One survey of earth scientists found that while 97 per cent of actively publishing climate scientists agree humans are changing global temperatures, only 47 per cent of economic geologists (those who study geology with a view to its commerical exploitation) concur. In fact, among all earth scientists, economic geologists are the most sceptical.
Similarly, in response to the consensus on global warming, the American Association of Petroleum Geologists "respects these scientific opinions but wants to add that the current climate warming projections could fall within well-documented natural variations in past climate and observed temperature data".
So climate scepticism seems strongest among geologists closely linked to the mining and fossil fuel industries. Perhaps the words of Upton Sinclair shine some understanding on the forces at play here: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it."
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.