Worldwide trading of greenhouse gas emissions more than doubled over the last year to about 71 million tonnes, with governments and multilateral agencies accounting for more than half of the purchases, according to a World Bank study released Thursday. That is a tiny amount of worldwide energy-related carbon emissions, which are expected to rise to a total 8.3 billion tons by 2010 from 6.6 billion tons in 1996, according to U.S. government data.