By Jim Steele
Focusing only on CO2’s warming effect distorts our understanding of how the atmosphere warms and cools. More importantly it ignores how CO2 is critical for cooling our atmosphere. Oxygen and nitrogen comprise 99% of our atmosphere. Those molecules do not absorb or emit radiation. Our air is heated by collisions with solar heated surfaces and warmer atmospheric molecules. So, we must understand how oxygen and nitrogen shed their absorbed energy, so it can exit back to space.
Only the sun’s radiation adds heat to our world. Of the arriving solar heat 30% is reflected without warming the earth. 20% warms the atmosphere directly and only 50% warms the earth’s surface.
About 25% of that surface heat (12 units) gets radiated back to space immediately at wavelengths that are not affected by greenhouse gases.
About 14% (7 units) of the surface heat warms the air above via surface collisions with that heated air carried away from the surface by rising convection.
Slightly less than 50% (23 units) of the surface heat causes evaporation with moist convection also carrying heat away from the surface.
The surface only radiates 16% (8 units) of absorbed heat away, which is however also intercepted by greenhouse gases, primarily water and CO2. But greenhouse gases redirect some of that heat back to the surface and slow the earth’s cooling rate.
Eventually at the top of our atmosphere (the troposphere) greenhouse gases must radiate about 58% of all incoming solar heat back to space.
This diagram illustrates the physics showing how the earth cools by radiating infrared heat back to space according to Planck’s Law.
The 320k dashed line represents how much heat via each infrared wavelength should theoretically radiate back to space if there is no interference from greenhouse gases. Here, its from the Sahara desert’s surface when the surface is heated to 47 Celsius. This theoretical line is based on solid reproducible & trustworthy science.
The other dashed lines represent how much heat each wavelength should radiate from cooler temperatures. These cooler temperatures happen at higher altitudes as seen in the table on the right.
The solid black line represents what wavelengths of heat that satellites actually detected leaving earth. The thin red lines are model predictions. Only infrared heat with wavelengths between 10 and 13 microns as well as between 8 and 9.5 match theory. That’s because greenhouse gases do not interfere with those wavelengths, allowing 25% of the surface heat to escape immediately.
Water vapor absorbs infrared wavelengths between 14 & 25 microns as well and between 6 & 8 microns. Satellites detect those wavelengths but with intensities suggesting the heat has radiated from cooler temperatures. Satellites can’t detect the latent heat absorbed during water’s evaporation at the surface, but satellites do see the heat once released when water vapor cools and condenses to form clouds and rain at 1 to 4 kilometers of altitude. Above water vapor’s condensation altitudes, water vapor provides an insignificant greenhouse effect as the air is very dry.
CO2 primarily absorbs infrared centered around 15 micron wavelengths but satellites do not detect any heat from 15 micron wavelengths being emitted from the surface or lower atmosphere. So, a second critical question is why don’t satellites detect 15 micron wavelengths until the top of the troposphere and stratosphere? Radiant heat proven to be absorbed and emitted by CO2.
Greenhouse theory argues infrared absorbed by CO2 is trapped near the surface according to the following model.
Surface heat radiates upward but some is intercepted by greenhouse gases.
Those gases absorb the infrared heat but then quickly emit it. However, about half is redirected back towards the surface, slowing the surface’s cooling rate.
The infrared that continues upwards is again intercepted sending half back towards the surface
The same dynamic continues so by 2 to 4 kilometers altitude, virtually no surface radiation absorbed and emitted by CO2 escapes. So, satellites simply don’t detect it.
The same diminishing dynamic reduces the amount of infrared emitted from higher altitudes that can reach the surface. So, most scientists believe warming via a greenhouse effect happens mostly in the lowest atmosphere.
But this dynamic only partially explains atmospheric warming.
That greenhouse theory does not account for how collisions, as illustrated by Newton’s Cradle, that transfer energy from greenhouse gases to non-greenhouse gases and vice versa.
Water and CO2 do not trap absorbed infrared heat, but quickly emit it in just milliseconds after it is absorbed. This is called the relaxation time.
Now compare well studied collision frequencies to relaxation times.
Near the surface, nitrogen molecules can collide with CO2 6 billion times in just one second. So, before a CO2 molecule can relax and emit any radiation, nitrogen will collide 600,000 times with CO2 and steal it’s absorbed energy and transfer it to other atmospheric molecules via more collisions.
Collisions with nitrogen and oxygen constantly steal heat from CO2 and because nitrogen and oxygen don’t radiate that heat, satellites don’t detect any wavelengths centered around 15 microns until CO2 emits that heat in the stratosphere.
Heat trapped by oxygen and nitrogen can only lose that collisionally absorbed heat by transferring energy back to a greenhouse gas via more collisions. Only then can greenhouse gases radiate that energy back to space. However, CO2 will only radiate heat back to space where heat stealing collisions are greatly reduced, and that only happens in the less dense upper troposphere and stratosphere, exactly where satellites detect 15 micron wavelength emissions back to space.
Heated air constantly convects to the upper atmosphere, but without CO2, there would be no shedding of the air’s absorbed energy. There would only be runaway warming.
