25.18% of U.S. greenhouse gases are being emitted by only 43 firms, according to figures tabulated by the Political Economy Research Institute (PERI), from corporate filings with the U.S. Environmental Protection Agency, as of the latest available year, which was 2011.
PERI, which is sponsored by the Economics Department at the University of Massachussetts at Amherst, lists the largest 100 emitters at its website.
Even just the top ten emitters account for 12.05% of U.S. greenhouse gases. In order, from the top, they are:
1: American Electric Power, 1.94%
2: Duke Energy, 1.89%
3: Southern Co., 1.76%
4: U.S. Govt., 1.16%
5: Berkshire Hathaway, 1.06%