Tuesday, July 5, 2011

On Warm Waves and Melting Ice

I'm usually pretty good about getting a couple of posts up by the end of the week. Instead, I spent my July 4th holiday weekend doing what most Americans do to celebrate Independence Day: Slept in late, went to a BBQ, swam in a lake, and blew up thinks that sparkle. I also applied copious amounts of sunscreen while roasting in the summer heat, which gave me the inspiration for today's post.

A new study in Nature Geoscience takes a look at the melting of the ice sheets in relation to increasing ocean temperatures. Now, if you Google "ocean warming" or "increasing ocean temperatures" you are likely to just confuse yourself with a myriad of websites on both sides of the climate change debate (however, if you feel you must, I recommend looking at .edu sites and reading the actual research). I'm not writing today to debate the this-says-that-says and he-says-she-says of ocean warming itself. Let's just start with the fact that the ocean is not only acidifying due to increasing atmospheric carbon dioxide (CO2) but it is also warming. This warming is expected to cause changes in currents, changes in oxygen levels, shifts in plant and animal habitats, and sea level rise.

Since this study focuses on ice sheets let's jump right to that. The Greenland Ice Sheet (GIS...not to be confused with Geographic Information Systems) and the marine-based West Antarctic Ice Sheet (WAIS) have both recently undergone rapid mass loss that is not explainable by atmospheric warming only. So, well, why? First you need to picture what's going on here. Glaciers are like very very slow moving rivers of ice. Ice streams and outlet glaciers are channelized glaciers, they flow more rapidly than the surrounding body of ice, and they drain an ice sheet or ice cap. Tidewater glaciers are valley glaciers that flow far enough to reach out into the sea. The point at which a tidewater glacier floats free of its bed is called the grounding line. In Greenland the fjords are where outlet glaciers terminate and can reach several hundred meters depth at these grounding lines. Here is also where "warm and salty North Atlantic subtropical waters could penetrate from the shelf into the deep fjords, remain in the subsurface layer year-round and flush rapidly through local wind-driven circulation, thereby giving a ‘warm bath’ to the ice sheet." To date, most of the simulations and projections of melting polar ice have focused on the effects of atmospheric warming on the surface of the ice. This study focuses on this subsurface warming and how it is contributing to ice mass loss.

The scientists here used 19 state-of-the-art climate models to examine future ocean warming around the periphery of the ice sheets in response to increasing greenhouse-gas concentrations. Normally, the periphery of these ice sheets (GIS and WAIS) are kept cold by the currents that flow around them. In the Northern Hemisphere the East and West Greenland Current (EGC and WGC) keeps Greenland frozen, and in the Southern Hemisphere the Antarctic Circumpolar Current (ACC) combined with an upwelling of cold deep water keeps Antarctica frozen.

They found that, given a midlevel increase in greenhouse gasses, the ocean depths between 200 and 500 meters (650-1650 ft) will be warming 0.5 and 2°C, an average of 1°C (1.8°F), by 2100. However, the results for the two ice sheets studied were not the same. In Greenland the warming could be twice as much with the subsurface ocean temperatures increasing as much as 2°C (3.6°F). In Antarctica it could warm less, with only a 0.5°C (0.9°F) increase. Why such a difference? It is all due to those currents that I mentioned. Greenland receives warmer waters from the Gulf Stream. This warm water affects the exposed tidewater glaciers, melting them from underneath and causing the now unsupported tops to break off into the sea. Also, as the warm water melts the undersides of the glaciers the meltwater acts as a lubricant, speeding the glaciers' movement into the sea. Eventually, the glaciers will melt so much that they will not reach the sea. Antarctica has the ACC and the cold deep water upwellings to prevent, or at least slow, the warm water in the south. Unlike Greenland where the ice flows out into the sea, Antarctic ice is based on land that is already below sea level. This means that as the ice sheet melts the leading edge will continue to be underwater. Regardless of the differences between continents, the sea level is expected to rise by about 1 meter (~3ft) by the end of the century.

Looks like my inland Florida relatives will have beachfront property. Perhaps I should invest in a surfboard.

Here is the paper:
Jianjun Yin, et al. (2011) Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica. Nature Geoscience: published online 03 July. (DOI: 10.1038/ngeo1189)

Learn more about glaciers here:

and on this study...
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