Friday, 4 October 2013

On the rocks, no ice

A swift trawl through some of the Open Access Journals has brought up this gem from The Cryosphere:

Global glacier changes: a revised assessment of committed mass losses and sampling uncertainties

S. H. Mernild1,2, W. H. Lipscomb3, D. B. Bahr4,5, V. Radić6, and M. Zemp7

Abstract. Most glaciers and ice caps (GIC) are out of balance with the current climate. To return to equilibrium, GIC must thin and retreat, losing additional mass and raising sea level. Because glacier observations are sparse and geographically biased, there is an undersampling problem common to all global assessments. Here, we further develop an assessment approach based on accumulation-area ratios (AAR) to estimate committed mass losses and analyze the undersampling problem. We compiled all available AAR observations for 144 GIC from 1971 to 2010, and found that most glaciers and ice caps are farther from balance than previously believed. Accounting for regional and global undersampling errors, our model suggests that GIC are committed to additional losses of 32 ± 12% of their area and 38 ± 16% of their volume if the future climate resembles the climate of the past decade. These losses imply global mean sea-level rise of 163 ± 69 mm, assuming total glacier volume of 430 mm sea-level equivalent. To reduce the large uncertainties in these projections, more long-term glacier measurements are needed in poorly sampled regions.

In layman's terms, the authors are suggesting that (accepting high uncertainty) the World's glaciers and ice caps are shrinking faster more out of balance than previously estimated, with a consequential increase in their contribution to global sea level rise. This conclusion is premised on a future climate resembling that of the last ten years (so no projection included for increasing mean annual temperatures).

I'd be interested to get Stoat's response - it's his kind of thing, as well as those of similar inclination. My first thoughts are that here is more suggestive evidence that the synthesis estimates of future change represented by the AR5 tend to underestimate likely future climate-related trends. 

The niggling worry is that there may be a cumulative imbalance - for each metric a conservative estimate, each conclusion/estimate slightly understated - and, if it proves otherwise, the gross cumulative change is far more substantial than the projections. In this case, the paper draws our attention to something I have previously researched and have an interest in - global seal level rise. There are plenty of voices in the scientific community already saying that the AR5 estimates are conservative - here is another potential string to their bow. At it, friends...

(Indeed, Stoat responds here: http://scienceblogs.com/stoat/2013/09/30/ar5-cursory-review-of-chapter-4-cryosphere-mass-balance-of-antarctica/ see comments 8&9.) Thanks, William.

Further comment: since it is a well established and credible blog, the discussion continues at WC's site - no point in duplicating effort. Please go there and read on...

3 comments:

  1. Re shrinkage, over at Stoat you conflated shrinking and shrinking faster. They are certainly shrinking, but saying they're currently shrinking faster than other recent estimates involves an explicit comparison to those. Did they do that? Re the future, a larger mass balance deficit would imply faster shrinkage going forward, but it may be hard to compare to other estimates unless they also held climate steady. IIRC they don't generally do that. Also, don't forget that for this paper holding climate steady means deleting the thermal expansion contribution of future warming, which IIRC is larger.

    But all of this regards glaciers and ice caps, which as you know are small potatoes relative to the ice sheets. There are a number of recent papers that throw the WG1 conclusions as to the latter into serious question.

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    1. I thought I had read that they had assumed present temperature trends going forward, and a larger mass balance deficit under those conditions. Trouble is, I am a self-educated reader rather than a clomaie scientist so I sometime miss the nuances, especially when I'm consciously tring to simplify the message. Which is a bit bad, but mostly embarrassing. Always happy to be corrected.

      Did you spot the paper in 'The Cryosphere Discussions' (ie not yet cleared for publication) about the downscaling operation in the Greenland Ice sheet Model? There are some interesting figures at the end, figs. 10 - 14. Worthy of a comparison with the GMB projections used in AR5.

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  2. Should add: Thanks for the catch on this one. I try to keep up but had missed it.

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