In Part 1 I discussed the difference between skeptics and denialists. Not a few hours after I did that, I read a post over at Deltoid where Tim Lambert talks about a popular subject: Hotspots (no, nothing to do with geology).
Back in July, one David Evans published a piece in The Australian where he made a series of awful arguments against AGW, but the article was most famous for him coining the term ‘hotspot’ and claiming that such a hotspot doesn’t exist. The “hotspot” is the amplifying tropical tropospheric temperature relative to the surface. The argument goes like “models predict this hotspot with rising CO2, but it’s not there, so AGW must be wrong.” This is an important issue since formal attribution of temperature trends has always been based on spatio-temporal patterns, so it is necessary to discuss the observed and modeled evidence as well as their relevance and implications.
As Tim Lambert has shown, David Evans is very confused, and now it appears he doesn’t even understand what his own argument was. I will copy David Evan’s reply to Tim Lambert with piece-by-piece clarifications,
Tim (Post 14):
With your last reply, we have now identified the source of your disagreement: You say the hotspot is not part of the signature of enhanced greenhouse warming, I say it is.
I do not really like how either party has phrased this. You should get enhanced tropospheric warming with increased CO2, but that is not the complete argument. Tropospheric warming in the tropics is a signature of greenhouse warming, but it is more accurate to say that it is not a unique signature (i.e., you get this “hotspot” with all types of forcings). The ‘hotspot’ arises due to the moist adiabat. In the extra-tropics you do not don’t expect the lapse rate changes to be so dominated by moist convective effects. Some people have claimed this signature exists because “greenhouse gases do their absorbing at higher altitudes” but they also absorb outside the tropics so this cannot really be correct. The IR heating change is pretty uniformly distributed, in fact. Moreover, the whole troposphere is well mixed in heat, and is more or less constrained by convection to stay near the moist adiabat. The below graphs are model runs from GISTEMP, the first being a 2x CO2 scenario with a 100 yr. response, and the second being a 1.02x Solar increase case.
As seen, until you get to the stratosphere the responses are very similar.
Please look at Chapter 9 of the IPCC Assessment Report 4, 2007:
On page 675 is Figure 9.1, which shows “Zonal mean atmospheric temperature change from 1890 to 1999 (°C per century) as simulated by the PCM model” from five possible causes of global warming. Part (c) shows the signature of “well-mixed greenhouse gases”. Part (f) shows the warming pattern from “the sum of all forcings” (considered by the model). Both (c) and (f) show a large, intense hotspot over the tropics at about 190km — it is the most dominant feature in both diagrams
No disagreement, but again, it’s not unique. Climate forcings such as increasing solar irradiance, increasing, decreasing surface albedo, decreasing low cloud cover, will warm the ground and troposphere and will also warm the stratosphere. On the other hand, increasing GHG’s uniformily in the vertical, decreasing high-level ozone, and increasing stratospheric water vapor will act to “invert” the temperature anomaly profile above the tropopause and cause a cooling and contraction of the stratosphere while the surface and troposphere warm.
Here is another source, at the US CCSP 2006, chapter 1: http://www.climatescience.gov/Library/sap/sap1-1/finalreport/sap1-1-final-chap1.pdf. On page 25, Figure 1.3, there are “PCM simulations of the vertical profile of temperature change due to various” for Jan 1958 to Dec 1999, with similar results. The “Well mixed greenhouse gases” and “All forcings” signatures both show an intense, dominant tropical hotspot at 10km.
Or look at Figure 14 on page 59 in http://climate.snu.ac.kr/2005_new/pub/papers/p104.pdf, which shows the zonal mean distributions of temperature change for a CO2 doubling, as predicted by four atmospheric general-circulation models, for all forcings. Again, the hotspot is prominent in each one.
