over the last century managed to contribute to the flooding of the lower Manhattan subway tunnels when the region was struck by Hurricane Sandy last year.

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

Photo Credit: christine zenino via Compfight cc

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5 thoughts on “The Science of Melting Ice Sheets: New review in Nature

  1. Wow, Which one is correct? Ice 2sea or this one? Big Difference! I have already been told quite bluntly that most people care a lot more about Wheat and Rice yields apparently enhanced by additional Co2 concentration (Greening Desert Implication #1 as deserts would be last and least affected) than melting sea ice and snail shell thickness! Where is this all going?

  2. Ice2sea is not a study. This paper is not a study. They therefore can not say two different things. In any event, the Ice2Sea report is cited in this paper, and incorporated in it. It is not in conflict with it. Specifically, “Analytical solutions are now available to test and verify marine ice-sheet models, so that the numerical error associated with predicting grounding-line motion can be reduced significantly to the level of parameter uncertainties39: models that attempt to account for grounding-line dynamics should incorporate horizontal stress transmission across the grounding line, so that the grounded ice sheet realistically feels the influence of floating ice (Box 2). Furthermore, the grounding line needs to be resolved at a sufficiently high spatial resolution.”

    You need to start paying more attention.

    Your comments about wheat and rice are out of the blue and hardly relevant to the science we are discussion.

    Where this is all going is that you are getting annoying.

    (“I’m about to be censored because I speak the truth” in 3 … 2… 1…)

  3. Wait a minute, Greg, I thought I asked a legitimate question about Ice melt Predictions and sea level rise. I came to this part of your blog because I thought you said sea levels are rising at twice the rate from ten years ago. Sea levels are supposed to have risen seven inches in more than a century .This predicts a foot in another century or five inches past background if I read it correctly. As far as Sandy goes in the tunnel flooding, It also came in at high tide with Lunar Perigee,did it not? If I have to have a PhD to understand all this Fine. I had hoped you could answer questions being asked all over.

  4. Socalpa, you might enjoy the book “rising seas” (see right side bar near the top for a link to it).

    Sandy’s would not have flooded the subways had it not been for prior sea level rise.

    And yes, again, sea level rise rate doubled. There is not a “background” rate of sea level rise. We might expect the rate to increase further, and no current study effectively addresses some of the most likely cause of large scale quick melting of continental glaciers or explains the quasi-periodic sudden melting and sea level rise events we’ve seen in the past.

  5. Here a video how these pulses might happen =>

    Also see James Hansen’s new meta study, “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming is highly dangerous”, available here (open access) =>

    And here his blog post on the issue => “Disastrous Sea Level Rise Is an Issue for Today’s Public — Not Next Millennium’s”

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