global warming... really?

I've been becoming increasingly skeptical of the whole idea of human induced global warming or even that natural global warming is much of a long term threat. Part of this skepticism might stem from the fact that I'm reading Michael Crichton's novel "State of Fear". He has that book pretty extensively footnoted and I must say that a lot of the arguments against global warming make a lot of sense. For a long time, I have been skeptical of it just based on my knowlege, as a geologist, of how much climate has varied over geologic time. Heck, just in recorded history it has varied pretty significantly. There was the Medieval Warm Period when the Vikings colonized Greenland and there were healthy vineyards in England and this was followed by the Little Ice Age when it got so cold that during the Revolutionary War, General Henry Knox was able bring the cannons from Fort Ticonderoga over a frozen solid Hudson River. So it is really only since the latter part of the 19th century that temperatures have risen. Even over that time period there have been variations. So to say that it is human-induced seems bogus to me. What really clinches it for me is the fact that the Mars Orbital Camera (MOC) on-board the Mars Global Surveyor has been tracking the retreat of features on the south polar cap of Mars since 1997 (press release). So I think it would be pretty hard to pin that warming of Mars on human activity! Increased solar activity seems to be the culprit in any global warming that we're seeing on Earth (and on Mars).