Greenhouse Gases’ Heat Trapping Ability Hasn't Saturated As Some Predicted – But Why?

Anytime global warming or climate change get mentioned online, you can expect a bunch of comments claiming to be able to disprove the whole concept, at least in relation to human activity. Most of these have been debunked, not just in detailed scientific papers, but by plenty of popular science websites, IFLScience included. There’s at least one idea, however, for which explanations are much harder to find for those without some expertise in the field, and it’s also got one of the most interesting histories: greenhouse gas saturation. Perhaps this will be a useful resource if you ever see the argument presented.

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The claim is that greenhouse gases do warm the planet up to a certain point, but that this effect saturates, and as a result, once you pass a certain point, adding more to the atmosphere has little to no effect. Conveniently, this point is stated or implied as being one we have either passed or will reach soon, so we can keep on burning fossil fuels to our hearts’ delight.

In one sense, it is fairly easy to see that this claim is wrong: if it was right, the world would not currently be warming faster than any period we can find in the palaeontological record. Moreover, if there was much to it, we’d be seeing it a great deal more often, instead of all the other claimed disprovals of global heating you can find answers to at places like Skeptical Science. 

On the other hand, it’s not so easy to see why the saturation argument is wrong, and indeed, it took scientists decades to spot the flaw in the reasoning.

A brief history of carbon dioxide saturation

The first indication the Earth had a greenhouse effect emerged in 1824, when Joseph Fourier noted that based on the amount of heat the Earth received from the Sun, it ought to be so cold we’d be bound in ice. Something clearly was warming the planet up, and Fourier figured it must be the atmosphere.

Three decades later, Eunice Foote and John Tyndall found part of the answer (possibly independently, possibly from Tyndall stealing Foote’s work). They showed that carbon dioxide traps heat, and indeed does it so powerfully that it will produce measurable differences in the temperature of jars placed in sunlight, depending on how much of the gas they contain. 

We now know this occurs because carbon dioxide molecules are particularly good at capturing photons of particular infrared wavelengths, which would otherwise escape into space. Most of these photons are emitted by the Earth in response to direct heating from solar radiation at shorter wavelengths.

We now also know there are a lot of other gases involved, but CO₂ is the most important one.

Four decades after that, the Swedish physicist Svante Arrhenius noted that humanity was burning a lot of coal (and starting to burn oil) and that carbon dioxide was released in the process. Might all this carbon dioxide eventually heat the planet, Arrhenius wondered? For him, this wasn’t something to worry about. Remember, he was Swedish; he thought warmer weather sounded rather nice, forgetting that 99 percent of the world’s people lived in hotter places, and all the other consequences like rising sea levels.

Nevertheless, Arrhenius did think this was interesting, and did some calculations on the effect of doubling atmospheric CO₂. He left out a great many factors now included in global warming models, but interestingly, his result was not too far from current estimates, because many of these factors largely cancel out.

Arrhenius wasn’t calling for people to replace coal with something else, so his work didn’t threaten vested interests, other than one. Knut Ångström was also a great Swedish physicist, and a rival of Arrhenius, who always wanted to prove his enemy wrong, so he went looking for a flaw in Arrhenius’s work.

Ångström thought he found it in saturation. He argued that carbon dioxide was already capturing most of the infrared radiation reflected off the Earth that would otherwise escape into space at the relevant wavelengths. Other wavelengths would never be captured, no matter how much CO₂ there was, so atmospheric increases could have only small effects.

Not everyone accepted this argument – you may have seen the article published in 1912 warning of disaster if coal-burning continued long enough – but most scientists were uncertain.

It was only decades later that the flaw in Ångström’s arguments was identified.

Atmospheric height

What Ångström had missed, it was pointed out, was that the atmosphere is not infinitely thin. Carbon dioxide capturing heat near ground level has different consequences from that heat being captured much higher up. You could think of it as being like pulling your blanket more tightly around you if you’re cold in bed – the heat is prevented from escaping closer to the source, and therefore makes you warmer.

Ångström’s calculations had assumed that it didn’t matter whether infrared radiation emitted from the Earth was captured at just above ground level or high in the stratosphere, but that’s not true. As more carbon dioxide is released, there will be more in the troposphere, the lowest layer of the atmosphere. This means that it is more likely a photon of infrared radiation emitted by the Earth will be captured in the troposphere, rather than reaching the stratosphere and possibly being captured there.

Consequently, one of the key predictions of global climate models is that as carbon dioxide levels rise, the troposphere should warm, but the stratosphere should get colder. Some of the heat that was previously escaping the troposphere is now not reaching the stratosphere, cooling the upper atmosphere down.

Since the only time we venture into the stratosphere it’s encased in metal cages called airplanes, a cooling stratosphere doesn’t affect us much, but the warming troposphere definitely does.

Just as expected

When Arrhenius and Ångström were arguing, we didn’t know much about the Earth’s climatic history, and of course, neither knew what was to come. Now we have an abundance of evidence of past ice ages and hot-house eras, and we know these have been associated with swings in carbon dioxide levels. The changes cannot be explained without allowing for the extra heat trapped by carbon dioxide concentrations at, and well above, modern levels.

We’ve also seen Earth’s temperatures rise sharply in the troposphere over the last 50 years as carbon dioxide has soared, something that would not have happened if the saturation hypothesis had been right.

Meanwhile, satellites and weather balloons have measured an increasingly cold stratosphere. 

A far more detailed and rigorous analysis of just how much of the warming we have experienced was published in the Journal of Climate five years ago. This paper estimates that 90 percent of the direct warming effect from increases in atmospheric CO₂ in recent decades is from changes in the height at which trapped heat is emitted. 

Carbon dioxide also has an indirect effect by allowing the atmosphere to hold more water vapor, and for this, height contributes around 60 percent of the effect. The authors note that their calculations are for cloudless skies, so may not completely capture reality.

We also have evidence from other planets, which the 19^th-century pair did not, with Venus’s extreme heat a product of its thick 96 percent carbon dioxide atmosphere. This proves that more CO₂ really can keep raising temperatures to staggering levels, far beyond what Ångström thought possible.