People watch a video on global warming, projected onto a floating cube, on the opening day of the Climate Conference in Copenhagen, Denmark, on Monday. The scale of the CO2 cube installation represents one metric ton of carbon dioxide, the amount an average person in an industrialized country emits each month. The largest and most important United Nations climate-change conference in history opened with organizers warning diplomats from 192 nations that this could be the last best chance for a deal to protect the world from calamitous global warming.