Almost all info on global warming is speculation. Sure its gotten a bit hotter since the industrial revolution, but nobody can work out how much. It could be proven in future years that our influence was negligible, and cosmic effects such as sunspots, and solar winds had most of the influence.
Still want extra proof? Dinosaurs were reptiles, cold blooded, and heavily relied on the sun and heat to survive, yet they managed to thrive around the entire globe, meaning it must have been pretty hot back then, and since creatures that size probably couldn't hibernate succesfully, Id be willing to bet the winters werent exactly cold.