Glaciers and Sea Ice Are Melting
Ice sheets, glaciers, and Arctic sea ice are melting at increasing rates. This is important for two reasons. First, ice tends to cool the planet, by reflecting back solar radiation and by absorbing heat energy as it melts. White is more reflective than darker colors, like land or deep blue sea, which tend to absorb heat, as you know if you sit in the sun with dark trousers on. This reflectivity is called albedo; white has a high albedo or reflectivity. Second, ice currently on land when melted will raise sea level. Of course ice floating in the sea, or sea ice, will not raise sea level as it melts. It is already in the sea. Think of the ice in your gin and tonic. If you fill the glass with ice and then liquid to the top, the glass doesn’t overflow as the ice melts.
Arctic Sea Ice
The Arctic sea ice is melting farther and farther north as seen in the figure from the National Snow and Ice Data Center.
Click on figure to enlarge.
Although the Arctic sea ice will not raise sea levels, melting of the ice is very important for the overall climate because of its effect on cooling the planet as described above, and because continued cooling of the area will accelerate melting of methane hydrates, which can add significantly to greenhouse gases in the atmosphere. Although with warming there will be more vegetation, which will tend to take up CO2, carbon released from from permafrost will “win handily“.
Opening of the Arctic Ocean would also open up new areas for oil and gas and minerals exploration, as well as the fabled northern passage. Russia and Canada are currently at odds as to who owns what.
Antarctic Ice Sheet and Sea Ice
Why the sea ice is increasing in area is not because Antarctica is cooling, but primarily because the melting of land ice. How does that work? Glaciers are formed of fresh water, so when they melt and that water flows into the sea, the salinity of the ocean at that site drops. Fresh water is less dense than sea water so it stays near the surface. Fresh water also freezes at a higher temperature than sea water. Sea water of typical salinity freezes at 28°F (-2°C). So the fresh water at the surface freezes at warmer temperatures than sea water, resulting in formation of more floating ice. Other components, are also causing increasing sea ice, including changing wind patterns associated with the closing of the ozone , changing cyclonic, and as air temperatures warm, clouds and precipitation increase which further freshens the surface ocean waters (see Thompson and Solomon, 2003, Gillett and Thompson, 2003, and Turner et al., 2009).
The land ice (ice sheets) are melting. Gravity data collected by NASA‘s GRACE (Gravity Recovery and Climate Experiment) satellite show that Antarctica has been losing more than a hundred cubic kilometers (24 cubic miles) of ice each year since 2002.
Most of the ice loss is from the Western Antarctic Ice Sheet (WAIS), which is the most vulnerable to melting. The WAIS lies over a series of islands with ocean between, the ice sitting on the floor of the ocean. As this “grounding” of ice is lost, glacier melt accelerates since there is nothing stopping it from sliding into the ocean. We are seeing this in the Amundsen Sea area where the ice loss is considered “unstoppable”. If the entire WAIS melts it will add about five meters of sea level rise. By contrast, the giant East Antarctic Ice Sheet (EAIS) has enough ice to raise sea level by 50 meters, although the EAIS ice will probably take centuries to melt.
Another issue with the Antarctic Ice Sheet is loss of ice shelves, which “buttress” the on-land ice sheets. These ice shelves can disintegrate in a hurry. The figure on the right shows the Larsen B ice shelf as caught in 2002 by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) imaging instruments on board its Terra and Aqua satellites.
It took just three weeks for the 12,000-year old ice shelf to crumble.
Greenland Ice Sheet
Greenland ice is also on land and so will raise sea level as it melts. It too is melting at increasing rates. Calculating the melt rates in either Antarctica or Greenland is exceedingly difficult, but much study is underway. Particularly, GRACE satellite measurements using differences in gravity measurements, are improving our understanding. The figure on the right from Tedesco et al., 2015, shows a steady decrease in Greenland ice.
Click on figure to enlarge.
Greenland is further complicated because its topography under the ice is a large basin surrounded by mountains. How this configuration will affect ice melt is difficult to determine.
Total melt of the Greenland Ice Sheet would contribute about seven meters to sea level rise.