Contradictory Contrarian Claims, Part 1: CO2 is Both Starved and Saturated

How Increasing CO2 Affects Radiative Forcings and GMST

If you pay enough attention to contrarian climate influencers, you may begin to notice how frequently they affirm conflicting opinions and/or flat out contradict themselves. I think they hope that as long as they are careful to word contradictory claims in sufficiently different ways without saying both within the same minute or so, you won't notice the conflict. So I thought it might helpful to expose some of these contradictory claims. I'll start with my personal favorite: CO2 is both low/starved and high/saturated. 

This contradiction appears to be particularly common among those who speak for the CO2 Coalition, like William Happer, John Shewchuk and Gregory Wrightstone. You can find these influencers saying both of the following:

  1. CO2 concentrations are so how that the planet is starved of CO2. We're in a "CO2 famine."
  2. CO2 concentrations are so high that its effect is "saturated" in the atmosphere.

It's possible that CO2 concentrations are either too low and we're starved of CO2 or that they're so high that CO2 is saturated, but it excessively strains logic to imagine both of these claims are true. These influencers appear to try to avoid saying these things in ways that make the contradiction apparent, but I believe I have fairly represented their positions. For instance, here are examples of Happer & the CO2 Coalition making both these claims. 

Happer & the CO2 Coalition on CO2 Starvation:

  • "We are in a CO2 famine now, compared to what is normal for plants. And just about any plant, if you give it more CO2 and a lot more, it will do better.... The clearest evidence that Earth is in a CO2 famine, compared to what most plants would prefer, is that growers of flowers, fruits and vegetables continue to pay the substantial costs of tripling or quadrupling the concentrations of CO2 in their greenhouses." From SkyNews Australia in Sept 2023.
  • "Few realize that the world has been in a CO2 famine for millions of years, a long time for us, but a passing moment in geological history... Pre-industrial levels of 280 ppm (parts per million), are not that far above the minimum level, around 150 ppm, when many plants die from CO starvation." From Data or Dogma in 2015.
  • "CO2 levels fell precipitously & steadily to within about 30 ppm of the 150 ppm 'line of death' below which plants can’t survive. Both the relatively short-term data from ice cores and much longer-term data going back 140 million years... show an alarming downward trend toward CO2 starvation." One of CO2 Coalition's "facts" (probably not authored by Happer).
Happer & the CO2 Coalition on CO2 Saturation:
  • "The greenhouse effect of CO2 is already saturated. That’s the jargon in science. It means it’s done all the greenhouse warming it can already, practically. And doubling it practically doesn’t make any difference. You could quadruple it. It also practically makes no difference. So the CO2 effects are strongly, strongly saturated." From How to Think about Climate, 2025.
  • "That's such a strong absorption that it's saturated now, so as you add more CO2, most of what you can do has already been done. There's still a little addition, but not very much.... If you have a barn, you know, and you want to paint it red. You know if you put a thin layer of paint, it may not be red enough, but if you put two or three then it really looks red. After that, if you add mor red, it doesn't make much difference. So that's sort of what CO2 is doing now. Most of the easy absorption has been done." From this Interview.
  • "The greenhouse effect is very insensitive to changes in CO2 concentrations. In the jargon of radiative transfer, the greenhouse effect is said to be 'saturated.'" Quote from Happer in one of CO2 Coalition's "facts."
The motivation to make both of these conflicting claims is obvious, given that the Happer and the CO2 Coalition actively support the political narrative of the fossil fuel industry. The narrative that the CO2 Coalition wants you to believe is that CO2 levels have been in a long term decline on geologic time scales, and we were headed towards dangerously low levels of CO2. But thanks to fossil fuel emissions, we are avoiding that catastrophic future. But in order to make that claim, you have to reject the notion that doubling CO2 causes significant warming, so Happer needed a way to minimize that effect. To do this, Happer and the CO2 Coalition are resurrecting a myth from the beginning of the 20th century by Knut Ă…ngström. The "saturation" myth sounds very science-like, and it's what they use to justify their claim that doubling CO2 won't make much of a difference in temperature. In fact, Happer has recently said it would make a difference of about 0.7°C. Clearly the idea is to promote both that:
  • CO2 concentrations are so low that the Earth is "starved" and we're in an "alarming" CO2 "famine." We need to add more CO2 because it's a very powerful "gas of life" that will cause wonderful greening, support crops, etc.
  • CO2 concentrations are so high (there are so many "layers of paint") that its warming effect is "saturated" and adding more won't change global temperature much at all. We can add more CO2 because it's a very week greenhouse gas, and the mild warming will be good for us anyway.
This appears to be the "gospel" of the CO2 Coalition. Rather than causing significant global warming that increases our risk of harmful consequences to our climate, the fossil fuel industry is actually saving humanity from a future apocalypse of starvation and death, and continued carbon emissions will provide greening, more crops and a healthier, only slightly warmer climate. 