Greenhouse warming in the lower atmosphere is balanced by CO2 cooling in the stratosphere. This explains why CO2 concentrations do not correlate with earth’s temperatures over millions of years!
Jim I think it would be more correct to say “…but without CO2 and H2O vapour…” and even that clarification ignores cloud tops emitting to outer space and they cover about 65% of the Earth’s surface although their biggest effect is reflection of solar SW.
You have brought up a very good point in this essay. At top of troposphere water vapor is below 10 ppm while CO2 is still 400 ppm….so it’s CO2 doing much of the radiation of the planet to space only 10 km. up where passenger jets fly.
DM,
A couple of questions that I don’t really have definitive answers to: First, according to Wijngaarden & Happer (and others), CO2 doesn’t emit to space below 84km, so is it really accurate to say that CO2 is emitting to space above 10km, i.e. over the entire region from 10km to 84km? Second, presuming that H2O has already done most of the heavy lifting below 10km, isn’t the modicum of radiation left over for CO2 above that level kind of a ‘so what’ exercise?
Frank,
I have read Wijngaarden & Happer and their belief that CO2 doesn’t emit to space until the stratosphere and mesosphere. I have talked to Will about that and he said it was due to their understanding of altitudinal effects on either vibrational or rotational changes. Here I am basing my arguments on the observed Planck curves showing emissions around +/- 220 K which is around 10-11 km altitude. Furthermore there is a narrow spike in 15 micron emissions around 7 to 8 km, that is likely induced by collision enhanced emission. So as much respect as I have for Happer, I lean towards trusting observations showing emissions at lower altitudes.
And yes I agree that H20 has done the “heavy lifting”, but the warmed dry air that a convects to the troposphere depends on CO2 to radiate the heat away as water vapor is virtually absent.
There is no “guessing” regarding where CO2 radiation escapes to space. CO2 is deactivated by collisions everywhere until it you reach the mesopause where the temperature is also around 220K. You cannot ignore that temperature increases in the upper stratosphere.
Insofar as water vapor is concerned, it cannot radiate effectively until its concentration drops to ~ 100 ppm or so, and it radiates strongly at lower concentrations due to collisional excitation. Different lines will begin to release energy to space at different concentrations dependent on each line’s radiative lifetime. The longer the radiative lifetime, the more likely it will be deactivated, and so the higher will be the emission altitude. It will be radiating to space at 10 ppm.
I did not see any comment to my post here:
https://wattsupwiththat.com/2025/08/21/how-co2-both-warms-and-cools-our-atmosphere/#comment-4108954
This explains some of the actual microphysics that take place in the atmosphere and why the radiative transfer equation can produce a “proper spectrum, despite the fact that it is based on the heuristic, phenomenological concept of “radiance.” You are free to ignore it, of course.
Nonsensical word salad. CO2 is not “deactivated”. It remains CO2, even at absolute zero. Are you really ignorant and gullible, or just pretending?
Yes, I’m laughing at you!
Happy that I could bring some levity to your day.
You’re welcome,
Jim,
Thank you for considering my questions and for sharing many informative articles on WUWT.
I make no pretense of being an ‘authority’, but having seen some of the work of Shula& Ott, have become skeptical that radiative transfer models accurately describe how thermal radiation from the Earth’s surface makes its way to space, i.e., I believe that convective heat transfer is a much bigger deal than the alarmists would have us believe.
Kudos, then, for giving some airtime to the impact of ‘collisions’ and how these relate to warming and cooling in the lower and upper troposphere, respectively. I believe Happer and van Wijngaarden also describe this phenomena in one of their ‘primers’, specifically as to why the Earth’s troposphere is not isothermal, but then go on to invoke Schwarzschild’s methodology to describe how heat is transported through the troposphere.
Hopefully, we’ll be seeing more articles on how collisional dynamics impact energy transfer from Earth to space in the near future.
DMacKenzie,
Yes it would be more correct over all to say “but without CO2 and H2O vapour” for overall emissions.
But I waffled on saying that because my focus was more on the upper atmosphere where, as we both agree, “At top of troposphere water vapor is below 10 ppm while CO2 is still 400 ppm….so it’s CO2 doing much of the radiation of the planet to space”
Great article. I have spent hours trying to show how warmists that heat is not trapped. Slower cooling means higher temperature for a longer time thereby radiating more heat over time. Planck calls this compensation.
So… heat is trapped for a while.
No, it is never “trapped.”
The claim that CO₂ “only emits at the top of the atmosphere” is misleading: in reality, CO₂ molecules absorb and emit infrared radiation at all altitudes, but the photons from lower layers are quickly re-absorbed so that only those from higher, optically thinner layers escape directly to space. Satellite spectra confirm this by showing CO₂ emission both from the cold stratosphere at the band center and from warmer, lower tropospheric layers at the band edges.
Nope.
https://wattsupwiththat.com/2025/08/21/how-co2-both-warms-and-cools-our-atmosphere/#comment-4108954
https://wattsupwiththat.com/2025/08/21/how-co2-both-warms-and-cools-our-atmosphere/#comment-4109713
The spectrum is widely misinterpreted.
