The Royal Society today publishes its latest layman’s summary of Climate Change, and thankfully manages to avoid several representational pitfalls that sceptic-deniers could have leapt on and said “See ! We told you !”
Unfortunately, to my mind, it still has a few chinks in the door that should have slammed shut and permanently sealed off the sceptic-denier “contributions” on the subject.
Let’s look at the Royal Society narrative of progress by degrees, for example.
In section 28, “Aspects of climate change on which there is wide agreement : Climate forcing by greenhouse gas changes”, it reads :-
“…Application of established physical principles shows that, even in the absence of processes that amplify or reduce climate change […], the climate sensitivity would be around 1 degree C, for a doubling of CO2 [Carbon Dioxide] concentrations [in the atmosphere]…”
The related material in section 36, “Aspects of climate change where there is a wide consensus but continuing debate and discussion : Climate sensitivity”, goes on to talk about how global warming causes changes in the hydrological cycle, and how water vapour builds up in the atmosphere because of global warming, leading to further global warming :-
“…The more complex climate models, supported by observations, allow climate sensitivity to be calculated in the presence of processes that amplify or reduce the size of the climate response. Increases in water vapour alone, in response to warming, are estimated to approximately double the climate sensitivity from its value in the absence of amplifying processes. There nevertheless remain uncertainties in how much water vapour amounts will change, and how these changes will be distributed in the atmosphere, in response to a warming. Climate models indicate that the overall climate sensitivity (for a hypothetical doubling of CO2 in the atmosphere) is likely to lie in the range 2 degrees C to 4.5 degrees C; this range is mainly due to the difficulties in simulating the overall effect of the response of clouds to climate change mentioned earlier…”
Section 28 is at fault in my mind in three aspects :-
1. The IPCC Working Group 1 review of the science, which is the first of only two formal references in the new Royal Society guidance, makes clear that the likely influence of doubling Carbon Dioxide concentrations, with no other contributions included, would be a global warming of 1.2 degrees C, not 1 degree C :-
“…In the idealised situation that the climate response to a doubling of atmospheric CO2 consisted of a uniform temperature change only, with no feedbacks operating (but allowing for the enhanced radiative cooling resulting from the temperature increase), the global warming from GCMs would be around 1.2 degrees C (Hansen et al., 1984; Bony et al., 2006)…”
2. The second reason that section 28 is at fault is because of what the IPCC says directly afterwards :-
“…The water vapour feedback, operating alone on top of this [the calculation from basic physics of the effect of doubling Carbon Dioxide], would at least double the response. The water vapour feedback is, however, closely related to the lapse rate feedback […], and the two combined result in a feedback parameter of approximately 1 W m–2 degrees C–1 [Watts per square metre per degree Celsius], corresponding to an amplification of the basic temperature response by approximately 50%. The surface albedo feedback amplifies the basic response by about 10%, and the cloud feedback does so by 10 to 50% depending on the GCM [General Circulation Model or Global Climate Model]…”
Section 28 should at least have mentioned that more water vapour residing in the atmosphere is an inevitable result of global warming, and that this will compound the warming.
Adding Carbon Dioxide to the atmosphere is not happening in isolation from any other effect. Because the Earth has an atmosphere, increased Carbon Dioxide in the atmosphere has a warming effect. This automatically and naturally has an effect on the composition of the atmosphere through changes in the cycling of water between the Earth surface, the oceans and the atmosphere. You don’t get one without the other.
It’s a bit ridiculous to suggest that a doubling of CO2 in the atmosphere could have an effect all by itself – it’s bound to cause an increase in the temperature of the Earth’s surface, which is bound to cause an increase in water vapour concentrations in the Atmosphere.
Global Warming from Carbon Dioxide is not going to be “in the absence of amplifying processes.”
This is established science, and should have been included in section 28, in my view.
3. The Royal Society guidance does not issue the standard caveat about “Climate sensitivity” – that by the time that mankind stops adding Carbon Dioxide to the atmosphere, the actual increase in concentration could be a lot more than double.
If CO2 levels more than double, the calculation of the actual temperature rise from the climate sensitivity will have to be adjusted by the relevant factor.