Dr. Frederick Seitz Past President, National Academy of Sciences and President Emeritus, Rockefeller University for one has set up a petition project to permit the voices of dissenting scientists to be heard. For an overview of the project visit the website: http://www.oism.org/pproject/s33p41.htm
One of the things about science is that, like the weather and climate, it is always in a state of flux. New theories challenge the old, become accepted, and are in turn replaced with others. Two major reasons to be skeptical of the "global warming" movement in particular are the smugness and intolerance of dissent displayed by its proponents, and the vast amounts of money involved.
I'd add a third. What can be generously considered amateurish errors in interpreting data (more likely intentional fraud) i.e. Mann's famous "Hockey Stick" chart which the entire IPCC and Kyoto is predicated on. If their data were as strong as their propaganda machine, there would be not need to falsify results, but they have been caught doing it over and over again.