Recent contributins of glaciers and ice caps to sea level rise

Yearly glacier and ice cap melt contributes sea level changes. How the rate of this contribution to sea level changes is very important in understanding changes in Earth’s climate. Jacob et al. (2012) use the Gravity Recovery and Climate Experiment (GRACE) satellite to calculate changing masses of glaciers and ice caps around the world from 2003 to 2010. These values allow the authors to calculate the changes in the sea level. Their findings show that there are increasing rates of loss in most areas of the world that are ice-covered. The full contribution of ice mass loss around the world to sea level change is about 1.48±0.26 millimeters per year. However, their data did show difference from previously published work in some regions, specifically in High Mountain Asia (Northern India and the Himalayas). Looking at their data and the previously published data in more detail revealed that their data were most likely accurate, as there were little reasons for GRACE to miscalculate the data in the region. Furthermore, the final rate of change is sea level rise is nearly identical (difference of 0.2±0.6 millimeters per year, which is not significantly different from zero) to other recent studies that focus on physical calculations of sea level change. This paper demonstrates the firm understanding of glacier and ice cap contribution to sea level change in the scientific community. –Mathew Harreld
Jacob, T., Wahr, J., Pfeffer, W.T., Swenson, S. 2012. Recent contributins of glaciers and ice caps    to sea level rise. Nature (Online) 1-5.

            A major concern in a changing climate is its the impact on sea level changes. The melting of glaciers and ice caps throughout the world mostly drives the changes in sea level. Rising global mean temperature could mean a faster rate of glacier and ice cap melting, which in turn will increase sea level. The big question, however, is how much. Many studies have been done recently to calculate the changing sea level due to glacial melt, but Jacob et al. reevaluate those studies with one of their own. Using the Gravity Recovery and Climate Experiment (GRACE) satellite to calculate changing sea levels due to changing masses of glaciers and ice caps in the 8 year period from 2003 to 2010.
            GRACE observes monthly, global gravity field changes, allowing the authors to calculate changes in mass on Earth’s surface. Using data compiled from GRACE the authors calculated the rate of loss or gain of glaciers and ice caps from around the world, and converted that data into rates of sea level rise. The data from GRACE were split into 175 small arbitrarily defined regions of Earth, called “mascons”. The 175 mascons were then grouped into 20 regions based on location on the Earth’s surface. GRACE does not have high enough resolution to separate Greenland and Antarctic ice sheets from their peripheral glaciers and ice caps, and therefore the main part of the study focused on results without the peripheral glaciers and ice caps. The authors, however, used a different source for the peripheral glaciers and ice caps for the sake of completeness in their final results.
            The results without the peripheral glaciers and ice caps showed increasing rates of loss in most areas of the world. The authors do note that certain areas show positive increases in rate, but that the increases are not significantly different from zero. The total mass of glacier and ice cap rate loss between 2003 and 2010, with the peripherals,was calculated to be about –536±93 gigatons per year. The peripheral glacier and ice caps contribute about –236 gigatons to the total amount, a significant amount. Since the peripheral glacier and ice cap amounts were not derived from GRACE, the amounts could be brought into question. The authors calculated that the –536 gigaton per year loss of glacier and ice caps contributed to an increased rate of 1.48±0.26 millimeters per year of the sea surface between 2003 and 2010. The peripheral glaciers and ice caps contributed about 1.06 mm per year to the total.
            The findings of this paper were similar to the results of other papers, except for the High Mountain Asia region (Northern India and the Himalayas), which the authors calculated to have a much lower rate of loss than reported in another paper. A paper used GRACE to evaluated glacier and ice cap changes between 2002 and 2009 and determined that the same region rate of loss was around –55 gigatons per year, whereas Jacob et al. determined it to be around –4 gigatons per year. Due to this large discrepancy the authors decided to evaluate the region in more detail. It is possible that changes in the tectonic process under the region could be causing Jacob et al.’s difference in data, but this seems to be unlikely for a number of reasons. The amount of tectonic uplift needed to cause the large discrepancy is unlikely to work on such a short time period, or occur. For GRACE to not pick up these changes is even for unlikely, for the broad spatial changes must occur on a hundred to thousand year timescale. Another possibility is the absorption of melt water by the ground, effectively showing no change in mass in the studied area, when in fact the glaciers have decreased in size. But this too seems unlikely to contribute a large enough offset to result in such drastically different data.
            The authors determined that their derived data were accurate enough to add validity to changes in sea level rise. Their final results suggest that ice-covered regions are contributing an increase in sea level rise at a rate of 1.48±0.26 millimeters per year. Their findings differ from previous findings based on water-based measurements by 0.2±0.6 millimeters per year, which is not significantly different from zero. This suggests we have an accurate understanding of sea level changes due to glacial and ice cap melt. Future work needs to work on consolidating the data, and region changes.

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