Here's the problem: Once someone comes up with a theory, (i.e. manmade global warming,) scientists start looking at other data and saying "Oh, yeah, that supports this theory!" and automatically assume that the original theory was correct. They get a bunch of evidence that
seems to support their theory, and yet they don't consider other possibilities.
Remember back before Louis Pasteur? People thought that all of the insects, bacteria, etc. in their rotting meat was actually being created
from the meat. They even did "experiments" that seemed to further prove their theory.
Then, of course, Pasteur came along and proved them all wrong.
Now, obviously I don't think that we should just stop taking care of our environment. I mean, we only have one Earth, so might as well take care of it, right? But, some people can take this "theory" of anthropogenic global warming
way too far. It is being used as a springboard for sales and politics. Most "environmentally friendly" products sell better. People are willing to pay extra money to "go green". Politicians get more support if they promise to help the environment. Etc., etc..
Also, by the way, I assume you guys have heard of
Climategate by now, right? Read the article if you haven't.

(TL;DR: There's a bunch of evidence that scientists tampered with the temperature data to fit their theories.)
I could seriously talk for hours on this subject, but it's really late. I gotta get up in like 4 hours. >.< G'night all.