Cross posted from Prometheus: The Science Policy Blog

Remarkably, with all of the talk of targets and timetables for emissions reduction and expansion of renewable energy technologies, only a few analysts have worked the problem from the other direction, which is to ask, how fast can a big economy decarbonize?

The answer to this question has more to do with the simple math of energy, technological possibilities, and economic and political realities, and less with aspirational goals, over-the-top rhetoric, and wishful thinking. Tuesday's New York Times provided the perspective of major fossil fuel companies on the time scale of decarbonization:

"In my view, nothing has really changed," Rex W. Tillerson, the chief executive of Exxon Mobil, said after the election of President Obama.

"We don't oppose alternative energy sources and the development of those. But to hang the future of the country's energy on those alternatives alone belies reality of their size and scale."

The administration wants to spend $150 billion over the next decade to create what it calls "a clean energy future." Its plan would aim to diversify the nation's energy sources by encouraging more renewables, and it would reduce oil consumption and cut carbon emissions from fossil fuels.

The oil companies have frequently run advertisements expressing their interest in new forms of energy, but their actual investments have belied the marketing claims. The great bulk of their investments goes to traditional petroleum resources, including carbon-intensive energy sources like tar sands and natural gas from shale, while alternative investments account for a tiny fraction of their spending. So far, that has changed little under the Obama administration.

"The scale of their alternative investments is so mind-numbingly small that it's hard to find them," said Nathanael Greene, a senior policy analyst at the Natural Resources Defense Council. "These companies don't feel they have to be on the leading edge of this stuff."

Perhaps not surprisingly, most investments in alternative sources of energy are coming from pockets other than those of the oil companies.

In the last 15 years, the top five oil companies have spent around $5 billion to develop sources of renewable energy, according to Michael Eckhart, president of the American Council on Renewable Energy, an industry trade group. This represents only 10 percent of the roughly $50 billion funneled into the clean-energy sector by venture capital funds and corporate investors during that period, he said.

"Big Oil does not consider renewable energy to be a mainstream business," Mr. Eckhart said. "It's a side business for them."

Shell, for example, said it spent $1.7 billion since 2004 on alternative projects. That amount is dwarfed by the $87 billion it spent over the same period on its oil and gas projects around the world. This year, the company's overall capital spending is set at $31 billion, most of it for the development of fossil fuels.

Industry executives contend that comparing investments in oil and gas projects with their research efforts in the renewable field is misleading. They say that while renewable fuels are needed, they are still at an early stage of development, and petroleum will remain the dominant source of energy for decades.

I'd welcome pointers to analyses and literature focused on how fast a big economy can decarbonize. Are the energy companies wrong?