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 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. So I thought it might helpful to expose some of these contradictory claims. I'll start with my personal favorite: CO2 concentrations are 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 and propagandists 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.

This is clearly conflicting statements given that these influencers admit the relationship between CO2 and temperature is logarithmic, meaning that the more "starved" the planet is of CO2, the more warming potential CO2 has. Because preindustrial levels is a "low" 280 ppm, we can much more easily double CO2 concentrations from a 280 ppm starting point than we could if we were starting at 2800 ppm CO2.  Saying CO2 concentrations are so low that they are "starved" is also saying that the warming potential of increasing CO2 is large. The statements from these influencers can both be false, but they cannot both be true. CO2 concentrations are either so low that we're starved of CO2 or they're so high that CO2 is saturated, but not both. These influencers appear to try to avoid saying these things in ways that make the contradiction apparent but I believe I have fairly represented both of their claims. For instance, here are examples of CO2 Coalition making both these claims. The following quotes are from William Happer unless otherwise indicated.

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 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. To minimize the warming effect of CO2, Happer and the CO2 Coalition are attempting to resurrect a myth from the beginning of the 20th century associated mostly with Knut Ångström. The "saturation" myth sounds very science-like, and it allows them to sound like they can justify that fact that doubling CO2 from now on won't make much of a difference. In fact, Happer has recently said it would make a difference of about 0.7°C. The CO2 Coalition wants us to believe both that:

• CO2 concentrations are so low that the Earth is "starved" and we're in a 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 that its warming effect is "saturated." The warming potential of CO2 decreases as concentrations increase, since the relationship between CO2 and temperature is logarithmic. Adding more CO2 above current levels 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.

Of course this is alarmist nonsense and propaganda designed to convince people to put their faith in the fossil fuel industry that continued carbon emissions are good for humanity. For them, the combustion of fossil fuels isn't 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 emissions will provide greening, more crops and a healthier, only slightly warmer climate. 

You could try to argue that I'm being unfair. They say CO2 is saturated in terms of CO2's radiative properties and starved in terms of its biological benefits, so this isn't a full contradiction. But this misses the point. They claim we are "starved" of CO2 only because concentrations are deemed to be low. CO2 has been much higher in the past, so we're "starved" of it now. We're also told that CO2 is a "trace gas" and at trace concentrations it is impotent to cause warming, but the log relationship between CO2 and temperature means that CO2 would be most potent at low concentrations. The more "starved" we are of CO2 the more potent it would be to cause warming.

While it strains logic to imagine that both claims are 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, and 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.
6. The PETM was a time of geologically rapid warming of 5-6°C warming over about 200K years, caused by biogenic carbon indicated by a very large negative carbon isotope excursion (nCIE). A study published this year[1] concluded that the global warming associated with the PETM "exceeded the adaptation capacity of vegetation systems, impacting the efficiency of terrestrial organic carbon sequestration and silicate weathering." They concluded that "global warming of the magnitude as during the PETM could exceed the response capacity of vegetation systems and cause a long-lasting decline in the efficiency of vegetation-mediated climate regulation mechanisms." In other words vegetation doesn't adapt quickly to climate changes. Current global warming is occurring at rates likely exceeding the most rapid warming of the PETM, and so it's likely that recent warming will likewise stress vegetation.

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. A video rebuttal of this is here.
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 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

References:

[1] Rogger, J., Korasidis, V.A., Bowen, G.J. et al. Loss of vegetation functions during the Paleocene–Eocene Thermal Maximum. Nat Commun (2025). https://doi.org/10.1038/s41467-025-66390-8