OK, despite what you might think, it’s actually quite easy.
Step one: Establish the Default Position
The first thing you have to do is point out the obvious fact that it is *not* your job to prove them wrong. Rather, it is *their* job to prove that they’re right. Remember, it’s not you who’s asking them to hand over their hard-earned cash. That’s what they’re doing and if they want your money, they have to demonstrate that they’re correct.
So if they say “Prove us wrong” you simply respond with “I don’t have to, I’m happy to keep my money in my pocket unless you can prove there’s something to worry about.” If they say “What do you say is causing it then?” you respond with “I don’t care, but I’m not paying you anything unless you can prove that your theory is correct.”
Thus, you must establish that the “Default Position” is that there is no problem and that it is up to the alarmists to demonstrate otherwise.
Step two: Ask for Evidence.
Remember, this is supposed to be a *scientific* debate. Science works on empirical evidence; so ask for the unequivocal, empirical, scientific evidence that supports the following:
-That mankind’s CO2 emissions have had a *significant* effect on global temperature.
-That global temperature will rise as much as predicted in the future as a result of mankind’s CO2 emissions.
-That any warming that does occur will cause harmful events. (If the warming isn’t going to be harmful, we don’t need to do anything.)
-That it would be cheaper to try and prevent those harmful events *now* rather than simply dealing with them *if* and when they happen.
Now, you have to be careful here and make sure any evidence that they provide *is actually* evidence for what you’re asking for. For example:
Evidence that the world is warming (Temperature records, ice caps melting, etc., etc.) is *not* evidence that mankind is causing it.
Evidence that the amount of CO2 in the atmosphere is rising is *not* evidence that it’s causing *significant* warming.
Evidence that mankind is causing the rise in CO2 is *not* evidence that it’s causing *significant* warming.
Evidence that CO2 is a greenhouse gas is *not* evidence that it’s causing *significant* warming.
Evidence that the extra CO2 in the atmosphere is indeed behaving in exactly the way that they predicted is *still not* evidence that it’s causing *significant* warming.
While it’s perfectly reasonable to assume that the extra CO2 will cause *some* warming, the $64,000 questions are: “How much?” and “Should I be worried?” and *that’s* what you need evidence for.
And remember, it’s unequivocal, empirical, scientific evidence that you’re asking for, and Climate models are *not* empirical. Many aspects that affect the climate of planet Earth are poorly understood. What values do we use in the climate models for those poorly understood aspects? Answer? We guess. And guesses, let’s be clear, are merely expressions of bias. If you want to predict a catastrophe, you guess at values that produce that result. And vice versa.
Finally, you can also point out that there is evidence to suggest that there is less cause for alarm than the alarmists are suggesting. For example; CO2 is rising as fast as ever, but there’s been no warming at all for the last decade. According to the temperature data from the Climate Research Unit in the UK (the data that the IPCC uses) the trend since 2001 is -0.05 per decade (note the minus sign). (Other temperature data sets show warming, of course, but this just demonstrates the high degree of uncertainty in this debate – You need more certainty before you’ll part with your money!) Also, the high temperature rise predicted by the alarmists is a result of amplification by climate feedbacks – the most significant of which is water vapour. However, the water vapour feedback is supposed to cause a tell-tale hotspot high in the atmosphere that not one, but three separate sources of temperature data say is simply *not* there. So that’s real, observation evidence from multiple sources that suggests that the alarmists’ predictions are wrong.
End of scare.