One of the greatest problems associated with rising temperatures is one that is often overlooked in climate-related discussions: permafrost. Where temperatures drop well below freezing for a long period of time, the ground freezes solid. In the upper middle latitudes, as temperatures begin to warm, the frozen ground thaws. In colder northern latitudes the surface thaws, but at a depth the ground remains frozen. It is this permanently frozen ground that scientists call permafrost. It occurs in much of Canada and Alaska and in a broad belt across northern Europe and Siberia.

Under warming conditions, the active (upper) layer of permafrost thaws out to a greater depth. This creates a deeper "spongy" surface that can cause tremendous damage to structures. Roadways and railroads, for example, begin to buckle as they sink into the mire. Buildings, if not protected, also begin to sink. Protecting against melting permafrost was a huge problem engineers had to overcome in designing the Trans Alaska Pipeline System.

Several methods are used to protect against permafrost. Buildings can be constructed on pilings that extend deeply into the permanently frozen ground and thereby offer support. Another method is to remove the upper active layer of earth and replace it with rock or gravel that will neither freeze nor thaw. This method is used frequently with roads and railroads. Finally, in some instances, the ground is kept frozen artificially during the summer months. All of these methods are extremely costly. If warming continues, some estimates place the cost of protecting against permafrost at trillions of dollars.

Some scientists have asked a very interesting question: Is permafrost melting because of global warming, or is melting permafrost the chief cause of rising temperatures? As permafrost thaws, greenhouse gases stored within the formerly frozen ground are released. The amount of carbon that is now trapped in permafrost is much more abundant than first thought. In fact, according to some estimates there may be 30 to 100 times more carbon released into the air from melting permafrost than by the burning of fossil fuels! As permafrost thaws, methane, another greenhouse gas, is also released in increasing amounts. Regardless of the cause, if the recent warming trend continues, released carbon dioxide and methane could cause a rapid acceleration of global warming.

Some scientists believe that further warming could start another ice age, at least in Europe. Today, Western Europe is much warmer than its latitudinal position would suggest. This condition results at least in part from the transfer of water heated in the tropical Atlantic. The heat is transported into northern waters by the Gulf Stream and North Atlantic Current. As ice melts from the Greenland ice cap and elsewhere in the Arctic region, a layer of freshwater forms over the North Atlantic. (Freshwater is less dense than saltwater.) As this happens, it is feared that the warm saline current would dip below the freshwater and descend into the ocean depths far to the south of its present location. In the absence of moderating warm ocean water, temperatures throughout much of Europe would plummet.

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