Again, same thing. Tim Lambert probably led David Evans to believe there should not be an amplicative effect in the tropical troposphere, but this is not the case, just that the argument needs to be completed to all forcings. We can just as well say the hotspot for increasing the sun is missing, but the most unique fingerprint is that the “cold spot in the stratosphere” for the sun is missing. Indeed, even the mesosphere and thermosphere cool with increased CO2. The lapse rate in the troposphere is set by convection and other vertical mixing processes; in the stratosphere, the lapse rate is set by radiation balance, so the reasons why the stratosphere cool depend on upwelling and emitted radiation but also on ozone depletion. The stratospheric balance is heating by solar absorption vs. infrared cooling. If you increase the greenhouse gas concentration then the stratosphere is being shielded more strongly from upwelling radiation from below by the increased opacity in the 15 micron region and you get more cooling balancing the same solar heating, hence the stratosphere can get cooler.
Tim, you quote the Executive Summary of the IPCC AR4 Chapter 9, from page 665, “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.”
Tim this does not say anything about a hotspot, or imply that the signature of enhanced greenhouse warming does not include a hotspot. As noted in my article, the combination of broad stratospheric cooling and broad tropospheric warming is one of the two features of enhanced greenhouse warming (the other is the hotspot), and is the sole feature of ozone depletion (see the first two sources quoted above for verification). And this combination has been observed — which is what your quote is commenting on.
Keep in mind that the lower stratospheric is influenced heavily by ozone depletion, whereas the mid-stratosphere and above is influenced by greenhouse gas increases.
Tim, your quote attributes the observed stratospheric-cooling and tropospheric-warming to extra greenhouse bases and ozone depletion, just as I do. And no mention of the hotspot.
Everyone agrees about this “hotspot.”
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.)
None of this is correct. Warming the troposphere or cooling the stratosphere tends to increase tropopause height. If this were the whole argument, the tropopause height increase is real. But the “hotspot” refers to the fact that the surface-to-troposphere temperature gradient decreases. Interestingly, since the greenhouse effect depends on the lapse rate, models produce a negative feedback to this change in temperature gradient. If David Evans is correct about the hotspot not being real, then the most intuitive implication would be toward a heightened climate sensitivity. As Tim Lambert also mentions, water vapor certainly does not decrease with more CO2. I believe this came from Ferenc Miskolczi’s flawed paper on atmospheric optical depth and the greenhouse effect, but like most neo-Galilean pieces, it didn’t work, and neither did that particular claim.
In other words Tim, a fuller quote reveals that the IPCC AR4 is saying that it thinks a hotspot should be there. But it does not claim to have found it — if they could they would have explicitly claimed it, and trumpeted it around the world. The IPCC is encouraged by the observed stratospheric-cooling and tropospheric-warming because if it hadn’t been observed either then there is no hope for AGW. They are playing up the positives: at least half the signature of enhanced greenhouse warming might have been found! But notice that they acknowledge that the half signature observed could be due to ozone depletion. The IPCC quotes are fully consistent with ozone depletion as the sole cause of the observed stratospheric-cooling and tropospheric-warming.
This is nonsense. Much of the cooling above the tropopause is due to GHG’s, while MSU4/TLS lower down does respond to ozone changes which are levelling off. This paper may be instructive.
Sorry Tim, the text in these reports is pored over by teams of bureaucrats and parsed endlessly, which is why it ends up so dense and so hard to read. It is easy to be misled.
Tim, climate scientists have been debating for years over the (some would say apparent!) non-observation of the hotspot. If the answer was as simple as noting that the hotpot is not part of the signature of enhanced greenhouse warming then they could all have saved themselves a lot of time and work.
Now this is a real issue. The real issue is what is actually going on in the tropical troposphere, because there’s a lot of uncertanties in both observations and modelling. Part of the reason involves sampling issues, contamination of stratospheric cooling and incoming sunlight with sensing devices, and standarization problems. There’s also a lot of “noise” in the troposphere.The troposphere is certainly warming, but the issue of how big the signal is becomes a difficult problem. In the extratropics, observations are as expected. Based on many recent papers, such as Tropical vertical temperature trends: A real discrepancy? by Thorne et al, Robust tropospheric warming revealed by iteratively homogenized radiosonde data, by Sherwood et al, and the recent Santer et al. paper, it is not obvious a real model-observation discrepancy exists, so to ignore these papers and the uncertainty in the data is not going to get Evans very far. Realclimate has a much more thorough discussion of those, and more papers on this topic here, Part 2, and Part 3
How Evans continues to go about this “hotspot” issue should be very revealing.