An argument from authority is not a fallacy as long as the authority is a legitimate expert on the subject and there is a consensus among the majority.
And how many authorities are there who are legitimate authorities in ALL of the required fields to declare that human CO2 emissions have caused the last century of warming? It spans virtually every scientific discipline that there is. There is meaningful individual consensus on specific, isolated facts. It has indeed been warming over the last 100 years. Human's have indeed been releasing measurable levels of CO2. CO2 is indeed a GHG that contributes to warming. I'm afraid however, that there is no legitimate expert and majority consensus that those individual pieces prove unprecedented and potentially catastrophic anthropogenic global warming is underway.
The legitimate experts that tell us the last 100 years have been warming equally have records showing similarly high temperatures within the last 2k years, with temperatures previously dropping as quickly as they are warming today(see even Mann's own corrections to his previous work). That seems to argue against the last 100 years of warming indicating catastrophic AGW is underway.
The legitimate experts that tell us that humans are emitting measurable levels of CO2 each year also tell us that those are a mere 3% of the natural emissions our planet produces every year. That seems to argue against our CO2 emissions being proof that catastrophic AGW is eminent.
The legitimate experts that tell us that CO2 is a powerful GHG also tell us that it is responsible for 10-25% of the greenhouse effect, while water vapor accounts for better than 70. Combine that with the small contribution humans make to natural CO2 emissions, and it is hardly compelling that catastrophic AGW is upon us.
The tricky part about science is cross-disciplinary studies and a failure to recognize uncertainties in results that get picked up as input from another discipline. The IPCC has tried to get around that with committees of experts, but it seems to have only managed to add the additional problem of politics into the mix.