Only 20 years left to stop serious global warming?

"We have to make deep, deep reductions in emissions within the next 20 years," says Tim Flannery, a respected Australian scientist and author. He further states the world's economic powerhouses must take drastic measures over the next two decades before Earth's climate is irreversibly altered. "We will have won or lost the battle for climate stability in that time." (more)

Of course the naysayers will claim that global warming is just some scientific conspiracy. Yet, the evidence that the world is rapidly changing is abundantly clear. Perhaps human action is having little effect in this 'natural' change. Still, considering the ramifications of this change, isn't it worth making sure that it isn't humans causing the problem?