notes:   This is from a post by John Cook, creator of Skeptical Science, rebutting petroleum geologists who argue that since Earth's climate has changed in the past from natural causes, therefore the current change is likely also due to natural causes, not human activity.     Extended quote:

"When geologists bring up past climate change, they're actually citing evidence for climate feedback. Dramatic swings in global temperature, dragging the planet in and out of ice ages, are possible because of these feedbacks.

"Renowned paleoclimatologist Wally Broecker sums it up beautifully:  'The paleoclimate record shouts out to us that, far from being self-stabilising, the Earth's climate system is an ornery beast which overreacts to even small nudges.'

"We have already given our climate a big nudge. 

"How do we know it's us causing the warming and not natural causes? Because we've directly measured it. Satellites measure reductions in heat escaping to space - direct empirical evidence that carbon emissions are trapping heat. Surface measurements measure more heat returning to Earth, confirming the increased greenhouse effect. We see many signatures of greenhouse warming such as winters warming faster than summers, cooling upper atmosphere with warming lower atmosphere and nights warming faster than days. The case for human-caused warming is based on many independent lines of evidence.

"The feedbacks that amplified past climate change are now amplifying the warming caused by our carbon emissions. We're measuring more water vapour in the atmosphere, a strong feedback. Arctic sea ice is disappearing and satellites measure less sunlight reflected back to space - another significant feedback. The Earth's past and modern measurements all paint a consistent picture - our climate is already overreacting to our 'nudge.'

"The peer-reviewed literature on past climate change sends a strong message, in stark contrast to what we hear from petroleum geologists.  Past climate change is not a source of comfort. It's a cause for concern."

image source:   http://www.cosmographicresearch.org/prelim_glacial_maximum.htm and  http://www.exo.net/~pauld/workshops/globalclimate/atmospherecarbondioxide.htm