Turning off your lights for an hour shows the world's leaders that WE WANT CHANGE!
it's just one hour, no need to feel depressed, organize something with ur friends, take pictures and send them to the earth hour website, and ur photo will be shown to the world, and the leaders, that we want to stop global warming!
of course this wont affect the global warming directly, but it will in one way or an other .. trust me!