HUMANITY is heading for a hotter world than experts projected last year — and the United States is a big reason for that, a new United Nations report says.If all the countries do what they promise to reduce heat-trapping emissions, Earth will likely still warm another 1.2°C above current temperatures, according to today’s update of climate calculations by the United Nations Environment Programme.That's about 0.1°C more than last year’s projections by the same science team.