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At a glance - Is the CO2 effect saturated?

Posted on 9 January 2024 by John Mason, BaerbelW

On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added "At a glance" section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a "bump" for our ask. This week features "Is the CO2 effect saturated?". More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.


At a glance

This myth relies on the use (or in fact misuse) of a particular word – 'saturated'. When someone comes in from a prolonged downpour, they may well exclaim that they are saturated. They cannot imagine being any wetter. That's casual usage, though.

In science, 'saturated' is a strictly-defined term. For example, in a saturated salt solution, no more salt will dissolve, period. But what's that got to do with heat transfer in Earth's atmosphere? Let's take a look.

Heat-trapping by CO2 in the atmosphere happens because it has the ability to absorb and pass on infra-red radiation – it is a 'greenhouse gas'. Infra-red is just one part of the electromagnetic spectrum, divided by physicists into a series of bands. From the low-frequency end of the spectrum upwards, the bands are as follows: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Gamma rays thus have a very high-frequency. They are the highest-energy form of radiation.

As our understanding of the electromagnetic spectrum developed, it was realised that the radiation consists of particles called 'photons', travelling in waves. The term was coined in 1926 by the celebrated physicist Gilbert Lewis (1875-1946). A photon's energy is related to its wavelength. The shorter the wavelength, the higher the energy, so that the very high-energy gamma-rays have the shortest wavelength of the lot.

Sunshine consists mostly of ultraviolet, visible light and infra-red photons. Objects warmed by the sun then re-emit energy photons at infra-red wavelengths. Like other greenhouse gases, CO2 has the ability to absorb infra-red photons. But CO2 is unlike a mop, which has to be wrung out regularly in order for it to continue working. CO2 molecules do not get filled up with infra-red photons. Not only do they emit their own infra-red photons, but also they are constantly colliding with neighbouring molecules in the air. The constant collisions are important. Every time they happen, energy is shared out between the colliding molecules.

Through those emissions and collisions, CO2 molecules constantly warm their surroundings. This goes on all the time and at all levels in the atmosphere. You cannot say, “CO2 is saturated because the surface-emitted IR is rapidly absorbed”, because you need to take into account the whole atmosphere and its constant, ongoing energy-exchange processes. That means taking into account all absorption, all re-emission, all collisions, all heating and cooling and all eventual loss to space, at all levels.

If the amount of radiation lost to space is equal to the amount coming in from the Sun, Earth is said to be in energy balance. But if the strength of the greenhouse effect is increased, the amount of energy escaping falls behind the amount that is incoming. Earth is then said to be in an energy imbalance and the climate heats up. Double the CO2 concentration and you get a few degrees of warming: double it again and you get a few more and on and on it goes. There is no room for complacency here. By the time just one doubling has occurred, the planet would already be unrecognisable. The insulation analogy in the myth is misleading because it over-simplifies what happens in the atmosphere.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Click for Further details

In case you'd like to explore more of our recently updated rebuttals, here are the links to all of them:

Myths with link to rebuttal Short URLs
Ice age predicted in the 1970s
It hasn't warmed since 1998
Antarctica is gaining ice
CRU emails suggest conspiracy
What evidence is there for the hockey stick
CO2 lags temperature
Climate's changed before
It's the sun
Temperature records are unreliable
The greenhouse effect and the 2nd law of thermodynamics
We're heading into an ice age
Positives and negatives of global warming
Global cooling - Is global warming still happening?
How reliable are climate models?
Can animals and plants adapt to global warming?
What's the link between cosmic rays and climate change?
Is Al Gore's An Inconvenient Truth accurate?
Are glaciers growing or retreating?
Ocean acidification: global warming's evil twin
The human fingerprint in global warming
Empirical evidence that humans are causing global warming
How do we know more CO2 is causing warming?
Explaining how the water vapor greenhouse effect works
The tricks employed by the flawed OISM Petition Project to cast doubt on the scientific consensus on climate change
Is extreme weather caused by global warming?
How substances in trace amounts can cause large effects
How much is sea level rising?
Is CO2 a pollutant?
Does cold weather disprove global warming?
Do volcanoes emit more CO2 than humans?
How do human CO2 emissions compare to natural CO2 emissions?
Climate scientists could make more money in other careers
How reliable are CO2 measurements?
Do high levels of CO2 in the past contradict the warming effect of CO2?
What is the net feedback of clouds?
Global warming vs climate change
Is Mars warming?
How the IPCC is more likely to underestimate the climate response
How sensitive is our climate?
Evidence for global warming
Has the greenhouse effect been falsified?
Does breathing contribute to CO2 buildup in the atmosphere?
What is causing the increase in atmospheric CO2?
What is methane's contribution to global warming?
Plants cannot live on CO2 alone
Is the CO2 effect saturated?


