A bright idea to fight global warming will leave people in the dark: 8 p.m. Saturday is Earth Hour, when the World Wildlife Fund wants people to turn off their lights. From Chicago to Copenhagen, iconic high-rises will power down. Hundreds of McDonald's golden arches will turn black. And some restaurants will offer candlelight dinner — with a side of waiters wearing glow sticks. But critics say one hour won't save a "watt" of energy.