It is generally accepted that the earth's carbon dioxide levels have increased by about 30 percent. Carbon dioxide helps to trap the sun's heat as it radiates from the earth's surface in what is called the "greenhouse effect." While water vapor is responsible for between 80 and 90 percent of the earth's heat retention, carbon dioxide from the burning of fossil fuels is playing an increasing role. The result is that in the last century, the earth's temperature has risen by roughly 1 degree F., with the result that global temperatures in the first part of 2010 were the highest since records have been kept.
This increase in temperature is responsible for a corresponding rise in sea levels. In the Antarctic Peninsula, for example, the rapid (in geological terms) temperature increase has caused melting to increase by 140 percent in the last decade. The melting ice sheets expose glaciers. As glaciers are essentially rivers of ice, they melt into the oceans as well, speeding the process. These vast quantities of fresh water run into the ocean, where they contribute to the higher sea level. Ice loss is also of major concern in places like Greenland.
In addition to increasing the amount of water in the oceans, global warming is causing the Earth's existing water to take up more space. The expansion of any substance due to higher temperature is called thermal expansion. Water is no exception, and warm water takes up more space than cold water. As global warming increases the temperature in the ocean's upper layers, thermal expansion occurs. As the seawater expands, sea levels rise.
The cycle described above will reach a point where it feeds upon itself. As ocean temperatures increase worldwide, the oceans will evaporate more rapidly, feeding water vapor into the air. Water vapor is responsible for the majority of the atmosphere's heat retention. The resultant difference in air temperature will again cause more and faster glacier and ice sheet melt. It will cause the oceans to grow still warmer, speeding the thermal expansion of the oceans. Both thermal expansion and ice sheet runoff will contribute to future rises in sea level.