One commenter asked about CO2 absorption in the solar spectrum.
If CO2 absorbs incoming solar radiation then surely an increase in CO2 will reduce incoming radiation and balance any increase in longwave radiation.
The important factor is the usual question of quantifying the different effects.
Let’s take a look.
The solar radiation is overlaid. Well, more accurately, the Planck function for 5780K is overlaid (simply drawn using Excel). Note that the CO2 absorption spectrum is on a log graph, while the radiation is on a linear graph. For those not so familiar with logarithmic graphs, the peak absorption around 4.3μm is 10-18, while the two peak absorptions just below 1μm are at 10-26 – which is 100,000,000 less.
The value of seeing the solar radiation spectrum overlaid is it enables you to see the relative importance of each absorption area of CO2. For example, the solar radiation between 2 – 4μm is only 5% of the solar radiation, so any absorption by CO2 will be quite limited.
Here’s the comparison with the important 15μm band of CO2. A 6μm width is shown, overlaid (blue line) with the 12-18um longwave radiation of a 288K (15°C) blackbody:
Just a little explanation of this graph and how to compare it to the solar version.
The average surface temperature of the earth is 15ºC, and it emits radiation very close to blackbody radiation (watch out for a dull post on Emissivity soon).
The proportion of radiation of a 288K blackbody between 12-18μm is 28%. What we want to do is enable a comparison between the CO2 absorption of solar radiation and terrestrial radiation.
Averaged across the globe and the year the incoming solar radiation at the top of atmosphere (TOA) is 239 W/m2 and the radiation from the earth’s surface is 396 W/m2. This works out to 65% higher, but so as not to upset people who don’t quite believe the earth’s surface radiation is higher than incoming solar radiation I simply assumed they were equal and scaled this section of the earth’s terrestrial radiation to about 28% of the solar radiation on the earlier graph. We are only eyeballing the two graphs anyway.
So with this information digested, the way to compare the two graphs is to think about the absorption spectra of CO2 simply being scaled by the amount of radiation shown overlaid in both cases.
As you can see the amount of absorption by CO2 of solar radiation is a lot less than the absorption of longwave radiation. Remember that we are looking at the log plot of absorption.
Is That the Complete Story?
Really, it’s more complicated, as always with atmospheric physics. There’s nothing wrong with taking a look at the approximate difference between the two absorption spectra, but luckily someone’s already done some heavy lifting with the complete solution to the radiative transfer equations using line by line calculations. For more on these equations, see the CO2 – An Insignificant Trace Gas series, especially Part Three, Four and Five.
The paper with the heavy lifting is Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the IPCC AR4 by W.D. Collins (2006). There’s a lot in this paper and aspects of it will show up in the long awaited Part Eight of the CO2 series and also in Models, On – and Off, the Catwalk.
Solving these equations is important because we can look at the absorption spectrum of CO2 in the 15μm band, but then we have to think about the absorption already taking place and what change in absorption we can expect from more CO2. Likewise for the solar spectrum.
Here are the two graphs, which include other important trace gases, as well as the impact of a change in water vapor. Note the difference in vertical axis values – the forcing effect of these gases on solar radiation has to be multiplied by a factor of 1000 to show up on the graph. The blue lines are CO2.
You can also see that the CO2 absorption in shortwave is across quite narrow bands (as well as being scaled a lot lower than terrestrial radiation) – therefore the total energy is less again. The vertical scale is energy per μm..
From these calculations we can see that with a doubling of CO2 there will be a very small impact on the radiation received at the surface, but a comparatively huge increase in longwave radiation retained – “radiative forcing” at the tropopause (the top of the troposphere at 200mbar).
So Is That the Complete Story?
Not quite. If trace gases in the atmosphere absorb solar radiation, is that so different from the surface absorbing solar radiation?
Or to put it another way, if the radiation doesn’t strike the ground, where does it go? It’s still absorbed into the climate system, but in a different location (somewhere in the atmosphere).
But as one commenter said:
The other point [this one] you make is simply not true and/or also not proven. There is only so much energy that can be taken up by a molecule.
This is a theme that has arrived in various comments from various posts. So the concept of How much work can one molecule do? is worth exploring in a separate post.
Hopefully, it’s clear from what is presented here that increases in CO2 absorption of the solar radiation are very small compared with absorption of longwave radiation.