27 July 2016

Outgoing Longwave Radiation – the fuller story


For a couple of years or so I have been claiming [1, 2] that outgoing longwave radiation (OLR) should be decreasing if infrared was being curtailed by increasing levels of carbon dioxide in the atmosphere as per AGW theory. But there is more to the story.

On a spectral line near 700 wavenumber (14.3 µm) there is indeed quite a reduction in radiation expected to be caused by an increase in CO2.

The following chart (data via the Modtran simulator) shows a deepening of the groove that appears around 700 to 770 wavenumber with surface temperature kept constant and atmospheric carbon dioxide increased from 150 to 1000 ppm.

Modtran simulation of earth spectral radiance of tropical atmosphere from 70 km looking down with carbon dioxide varying from 150 to 1000 ppm. Water vapour = 0. No clouds or rain.


But the satellites that measure OLR do not measure near this 700 wavenumber of CO2 absorption but at a slightly higher frequency range between 800 and 952 wavenumber (12.5 µm to 10.5 µm wavelength). There is also a slight reduction in radiation in this range with increased CO2 but it's less pronounced.

Here's the same graph as above in blink-gif form. The orange window is 12.5 µm to 10.5 µm:



Horizontal axis is wavenumber (cm-1)


There have been and are satellites that measure in a greater spectral range such as IRIS, IMG and AIRS. But in the case of the first two these measurements occurred over a shorter period of time such as months or days. 

The monitoring of OLR since 1974 in the 12.5 µm to 10.5 µm range by NOAA satellites are over a period of several decades and are more continuous in nature and hence useful for a longer term comparison with CO2.

My claim is true, that OLR should be decreasing, if the temperature is steady and the carbon dioxide is increased - which been the case for about 19 years now



Observations like the 19-year warming pause expose the weakness of the AGW theory.


Rather than decreasing, the OLR has varied up and down a bit but for the most part has increased rather than decreased since the warming pause started in 1998:

The Y-axis scale on the left is from Climate4you.com. The similar scale on the right is from my attempted reproduction of the data [1, 2] on Matlab. (Source. More climate4you.com graphs here.)


But if the earth were in fact warming as per the CO2 theory, my claim would be wrong: OLR in the 12.5 µm to 10.5 µm window should increase, not decrease, because the warming outweighs the curtailing effect of CO2.

AGW theory has it [12] that a doubling of CO2 results in ~3.7 (W/m2) more downward longwave radiation. The amount of warming produced by this is the subject of much conjecture [1, 2]. Let us give a modest value of 1.11C warming for this doubling of CO2. This results in a conversion rate of 1 W/m2 per 0.3 degree C.

It doesn't matter if this is an underestimate of the warming, because this amount of climate sensitivity is already enough to cancel the curtailing effect of CO2 in the measured range.  The point being that in the case with warming, my claim is wrong: OLR should actually increase in the 12.5 µm to 10.5 µm range. 

Here's a table I made in an Excel spreadsheet to deduce the temperature offset I should input to Modtran to account for the expected warming – the last line in red.

Excel worksheets 12 
With these CO2 and temperature offset parameters input to Modtran the following graphs are output. The warming offsets the CO2 curtailment in the 12.5 µm to 10.5 µm (wavenumber 800 to 952) range.  

(Graph with all six CO2 levels here.)
I selected four CO2 data points coinciding with the beginning and end of the 160-year HadCru temperature record plus two points coinciding with the IRIS and IMG satellite snapshots of earth emission spectrum in 1970 and 1997 (more here). These points are 280, 324, 361 and 400 ppm.

I hand drew the spray painted bit before the year 1969 as a simplification



We should according to AGW theory see that with the temperature kept constant, increasing CO2 should increase the curtailing effect.

It could be argued that the slight reduction in OLR the last 6 years or so is evidence of such a curtailment. But the fact that OLR has on the whole been rising the last 19 years is evidence that OLR varies naturally and is not tied to the CO2 rise nor to the curtailment in OLR expected in AGW theory.

The CO2 curtailment signal should be increasing with time, reflecting the slightly exponential shape of the Keeling Curve. But when this exponential shape is compared to the 160-year Hadley CRU temperature record we can see that for the last 19 years when CO2 is rising fastest the curtailment effect on OLR is decreasing with time when it should be increasing.





The last 19 years of warming stasis have offered us a remarkable opportunity to test whether CO2 is curtailing infrared in the measured range -- and that test is failing. It could be argued that the change expected would be very small, but never the less OLR does not inversely correlate to the Keeling Curve. This lack of correlation is therefore evidence against the theory of AGW.


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(Wavelength to wavenumber converter.)




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