A senior U.N. environmental official says entire nations could be wiped off the face of the Earth by rising sea levels if the global warming trend is not reversed by the year 2000. Coastal flooding and crop failures would create an exodus of ″eco- refugees,′ ′ threatening political chaos, said Noel Brown, director of the New York office of the U.N. Environment Program, or UNEP. He said governments have a 10-year window of opportunity to solve the greenhouse effect before it goes beyond human control. As the warming melts polar icecaps, ocean levels will rise by up to three feet,...