Advocates of the energy bill making its way through the House say that it will help America make the transition to a new, green economy. But first it must get past the powerful agriculture bloc.

Among many other things, the bill – known as Waxman-Markey after its champions, Reps. Henry Waxman, D-Calif. and Edward Markey, D-Mass. – requires that utilities derive 15 percent of their electricity from “renewable” sources by 2020.

Most people think of wind or solar power, but under the bill a big source of renewable energy would be “biomass” – everything from wood chips to algae that can be burned to produce electricity. Carbon is still emitted in the process, but, given the right measures, burning biomass is better than burning coal, since biomass gets regrown, a process that removes carbon from the atmosphere.

But it might not do much good, environmentalists warn, if biomass suppliers clear, say, carbon-gobbling forest land in order to grow their product. So Waxman-Markey contains provisions aiming to prevent materials harvested on sensitive lands from qualifying as renewable. The particulars chafe legislators from farm districts, who want a more liberal policy to open up lands for biomass cultivation so their constituents can take advantage of the new renewable electricity mandate. They also want to change rules on biofuels such as ethanol, which are governed by a separate mandate contained in the 2007 energy bill.

The first thing this fight highlights is how difficult it is for those who draft these laws to get regulation right, given the complexity of the issues and the parochial politics of Congress. An attraction of a well-designed carbon tax or cap-and-trade system is that it should keep rough-hewn regulation and its associated distortions to a minimum.

Nevertheless, Congress must ensure that it does not give biomass suppliers incentives to produce a fuel that is barely better – or that is perhaps worse – than fossil fuels. Sensitive lands should remain protected. The best way to do that is to require that qualifying biomass meet a fair, minimum carbon pollution standard that is calculated over the full life cycle of the fuel – from clearing the land to burning the biomass – instead of battling over precisely which classes of marginal land need to be protected to accomplish the same goal.