Technological progress is impossible to predict, but it's safe bet that we won't be flying solar- or wind-powered airplanes anytime soon. So the best hope of flying without emitting large volumes of greenhouse gases lies with biofuels.

This week, there's good news on bringing biofuels in the air. Beginning Wednesday, Alaska Airlines will fly 75 commercial passenger flights in the U.S. powered in part by biofuels. "This is a historic week for aviation," declared Alaska Air's CEO, Bill Ayer, in a press release. Today (Nov. 7), United Airlines make the first U.S. commercial flight using an advanced biofuel made from algae, according to Reuters.

Keith Loveless, vice president of corporate and legal affairs, who oversees sustainability, told me: "These fuels will make a meaningful contribution towards reducing the aviation industry's environmental impact, and towards reducing fuel volatility, which is an incredible problem for the airline industry."

But -- and you knew there would be a but -- biofuels remain way too expensive to replace jet fuels today. That's why Tom Vilsack, the agriculture secretary, got on the phone with me last week so that the Obama administration will do all it can to advance progress on aviation biofuels. "We are engaged right now in aggressively promoting research to determine the most efficient non-food feed crop that can be used," he said.

Biofuels remain controversial, of course. The U.S. Senate, in a symbolic vote, overwhelmingly expressed support last summer for an end to massive corn ethanol subsidies, in what the NRDC's Nathanael Greene called "a victory for taxpayers and the environment."

But both the industry and environmentalists say that biofuels for aviation make sense. That's because the only practical way to dramatically reduce CO2 emissions from planes (other than grounding them) is with biofuels. Plant-based jet fuels emits as much carbon pollution as traditional, petroleum-based jet fuel -- indeed, they are chemically almost indistinguishable -- growing new plants recaptures those CO2 emissions.

And airplanes matter. Aviation accounts for about 1.5 percent of global man-made GHG emissions per year, and gains in aircraft efficiency have been entirely offset by growing demand for air travel, according to the Pew Center on Global Climate Change.

The industry supports biofuels for business as well as environmental reasons, Loveless told me. Creating an alternative to traditional get fuel will increase supply, drive down costs and reduce price volatility. "We are probably more affected by the price of fuel than any other industry," he said.

"This will help create jobs in rural America," Vilsack told me. "It should also save money for the airlines, which will reduce the cost of flying.... and it'll reduce our reliance on foreign oil."

All true, but let's remember there's a long way to go before biofuels become mainstream. According to Loveless, Alaska Air will pay about $17 per gallon for a biofuel made from discarded cooking oil that will replace 20 percent of the jet fuel on the 75 flights, between Seattle and Washington, D.C., and Seattle and Portland, Oregon. Despite that boost in cost, the environmental benefits of a 20 percent blend will be modest, according to the airline:

Alaska Air Group estimates the 20 percent certified biofuel blend it is using for the 75 flights will reduce greenhouse gas emissions by an estimated 10 percent, or 134 metric tons, the equivalent of taking 26 cars off the road for a year.

The company also said:

The fuel was supplied by SkyNRG, an aviation biofuels broker, and made by Dynamic Fuels, a producer of next-generation renewable, synthetic fuels made from used cooking oil. The synthetic high-performance airliner fuel made by Dynamic Fuels -- a $170 million joint-venture between Tyson Foods Inc. (NYSE: TSN) and Syntroleum Corp. (NASDAQ: SYNM) -- meets aviation and military safety, sustainability and performance standards.

United Airlines, meanwhile, is using biofuels to power a flight from Houston (oil country!) to Chicago. According to Biofuels Digest, the fuel produced by Solazyme is a 40/60 blend of sustainable biofuel and traditional petroleum-derived jet fuel. (See my 2010 blogpost, Gee whiz algae! for more on Solazyme.)

To speed adoption of biofuels, Vilsack and the ag department this fall announced a $136 million research program of grants to universities in Washington, Iowa, Louisiana and Tennessee to develop next-generation biofuels from such feedstocks as switchgrass, woody biomass and sugarcane. Separately, the ag department, energy department and the navy have plans to invest up to $510 million in partnership with industry to produce advanced, drop-in biofuels to power military and commercial planes and ships.

Said Vilsack: "We may have to provide some encouragement [meaning money] at the beginning, until the market becomes a marture market."