Sir

Congratulations on your excellent Biofuels Special (Nature 444, 654; 2006 and Nature 444, 669–678; 2006). Not only is this an important topic for scientists and environmentalists, but also developing energy products that move the United States away from its over-dependence on oil is a critical national-security priority.

There are many exciting possibilities for expanding renewable fuels in the United States and around the world. Today's ethanol from corn and sugar will be complemented by cellulosic ethanol from many energy crops, as well as by next-generation biodiesel and perhaps other renewable fuels that have yet to be developed. Moreover, these will complement liquid fuels made from abundant coal reserves.

Although I agree that markets need to determine which fuels emerge as the most competitive, we should recognize that these new fuels are not competing on a level playing field with oil. The true costs of oil to our economy and national security are much higher than the price paid at the pump. Much of the world's oil is controlled by authoritarian regimes that increasingly use their oil resources for political gain. In addition, the price we pay for oil does not cover the risks to the economy from delivery disruptions, price spikes, environmental hazards and the vast cost of protecting supply routes and infrastructure. Milton R. Copulos, president of the National Defense Council Foundation, testified to the Senate Foreign Relations Committee last year that the minimum military subsidy Americans pay to protect Middle East oil trade routes is $50 billion a year (see http://www.senate.gov/~foreign/hearings/2006/hrg060330a.html).

Consequently, Congress is debating a variable price floor for alternative fuels so that oil producers cannot use temporary price cuts to undercut the new fuels that make America more secure. Such a floor should allow all fuels to compete, so the best ones may emerge.