At the end of the last ice age, parts of an enormous ice sheet covering Eurasia retreated up to a startling 2,000 feet per day — more than the length of the Empire State Building, according to a study released Wednesday. The rate is easily the fastest measured to date, upending what scientists previously thought were the upper speed limits for ice sheet retreat — a finding that may shed light on how quickly ice in Greenland and Antarctica could melt and raise global sea levels in today’s warming world.