Why build a supercomputer if you don't have to? That's the idea behind the BBC's climate change experiment, which asks volunteers to download a program that uses their PC's downtime to crunch statistics about the planet's temperature, precipitation, pressure and cloud cover patterns dating as far back as 1920. Based on the historical data, the system sends each participant's results to a lab in Oxford, U.K., where they are aggregated to predict weather patterns through 2080. With more than 180,000 participants worldwide (see map), the BBC estimates the grid is crunching numbers faster than the world's biggest supercomputer. All of which saves the time and expense of building an energy-gulping computer specifically for the task.