Floating Ice Contributes to Rising Sea Levels

Over the last century, global sea level rose by more than half a foot. The predictions about how much the seas will swell in the next century are much worse, with some models forecasting that the arctic will be entirely ice-less and that ocean levels will increase by a meter or more. 

Now a new study suggests that these already staggering predictions may, in fact, be too low because they do not account for the contribution of melt from floating ice.

Until now, climatologists have treated the melting of floating ice like the melting of ice cubes in a cup of water. When an ice cube melts, the water in the cup does not overflow because the total volume of water before and after the melting occurred remains the same.

But in the case of icebergs and the ocean, researcher Andrew Shepard from the University of Leeds in London proved that it is not the same. The composition of icebergs differs substantially from the ocean water they live in. Icebergs have a high amount of fresh water, which is less dense than the surrounding heavy salt water.

When a piece of iceberg melts, fresh water floats on top of salty, rather than mixing together. The layering results in a slight volume increase. 

Using satellite data and an ice-ocean computer model, Andrew Shepherd of the University of Leeds and his colleagues tracked the changes in floating ice volume around the world between 1994 and 2004. The ice produced enough melt to raise the oceans .049 millimeters per year, according to the team's article in the journal Geophysical Research Letters.

In comparison to the 1.6 millimeters per year added to the ocean as it warms, or the .8 millimeters per year coming from land-based glacier and melting ice sheets, the impact of floating ice is very small.

Over time, however, the amount adds up. Shepard estimates that if all the ice floating around the ocean melted, global sea levels would increase 1-2 inches. On a regional level, melting could be much greater.

An article in Scientific American reported that a one meter increase in global sea level would result in the loss of 861,000 square miles of land, affecting 145 million people.

As Shepherd argues in his study, it is crucial that sea level rise models start including the contribution of floating ice melt. Knowing exactly what sources are contributing to sea level rise and by how much is essential for protecting the string of vulnerable communities situated along the world's coastlines, from New York City to Bombay.

Image: DorkyMum, Flickr