When will oil production peak and begin to decline? Scientists, engineers and economists have debated the point for years, on the assumption that emissions of carbon dioxide into the atmosphere will decline when less oil is burned.

Not so, says Ken Caldeira, a climate scientist with the Carnegie Institution in Stanford, Calif. That assumes society switches to low-carbon fuel. But there's a good chance society will jump to the most abundant fuel around: Coal, which emits 25 to 50 percent more carbon dioxide per energy unit than petroleum, according to the Energy Information Administration.

Therefore, Caldeira said, the more important question - and one of the largest sources of uncertainty in climate models - is "will the end of oil usher in a century of coal, or will it usher in a transition toward low-carbon-emitting technologies?"

Speaking Wednesday at the American Geophysical Union's Fall Meeting in San Francisco, Caldeira reported on recent forecasts of how the climate would respond if the world completely stopped using oil today. In the one case, it is replaced with coal-based liquid fuels and in the other with renewable resources, such as wind, solar, or nuclear power.

The results are clear, Caldeira said. If liquefied coal powered the world's vehicles, produced its heating, and generated its electricity, Earth would warm 2º Celsius (3.6º Fahrenheit) by 2042, three years sooner than if society continued to use oil. If, however, society replaces oil with renewable energy, that 2º C rise would occur in 2056, 11 years later than with oil.

The reality, Caldeira said, is that we will never run out of oil. As it becomes scarcer and more expensive to extract, industry will switch to other fuels for economic reasons. The danger is that coal will likely appear to be the cheapest alterative.

So rather than view peak oil as a climate savior, he said, those scientists, engineers and economists should see the end of oil as a "new challenge" to efforts to cut carbon dioxide emissions.

This story originally appeared on www.dailyclimate.org and is authored by Harvey Leifert, who spent nine years as the public information manager of the American Geophysical Union before retiring in 2007. Like many reporters covering the meeting, he is a member of the union, but he is not representing the association nor is he writing about it.

The Daily Climate is the climate change news source published by Environmental Health Sciences, a nonprofit media company.