94. July 5th, 2008, 8:59 am
MeltyMan and others interested in the science:
I have read the paper, and I conclude that this particular piece of work is an important contribution to understanding the flow of ice streams, but that stretching it to base larger conclusions on the the effects of global warming is unsound. As with most science, this a piece of an ever more detailed puzzle but not a watershed discovery.
The authors of this paper (van de Wal et al., Science, in press) measured ice velocity by drilling stakes into the ice surface and annually recording their position using GPS equipment (thus the satellite measurement reference). Starting in 2005, permanent GPS equipment was affixed to each stake that measured and reported positions as frequently as hourly with an overall temporal accuracy of 1 day or better for each stake location. This was done in an effort to identify short-term changes versus seasonal averages in ice velocity.
They also collected data on surface ablation (melting) using sonic height ranging. A stake is again drilled into the ice surface to a depth that fixes it so it will remain unaffected by surface melting, and an ultrasonic height measuring device is mounted on it. Melting is measured from the changes in the height of the snow and ice in the area.
The work correlates changes in ice velocity with melting rate and serves to help understand the response of massive ice streams to the effects of increased lubrication caused by surface melting. A few limitations and related points deserve consideration, in my opinion.
First of all, the measurements were made on a transect that was oriented East-West at a latitude of 67 deg. N on the East coast of Greenland. This region has been identified by three separate bodies of measurement as being roughly in mass balance (no net melting or ice accumulation). The 2006 reports of acceleration of ice mass loss have been located on the West coast of Greenland, and thus the measurements made by these authors (that average ice velocities have declined slightly) do not represent a contradiction to the overall state of an increase in the rate of mass loss on the Greenland ice cap. The authors may have had a good reason for measuring here as opposed to West Greenland. For one thing, their data collection was established in 1991, more than a decade before GRACE measurements identified accelerating ice loss in Western Greenland. This work is a continuation of a long-term project.
Second, the ablation measurements made by the authors identify a clear increase in surface melting over the study period. Average surface losses increase about 17% (my calculation based on the authors' graphs) over the study period. This may be the early signs of a change in the long-term mass balance of East Greenland, but at this stage a conclusion of that nature would be premature.
Thus, these data serve to better understand the dynamics of ice-stream movement but do not, in my opinion, speak much to the overall state of ice loss in Greenland.
— Posted by Scott Wahlstrom
Melty Man:
My read of the satellite measurement is that the stake positions mentioned in my earlier post were located using GPS measurements (probably by researchers using portable equipment) for the first 14 years of the study. This was done once per season for an annual fix. The last three years used in situ receivers that transmitted positions daily for a better resolution data set. This advance is what made it possible to link local melting rates with changes in velocity at the same locale. The older data however provided a baseline measurement of average velocity on a year-to-year basis (which the researchers indicated held nearly constant or even decreased slightly over the study period).
E. O’Neal #132:
For clarity: Measurement of mass loss from Greenland has been made using laser altimetry, satellite interferometry, and gravitation anomaly measurements. The gravitational data supplied by the GRACE satellite fixed the overall mass loss at 239 +/- 23 cu. km based on measurements made from 2002 to 2005 (Chen et al., Science, 29 June, 2006). This measurement compares very well with another recent (2005) study based on the satellite interferometry technique that placed mass loss at 224 +/- 41 cu. km per year (Ringot & Kanagaratnam, Science, 17 February 2006).
We have two data points by independent teams using different technology that agree within the measurement error margins. What is more interesting is that older measurements suggest that this loss rate is increasing rapidly.
Between 1997 and 2003, laser altimetry measurements placed the average loss at 80 ± 12 km3/year (W. Krabill et al., Geophys. Res. Lett., 31). In 1996, satellite interferometry placed the loss at 91 ± 31 km3/year (Ringot & Kanagaratnam, 2006). Using these data as a baseline, an increase in melting of almost 250% in a decade can be calculated.
The GRACE data showed this trend over the three-year period of its study, with a substantial increase in mass loss in SE Greenland during the last year of the study as compared to the first.
These data are clearly not rounding errors, and the increases in mass loss identified should get anyone’s attention. At the same time, it is clear that follow on data are needed to duplicate and confirm the trends hinted at thus far.
I agree with your analogy as to the absolute size of losses of this scale compared to the total volume of the ice cap, but that is not really the issue of concern. Scientists and laypeople alike long considered these ice caps as stable and fixed features on the surface of the Earth with tremendous thermal inertia. This view is changing, and the measurements being made help to identify future changes and trends in our climate.
I will refrain from extrapolating the 250% increase out into the future and eagerly await more data, but the existing information is consistent with the overall view that global warming is firmly established and accelerating.
— Posted by Scott Wahlstrom
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