The West Antarctic warmed in response to higher temperatures in the tropical Pacific, which itself has been warming due to weather patterns like a major El Nino event from 1939 to 1942 and greenhouse emissions from cars and factories, according to the study.
"An increasingly large part of the signal is becoming due to human activity," said the study's lead author David Schneider of the National Center for Atmospheric Research. The study appeared on Tuesday in the journal Proceedings of the National Academy of Sciences.
Previous studies had showed the West Antarctic had cooled partly due to winds caused by depletion of the ozone layer.
The El Nino pattern is a periodic shift in air pressure accompanied by oceanic warming in the tropical Pacific.
Scientists are interested in whether warming will destabilize the West Antarctic ice sheet, which covers a region the size of Mexico and averages about 6,500 feet deep. If it all melted, it would raise sea levels by 8 to 16 feet.
There are few historical records and little understanding of how ice sheets might react to rising temperatures due to global warming.
The study, supported by the National Science Foundation, showed the West Antarctic warned about 1.6 degrees Fahrenheit (0.9 C) over the 20th century, or slightly more than the global average of about 1.3 degrees F (0.7 C), though there was some uncertainty in the estimate.
The U.N. Intergovernmental Panel on Climate Change figured last year that Antarctica would not contribute to rising sea levels, and in fact predicted a growth of the big ice sheet the covers much of the continent from enhanced precipitation.
There are parts of Antarctica that are gaining snowfall and ice, Schneider said, but the overall trend for the continent is that the ice is diminishing.
(Reporting by Timothy Gardner, editing by Chris Wilson)