Reuters: Two of the top Democratic presidential contenders weighed in on the issue of how to limit emissions of carbon dioxide, a gas associated with climate change, during a debate on Saturday. Both Illinois Sen. Barack Obama and New Mexico Gov. Bill Richardson, a former U.S. Energy Secretary, said they agreed on that a cap-and-trade system, in which polluters have to pay for the right to emit gases and can sell emission rights they do not use, would be a logical approach. BILL …