We tend to talk about global warming as a future problem. But climate scientists are starting to use the present tense when they talk about it. Stephen Schneider is a researcher at Stanford University.

"From the days that we were predicting, theoretically, that there was going to be increased temperature, increase in heat waves, drought, flood, extremes, fires and melting of ice. It's all happening, now it's happening early and it still fluctuates a lot so skeptics say, 'Oh well it's just natural.' No, it's not natural."

Schneider says scientists are worried that political leaders aren't acting fast enough. Polls show that Americans feel no real sense of urgency. But some scientists and economists at the recent congress on climate change in Copenhagen said their research actually underestimates the threat.

But it's not all bad news. Many scientists at the conference say we still have an opportunity to change directions on the climate crisis. And create clean-energy jobs around the world.