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Greenland ice sheets losing mass faster and faster - NASA/ESA Study

An interview with Glenn Milne, Geophysicist, Earth System Dynamics Group, Ottawa University - 1/4/13


After reassessing 20-years of satellite measurements a team of scientists from around the world largely reconciled the differences between past estimates of the changing size of the ice sheets in Greenland and Antarctica, and in doing so, have produced a more accurate estimate of their contribution to sea level rise over this period.

The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE), a joint effort by NASA and the European Space Agency (ESA) was released on November 29, 2012. The project brought together 47 climate researchers from 7 countries and 26 different institutes - including Glenn Milne, a geophysicist with the University of Ottawa's Earth System Dynamics Group - to resolve years of divergent results obtained using various satellite-based methods for estimating changes in the ice sheets.

We talked with Ottawa University's geophysicist Glenn Milne on Thursday December 20.

Water Today

What exactly are ice sheets as opposed to ice caps, glaciers?

Glenn Milne
Ice sheets are continental size ice formations. There are only two, in Greenland and Antarctica (although Antarctica is actually comprised of two ice sheets). Whereas ice caps are smaller scale ice formations such as the Agassiz ice cap on Ellesmere Island in northern Canada, and glaciers are smaller still and generally flow down valleys - they are basically rivers of ice.

Water Today
If I understand correctly, three satellite-based techniques have been used to monitor ice sheets: altimetry, gravity and interferometry. Can you explain in simple terms what these refer to?

Glenn Milne
Well, to put it simply, altimetry measures the height of the ice sheet surface, gravity measures mass and interferometry measures the velocity of ice sheet surface. Knowing the latter, it is possible to estimate how much mass the ice sheet has lost through flow at its margins (as opposed to gained through snowfall in its interior). All of these measurements permit estimates of changes in the volume or mass of the ice sheet over time.

Water Today
With such a large group of scientists involved, how was a consensus reached?

Glenn Milne
A half year ago, the entire group was invited for an open discussion; focus groups were then set up to identify key issues. The measurements were reassessed ensuring that the data were analyzed using consistent methods and over identical time periods (e.g. 1992-2011). For example, consistent methodology requires using the same models when correcting for processes that influence the observations but are not related to changes in ice mass or volume. In the end, we were able to reach an agreement on the majority of the results obtained using the different techniques.

Water Today

What in your view is the major finding of this study?

Glenn Milne
Apart from the reconciliation of previous results, the finding that the Greenland ice sheet is losing mass five times faster in recent years compared to the early 90s is important. We don't know if this trend will continue but if it does it will have major repercussions. The study also confirmed that, overall, there has been a loss of ice in Antarctica: the West Antarctic ice sheet has lost mass while the East Antarctic ice sheet has increased its mass (but not enough to compensate for the mass loss from the west).

Water Today

What are the study's final conclusions as to the contribution of Ice Sheet mass loss to global Sea Level Rise?

Glenn Milne
Since 1992, we found that the Greenland and Antarctic Ice Sheets together lost mass that equated to a global rise in sea level of just over 11 millimeters, amounting to about one-fifth of the total sea level rise over the same period.

Water Today

A recent NOAA report offered four Sea Level Rise scenarios ranging from at least 8 inches to at worst 2 meters with maximum ice sheet loss and glacial melting factored in by 2100. If I project the IMBIE findings over a hundred years, your conclusions seem much more conservative. Is this correct?

Glenn Milne
Yes, but you have to keep in mind that such a (linear) projection is overly simplistic and that the IMBIE numbers do not include the effects of ocean warming and melting of ice caps and glaciers. On the other hand, a 2 m rise is quite an extreme scenario - not impossible but highly unlikely.

Water Today

Although sea level rise is a global phenomenon it is not uniform in all areas of the Earth because of the varying rates in land height change and sea-surface height change (due to different processes such as post-glacial uplift of land masses following the last glacial age). Which are the areas of Canada least and most at risk from sea level rise?

Glenn Milne
Post-glacial land uplift rates are at their highest in the Hudson Bay area, and so sea levels there have and are currently falling. However, the land is currently sinking in Nova Scotia leading to an amplification of sea-level rise in that region. This also holds true for other parts of the Canadian Atlantic coast including Prince Edward Island, parts of New Brunswick, Newfoundland and Labrador. I would say these are the areas most at risk in Canada.

NOTE: This interview was reviewed by Glenn Milne to ensure the accuracy of the scientific information.


Related Info
The IMBIE Assessment - ESA/NASA Report
Clearest evidence yet of polar ice losses - University if Leeds - Press Release
Global Sea Level Rise Scenarios for the United States National Climate Assessment - NOAA
Sea Level Rise & Crisis Management - Interview with USGS Chief Climate Scientist - Water Today










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