A donor to the Cornwall Alliance has asked if it is true, as Travis Kavulla wrote in “What Is the Green New Deal?” (National Review, February 21, 2019), that “The atmospheric concentration of greenhouse gases has not been higher since the dawn of human civilization than it is today. No debate about the niceties of climate science can eclipse this basic fact.”

The first sentence is probably true. (Probability is the best empirical observation can give us.) It’s not something that “global warming skeptics” or “climate realists” challenge. But it’s also pretty well irrelevant. Why?

First, because what’s relevant to climate change isn’t what’s happened only since “the dawn of human civilization,” it’s what’s happened throughout geologic history. The existence of human civilization hasn’t changed the laws of physics or the overall working of the lithosphere, hydrosphere, atmosphere, cryosphere, and biosphere.

The vast majority of those who embrace the view that rising atmospheric carbon dioxide concentration is driving dangerous global warming think the Earth is about 4 billion years old, and they interpret the evidence (mainly gas bubbles trapped in layers of ice they think were deposited annually over hundreds of millions of years) as indicating that at various times before “the dawn of human civilization” carbon dioxide concentrations have been much higher than they are today—as high as 8,000 parts per million compared with today’s roughly 410 parts per million.

(Those who believe the Earth is much younger interpret the evidence differently, arguing that the ice layers were deposited not one per year but much more rapidly. But what’s crucial in answering the global warming alarmists is that their own claims are inconsistent.)

Second, the same scientists who claim that rising atmospheric carbon dioxide concentration over the last couple of centuries has been the primary cause of rising global average atmospheric surface temperature (GAAST) also know that in the past the relationship between carbon dioxide concentration and temperature doesn’t support that theory. To support the theory, the evidence would have to show that carbon dioxide concentration consistently changed before temperature changed, with rising concentration leading to rising temperature and falling concentration leading to falling temperature. But the evidence actually shows that temperature often changes before concentration. This is because as ocean temperature rises, the oceans outgas carbon dioxide (just as carbon dioxide escapes more quickly when you open a warm can of soda than when you open a cold one), while as temperature falls, oceans absorb it.

This can be confusing, since carbon dioxide is indeed a “greenhouse” gas (the term is actually misleading—it doesn’t work the way a greenhouse does): it absorbs infrared (heat) radiating from Earth’s surface out to space and re-radiates it, sending some of it back toward the surface. According to simple physics, this should make the lower atmosphere warmer than it would be without the carbon dioxide, and the upper atmosphere cooler. But we must remember that carbon dioxide is only one of a number of factors that influence GAAST.

The amount of energy emitted by the Sun changes over time, as does the amount of solar magnetic wind it emits, and both of those affect GAAST, the energy directly, the magnetic wind indirectly by regulating the rate of influx of cosmic rays into the atmosphere, which in turn regulates cloud formation, which in turn regulates temperature by reflecting more or less of the Sun’s energy back into space before it reaches Earth’s surface. In addition, cyclical changes in Earth’s distance from the Sun as well as the tilt of its radius affect how much of the Sun’s energy enters our atmosphere and reaches the surface, and where, and cyclical changes in ocean currents (Pacific Decadal Oscillation, El Niño/Southern Oscillation, Atlantic Multidecadal Oscillation, and others) bring changes in cloud formation and thus GAAST.

And carbon dioxide is not the only “greenhouse” gas; the most important one is water vapor including, in its condensed form, clouds, which causes 95% of the “greenhouse effect” (the warming of the lower atmosphere and cooling of the upper atmosphere by infrared-absorbing gases); carbon dioxide causes only about 4.5%, with methane and a few others causing the remaining 0.5%.

This means that since these other factors greatly outweigh carbon dioxide concentration in regulating GAAST, carbon dioxide concentration can be rising even while temperature is falling, because the other factors could be causing temperature to fall more than carbon dioxide is causing it to rise.

Third, while carbon dioxide is a “greenhouse” (properly infrared-absorbing) gas, its effect is logarithmic. That is, the effect of each added molecule is less than the effect of the prior added molecule. This is because carbon dioxide absorbs infrared only in certain wavelengths, and as carbon dioxide concentration reaches the level necessary to absorb all the infrared in a given wavelength, adding more won’t absorb any more in that same wavelength. Bit by bit, all the different wavelengths that carbon dioxide absorbs get fully absorbed, so each addition of carbon dioxide is left with less infrared wavelength to absorb.

The basic idea can be illustrated this way: Imagine that you have a clear glass window, and you want to block all the light that comes through it. You could paint it over with white paint. That would block most of the light, but some would still come through. A second coat of paint would would block less light than the first, but most of what’s left. A third coat would block less than the second, but most of what’s left. Eventually you’d have applied enough coats that no light would come through. After that, no matter how many more coats you applied, you wouldn’t block any more light.

The best evidence is that Earth’s atmospheric carbon dioxide concentration is already at, or very near, “saturation,” that is, the point at which adding more won’t absorb any more infrared and therefore won’t warm Earth’s atmosphere near the surface or cool it in the stratosphere anymore. A highly technical paper by physicists W. A. van Wijngaarden and William Happer explains this. The authors have also published a less technical summary.

It’s unclear what Travulla meant by “No debate about the niceties of climate science can eclipse this basic fact.” Since his next sentence is “It is prudent to encourage the development of power plants that emit no greenhouse gases, or less of them,” it seems that he means it necessarily follows from the fact that atmospheric carbon dioxide concentration is higher today than at any time since the dawn of human civilization that that concentration necessarily is causing, or as it continues to rise soon will cause, more harm than would be caused by “the development power plants that emit no greenhouse gases, or less of them.” But it is not clear that it will cause any more harm at all, since it may not cause any more temperature increase, or if any, only an extremely slight amount. As you can see from the above, that is false. (At the same time, the biological benefits of increasing atmosphere carbon dioxide concentration are clear and large, since it makes plants grow better and thus makes more food available.)

You can learn much more about this from Dr. Roy W. Spencer’s Global Warming Skepticism for Busy People, available from our bookstore. Or, if you’d like, we’ll send you a free copy as our thanks for a 100% tax-deductible donation of any size if you request it; just go to our donation page, fill out the donation form, and in the comments field write “Please send me a free copy of Global Warming Skepticism.”