Recent melting of antarctic glaciers, while dramatic, is within normal ranges and cannot be attributed to human-caused global warming, a U.S. scientist says. In the last few decades glaciers at the edge of the icy continent have been thinning, and research has shown the rate has accelerated and contributed significantly to sea level rise. However, Eric Steig, a University of Washington professor of Earth and space sciences, says the thinning is at the "upper bound" of normal that could be attributed to natural climate variations. The majority of recent antarctic warming came during the 1990s in response to El Nino conditions in the tropical Pacific Ocean, Steig said, when conditions were not greatly different from some other decades -- such as the 1830s and 1940s -- that also showed marked temperature spikes. "If we could look back at this region of Antarctica in the 1940s and 1830s, we would find that the regional climate would look a lot like it does today, and I think we also would find the glaciers retreating much as they are today," he said. The implication is that rapid ice loss from Antarctica observed in the last few decades, particularly the '90s, "may not be all that unusual," he said in a university release. While changes in recent decades have been at the "upper bound of normal," Steig said, they cannot be considered exceptional and cannot with confidence be blamed on human-caused warming. "The magnitude of unforced natural variability is very big in this area," he said, "and that actually prevents us from answering the questions, 'Is what we have been observing exceptional? Is this going to continue?'"