Scientists should be biased to believe in favor of man-made global warming. There are many things to study: Co2 out put, deforestation, concrete coverage, agricultural dust, ozone depletion, automobile pollution, farm animal f@rts, and any number of a vast array of things that man may be4 doing to destroy the climate. Easy to say,"I have a theory, I need an immediate research grant, And immediately call and alert Senator Greenpants and the President of CNN for this breaking news!"
If you do not believe in man made global warming, your pretty much stuck with, "I don't know, nobody knows, could be affected somehow by man's activity or it could simply be cyclical natural forces. We do know that both conditions exist, but there is no conclusive evidence that man's activity is affecting the climate. On the other hand, we do know that cyclical events of nature affect the climate. I have therefore concluded that I am skeptical of man made global warming. I can study things if you like, but in truth I just don't know."
That is just a summary overview of my perspective of the bias involved in the situation from the scientist's position. But it gets worse. In the first case, there is a big news story and something that may require legislation to monitor or regulate. And nobody is going to be too enthused about granting money to finance "mr. i don't know, but...". So I can just see layer upon layer of bias.
That is primarily why I am a skeptic these days about man made global warming. Any past research for actual details in favor of man made global warming have ranged anywhere from inconclusive to ridiculously absurd for truth. I remain unconvinced.