WASHINGTON (AP) - The massive West Antarctic ice sheet may be headed for a complete meltdown in a process that a new study indicates was triggered thousands of years ago, not as a result of global warming. As scientists have been increasingly able to document melting and the discovery of icebergs breaking off from Antarctica in recent years, concerns have risen that human-induced climate change could be damaging the Antarctic ice sheet. But the future of the West Antarctic ice sheet "may have been predetermined when the grounding line retreat was triggered in early Holocene time," about 10,000 years ago, a team of scientists led by Howard Conway of the University of Washington reports in Friday's edition of the journal Science.