BOULDER, Colo. - - Behind a locked door on the ground floor of a new University of Colorado science center here, a laboratory outfitted with specially reinforced concrete floors sits dark and empty, like a dining room set for a guest who never arrived. In this case, the no-show is a $2 million, 12-ton machine that is vital to addressing global warming. The machine, a high-precision accelerator mass spectrometer, uses nuclear physics to detect the presence of a rare, heavy isotope of carbon. It enables scientists to distinguish fossil fuel emissions from all other sources of carbon dioxide in the...