It has become very difficult ...

It has become very difficult for anyone to argue that observed global warming is natural variability. We have good reason for being able to say that the world will be warmer by about a quarter of a degree in the next decade. It's the same reason we had 10 years ago when we said that the 1990s would be warmer than the 1980s: The planet is out of equilibrium.
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 James Hansen

Quotes from the same author

Imagine a giant asteroid on a direct collision course with Earth. That is the equivalent of what we face now with climate change, yet we dither.
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 James Hansen
If we fail to act, we will end up with a different planet.
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 James Hansen
The danger is that the compromises and special interests inherent in Kyoto-style targets and cap-and-trade will be accepted because of bureaucratic momentum.
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 James Hansen
The question is not whether to close the parks, but how to accomplish this goal.
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 James Hansen
...the global surface albedo [surface whiteness] and greenhouse gas changes account for practically the entire global climate change.
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 James Hansen