Bellow are some reasons people dispute the theory:
Co2 has never driven the climate in the past (it doesnt mean it couldnt now).
Co2 is a weak greenhouse gas and only accounts for 0.054% of the greenhouse layer, of which 3% of the co2 comes from man. Some find it hard a scarce weak greenhouse could considerably drive the climate.
Ice core samples have been shown to underestimate co2 levels.
The hocky stick graph has been debunked (as was used by Al Gore and the IPCC)
The IPCC predict a warming of the troposphere as the signature of global warming. This isnt happening so the signature isnt there.
It has been warmer in the past and co2 has been higher before man made co2 was an issue.
Models are not proof of the thoery and they ignore many effects such as positive feedbacks.
All the IPCC models are off, none failed to predict the current cooling.
It has cooled for 3 periods during rising co2, hence there is no direct correlation.
Solar activity fits with temperature variation much better than co2.
Temps have been returning to the 3000 year mean from a particuarly cold period so it impossible to seperate what is natural and man made.
There are also issues of political motivation as the IPCC were set up to prove a link between man made co2 and warming not to un-biasly investigation the cause of the warming trend this centuary.
There are many leading scientists in their field who disagree with the AGW theory.
There is a lot of misrepresentation of what the warming will actually mean.
There are more reasons, but these are the main ones.
Check my sources, I have provided links to documents and organisations who disagree with AGW so you read further in to their reasons for disagreeing with the theory.
also see the following documentary which explains some of the arguments against very well: