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Environment

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After naming global warming the top science story of 2004, Discover received numerous questions from readers about the science. Glaciologists Waleed Abdalati and Jay Zwally of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, answer two of them.

Not long ago scientists were worrying about another ice age. What happened?

The idea that we might be entering another ice age was put forward in the 1970s by some scientists who noted a cooling trend in global temperatures from the 1940s to the 1970s. At that time, scientists were gaining a better understanding of climate cycles that occur because of slow changes in the shape and orientation of Earth’s orbit with respect to the sun. Scientists knew about the warming effects of greenhouse gases, but proponents of global cooling argued that greenhouse warming would be more than offset by Earth’s orbital changes. Some also argued that the effects of aerosols and pollutants, which block sunlight and facilitate cloud formation, would enhance this cooling trend. Not long after that, however, scientific understanding of the effects of greenhouse gases and aerosols advanced greatly. By the late 1980s, temperatures had increased significantly, as predicted by improved scientific models. Global warming, not cooling, was the real trend.

How can polar ice melt when it’s well below freezing in these regions?

Ice melting occurs during the summer when temperatures rise above freezing in some places, depending on how high the ice is above sea level and how close it is to a pole. The great ice sheets of Greenland and Antarctica, which rise to over 13,000 feet above sea level, accumulate ice over most of their surfaces and melt only at their lower elevations near the edges. Melting near the edges of the Greenland ice sheet, where the surface is below 4,000 feet, causes about half of its annual ice loss. In Antarctica, which is closer to a pole and colder, only a small percentage of the surface melts each summer. Most of the ice loss from Antarctica is from the flow of glacier ice into the ocean. Much of the polar sea ice (frozen ocean water) that blankets the Arctic Ocean and surrounds the Antarctic continent in winter melts during the summer as temperatures do get above freezing. Sea ice and icebergs also melt as ocean currents carry them to warmer places farther from the poles.

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