Back in July, David Evans had on opinion piece in the Australian claiming:
The greenhouse signature is missing. … The signature of an increased greenhouse effect is a hot spot about 10km up in the atmosphere over the tropics.
This is wrong. The hot spot is not the signature, since you get a hot spot, no matter what the cause. The signature is stratospheric cooling combined with tropospheric warming and that has been detected.
the hotspot was not a signature of the greenhouse effect – it is a signature of warming from any source
Evans just ignored this as Brooks relates:
Unsurprisingly, [Evans] deploys the standard non-greenhouse theorist approach of yet again blithely ignoring any refutation and simply repeating the exactly the same arguments again in a third forum. So, yet again, a climate scientist had to patiently refute this.
So guess what we find on ABC Unleashed on Friday? Evans repeating all the false claims from his earlier pieces. (Though he has cranked up the rhetoric and is now making defamatory statements about climate scientists.) And, of course, he includes this:
The signature of an increased greenhouse effect consists of two features: a hotspot about 10 km up in the atmosphere over the tropics, and a combination of broad stratospheric cooling and broad tropospheric warming.
I suppose we should count our blessings that he deigns to mention the stratospheric cooling thing.
Evans volunteered to defend his piece at Club Troppo. I pressed him on his claims:
The hot spot is not the signature of greenhouse warming. The IPCC report you cite does not say that it is. I quoted the relevant part above. Here it is again:
The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the influence of anthropogenic forcing, particularly greenhouse gases and stratospheric ozone depletion.
Now Tim, look at the sentence after the one you quoted: “The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause.” That increase in the height of the tropopause IS the hotspot! The hotspot arises in AGW theory because an increase in greenhouse gases (of CO2 due to humans, and of water vapor due to increasing temperature) pushes the top of the tropopause higher, thus replacing cold stratosphere with warmer troposphere at the top of the troposphere — which is at about 10km over the tropics. (Btw, it now appears that the atmosphere just drops out water vapor as it is replaced by CO2, to keep the total greenhouse effect about constant. This is the nub of the argument, and where AGW went wrong.)
Despite going on endlessly about the hot spot for a half a year, Evans doesn’t even know what it is. The hot spot is not an increase in the height of the tropopause. You’d get that from uniform heating of the troposphere as well. The hot spot is a decrease in the lapse rate, as the team at RealClimate explains:
[The hot spot] really has nothing to do with greenhouse gas changes, but is a more fundamental response to warming (however caused). Indeed, there is a clear physical reason why this is the case – the increase in water vapour as surface air temperature rises causes a change in the moist-adiabatic lapse rate (the decrease of temperature with height) such that the surface to mid-tropospheric gradient decreases with increasing temperature (i.e. it warms faster aloft). This is something seen in many observations and over many timescales, and is not something unique to climate models.
And increasing CO2 does not decrease water vapour. See, for example Dessler, A. E., Z. Zhang, and P. Yang (2008), Water-vapor climate feedback inferred from climate fluctuations, 2003-2008, Geophys. Res. Lett., 35:
Warming temperatures evaporate water, increasing humidity. This increase in humidity has the potential to further warm the atmosphere because water vapor is a potent greenhouse gas. This water vapor feedback has the capacity to about double the direct warming from greenhouse gas increases. Using satellite data, Dessler et al. (2008) observed and quantified the behavior of atmospheric water vapor and the water vapor feedback during variations of the Earth’s climate between 2003 and 2008. They found that global averaged surface air temperatures on Earth varied by 0.6Â°C during the years analyzed, with specific humidity over most of the troposphere increasing with rising global surface temperature averages. Relative humidity increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water vapor feedback implied by these observations is strongly positive, similar to that seen by climate models. The magnitude of the feedback is similar to that obtained if the atmosphere maintained constant relative humidity everywhere.
I understand that the water vapor treatment of the models used to be simply to assume constant relative humidity. So in the models, as temperature rose there was more water vapor. Water vapor is a greenhouse gas, so creates a hotpot. (Which is why those models show a hotpot for any source of warming, and why a hotspot is so associated with an enhanced greenhouse effect.) However observations show that since the 1940s the relative humidity has been steadily dropping, pretty much everywhere as I recall (sorry, do not recall link).
Not only does Evans not know what the hot spot is, he doesn’t know what causes it. More water vapour means that there is more condensation up in the atmosphere. Condensation releases latent heat. That latent heat creates the hot spot. And notice the way he counters a link to a peer-reviewed paper that shows that relative humidity stays the same as the planet warms, with a “do not recall link” cite.