---------------------------------------
From: Michael Mann [mem45]
Date: Tue, May 10, 2011 at 4:10 PM
Subject: Re: climate
To: denis rancourt
I will be away from my email through May 14, 2011.
Any email sent before then may remain unread and be discarded. If your message is important, you will need to resend after that date.
-------------------------------
From: Raymond P.
Date: Tue, May 10, 2011 at 4:24 PM
Subject: climate
To: Denis Rancourt
Cc: allrc
Dear Mr. Rancourt,
Your discussion of energy balance starts off soundly, but then you get mired in a nest of mistakes, the most critical of which is your misunderstanding of Kirchoff's Law. Kirchoff's law only requires that emissivity and absorptivity be equal at the same wavelength. Since Earth's incoming radiation is shortwave, but outgoing is longwave (infrared), there is no contradiction between a surface emissivity near unity and an albedo of around 0.3 . Your calculation erroneously attributes the effect of atmospheric infrared opacity to surface emissivity.
By the way, climate models do not assume surface infrared emissivity to be unity. The deviations from unity are not a major effect in the surface budget, but the emissivity of common Earth materials can be easily measured, and in full GCM's the emissivity is taken into account in the lower boundary condition for radiation models.
Thank you for your interest in this subject.
--Ray Pierrehumbert
--------------------------------
From: Denis Rancourt
Date: Tue, May 10, 2011 at 5:05 PM
Subject: Re: climate
To: "Raymond P."
Cc: allrc
Dear Mr. Pierrehumbert,
Thank you for your critique. I will post it. You are correct that Kirchoff's Law strictly applies for a given wavelength and that I took it to apply at all wavelengths for a given material. (I took albedo and emissivity to be wavelength-independent. This could indeed be rather off, depending on the material.)
Could you point me to a paper where shortwave albedo and longwave emissivity were measured in a common Earth material?
Do you believe that a correct longwave emissivity for Earth is 1 and that therefore the atmosphereless Earth would be at -19C? This is stated on the RealClimate web site.
Also, it would be useful if you could state what other errors you have found in the article, since you have already ascertained these errors.
Sincerely,
Denis Rancourt
---------------------------------
From: Raymond P.
Date: Wed, May 11, 2011 at 12:11 AM
Subject: Re: climate
To: Denis Rancourt
Cc: gschmidt@giss.nasa.gov, mann@psu.edu, ammann@ucar.edu, rasmus.benestad@met.no, rbradley@geo.umass.edu, rahmstorf@ozean-klima.de, rahmstorf@pik-potsdam.de, steig@u.washington.edu, d-archer@uchicago.edu, garidel@marine.rutgers.edu, jrbouldin@ucdavis.edu, wconnolley
Emission-weighted emissivity is one of the easiest quantities to measure. It is even routinely used in engineering studies. Detaied spectrally resolved emissivity measurements are routinely reported in Icarus and JSQRT, since they are crucial to the use of infrared spectroscopy in estimating mineral composition of planetary surfaces. The literature is far too vast to summarize, but an example of some typical emissivities at various temperatures for common building and terrestrial materials is listed, for exampe, at:
http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Emissivity.htm&ID=254#anchor4
Also emissivity can be computed in a straightforward way from measured absorption coefficients of condensed substances. Detailed data for water, water ice and CO2 ice is given in Chapter 4 and 5 of my book, Principles of Planetary Climate. Data sets and references are found on the book web site. (go to geosci.uchicago.edu/~rtp1 and follow links to ClimateBook).
The emissivity of ice and water, which makes up most of the surface of the Earth, is very accurately known. The emissivity of land varies more, especially in response to vegetation cover, but it is known well enough for the purposes of determining energy balance; there can be problems with microwave emissivity of land surfaces which complicate doing microwave retrievals over land in some circumstances.
I do not actually agree with the statement that the Earth's temperature would be -19C if you removed the atmosphere. This is a true statement of what you would get if you replaced the atmosphere with an IR-transparent atmosphere (e.g. N2), which could still transport heat, and if moreover you could prevent the ocean from freezing over. In reality, if you did that, the oceans would freeze, and the mean temperature would be much colder than -19C (see my AREPS Neoproterozoic review article for numbers). But if the Earth had no oceans and you took away the atmosphere, first of all, you would also take away the clouds (which changes the albedo) and second of all you would take away heat transport. In that case, the temperature distribution would be rather like the Moon -- 100C at the subsolar point, maybe -100C on the night side. (Slightly warmer than the Moon on the night side, because the Earth is more rapidly rotating, so there's less time for the surface to cool down).
But there is absolutely zero question that removing CO2 from the Earth's atmosphere would make it much colder than -19C, and probably cause the Earth to turn into a snowball. The radiation physics of this is unassailable, and based on more than a century of work by some of the best physicists around, backed up by thousands of laboratory and satellite measurements. You are really barking up the wrong tree.
--Ray Pierrehumbert
-----------------------------------
From: Denis Rancourt
Date: Wed, May 11, 2011 at 8:40 AM
Subject: Re: climate
To: "Raymond P."
Cc: gschmidt@giss.nasa.gov, mann@psu.edu, ammann@ucar.edu, rasmus.benestad@met.no, rbradley@geo.umass.edu, rahmstorf@ozean-klima.de, rahmstorf@pik-potsdam.de, steig@u.washington.edu, d-archer@uchicago.edu, garidel@marine.rutgers.edu, jrbouldin@ucdavis.edu, wconnolley
Thank you for your detailed response.
From now on, whenever I describe the simple radiation balance equations I will stress that the albedo is for shortwave whereas the emissivity is for longwave (and that Kirchoff's Law therefore is not relevant). That arbitrary step of setting emissivity equal to 1 in textbook descriptions (without explaining that water and vegetation emissivities are close to 1) always bothered me, since I was 18 in first year.
I will make a revised article and I will also correct others who may make the same error about Kirchoff's Law.
Thanks.
Denis Rancourt
-------------------------------------
From: Raymond P.
Date: Wed, May 11, 2011 at 8:55 AM
Subject: Re: climate
To: Denis Rancourt
No problem. Radiation balance and planetary temperature is a really fascinating subject, and I'm always happy when people take in interest in it. You might be interested in some of the classic papers Dave Archer and I compiled (with interpretive essays) in our book, "The Warming Papers" -- particularly the things Arrhenius did or didn't get right, the false steps along the way (e.g. Plass getting mired in a surface budget fallacy) and the way Manabe finally got everything right in the 60's. It's pubished by Wiley/Blackwell. The pricetag unfortunately got rather high, but that was out of our control, since most of the cost of the book was in copyright fees payed to copyright holders for permission to reprint.
--Ray
---------------------------------------
...
---------------------------------------
From: Denis Rancourt
Date: Thu, May 12, 2011 at 3:57 PM
Subject: Re: climate
To: gschmidt@giss.nasa.gov, mann@psu.edu, ammann@ucar.edu, rasmus.benestad@met.no, rbradley@geo.umass.edu, rahmstorf@ozean-klima.de, rahmstorf@pik-potsdam.de, steig@u.washington.edu, d-archer@uchicago.edu, rtp1@geosci.uchicago.edu, garidel@marine.rutgers.edu, jrbouldin@ucdavis.edu, wconnolley
Revised version posted:
http://climateguy.blogspot.com/2011/05/radiation-physics-constraints-on-global_12.html
Please provide any feedback.
Denis Rancourt
----------------------------------------
Note: The June 2011 (third and final) version of Rancourt's paper, corrected and largely augmented is LINK-HERE.
No comments:
Post a Comment