Antarctic Ice Sheet Deterioration Study Left Out Important Factors

A few days ago a team of climate scientists (Catherine Ritz, Tamsin Edwards, Gaël Durand, Antony Payne, Vincent Peyaud, and Richard Hindmarsh) published a study of “Potential sea-level rise from Antarctic ice-sheet instability constrained by observations.”

The study asked how much Antarctic ice sheets might contribute to global sea level by 2100 and 2200 AD. The results contradicted some earlier estimates which are on the high end, but conformed very closely to the current IPCC estimate, raising that number by a negligible amount.

The authors note that rising seas due to global warming is a significant problem. In other words, this research could be good news on one way, in that the highest estimates were not supported. But it is bad news in another way, in that the Antarctic ice sheet will contribute enough that when added to other sources of sea level rise, coastal regions will be seriously affected.

One of the study’s authors, Tamsin Edwards, wrote a summary of the paper in The Guardian. That essay provides a useful summary of the history of Antarctic ice-sheet research, and places the new research in perspective. In particular, Edwards notes,

We’re not the first to predict the consequences of Antarctic instability. So what’s new? We are the first to use all three elements I think are essential for climate predictions: physics, observations, and statistics.

I’m not sure if this is the first study to use data, physics, and statistics, but if it is, wow. However, there may be one very important thing missing from Ritz and Edwards Et Al: A full consideration of the factors involved in ice sheets turning into ocean because of global warming.

The study involved developing a computer model simulating the behavior of the ice sheet. This model was refined by comparing results of different runs, each using slightly different values for the relevant variables, with observations, in order to weight the model variants to get a more plausible set of results. Several thousand runs of the model were evaluated in this way.

My impression of the study, which I partially wrote up here, was that there were two possible problems. One derives from those earlier higher-end estimates that the new study contradicts. Some of those estimates are based on paleo data, which attempt to link either CO2 levels or global temperatures with known sea levels contemporary with those values. Looking at sea level from a paleo perspective, one could argue that current levels of atmospheric CO2 should be associated with much higher sea levels than we have today. Since added CO2 takes decades to be realized as surface warming, and surface warming takes, we assume, considerable time to be manifest as polar ice sheet melting or deterioration, the timing of sea level rise is very much an open question. In other words, a paleo-based estimate of many feet of sea level rise does not necessarily conflict with the results of this paper, which predict “that the Antarctic ice sheet will contribute up to 30 cm sea-level equivalent by 2100 and 72 cm by 2200.” Both could be right, because it may simply take several hundred years for sea levels to reach an equilibrium consistent with between 400 and 500 or so parts per million of CO2 in the atmosphere.

The second problem concerned me a bit more. This is the idea embodied in the “Ice swan analogy” I outlined in my post. The transformation of a continental ice sheet (and its nearby sea-situated parts) into ocean water could be somewhat over simply characterized as having two parts. One is simply the melt of ice being greater than the replacement of ice from precipitation and cold conditions. The other is the physical collapse of parts of the glaciers, causing large amounts of ice to slough off into the sea where they will quickly melt and contribute to sea level rise. It is likely that the latter would affect the former, so melting would increase because of changes to the structure and position of ice after physical collapse of large parts of it. Removing the distal part of a glacier’s tongue may unplug upstream sources of meltwater, and cause further rapid deterioration by destabilizing the ice sheet’s structure.

If the catastrophic deterioration of parts of the ice sheet (catastrophic in the sense that nothing happens, then more of nothing, then still more, then suddenly a threshold is reached huge chunks fall of for a time, then back to nothing again) is not accounted for, or insufficiently accounted for, in a model, then the model may be underestimating total ice sheet contribution to sea level rise, and the rate at which that may happen.

The possibility that large scale or at least rapid deterioration of parts of the ice sheet could happen has potentially important consequences. First, if such a thing does occur in large scale, the rate of sea level rise could be very rapid for a period of years. A sea level that goes up a few millimeters a year is potentially different, as a problem to which we must adapt, than one that rises in fits and starts. Second, the total contribution of Antarctic ice sheets to sea level rise may be both larger, and less predictable.

