A truth that floated to the surface during the BP energy company's Deepwater disaster in the Gulf of Mexico, along with hundreds of millions of liters of oil, is that the world does not have a ready replacement for conventional forms of fuel such as crude oil and likely will not have one for some time, particularly as demand for energy grows worldwide. Cleaner energy alternatives, including natural gas, wind, solar, nuclear and biofuel, have gained ground on greenhouse gas–producing oil (as well as coal), but there is still a long way to go before these inexpensive, efficient fuel sources can be phased out, according to a keynote panel assembled recently at Technology Review's Emerging Technologies (EmTech) conference in Cambridge, Mass. (Technology Review is published by Massachusetts Institute of Technology, where the conference is held.)

Fossil fuels (in particular oil and coal) accounted for nearly 90 percent of global energy consumption in 2000 and are projected to remain the dominant energy source throughout the 21st century, supplying 70-to-80 percent of energy demand in 2100, according to a 2007 U.S. Climate Change Science Program report (pdf) prepared by the Climate Change Science Program Product Development Advisory Committee for the U.S. Department of Energy.

Keeping in mind the enormous stake that panel members ExxonMobil and Shell have in the oil, natural gas and coal industries, here is a look at the panel's take on why oil and coal have been so difficult to replace by the following alternative energy sources:

Natural gas
ExxonMobil favors boosting the U.S.'s consumption of natural gas, in part, because it produces at least 50 percent less greenhouse gas per hour when burned compared with coal, Nazeer Bhore, ExxonMobil senior technology advisor, said during the panel. Of course, Exxon has a major investment in natural gas as one of its largest global producers and as recent purchaser of XTO, a firm which is active in developing ways to extract natural gas from the Marcellus Shale Formation in the Appalachian Basin. In addition, even though natural gas is not a renewable resource, there is an abundance of it. "Shale gas will revolutionize energy in this country in the short term," Bhore said. Under out feet is "the biggest energy revolution in the past 10 years, but people don't recognize this."

Increased use of natural gas is the best bet for cleaner energy in the near term, agreed fellow panelist John Reilly, a senior lecturer at M.I.T.'s Sloan School of Management and co-director of the school's Joint Program on the Science and Policy of Global Change. Reilly is also one of the co-authors of the U.S. Climate Change Science Program report.

Still, although natural gas is already in wide use and less of an "alternative" than other options, finding new sources to meet growing demand is not without controversy. In particular, concerns that the hydraulic fracturing process (called "fracking") used to capture natural gas could pose a threat to the environment and public health if chemicals used in the operation contaminate underground drinking water.

Wind energy is much more cost-competitive than solar when compared with coal. "We like wind better than solar in the near term because we don't see how solar's cost will come down to compete with wind," ExxonMobil's Bhore said.

The problem with wind continues to be its intermittent power generation and short-lived energy storage capacity. It is also unclear at this time what impact massive numbers of wind farms might have on lower-level air currents and whether this itself could impact climate change, Reilly said.

The amount of solar energy that reaches Earth has the capability to provide terawatts (trillions of watts) of power, according to Harry Atwater, a professor of applied physics and materials science at California Institute of Technology. Atwater pointed out during a presentation following "The Future of Energy" panel that solar still represents less than one percent of energy generation today. (Global energy consumption is currently about 15 terawatts.) The dilemma with solar power is that silicon-based photovoltaic cells are fairly efficient but also highly expensive, whereas thin-film cells are relatively inexpensive but inefficient.

For unsubsidized solar power to be competitive with coal- or natural gas–powered electricity, it needs to cost $1 per watt—today, solar is three to five times more expensive than fossil fuels, Atwater said. He is researching ways to meet this goal through the use of materials including amorphous silicon (a-Si), copper indium diselenide (CIS) and cadmium telluride (CdTe) to exploit silicon's efficiency and keep down costs.

Nuclear power met nearly 14 percent of the world's electricity demand in 2009, according to the International Atomic Energy Agency.

After decades of steady growth, however, nuclear power generation has actually declined in recent years as plants have been shut down (such as in Japan, due to earthquake concerns) and construction has been delayed on new plants (most notably, Finland's Olkiluoto facility). The nuclear option (that is, using nuclear fission reactions to heat water, which produces steam that turns turbines to generate large amounts of electricity) is expensive to start up—building a nuclear station can cost more than twice as much as building a pulverized-coal plant, Reilly said.

Shell is particularly interested in biofuels, especially algae, as is ExxonMobil, which last year invested $600 million in Craig Venter's company Synthetic Genomics. "What makes a sustainable biofuel is an interesting question," Shell Global Solutions chief scientist Jose Bravo said during the EmTech panel. "It changes from region to region."

Biofuels may provide a supplement to gasoline and diesel, but to completely replace them with biofuels would require an impossibly large one billion hectares of land, Reilly said, adding, "Just because biofuel can be done well in a lab doesn't mean that it can be done at scale."