Hogs Farmers Against Global Warming

Brendan Miniter in the WSJ ($$$):

North Carolina's global-warming activists are in hog heaven. Late last month, Gov. Mike Easley, a Democrat in his second term, signed legislation mandating that more electric power in his state come from "green" sources such as wind, solar energy, and hog and chicken waste.

Today, North Carolina gets about 2% of its electricity from "renewable resources." By 2021, under the new mandates, Progress Energy and Duke Energy will have to find 12.5% of the power that they sell to Tar Heel residents from renewables. Hog-waste-generated power -- as required by the new law -- will nearly triple to 0.2% of the electricity used in the state over the next decade as farmers capture and sell the methane gas given off from tons of decomposing manure.

Sounds like the kind of idea that might be relevant to a national energy-environmental policy. Do such mandates make any economic sense?