Three years after unveiling his plan for U.S. energy independence, which won praise from environmentalists for its reliance on wind power, Texas oilman T. Boone Pickens is back with a proposal to convert the U.S. trucking fleet to natural gas. But as his new plan gains traction, questions arise over how green it really is.

Energy companies are rushing to develop unconventional sources of oil and gas trapped in carbon-rich shales and sands throughout the western United States and Canada. So far, government officials have shown little concern for the environmental consequences of this new fossil-fuel development boom.

Many energy experts contend natural gas is the ideal fuel as the world makes the transition to renewable energy. But since much of that gas will come from underground shale, potentially at high environmental cost, it would be far better to skip the natural gas phase and move straight to massive deployment of solar and wind power.