More than 20,000 people around the world are donating their spare computing power to aid a scientific experiment that is attempting to forecast the climate of the 21st century. The Casino-21 project asks volunteers to keep their personal computers running around the clock so that when they are not otherwise in use, the PCs can help crunch climate data. Casino-21, based at the Space Science and Technology Department of the Rutherford Appleton Laboratory in England, is the world’s largest climate-simulation project and could produce the most accurate models. Bob Walker, a participant from Ottawa, Canada, says, “My reason for volunteering is to help, even in some small way, in accumulating the evidence we need to preserve our environment for my children’s grandchildren.”