Yes, by you it is!
In case you missed it.
https://wattsupwiththat.com/2025/08/21/how-co2-both-warms-and-cools-our-atmosphere/comment-page-2/#comment-4109865
The claim that CO₂ only emits “at the top” or that its role is negligible ignores a fundamental fact of radiative physics: CO₂ molecules absorb and emit infrared radiation at all altitudes, with collisions maintaining local thermodynamic equilibrium. In dense lower layers, photons are re-absorbed quickly, so we don’t see them from space — but this constant exchange still shapes the energy balance and lapse rate. Satellite spectra confirm that CO₂ emission comes from multiple layers (cold band centers from higher altitudes, warm band edges from the lower troposphere), exactly as radiative-transfer theory predicts. Water vapor and convection are indeed dominant players, but dismissing CO₂’s radiative impact contradicts both laboratory spectroscopy and direct atmospheric measurements.
Respectfully, CO2 plays no role in the lapse rate. The moist lapse rate is the result of condensation of water vapor, releasing latent heat. The collisional excitation and de-excitation of IR active species in the atmosphere does result in some spontaneous emission, but absorption/emission processes are orders of magnitude less than collisional excitation and de-excitation from the surface to the mesopause . There is no net transport of energy via radiation except at the respective emission altitudes of each of the IR active species.
CO2, because of the long radiative lifetime of the 15 μm excited state, is collisionally deactivated below the mesopause. That is where its emissions can escape tho space.
The “notch” in the spectrum is due to partial absorption of water vapor emissions by CO2 which is quickly deactivated by collisions giving back that energy as sensible heat in the atmospheric pool. H2O emits vigorously in a continuum from the mid to upper troposphere, at wavelengths below 13 μm, and its peak emissions completely overlap the extended rotovibrational Q-branch of CO2. The area under the “notch” is not from CO2 emission, it is from water vapor emissions that are not absorbed by CO2.
The only EMISSION from the Q-branch of CO2 in the spectrum is the tiny peak in the center of the notch, CO2 in emission at the mesopause.
See van Wijngaarden and Happer Figure 6b, and the descriptions on page 26 where they say, “Fig. 6b shows that for a frequency ν=667/cm, in the middle of the stronger Q branch of the CO2 bending mode band, the emission altitude is near the top of the atmosphere, ze = 85.3 km.”
That would be vW&H (2023).
https://arxiv.org/pdf/2303.00808
Apologies for forgetting to note which paper in the above post.
The lapse rate is just the decrease of temperature with altitude, following basic laws of physics.
Quite reasonable, considering the bottom the atmosphere is between 90 C and -90 C, and outer space is around -270 C or so. What else would you expect?
🙄
Sorry Tom, English is my mother tongue, and I can get by in one or two other languages.
I don’t speak Emoji.
I don’t know anyone who speaks emoji. You are not alone.
Sometimes words just aren’t enough.
It’s not entirely true that the only way a non-greenhouse gas molecule can get rid of its kinetic energy is by a collision with a greenhouse gas molecule which then radiates it out to space…
It can also get rid of it by a collision with the Earth’s surface which then radiates it out to space.
Which is why your conclusion of a runaway warming without greenhouse gases is completely wrong. Without greenhouse gases, thermal energy would go equally from the Earth’s surface to the gas molecules and vice versa. And the Earth would also radiate thermal energy, which would pass right through the atmosphere as if it weren’t there. IN would equal OUT. The atmosphere would only redistribute the warmth from warmer areas to cooler ones. Everybody happy. (Unless it would be too cold for everybody to exist.)
Not sure you can claim CO2 causes warming and cooling.
1) The one and only mechanism by which CO2 can affect climate change is through the thermalization of 15 Micron LWIR (Band 13 to 17 Micron). That is what your absorption graphic shows.
2) H2O and CO2 absorb those wavelengths, so with or without CO2, the absorption of those wavelengths is saturated in the lower atmosphere if H2O is present.
3) 15 Micron is radiated by dry ice of temp -80 C. How do you warm a body of 15 C by adding -80C energy to it?
4) If CO2 could actually cause warming, we would simply bubble wrap our homes in CO2 to warm them.
5) The relationship of CO2 to Temperarure is actually W/M^2 to Temperature, and that relationship shows a log decay, so it takes a whole heck of a lot of CO2 to materially impact temperature.
6) Submarines have CO2 of 10,000 ppm and things are just fine.
Simply look at the temperature data for a dry hot or cold desert where H2O isn’t present in the atmosphere. You will find there has been no warming over the past 120 years even as CO2 has increased by over 30%.
“3) 15 Micron is radiated by dry ice of temp -80 C. How do you warm a body of 15 C by adding -80C energy to it?”
Incorrect, 15μm radiation is emitted by blackbodies at all temperatures, the connection with -80ºC is that a blackbody at that temperature has its maximum emission at 15μm. A blackbody at 30ºC emits ~6 times as much 15μm radiation as one at -80ºC,
Well said.