The Contribution of Glaciers and Ice Caps to Sea Level Rise

The contribution of glaciers and ice caps (GICs) to global sea level rise is significant. However, most studies focus on the Greenland and Antarctic glaciers and their impact on sea level rise (SLR) while GICs for areas such as the Himalayas, Alaska, and the Alps are usually interpolated from more sparse mass balance measurements. This study used a combination of Gravity Recovery and Climate Experiment (GRACE) satellite data between 2003 and 2010 and analyzed global ice-mass changes in GICs. The data revealed that mass loss in GICs excluding Greenland and Antarctica was about 30% smaller than previous models predicted, or around 148 ± 30 Gt yr­–1. This equates to a sea level rise of about 0.41 ­­± 0.08 mm yr­–1 (Jacob et al. 2012). Although the data agreed with other measurements of sea level rise (SLR), the scientists note that the short study period, along with the high interannual variability during this time, should therefore encourage further investigation.¾Olivia Jacobs
Jacob, T., Wahr, J., Pfeffer, W.T., Swenson, S. 2012. Recent contributions of glaciers and ice caps to sea level rise. Nature 482, 514-518. 

                  Jacob et al. (2012) used Gravity Recovery and Climate Experiment (GRACE) satellite images to calculate the mass changes over all ice-covered regions greater than 100 km2across the world. They used data between 2003 and 2010, and divided 20 glacial and ice-covered regions into 175 smaller ‘mascons’ and evaluated the mass changes in each small area to analyze local changes. The areas studied included Iceland, High Mountain Asia, Alaska, Northwest America, Baffin Island, Scandinavia, Patagonia, and more. While Greenland and Antarctica were not the focus of this study, Jacob et al.used data from these areas to estimate the total contribution to sea level rise (SLR) from glaciers and ice caps.
                  By taking more accurate measurements of GICs across the world, Jacob et al. found that the mass balance changes in these regions was about 30% smaller than other studies had calculated. Globally, the GIC mass balance excluding Greenland and Antarctica was 148 ± 30 Gt yr­–1, contributing 0.41 ­­± 0.08 mm yr­–1 to SLR between 2003 and 2010. There have not been other estimates over this exact time frame, but similar studies indicate changes of 1.41 ±0.3 mm yr­–1 between 2001 and 2005 and 0.98 ± 0.19 mm yr­–1 between 2001 and 2004.
                  Jacob et al.focused attention on the High Mountain Asia (HMA) GICs as these calculations were significantly different than a recent GRACE study between 2002 and 2009. Previous data predicted a change of –55 Gt yr­–1 over the entire region and –29 Gt yr­–1 over the eastern Himalayas alone, but the data from this study predicted a change of –4 ± 20 Gt yr­–1 across the HMA region. To verify it, Jacobs et al. ruled out many other factors that could have given a low rate of loss. They considered groundwater loss by analyzing smaller mascons in the HMA plain region and found that loss happened at a rate consonant with their findings. They also considered tectonic processes as contributors to the low predicted rates of loss, and calculated that crustal uplift would have to be occurring at a rate of about 1 cm yr­–1 to account for their data, whereas Global Positioning Systems indicate long-term uplift rates of 0.5–0.7 cm yr­–1.
Jacob et al. thought it might also be possible that meltwater in the HMA might be sinking into the ground and remaining in the area, which would also create no change in GRACE imaging. However, local storage capacity in the HMA region is small due to thick permafrost layer, and thus it is unlikely that water could be stored here. Water may also be diverted for irrigation, but the irrigated areas lie well outside the HMA mascons, and thus GRACE would show a mass loss even if this water were being directed toward irrigated areas. By ruling out these potential factors of causation, Jacob et al. were able to further verify their findings and support the lower rate of loss in the HMA region.
Although this study focused on GICs rather than Greenland and Antarctica, Jacob et al. also compiled data from Antarctica and Greenland to calculate a total SLR contribution of 1.48 ± 0.26 mm yr­–1 between 2003 and 2010. This value compares well with other estimates of total SLR, but regional GIC data varied greatly between estimates and the scientists suggest that these differences should be further studied.
                  Additionally, this study was done on a very short time scale, and thus the results should be heeded cautiously. Interannual variability (due to seasonal changes) was very high during this time period, especially in regions such as High Mountain Asia, and Jacob et al. therefore suggest that this type of study should be extended to longer time periods in order to get a more accurate prediction of GICs and their impact on global SLR. 

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