Yes the climate has changed in the past and the Greenland ice cores provide exactly the evidence John says. What is not mentioned is that the changes in the past took place over 1000's of years, not the couple of hundred we are witnessing now.
The climatic effects we are witnessing fit very precisely within computer models of increasing greenhouse gases since the start of the industrial revolution. As in any scientific theory, abolute proof does not exist - we should work on the balanceof probability - which is overwhelmingly that it has a human cause. The few scientists still saying it is nothing we have done but just a natural cycle are mostly sponsored by oil companies and other industries, concerned not about the planet but profits.
We should accept that we are very probably responsible and do what we can to lesson the effects. Should time prove the small minority right, and it is totally natural, we may have done little for the long term health of the planet. Should we not try to avert climate change, and it is our fault future generations (if there are any) will suffer for our complacency.