Exxon Mobil, which had long been skeptical of global warming, offered its own suggestions. One of the company’s top executives, Michael J. Dolan, said that Exxon would back a tax on carbon, while criticizing a so-called cap-and-trade approach.

Under the cap-and-trade formula, which has considerable momentum in Washington, the government would set a ceiling on how much carbon dioxide could be emitted into the atmosphere each year. It would then give or sell permits that companies would be allowed to trade to meet their limit. Emitting carbon dioxide at present involves no penalties in the United States, meaning that companies have little incentive to curb their pollution.

Mr. Dolan, a senior vice president at Exxon, said that a carbon tax would be simpler and less subject to manipulation than a trading system. “A carbon tax reduces policy risks for businesses and investors in a way that cap-and-trade schemes do not,” Mr. Dolan said during his address at the industry conference, organized by Cambridge Energy Research Associates, a consulting firm.