Risk of rising oceans: How fast are the Arctic ice caps changing?

As the arctic ice caps are melting causing the risk of rising sea levels, Professor Martin Sharp of the University of Alberta discusses the speed of change

A major concern related to melting of Arctic ice caps and glaciers is how this will alter the global mean sea level. Ice caps and glaciers shrink because climate warming increases how fast they melt, and/or reduces how much snow accumulates on them. Snowfall decreases in a warming climate as more precipitation falls as rain. Where glaciers terminate in the ocean, iceberg calving also contributes to mass loss. As much of the water produced by melting and iceberg calving ends up in the ocean, global mean sea level rises. Thermal expansion of warming seawater adds to this effect.

Given the implications of rising sea levels, we must understand how rapidly ice caps and glaciers add water to the ocean. This depends on how their mass is changing. Since the 1940s, scientists have measured the mass balance of target glaciers across the Arctic every year. However, these measurements target only a small fraction of the region’s glaciers, so it can be hard to tell how well they represent the behavior of the entire population of glaciers.

A simple way to characterize glacier change is to calculate the cumulative change in the mass of all the monitored glaciers in a region over the measurement period. Here we use measurements of the annual mean mass balance (kg m^-2 yr^-1) of all monitored glaciers in each region for each year of the record to create an index of the cumulative change in mass per unit glacier area in a given region over the period of record. To do this, we sum all the annual mean rates of mass change measured on glaciers in that region (Table 1). From 1988 to 2015/17, cumulative mass loss was greatest in Alaska and least in Arctic Canada.  To obtain an index of the total mass change in each region, these rates of change must be multiplied by the total area of glaciers in the region. Unfortunately, there are no recent in situ measurements of rates of mass change for glaciers in the Russian Arctic Islands.

For more recent times we can estimate the total annual and longer-term changes in the mass of ice caps in a region (in Gigatonnes of water) using satellite measurements of changes in either the surface elevations of ice caps in a region (satellite altimetry), or of the Earth’s gravity field in the region of interest (satellite gravimetry – an approach that effectively measures changes in the weight of the ice caps in the region). Bert Wouters of Utrecht University recently produced regional mass balance time series for glaciers in Iceland, Svalbard, the Russian Arctic Islands, and Arctic Canada for the period 2010-2017 using radar altimetry data from the CryoSat2 satellite.  The results indicate total mass losses over the seven-year period of 16.3 Gt, 93.2 Gt, 131.1 Gt and 416.5 Gt from glaciers in Iceland, the Russian Arctic Islands, Svalbard and Arctic Canada respectively. Wouters’ analysis takes into account the differences in the area covered by glaciers in each region. Thus, while Arctic Canada has the lowest rate of mass change per unit area of glacier ice in the five regions considered in Table 1, it has the highest total mass change measured by satellite gravimetry (Table 2). This reflects the large area covered by glaciers in Arctic Canada.

Reference: Sharp, M., Wouters, B., Wolken, G., Andreassen, L.M., Burgess, D., Copland, L., Kohler, J., O’Neel, S., Pálssson, F., Pelto, M.S., Thomson, L., and Thorsteinsson, T. 2018. [Arctic] Glaciers and Ice Caps outside Greenland. [in “State of the Climate in 2017”]. Bulletin of the American Meteorological Society, S156-S159.