(Fortune) -- What do Oreo cookies made by Nabisco, Cheez-It crackers from Kellogg's or General Mills' Fiber One Chewy Bars have to do with global warming and the destruction of tropical rainforests? A lot, say environmental activists. The link between the supermarket shelf, climate change and shrinking rainforests is palm oil, a controversial ingredient that may now be the most widely-traded vegetable oil in the world. Here's the problem: Demand for palm oil, which is found in soaps and cosmetics as well as food, has more than doubled in the last decade as worldwide food consumption has soared. Farmers, in...