While it strains logic to imagine that both claims could be simultaneously true, they can both be false, and the evidence is overwhelming that this is the case. I've already written on both of these claims, so I don't think I need to do a full rebuttal of them here. To save on the length of this post, I'll just make bullet points as summary rebuttals of each claims and link to fuller posts and arguments.

On CO2 Starvation

  1. The idea behind this myth is that there's a "line of death" around 150 ppm below which photosynthesis stops and all plant life dies. That is wrong. The planet actually spent at least a million years at ~100 ppm CO2.
  2. The claim that we're on a 140 million year decline in CO2 leading to starvation is based on an outdated climate model and not based on the best proxy evidence. CO2 has been declining in the Cenozoic, but we're not on track towards dangerously low levels of CO2. That's contrarian alarmism.
  3. There is plenty of evidence that life can and has adapted to low CO2 levels during the glacial cycles of the Quaternary, including both the C3 and C4 pathways. Given enough time, species adapt. In fact, the large majority of species existing today evolved following the beginning of the Quaternary, and so they are adapted to low CO2 (below 300 ppm). The Earth is not starved for CO2; it has adapted to low CO2.
  4. Certainly greening has occurred with elevated CO2, but this is not expected to be a permanent benefit, since warming with elevated CO2 increases vapor pressure deficit.
  5. Greenhouses often do add CO2 because they are enclosed spaces, so daytime photosynthesis can draw down CO2 levels well below the ambient air. There are additional benefits to doing this, but you have to be careful to control for moisture, weeds, pests and disease.

On CO2 Saturation

  1. Happer's saturation analogy fails address how the greenhouse effect actually works. Adding layers of paint to a barn until it can't get any more red fails to account for the fact that the surface temperature is set at the emission altitude, and the surface warms according to its lapse rate.
  2. Happer also fails to address observed "pressure broadening" with increased CO2 concentrations. In fact, this has been understood since the 1950s with the writings of Gilbert Plass.
  3. A recent paper by Kubicki tried to demonstrate that CO2 was saturated, but that paper was so bad it was retracted. I have a review of that paper here.
  4. The limit of ln (x) as x→∞ is ∞. It's a simple mathematical fact that ln(x) doesn't saturate. The more you increase CO2, the higher the temperature.
  5. Happer's calculation for sensitivity is irrationally low, assuming no positive feedbacks, despite the fact that as recently as 2020, he admitted that the water vapor feedback amplified ECS by 1.6x. The best estimates for ECS bunch up around 3°C for 2xCO2.
  6. On geologic time scales, we have good proxy evidence for GMST and CO2, with CO2 reaching concentrations near 1300 ppm, and GMST warms at a rate of 8°C for 2xCO2. There is no physical evidence that CO2 "saturates" and adding more CO2 has little to no effect for any CO2 concentration observed in the Phanerozoic.
CO2 Doesn't Saturate

My main goal here is simply to highlight the logical conflicts and contradictions that contrarian influencers are forced to promote, not to write a full rebuttal of the claims. But the links provided in the bulleted list above provide those full rebuttals for those who are interested. 

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