Much in the news at the moment is the story about the dramatic loss of ice in the Arctic Ocean, and the likelihood that within the next twenty years it could be entirely free of ice in summer. The findings came from Professor Peter Wadhams at Cambridge University, who has been studying Arctic ice since the 1960s.
It's thought that loss of Arctic ice could itself lead to an acceleration in global warming. Firstly, ice is important in reflecting sunlight back into space, and so the less of it that we have, the more the sun's energy is absorbed into the sea, causing additional warming to occur in a well understood feedback loop.
More worryingly, research conducted by David McGuire of the U.S. Geological Survey show that the Arctic has been a carbon sink since the end of the last ice age which over time has accounted for between zero and 25 percent, or up to about 800 million metric tons, of the global carbon sink. On average, says McGuire, the Arctic accounts for 10-15 percent of the Earth's carbon sink. But the rapid rate of climate change in the Arctic could eliminate the sink and possibly make the Arctic a source of Carbon dioxide.
Although this year's Arctic ice has recovered in extent compared to the last couple of years, there is a clear trend towards less ice. Most scientists agree that man-made global warming has caused this loss of ice over the last few decades, and some would say it's happening even quicker than some climate models had predicted.
But in an unusual move, the Met Office sought to distance itself from Professor Wadhams in a press release issued on the same day.
In it they confirmed that Arctic ice decreased dramatically in 2007, but said this may have been wrongly attributed to global warming. 'Analysis of the 2007 summer sea-ice minimum has subsequently shown that this was due, in part, to unusual weather patterns. Arctic weather systems are highly variable year-on-year and the prevailing winds can enhance, or oppose, the southward flow of ice into the Atlantic', they said. Importantly they don't expect the Arctic to be ice free until the earliest 2060 - at least four decades later than Professor Wadhams' forecast, and conclude, 'The observed temporary recovery from the 2007 minimum in 2008 and 2009 indicates that the Arctic ice has not yet reached a tipping point, if such exists'.
But, on the other side of the world, something rather odd seems to be happening in Antarctica. The ice is actually increasing in extent, not decreasing.
So how does this tie in with warmer global temperatures? Well, as odd as it may seem, global warming may well be responsible, according to Jinlun Zhang, an oceanographer at the University of Washington.
He has pieced together a complex computer model that helps explain why Antarctic sea ice is growing even with signs that ocean and air temperatures are on the rise. The key is that warming temperatures can lead to more stratified ocean layers.
In the Southern Ocean, there's a layer of cold water near the surface and a layer of warmer water below. Normally, convection causes the two layers to mix and exchange water, a process that brings heat from the lower layers to the surface layer and ultimately helps keep sea ice expansion in check. But if the air gets warmer, the model indicates that the amount of rain and snowfall could increase, and surface waters could freshen. Since fresh water is less dense and less apt to mix with the heavier, saltier, and warmer water below, the layer at the ocean's surface could become more stratified and mix less. This, in turn, would reduce the amount of heat flowing upward, allowing surface ice to expand.
It's although thought that because of the hole in the ozone layer, weather conditions have changed. According to NASA Goddard scientist Josefino Comiso, the loss of ozone has caused lower atmospheric pressure over the Amundsen Sea. This causes colder, stormier and faster winds over the waters around Antarctica, especially in the Ross Sea, where ice growth is most rapid. The winds create open water near the coast that promotes sea ice production.
Water-logged sea ice also may explain why sea ice in the Antarctic is increasing. This occurs when the weight of accumulated snow presses down on a slab of sea ice until it's nearly submerged. When that happens, waves cause ocean water to spill on top of the ice and into the snow, forming a layer that eventually freezes and becomes "snow ice."
So, global warming is not only thought responsible for the melting of Arctic ice over the last few decades, but the observed increase in the ice at Antarctica, too.
Other parts of the Antarctica continent do seem to be showing the more straight forward impacts of global warming. As reported by David Shukman in August, one of the largest glaciers in Antarctica is thinning four times faster than it was ten years ago.