BackgroundI've written about this topic previously, I think in conversations with Chic Bowdrie. Rather than try to dig that up again, I give the present inspiration for writing this note dedicated to the concept. From Dr. Curry's Climate Etc. blog, we read these two comments:
captdallas2 0.8 +/- 0.3 | May 7, 2016 at 3:13 pm |
[Steven Mosher] “C02 does it’s work ABOVE the ERL.”
What’s the effective temperature of that effective radiant layer again?
captdallas2 0.8 +/- 0.3 | May 7, 2016 at 5:26 pm |
RiHo08, “Is this the battle ground where CO2 and Ozone duke it out; one for cooling and the other warming?”
All of the greenhouse gases have temperature and pressure “sweet spots” where they are most effective at doing the greenhouse thing. Below that sweet spot they become less and less effective as noted in the Antarctic where increasing CO2 likely increases cooling.
FacepalmHere is my initial response:
brandonrgates | May 7, 2016 at 7:07 pm |I further comment here: it's a pity that captaindallas mucks up such an interesting conclusion with bad physics. Hopefully I haven't committed the same sin by explaining it in terms of my coinage "local ERL", but I think my understanding has the better fidelity with literature.
captdallas2 0.8 +/- 0.3,
What’s the effective temperature of that effective radiant layer again?Benestad (2016) puts it at 254 K and an altitude of 6.5 km:
That’s a global average for cloudy/cloud-free regions, and represents the temperature and altitude at which bulk heat loss = bulk heat gain as the net of all radiative transfers, including solar SW. It is of course not constant due to local weather conditions, and long-term climatically-relevant means are also sensitive to latitude — ERL tends to be greater at low latitudes than at higher latitudes. Which brings us to …
All of the greenhouse gases have temperature and pressure “sweet spots” where they are most effective at doing the greenhouse thing. Below that sweet spot they become less and less effective as noted in the Antarctic where increasing CO2 likely increases cooling.You’re correct that rising CO2 may indeed cool central Antarctica, but the mechanism you invoke is … not even wrong. It’s true that any LW-active species’ spectral properties are sensitive to temperature and pressure. How I understand it — with the caveat that I’m doing some synthesis here — whether increased concentrations result in a heating or cooling trend as a result is a function of vertical position relative to what I call the “local ERL” for any given layer. The temperature/pressure sensitivity determine a given species’ efficacy in doing either, NOT whether they have a net heating/cooling effect.
With that in mind, the central highlands of Antarctica are an unusual (and interesting) case because its mean surface temperature is cooler than the stratosphere above it. Schmithüsen et al. (2015) explain:
CO2 is the strongest anthropogenic forcing agent for climate change since preindustrial times. Like other greenhouse gases, CO2 absorbs terrestrial surface radiation and causes emission from the atmosphere to space. As the surface is generally warmer than the atmosphere, the total long-wave emission to space is commonly less than the surface emission. However, this does not hold true for the high elevated areas of central Antarctica. For this region, the emission to space is higher than the surface emission; and the greenhouse effect of CO2 is around zero or even negative, which has not been discussed so far. We investigated this in detail and show that for central Antarctica an increase in CO2 concentration leads to an increased long-wave energy loss to space, which cools the Earth-atmosphere system. These findings for central Antarctica are in contrast to the general warming effect of increasing CO2.
They don’t explicitly state it in the body of the paper (it’s open access, and well worth a read), but the way I interpret their argument is that the central plateau of Antarctica is above the “local ERL” for significant portions of the year, sufficient enough that CO2 has a “negative greenhouse” effect at the surface in terms of the annual average.
Fascinating work; however, not confirmed by observation so far as I can tell from reading the paper (it’s a model study).
The Picture Worth 1,000 WordsSchmithüsen et al. (2015) Figure 2 tells a compelling story:
This might be easier to visualize if calculated Stefan-Boltzmann curves were shown for relevant absolute temperatures. However, we can still derive the correct interpretation. In the top figure representing a satellite view of spectral radiance, we see that the 15 micron wave band is below other spectral bands, indicating that the CO2 "notch" has been emitted at a lower temperature corresponding to a higher atmospheric layer. IOW, significant portions of the spectrum were emitted from ground level, or close to it.
In the lower plot, we see the inverse, which is consistent with a cooler ground and surface layer, with photons emitted by CO2 occurring by the warmer stratosphere above Antarctica's central plateau in March as described in the paper.
I will be updating this note with further details from both papers I reference. For now, I will publish so I can use the above image for a postscript to captaindallas over at Dr. Curry's.