WHEN PORTLAND’S 390,000 NW Natural gas customers crank up their thermostats this winter, each home will pump out about three tons of earth-heating CO2 before spring. Users who want to offset their global warming impact can invest $6 per month in NW Natural’s Smart Energy program, a campaign launched in September to curb the world’s climbing temps by harnessing a different kind of natural gas: the kind cows pass. Why bother with Bessie’s bum? Because methane, released by cow manure, traps heat in the atmosphere 21 times more effectively than CO2. After the manure is collected from the pasture, a gizmo called a biodigester converts the methane into electricity or even into (stench-free) natural gas. Smart Energy’s biodigesters aren’t yet built—NW Natural needs to raise at least $1 million first—but once they are, the process from pies to power will look something like this.