Its not a scam, there was a scientific basis to the hypothesis, but as our knowledge of the climate has increase elements of it have been shown to be wrong or unlikely and new theories have emerged with stronger evidence to support them. The main issue is the global warming theory has been used for environmental, political and finincial gain, often relying of unjustified alarmist claims (see Al Gores movie for example). Even Realclimate.org recently said they would do more to out these unjustified scare stories as they do no one any favours, especially not science as a whole.
Science has always progressed through paradigm shifts, it didnt mean the previous understanding was fraudulent, but that it was based on the wrong information, at its conception it may have been based on the best available knowledge.
To quote some of the politicians / IPCC scientists:
“Each of us has to decide what the right balance is between being effective and being honest” Stephen Schneider, lead IPCC author
“We've got to ride this global warming issue. Even if the theory of global warming is wrong” Timothy Wirth, Head of the UN Foundation
“A global warming treaty must implemented even if there is no scientific evidence to back the greenhouse effect.” Richard Benedick, Deputy Assistant Secretary of Sate, U.S. State Policy Planner
“Unless we announce disasters, no one will listen”
John Houghton, 1994, IPCC Second Chairman & Lead Author