Compounding problems for sea level rise…

28 01 2015

Another guest post by Mark Cochrane…..

One of the larger concerns in recent years has been the question of just how fast sea levels might rise due to collapsing ice sheets of Antarctica and Greenland. In the IPCC AR4 report (2007) there was considerable furor because the 2005 cut off for literature and the natural conservative nature of the ‘consensus’ interpretation resulted in estimates of sea level rise that were known to be too low at the time of publication: specifically, from 0.18 to 0.59 m by 2100, depending on which scenario you chose and the low-to-high extremes. In the more recent AR5 report (2013), they conclude that for the best of emissions cases, if we start immediate and extensive carbon emission reductions (RCP 2.6), sea level is expected to rise by 28-61 cm by 2100, while in the worst of cases (RCP 8.5) sea level rise is expected to be 52-98 cm. This is still conservative but much better than the AR4 estimates.

The real question is whether sea level rise occurs at close to a linear rate (fixed amount per year) that is slow and easily projected, or if it is increasing at a nonlinear rate (fixed percentage per year) that could yield unpleasant surprises in future years? Dr. Richard Alley (2010) compared projections of sea level rise going forward and basically found that most included 1m within their error range, with the exception of one serious outlier at 5m made by Dr. James Hansen (2005, 2007, 2012). Hansen’s predictions have not been well-received by the community of experts on ice sheet dynamics. They point out that, so far, there has been nothing like the amount of sea level rise observed that would be necessary to reach 5m in a linear fashion. Hansen however premises his ideas of rapid ice sheet collapse on nonlinear phenomenon caused by things like glacial melt water being transported to the base of the ice sheets and acting as a lubricant to speed their movement dramatically.

In the mean time, more traditional approaches to looking at glacial melting rates have had values centered more on 1 meter, to maybe 2 meters under worst conditions, of sea level rise by 2100 (NOAA 2012). Hansen has been intransigent in his estimations and the rates of sea level rise keep exceeding the best estimates of the ‘experts’. Glacial melt within dynamic ice sheet models has typically been modelled based on simple top down melting with unchanging processes for explaining the ongoing flow of ice sheets. However, the accelerating rates of observed ice sheet flow and disintegrating ice shelves have led to reappraisals of what is going on. As I recently detailed (post #2340), warmer ocean waters have been melting ice sheets from underneath in some regions, removing the stable grounding lines and now the West Antarctic Ice Sheet (WAIS) is in irreversible collapse (Rignot et al 2014, Joughlin et al. 2014).

Similarly, Pollard et al. (2014) have recently tried to improve continental ice sheet models by adding the processes of oceanic melting and hydrofracturing (melt water from the surface pouring into cracks and forcing them further apart) and also account for ice cliff failures (when they get so large the ice face crumbles). Both processes they added are based on observations made in the field in recent years. They looked at the effects on both the WAIS and the Eastern Antarctic Ice Sheet (EAIS). The interesting thing (to me at least) is that cliff failure and hydrofracture combine to cause very large changes in expected sea level rise that either process alone does not create. By itself, cliff failure does little to accelerate collapse over the standard model representation. Conversely, hydrofracturing, by itself, causes expected sea level rise to roughly double from 2 to 4 m over thousands of years. When both processes are included though, the sea level rises by 17 m, with about 4 m happening in the first 100 years! Clearly the two processes interact to strongly enhance the collapse rate.  The EAIS collapses slowly over thousands of years but the WAIS collapses in decades.

The Pollard et al (2014) paper is not expressly addressing our future as it was aimed at explaining formerly unexplainable sea level rises during some previous interglacial periods – which their results ended up matching fairly closely. They forced their model using 400 ppm CO2 so it isn’t wildly different from what we currently have though.  In the model, roughly 3 m of sea rise come from the WAIS alone, within 100 years. If you add the much slower response of the EAIS and the undiscussed but very similar ice sheet collapse from Greenland, suddenly Hansen’s 5 m sea level rise call doesn’t look so outlandish after all. Interestingly, the senior author on the Pollard et al. paper is none other than Richard Alley who previously did not see how such rapid ice sheet collapse could be occurring.

This still doesn’t mean that we definitely will get 5 m of sea level rise in this century (let’s pray that we don’t!) but it certainly increases the perceived risks of much larger sea level rises than the IPCC AR5 report states (again). It also helps explain the increasing rates of sea level rise from 1.0 mm/yr to 3.0 mm/yr in recent decades. Things seem to be proceeding in a decidedly nonlinear way.




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