Brumar, actually it has been warming up for the past, oh, 15-20,000 years or so, since the end of last glacial period. At least if we look at orbital cycles and the extent of ice coverage on the planet. There have been a few mini ice ages since then, but nothing that lasted more than a 1000-1500 years or so, and nothing that returned us to the condition of the actual glacial period. That, however, has nothing to do with why climate is changing right now. In fact, if anything, it should probably be getting to be a colder climate, based on these longer term orbital cycles. It is close to time for this interglacial to end, if it was following the pattern of long glacials (90-110,000 years or so) and shorter (10-20,000 year interglacials) that scientists have pieced together of the past 800,000 years or so.
Yes, climate fluctuates naturally. One of the reasons people have been studying this issue so intensively for the past 30 years is that they wanted to try to separate those natural variations from human influenced ones. That was one of the key questions they asked. Scientists from a wide variety of disciplines have studied this, and most of them have come to the conclusion that there is a very high probability--greater than 95%, I think they said in the last IPCC report--that the main cause for current observed changes is human activity. Now, you can disagree with that all you want if you like, be my guest. Personally I don't believe that either one of us--or most likely anyone who reads this board--has the expertise to actually judge that conclusion. I think that they didn't come to it lightly, that the evidence for it was very strong, that it isn't a political conclusion (as you have said you believe), and that the people involved were as careful as they could be in arriving at their conclusions.
As I've said before, you should read Spencer Weart's essays to get a history of how we arrived at these conclusions. It is both lucid and comprehensible for lay people to read.
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