The amount of heat-trapping carbon dioxide in the atmosphere has shot past a key milestone — more than 50 percent higher than pre-industrial times — and is at levels not seen since millions of years ago when Earth was a hothouse ocean-inundated planet, federal scientists announced Friday. The National Oceanic and Atmospheric Administration said its long-time monitoring station at Mauna Loa, Hawaii, averaged 421 parts per million of carbon dioxide for the month of May, which is when the crucial greenhouse gas hits its yearly high.