The only large bodies of frozen water that, if melted, would measurably increase the height of the oceans are in Greenland and Antarctica. The glaciers in Greenland are surrounded by mountains that block them from sliding suddenly into the sea, and their melting would take centuries. The Antarctic is covered with glaciers thousands of feet thick. The West Antarctic ice sheet could be discharged into the sea, a development that might raise ocean levels by sixteen to twenty feet within a hundred years and thus be extraordinarily costly. Fortunately, the experts believe that it is also extremely unlikely.
Ice sheets take thousands of years to respond to changes in surface temperature.
We are not only uncertain about the direction of sea levels but unclear about the future growth in greenhouse gas emissions. There is little doubt that the concentration of CO2 in the atmosphere has been rising for well over a hundred years. In 1990, the concentration of carbon dioxide in the atmosphere was measured at 353 parts per million by volume, a rise of 25 percent from the pre Industrial Revolution figure of 280 parts per million. Human activity, especially burning fossil fuels, has contributed to this change. What is uncertain is the future.