U.S. greenhouse-gas emissions hit 8 billion tons of carbon dioxide equivalent in 2007, up 1.4 percent from the year before and up about 16.7 percent from 1990 levels, according to the Energy Information Administration. The agency said much of the increase was due to two main factors: “unfavorable weather conditions, which increase demand for heating and cooling in buildings, and a drop in hydropower availability that led to greater reliance on fossil energy sources for electricity generation.”