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Comments 1 to 3:

  1. The sceptics claim that the CO2 warming effect is already saturated or is close to being saturated is clearly false. It seems to originate with the assertion that the surface receives about 3.7 W/m2 more energy each time CO2 is doubled ie: a logarthmic curve. This means eventually it would take a huge volume of additional CO2 (presumably over a long time period) to add an extra 3.7W / m2. Which they suggest means the effect is essentially then saturated, but without specifying a precise number (how convenient of them).

    I see that the article mentions that this assertion about a logarithmic relationship may have been proven false by He et al. 2023, (?) and it also depends on emissions trajectories and other factors, but assuming the logarithmic relationship is simplistically true a look at radiative forcing versus CO2 concentration below and a bit of maths and its obvious the warming effect is not saturated and we are not yet near saturation:

    My back of envelope maths: In 1960s CO2 was 320 ppm ppm so doubling would be 640 ppm at around roughly year 2100 assuming BAU emissions. This coincides with the IPCC warming projection of 3 - 5 degrees C by 2100.

    The next doubling is from from 640 to 1280 is a larger volume of CO2 and would presumably take longer to around year 2300 assuming the same BAU CO2 growth trend and other things being equal. Projections of warming by the IPCC by 2300 are not surprisingly around 8 - 10 degress C for this further doubling of CO2. Such a quantity of CO2 is large but may possibly be feasible given reserves of fossil fuels and uncertainties arount that.

    The next doubling from 1280 to 2560 would lead to something like 15 degrees C and is a huge volume of CO2 that would take many centuries and would almost certainly exhaust reserves of fossil fuels, and most probably well before 15 degrees is reached. We could say this is the point of saturation in a practical sense. Its of no comfort because we would have had at least 5 degrees of warming and probably more, and not even factoring in tipping points.

    Someone check my maths its very rough, but the sceptics claims are clearly false and meaningless.

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  2. I think this is where data from paleoclimatology can help as well.  Three recent studies have looked at the earth's temperature vs CO2 during the Cenozoic period, Rae et al.Honisch et al., and Tierney et al. .  Each of those show that the temperature of ancient earth continues to rise as CO2 increases.  As I understand it the first two are based solely on proxy data while the Tierney effort includes modeling to try and correlate the data geographically and temporally.

    All of these are concerned with earth system sensitivities that include both short term climate responses plus slower feedback processes that can take millenia, e.g. growth and melting of continental ice sheets. Both Rae and Honisch include reference lines for 8 C / doubling of CO2. In both cases, almost all the data lie below those reference lines suggesting that 8 C / doubling is an upper bound or estimate of earth's equilibrium between temperature and CO2. Also notice that there quite is a bit of spread in the data.

    In contrast, when Tierney et al. include modeling they get a much better correlation of T and CO2. They find that their data is best correlated with 8.2 C / doubling, r = 0.97.  Again, this represents an equilibrium that can take millenia to achieve but does to my way of thinking represent "nature's equilibrium" between T and CO2. 

    In these comparisons, the researchers define changes in temperature relative to preindustrial conditions, CO2 = 280 ppm. For Tierney's correlation then on geological timescale, the temperature would increase by 8.2 C at 560 ppm.  At our present value of 420 ppm there would be 3.7 C of apparent warming potential above our 1.1 C increase already achieved as of 2022, i.e., global warming in the pipeline if you will.

    Bottom line, based on paleoclimatological data, there is no apparent saturation level of CO2.

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  3. When I first sat up and took notice of global warming was when I started reading palaeoclimate studies (a very long time ago now). As a geologist trained to read 'what rocks have to tell', I quickly realised we were heading straight for trouble. Big trouble.

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