3) The ability to make forecasting models with a track record of accurate predictions over the very long term.

We don’t know whether current warming departs from natural variation, nor have scientists proven the underlying mechanisms by which humans could cause such an increase. But even if we did know these things, we would have to be able to forecast with reasonable accuracy how big the effect is going to be. A very small warming may not even be noticeable or may have mostly salutary effects, such as a slightly longer growing season, whereas the impact of a much larger warming is likely to cause greater disruption.

I should also point out that the “catastrophic” part of “catastrophic anthropogenic global warming” is a much larger question that is even harder to forecast. For example, global warming was supposed to lead to more hurricanes, which is why movie posters for Al Gore’s An Inconvenient Truth featured a hurricane emerging from an industrial smokestack. Then hurricane activity in the Atlantic promptly receded to historical lows.

It’s pretty clear that scientists aren’t any good yet at making global climate forecasts. Current temperatures are at or below the low range of all of the climate models. Nobody predicted the recent 17-year-long temperature plateau. And while they can come up with ad hoc explanations after the fact for why the data don’t match their models, the whole point of a forecast is to be able to get the right answer before the data comes in.