Though city dwellers may not realize it, agriculture is a big source of carbon emissions. That’s because of livestock’s production of methane, how manure is handled, and soil management (something as simple as tilling the soil releases greenhouse gases). Dairy geniuses Mike and Sue McCloskey, partners in one of the country’s biggest dairy operations, have come up with an elegant approach to tackling several of these problems at once in the hopes of creating a zero-carbon footprint dairy farm. At the heart of the operation: a process that turns their dairies’ tons of cow manure into natural-gas fuel. Here’s how it works.