WASHINGTON, Feb. 27 -- The American Geophysical Union issued the following news release: The Arctic Ocean could become ice-free in the summer in the next 20 years due to a natural, long-term warming phase in the tropical Pacific that adds to human-caused warming, according to a new study. Computer models predict climate change will cause the Arctic to be nearly free of sea ice during the summer by the middle of this century, unless human greenhouse gas emissions are greatly reduced. But a clo  more