The problem: poor mountain families in China have to haul huge loads of firewood for their cooking stoves. The solution: a new type of stove that runs on straw and other crop waste — first by heating them, then burning the resulting gas. And since Beijing Shenzhou Daxu Bio-energy Technology's $50 stove emits eight fewer tons a year of carbon dioxide than a coal stove does, the device is easing deforestation while helping China, the world's largest greenhouse-gas producer, reduce its heavy carbon load.