There has recently been some discussion, already mentioned in ‘The Observatory,’ about a finding that the Greenland Ice Cap and part of the Antarctic Ice Cap are not melting as fast as once suggested. The conclusion was based on a new method of analysing data from the pair of GRACE gravity measuring satellites. However, it is a good example of how a simple conclusion can enter the climate change debate and be accepted as a fact when a closer look at the data shows much uncertainty. It illustrates that standing back and looking at the bigger picture, often reveals something quite different.
According to the new paper by Wu et al (2010) published in Nature Geoscience the Greenland and West Antarctic ice caps are melting at half the rate previously measured.
The melting of the Greenland and Antarctic ice caps has been monitored since 2002 by the two GRACE satellites that detect small changes in the Earth’s gravitational field that are related to the distribution of mass on Earth, including ice and water. When ice melts, or moves it has a small but measurable effect on the gravitational field that shows itself as minor perturbations in the orbits of the two GRACE satellites. The problem with the GRACE data is that it is extraordinarily difficult to calibrate requiring very sophisticated mathematics and tiny measurements in a sea of noise and undoubtedly many unknown effects.
Data has been extracted from GRACE and most scientists understand that they are feeling their way towards a better understanding of the satellites and their data. One study of the polar ice caps published in 2009 that has received much comment calculated that for Greenland the ice was melting at an accelerating rate. In 2002 – 2003 it was 137 Gigatons per year, whereas by 2007-2009 it had increased to 286 Gigatons a year.
For West Antarctica, the estimate was 104 Gigatons per year in 2002 – 2006 and in 2006 -2009 it had increased to 246 Gigatons per year.
But the new results by Wu at al (2010) change those figures drastically. According to Wu et al it appears that the previous results were not properly corrected for glacial isostatic adjustment, which is the rebound of the Earth’s crust as a result of the melting of the massive ice caps from the last major Ice Age about 20,000 years ago. These crustal movements have to be included in the calculations, since they alter the Earth’s mass distribution and affect the gravitational field. Wu’s figures are Greenland (2002 – 2008) 104 +/- 23 Gigatons a year and 64 +/- 32 Gigatons a year for West Antarctica. Wu’s figures are approximately half of those previously published. How can they be reconciled with the other observations?
Compare Wu’s figures to another recent study published in Nature Geoscience by researchers at the University of Texas at Austin in Austin, Texas. They conclude that the mass loss for Antarctica – 190 +/- 77 Gigatons a year averaged over 2002 – 2009. This shows the spread of ice loss measurements for Greenland and Antarctica and is much larger than previous estimates (although consistent with the most recent Synthetic Aperture Radar estimates of 196 +/- 92 Gigatons a year.)
Researchers have told me that the new analysis technique being developed by Wu et al is promising, and will no doubt be very useful in the future, but at present it is throwing up contradictions.
It produces a figure for Greenland’s Glacial Isostatic Adjustment (GIA) that is 12 times larger than that predicted by the models that have observational support from geological changes in sea levels.
The large present day GIA required by Wu et al would have to have a large ice mass loss occurring in the Holocene period. There is no evidence for this in geological ice-core data.
Wu et al predicts vigorous thickening of the central region of the Greenland Ice Cap, major ice losses in the southwest and thinning in the north. None of these things are seen.
Clearly, the GRACE approach to evaluating ocean mass and sea level trends still has a long way to go, and must develop a longer period of data acquisition, before it can ever be considered a reliable means of providing assessments of ocean mass and sea level change that are accurate enough to detect an anthropogenic signal that could be confidently distinguished from natural variability.
Consequently, is would be unwise to make too much of the new figures. Such caveats are not present in the associated NASA press release detailing this research.
Finally, the ice mass loss rates must be put into context. Clearly their rate of change is all-important, as is region from which the loss is occurring and its contribution to sea level change. However, a loss of 200 Gigatons per year from Antarctica seems a great deal. In absolute terms however it is tiny. The mass of the Antarctic ice sheet is estimated at 33 million Gigatons.
I am grateful to several glaciologists and climatologists for discussions about this work. It was however initially not possible to obtain a copy of the Wu et al (2010) Nature Geoscience paper for detailed scrutiny as the lead author refused to send a copy to me, and was only willing to share it with “colleagues.”