David Middleton points to a fascinating thought experiment that ought to be applied very widely to climate change models and theories. There’s this collaborative effort to offer a dispassionate balanced explanation that climate change is real, manmade and urgent and somebody asked them what the temperature would be if humans weren’t creating GHGs. He found their answer muddled and so might you. But at its core is the claim that it wouldn’t change.

The answer wanders about, describing the atmosphere and criticizing humans before saying if we don’t stop emitting CO2 due to massive government intervention, aka “Short-term and scattered climate policy will not be sufficient to support the transitions we need” we’re all going to fry. But along the way it bites the bullet, saying “If people had not altered the composition of the atmosphere at all through emitting greenhouse gases, particulate matter and ozone-destroying CFCs, we would expect the global average temperature today to be similar to the pre-industrial period – although some short-term variation associated with the Sun, volcanic eruptions and internal variability would still have occurred.”

For greater certainty, it says “As a result of continued increases [in atmospheric CO2 due to humans], the global average temperature has climbed by just over 1 °C since pre-industrial times.” And since that’s the entire increase, it means we’d still be in the Little Ice Age if Watt or some other meddler hadn’t invented the steam engine.

It’s a curious answer. Why do they suppose the planet would not have returned to the conditions of the Medieval Warm Period, assuming they grant its existence? Or the Roman Warm Period? Why do they think the planet cooled from the Holocene Climatic Optimum?

OK, the answer was short and rambly. But never mind what this one group of people said. We should ask everyone: Putting aside measurement issues and granting for purposes of argument a 1 °C increase since Watt’s time, and an increase in atmospheric CO2 from 280 to 420 ppm, how much of the increase in CO2 was due to human influence not natural chances in the carbon cycle and how much of the temperature increase was due to our share of the CO2? If the answer is “all of it” we’d like you to run your model with inputs from 1750 and see where it goes. And from 1350. And from 49 BC.

If the answer is “most of it” we’d like to know how you evaluate the contributions of “the Sun, volcanic eruptions and internal variability”, whatever the latter means. And what it means for ECS.