Global warming has been with us for 400-500 years. Centuries ago, the Thames river froze year round for so long that many buildings were erected upon the ice(!). The earth, it is believed, has been emerging from a "little ice age" during that 400-500 years.

http://www2.sunysuffolk.edu/mandias/lia/little_ice_age.html

The warming observed within the 20th century falls onto that trendline.

Some people think that human activity, which is responsible for somewhere between .5 and 1.5% of the total 'greenhouse' gases estimated to be in the atmosphere, have caused this warming, and that this warming is therefore an industrial age phenomena. There are several real technical problems with this particular viewpoint, among them the rather poor correlation between the level of atmospheric greenhouse gases observed in the geological record and the estimated surface temperature corresonding to the respective time periods, the widely varying amount of natural greenhouse gases (variations which far exceed the total contribution of human activity), and lack of a clear understanding of how greenhouse gases actually behave in the real atmosphere(like the water vapor feedback problem), and the imperfect surface temperature estimations, based as they were on flawed data.

http://www.freerepublic.com/focus/fr/922376/posts

This leads to very inaccurate mathematical modeling of these phenomena, which is OK as long as we are only theorizing and not trying to make important decisions based on such flawed models.

The cosmic ray thoery certainly correlates far better with the gelogical record.

http://tinyurl.com/9mufe

Many "real scientists" believe the greenhouse gas theory is not plausible.

http://tinyurl.com/axkzk

http://en.wikipedia.org/wiki/Richard_Lindzen

Actually, even the "land usage change" theory is more plausible than the "greenhouse gas" theory.

http://www.discover.com/educators-guide/jan-04/guide3

The proponents of the greehouse gas theory tend to downplay the importance of volcanism even though such activity does produce climatologically significant amounts of both greehouse gases and particulates. This is yet another a big technical problem with this theory. The contribution of soot is yet another one.

If this theory were such a 'slam dunk', there would really be no room for debate. That is far from the reality. The reality is that as science learns more about climate, the greenhouse gas/global warming theory seems less plausible, not more.

http://www.wilsoncenter.org/index.cfm?fuseaction=wq.essay&essay_id=33083

This last one is long but VERY good. Here's a short quote:

"A great deal of global warming rhetoric gives the impression that science has established beyond doubt that the recent warming is mostly due to human activities. But that has not been established. Though human use of fossil fuels might contribute to global warming in the future, there’s no hard scientific evidence that it is already doing so, and the difficulty of establishing a human contribution by empirical observation is formidable. One would need to detect a very small amount of warming caused by human activity in the presence of a much larger background of naturally occurring climate change—a search for the proverbial needle in a haystack."

Jimbo