HOUSTON (Reuters) - Worldwide emissions of carbon dioxide, considered a culprit in global warming (news - web sites), are expected to increase by 3.5 billion tonnes, or 50 percent, annually by the year 2020, an executive for ExxonMobil Corp said on Wednesday. At the same time, global demand for energy will rise by 40 percent as the world population increases and economies grow, said Randy Broiles, global planning manager for Exxon's oil and gas production unit. "Between now and 2020 we estimate increases of some 3.5 billion tonnes per year of additional carbon emissions, so it's definitely increasing," Broiles said...