the epa estimates that if every household replaced its nine most used lightbulbs with cfls, we'd save as much in CO2 as if we eliminated 10 million cars. The bad news is, we need to cut about 17 times that much; the good news, according to a recent report by mega consulting firm McKinsey, is that we could trim the nation's ghg footprint by almost 30 percent over the next 25 years by getting business to invest in efficient cars, appliances, and buildings as well as cleaner energy, with incentives including tax credits, subsidies, offsets, and fewer "regulatory hurdles." The cost for the first 7 percent: about $50 billion a year, or about $50 per ton of greenhouse gases. And what about taking shorter showers? "Consumer conservation is important," says Jon Creyts, one of the report's principal authors, "but it was more practical for us not to factor individual choice into our methodology." So we did the math ourselves. Below, a few samples of what individual change and bigger policy shifts can shave off our 15.6-trillion-pound total.