Richard Alley is a climate scientist at Penn State who studies ice, glaciers, sea level change, and abrupt climate change. I asked him for his opinion on the Ritz, Edwards, et al. paper. I am happy-sad to say that many of his remarks mirrored my own thoughts. Happy because it is always nice to have one’s ideas about complex science confirmed by an expert to not be completely wrong. Sad, because the Ritz, Edwards et al paper does look like it may be underestimating the total amount and rate of Antarctic ice sheet contribution to sea level rise.

Alley is concerned about the lack of attention in the Ritz, Edwards et al study to important relevant mechanisms.

Alley told me that among the many factors that contribute to sea level rise (melting of mountain glaciers transferring water from the land to the ocean, expansion of ocean water as it warms, possibly from mining of groundwater exceeding water trapping from dams and other human activities) that “the largest uncertainties are attached to the ice sheets. For the 20 years leading up to the IPCC Fifth Assessment Report, the Shepherd et al. IMBIE assessment (Science, 2012) found an accelerating contribution to sea-level rise from the ice sheets, but with an average of only ~0.6 mm/yr out of the ~3 mm/yr total. At that rate, loss of all the ice sheets would require just over 100,000 years; the rate of loss of 0.001%/yr is equivalent to me as a professor losing 1/3 of one potato chip per year on a diet. Both I and the ice sheets could lose weight more rapidly; we generally would consider my weight loss to be good and that of the ice sheets to be bad.”

Alley notes that the projections made by the IPCC are a good starting point for understanding sea level rise, but that work done since the IPCC projections were solidified for the most recent report tend to indicate slightly higher rates. As with other features of climate change such as climate sensitivity, the distribution of possible sea level rise rates has a long tail at the high end. This means that rates below the average estimate are highly unlikely, but higher rates are not as unlikely, and there is a small possibility of much higher rates. The tail at the high end of the distribution is lengthened primarily by uncertainty with what will happen in Antarctica. This problem is central to current research on the contribution of Antarctica to sea level rise.

Alley notes, “Because the ongoing changes are relatively slow in their contribution to global sea-level rise, and based on other research showing how some of the processes involved in ice-sheet shrinkage cannot accelerate hugely, there has been some optimism that the long tail won’t be realized. However, a small but growing body of scientific literature has looked at the possibility that fracturing could greatly speed shrinkage; meltwater can wedge open crevasses on ice shelves or non-floating ice near the coast, thinning beyond some threshold tends to lead to complete ice-shelf loss, giant icebergs calving off the resulting ice cliffs can move the grounding line back rapidly especially if aided by meltwater wedging, and theoretically estimated limits on cliff heights suggest that much faster iceberg loss and cliff retreat are possible.”

Alley was co-author of a review here that addresses this topic. Here’s the abstract from that paper:

Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence.

Alley lauds the Ritz, Edwards, et al paper as representing “a great amount of careful work, and provid[ing] a particularly broad exploration of some of the poorly known parameters that control the ice sheet.” However, he finds that the study did not address some important mechanisms.

…the model does not allow loss of any ice shelves, does not allow grounding-line retreat from calving of icebergs following ice-shelf loss, and does not allow faster retreat from breakage of cliffs higher than those observed today, especially if aided by meltwater wedging in crevasses. The model restricts grounding-line retreat to the rate given by thinning of ice during viscous flow of an unbuttressed but still-present ice shelf, with a specified upper limit enforced on the rate of that retreat. The model also does not allow retreat up a sloping bed under forcing, something that is widely observed. The Supplementary Information includes discussion of checks that the authors did to assess the importance of these assumptions, which the authors argue justify omitting the mechanisms. However, it remains that with the model not allowing very rapid retreat, not allowing ice-cliff crumbling after ice-shelf loss, and not allowing retreat up sloping beds, the model cannot exhibit some possible behaviors that could cause rapid ice-sheet shrinkage.

So, I view this as an important step forward for the scientific community, but the qualification in the last sentence of the paper leads to additional information showing that we cannot yet confidently place quantitatively reliable limits on the possible sea-level rise from the Antarctic ice sheet. I personally hope that the new paper is right, but I will continue research on this topic in the hope of providing improved estimates. Until such work is successful, I do not believe we can exclude the possibility of faster sea-level rise than suggested in the new paper.

I did ask Edwards questions about these missing elements, but have not heard back yet. If I do, I’ll either post her response as a separate item or add them here, as seems appropriate.