How Do We Know?

Al Gore seems to think that climate change is responsible for Hurrcaine Katrina. How does Gore, or anyone else for that matter know the answer to the following questions:

1. How can we know for certain that human activity causes global wamring?
2. How do we know that global warming (no matter its source) isn't ultimately beneficial to the Earth? Warmer climates mean better growing seasons which means higher food output.
   
3. Assuming that we are causing global warming and that it is actually harmful, how do we put a stop to it short of totalitarian rule of the Earth?