The new article in Nature on “Recent contributions of glaciers and ice caps to sea level rise” (Jacob et al. 2012) is in stark contrast to what has gone before. It is far from the previous claims.
The main estimates before Jacob et al. 2012 were:-
- The Himalayan Glaciers will disappear by 2035. (UNIPCC AR4 2007) Changed to the Himalayan Glaciers may disappear by 2350. (UNIPCC 2010)
- The Grace Satellite data shows that the polar ice caps are not only melting, but the melt rate is accelerating. Velicogna 2009 claimed that the acceleration in Greenland was −30 ± 11 bnt/yr2 to 286 bnt/yr-1 in 2007 to 2009, and in Antarctica was −26 ± 14 bnt/yr2 to 246 bnt/yr-1 in 2007 to 2009. Concentrating on the period from 2006 to early 2009 for Antarctica only , Chen et al. 2009 estimated that the continent was losing ice at the rate of 190 ± 77 bnt/yr-1, two-thirds is of which comes from West Antarctica, covering about a quarter of the total land surface area. By 2010, the loss from both polar caps would, by Veligona’s estimate be 600 to 650 bnt/yr-1.
- The average of these two articles was that in 2010 there would be around 600 bnt/yr-1 loss per year.
One of the articles’ authors, Prof John Wahr of University of Colarado, Boulder, had previously stated that the Grace measurements indicate an accelerating trend in Greenland. The current graph at Wahr’s website for Greenland shows a distinct accelerating trend through to the start of 2010.
Mass variability summed over the entire Greenland Ice Sheet, monthly Gravity Recovery and Climate Experiment (GRACE) results (black line; the orange line is a smoothed version) April 2002 and December 2009.
Prof John Wahl’s graph of Greenland Ice sheet loss, indicating a doubling of the rate of loss over the period to around 150 bnt/yr-1 in 2009.
- In Zwally and Giovinetto 2011, using three separate estimation techniques, and including the pre-satellite data from 1992 to 2002, estimated the range of +27 to -40 bnt/yr-1.
The new paper in Nature:-
Estimates no net loss from the Himalayas in the period 2003 to 2010. When the claim that the Himalayas would lose their glaciers by 2035, Rajendra Pauchari, head of the UNIPCC said the doubts were “voodoo science”. Now even the more moderate claim of melting over hundreds of years looks to be in doubt. Josh has penned a cartoon to illustrate this point.
- Velicogna 2009, seems somewhat extreme. The Nature paper would estimates a loss of 50% to 75% Velicogna estimate for 2010.
- Most importantly, there is no mention of acceleration of ice melt from the polar ice caps. This sudden turn-around might be to a sudden change in the data. The sea level rise appears to have stalled in the last 18-24 months, so the sea ice melt (which the Nature paper estimates accounts for 40% of the sea level rise) may have stalled as well. (See Appendix 2). It is necessary to re-run the Nature paper numbers for 2011 data to confirm if this is the case.
In conclusion, it looks that the new nature paper reaches a more moderate position than previous papers using the GRACE satellite data, as it uses a longer period, and subjects the data to a more detailed breakdown. However, in terms of the polar ice melt, it still more extreme than a paper that uses a longer timeframe and three distinct methods of calculation.
Appendix 1 – Leo Hickman in the Guardian has a breakdown of the figures, that nicely puts the issue in context.
|Ignore||Region||Rate (Gt yr-1)|
|3||Franz Josef Land||0.±.2|
|6||Siberia and Kamchatka||2.±.10|
|8||High Mountain Asia||-4.±.20|
|8b||Pamirs and Kunlun Shan||-1.±.5|
|8c||Himalaya and Karakoram||-5.±.6|
|8d||Tibet and Qilian Shan||7.±.7|
|13||Northwest America excl. Alaska||5.±.8|
|15||Ellesmere, Axel Heiberg and Devon Islands||-34.±.6|
|16||South America excl. Patagonia||-6.±.12|
|19||Greenland ice sheet.+.PGICs||-222.±.9|
|20||Antarctica ice sheet.+.PGICs||-165.±.72|
|GICs excl. Greenland and Antarctica PGICs||-148.±.30|
|Antarctica.+.Greenland ice sheet and PGICs||-384.±.71|
|Total contribution to SLR||-1.48.±.0.26|
|SLR due to GICs excl. Greenland and Antarctica PGICs||-0.41.±.0.08|
|SLR due to Antarctica.+.Greenland ice sheet and PGICs||-1.06.±.0.19|
Appendix 2 – University of Colarado Sea level Rise Estimates