In one recent climate thread on Google+, I was informed, by three different people, that global warming would:
Create two billion climate refugees
Flood most of the world's large cities
The simplest rebuttal to such claims is the latest IPCC report. The IPCC, after all, is doing its best to persuade people to support action to slow global warming, so unlikely to minimize the problem. But if you look not at the rhetoric but at the factual claims, the impending climate catastrophe looks like a wet firecracker. For instance:
There is no evidence that surface water and groundwater drought frequency has changed over the last few decades, although impacts of drought have increased mostly due to increased water demand.
Economic losses due to extreme weather events have increased globally, mostly due to increase in wealth and exposure, with a possible influence of climate change (low confidence in attribution to climate change).
Some low-lying developing countries and small island states are expected to face very high impacts that, in some cases, could have associated damage and adaptation costs of several percentage points of GDP.
... most recent observed terrestrial-species extinctions have not been attributed to recent climate change, despite some speculative efforts (high confidence).
With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income ... .
Oddly enough, precisely the same evidence can be used against nuts on the other end of the spectrum, such as the gentleman in one thread who claimed that the IPCC had a budget in excess of twelve billion dollars. Pointing out to him that the actual budget, available on the web, came to about eleven million dollars had no effect. If, as he confidently believes, the IPCC is a massive fraud, inventing global warming out of thin air for its own sinister purposes, why should I be so naive as to believe their account of their budget? Pointing to webbed data on global temperature, which has increased by about one degree C since 1910, would be no more effective, since he can, and probably would, claim that the data are fake.
What he cannot explain away so easily is the modesty of the IPCC's claims. If, as he (correctly) believes, they are trying to scare people into doing something about global warming and if, as he (incorrectly) believes, they are unconstrained by actual evidence and science, why don't they tell a better story? Two degrees of warming over the next 86 years, a foot or two of sea level rise, are not very impressive threats, despite all the rhetorical efforts of the IPCC and its supporters. Why don't they make it ten degrees and ten feet? Twenty feet? A hundred feet? Why don't they tell something closer to the story that their nuttier supporters want to believe?