Hmm… Dr Dommenget seems to have vastly overshot the mark. Do people in the street really want to compile and run simplified climate models or do they want to know what, say doubling atmospheric CO2, might realistically do?
Most would likely be astonished to find that the realistic estimate is a mere +0.4 °C warming but that is what the IPCC and Trenberth et al demonstrate to be the upper limit. On that point climate models really mess up badly and so, apparently, does Dr Dommenget’s.
In fact climate models don’t produce anything like real world results and we can show this with a couple of links and “on the fly” calculations.
From the IPCC’s simplified expression for radiative forcing we know that doubling atmospheric carbon dioxide should deliver 3.7 Watts per meter squared (W/m2) of additional forcing (doesn’t matter if you double from 280 ppm or 300 or 500, the result will always be the same).
The IPCC also tell us that the climate sensitivity parameter (λ) is derived by the change in surface temperature (ΔTs) divided by the change in forcing (ΔF), thus ΔTs/ΔF = λ.
So how much will that 3.7W/m2 warm the surface? Too easy, +0.37 °C and we know that because Trenberth et al have been measuring and calculating energy flows through the atmosphere for decades and they have presented them in a lovely graphic with numbers:
Note the “Back Radiation” figure – that’s greenhouse effect including all the positive and negative feedbacks in the climate system and anyone who’s done their homework knows that’s generally reckoned on being 33 °C (the difference between earth’s calculated absolute temperature of 255 kelvins (K) without greenhouse and the 288 K we find with it).
Substituting earth’s greenhouse effect of 33 °C for ΔTs and Trenberth’s ~330 W/m2 (back radiation) ΔF then λ (our climate sensitivity value) is approximately equal to 0.1 °C for each Watt per meter squared (33 °C/(~330 W/m2) = 0.1).
Makes you wonder how climate modelers come up with such bizarre results as 2.5-6 °C for 2xCO2 doesn’t it. Not to mention why very few people laugh at Hansen’s climate sensitivity of 0.75 °C ± 0.25 °C (Can we defuse the global warming time bomb?).
Nonetheless, Dommenget sallies forth:
“Almost everyone has an opinion on climate change, but most people don’t know anything about it,” Dr Dommenget said.
“This has inspired me to develop a climate model suitable for use on home computers, so that everyone will be able to find answers to questions about everything from emissions to rainfall changes.”
In his upcoming lecture, Understanding the Physics of Climate Change, Dr Dommenget will delve into anthropogenic – or human impacted – climate change.