Emissions of carbon dioxide in the U.S. remained almost flat last year, rising only 0.4 percent, despite a booming U.S. economy that grew almost 4 percent and gasoline prices that hit record lows. These new findings are punching a hole in the theory that economic growth is linked to energy consumption, and are giving credibility to those who say that efforts to combat global warming won’t be hard on the economy. Some experts attribute the drop to the growing information sector, with companies that are light on CO2 emissions, and to the increasing use of more energy-efficient machines and appliances.