In Part Seven we looked at progressively wider wavelength ranges to see where doubling of CO2 had an impact.
We also compared the top layer of the atmosphere (in that model) with the bottom layer – again seeing very significant differences.
This series is aimed at helping people understand how “greenhouse” gases have the effect that they do.
Now we have a model (described in Part Five) which incorporates the actual line by line absorption and emission of various GHGs in a realistic atmosphere we can play around with “what if” scenarios.
Usually as we go up in altitude and pressure drops, the absorption lines get narrower. Around the tropopause the lines are almost 1/5 of their surface width.
I introduced a new on/off parameter into the code which “turns off” this physics and allows us to keep the lines the same width as at the surface. Then compared 280 to 560 ppm of CO2 under one clear sky condition for the case with and without absorption line narrowing.
The top graph is the difference in TOA spectrum with correct physics. The bottom graph is the same but with all absorption lines at their surface width:
Click to expand
The results surprised me. I expected that the effects of absorption lines narrowing would be more significant for this “increased GHG” scenario.
The difference in outgoing radiation (OLR) across this band (which is most but not all of the CO2 effect) is only 0.1 W/m².
[Update shortly afterwards inspired by comment from Hockey Schtick]
The original article might give the false impression that the narrowing of lines has little effect on TOA flux. Actually it has a significant effect. For example, for the case 280 ppm the difference in total TOA flux for correct – incorrect physics = 23.5 W/m². It’s just that for 560 ppm the difference in total TOA flux for correct – incorrect physics = 24.8 W/m².
The values annotated on the graph are the flux for that wavenumber region only.
TOA flux in the list below is across all wavelengths.
CO2 ppm Line Width TOA flux
280 Narrows with lower pressure 268.4 W/m²
560 Narrows with lower pressure 262.2 W/m²
280 Constant 244.9 W/m²
560 Constant 237.4 W/m²
Difference 280 ppm correct – incorrect physics = 23.5 W/m²
Difference 560 ppm correct – incorrect physics = 24.8 W/m²
Difference 280 ppm – 560 ppm (correct physics) = 6.2 W/m²
Difference 280 ppm – 560 ppm (incorrect physics) = 7.5 W/m²
DLR (downward longwave radiation = atmospheric longwave radiation incident on surface) for the same conditions:
CO2 ppm Line Width DLR flux
280 Narrows with lower pressure 374.7 W/m²
560 Narrows with lower pressure 376.9 W/m²
280 Constant 378.1 W/m²
560 Constant 380.6 W/m²
Difference 280 ppm correct – incorrect physics = -3.4 W/m²
Difference 560 ppm correct – incorrect physics = -3.7 W/m²
Difference 280 ppm – 560 ppm (correct physics) = -2.2 W/m²
Difference 280 ppm – 560 ppm (incorrect physics) = -2.5 W/m²
Part One - some background and basics
Part Two - some early results from a model with absorption and emission from basic physics and the HITRAN database
Part Three – Average Height of Emission - the complex subject of where the TOA radiation originated from, what is the “Average Height of Emission” and other questions
Part Four – Water Vapor - results of surface (downward) radiation and upward radiation at TOA as water vapor is changed
Part Five – The Code - code can be downloaded, includes some notes on each release
Part Six – Technical on Line Shapes - absorption lines get thineer as we move up through the atmosphere..
Part Seven – CO2 increases - changes to TOA in flux and spectrum as CO2 concentration is increased
Part Nine – Reaching Equilibrium - when we start from some arbitrary point, how the climate model brings us back to equilibrium (for that case), and how the energy moves through the system
Part Ten – “Back Radiation” - calculations and expectations for surface radiation as CO2 is increased
Part Eleven – Stratospheric Cooling - why the stratosphere is expected to cool as CO2 increases
Part Twelve – Heating Rates - heating rate (‘C/day) for various levels in the atmosphere – especially useful for comparisons with other models.