There are plenty of people out there who contend that anthropogenic climate change is all just a big story. The real question is: How would such a person be convinced that anthropogenic climate change is real?
We can break this down into proof that climate change is occurring; proof that climate change is caused by human activities; and I guess also important are: proof that reducing fossil CO2 emissions will slow climate change and proof that doing so is economically beneficial.
What would prove that climate change is happening? We need some measure of global temperature, say ice caps. If we could show that there is a large amount of polar ice that has been there for say a million years, and then it all melted, that would seem fairly convincing proof that global temperature has increased the most in a million years.
This hasn't happened yet, but if it did, it would be strong evidence indeed that something has changed in the climate that hasn't happened for a million years.
But this might be due to the sun getting hotter, or cosmic rays or something. (It would be surprising indeed for such an event to occur exactly as humanity starts to be able to measure such things though!).
How could we show that such climate change is caused by human activity rather than some random statistical blip? This seems harder, and I think this is where most climate realists/skeptics/deniers are building their sand castles on the beach.
One way to prove that humans are causing it would be to modulate the CO2 emissions at a rate slower than the time constant of the climate. Perhaps we could stop using fossil carbon for a decade, then start using it again, then stop. This seems a little impractical.
Mental suggests looking at the ocean pH. Apparently we know the ocean pH quite accurately for at least 300 years. Unfortunately, it might be that the statistical blip in the climate actually causes ocean pH to fall. But certainly the pH has dropped measurably in the last 300 years.
I think this question is the one that is going to be the sticking point for some time.
Incidentally, the evidence so far is that serious commitment to reduced CO2 emissions is not a heavy burden - France and Denmark produce $6k and $4.5k per tonne of CO2 resp., compared with the US and Australia at $2k. Yet Denmark and France have higher GDP per capita than the US and Australia (and work fewer hours). (The US has for a long time had a remarkably high GDP and hence a high GDP per capita, but I think this is more due to dominance of the US dollar — perhaps due to things like oil money. This advantage has dropped dramatically in the last few years as the US heads into recession and the US peso loses value)
In any case, given that a certain percentage of the population believes in the hogfather, the tooth fairy or homeopathy, what percentage of the population needs to be convinced of the need to act to actually act?