by James R. Barrante, Ph.D.
There has been a great deal of controversy over the years concerning CO2 sensitivity, the increase in global temperature associated with the doubling of CO2 gas level in the atmosphere. The reason that one considers doubling the CO2 level is because the relationship between the change in global temperature and the increase in CO2 level is not linear. A linear relationship also is known as a direct proportion. For example, let’s say that for every increase in CO2 level of 100 ppm the global temperature goes up 0.8°C. A linear relationship would require that if the CO2 level went up by 50 ppm, temperature would increase by 0.4°C, or if the level went up by another 100 ppm, the temperature would increase by another 0.8°C.But that’s not how it works. The relationship between CO2 level and global temperature is not linear, it is logarithmic. That drastically changes things. Using the same example, we notice that increasing the CO2 level by 100 ppm increases global temperature by 0.8°C. What happens if we increase the CO2 level by another 100 ppm? A logarithmic behavior requires the temperature to go up only 0.47°C. In order to get the temperature to go up by 0.8 degrees, we must double the level. That is, the CO2 would have to increase from 200 ppm to 400 ppm. Another 0.8-degree rise would require the level of CO2 to increase from 400 ppm to 800 ppm, etc. So, you can see that increasing the CO2 by a fixed amount results in a smaller and smaller increase in global temperature. Using the above example, increasing CO2 level by 100 ppm raises temperature 0.8°C, another 100 ppm increase raises temperature by 0.47°C, another 100 ppm increase by 0.33°C, etc.
Why does CO2 behave this way? It is because as the CO2 gas absorbs the infrared radiation emitted in a band centered around 15 microns, the intensity decreases. It is the way light interacts with matter. Think of it this way: adding a shade to a window decreases the intensity of light coming through. But some still gets through. Adding another shade decreases the intensity a little more, another shade, a little more, etc. Eventually, adding another shade will have no further effect.
Now if that were all there was to it, I would be a happy camper. But Nature has to complicate things, as evidenced by the historical record of CO2, which you never hear about. It’s pretty clear that at some point CO2 is going to be an ineffective greenhouse gas. The level will reach a point, like the final shade described above, where increasing it will cause no further warming. We don’t know where that point is — or do we? History might be able to show us where it is. We know that ice-core research has shown that there is a strong correlation between CO2 level and global temperature. However, in all that data CO2 levels have never increased above about 280 ppm. There is other data showing CO2 levels as high as 6000 ppm on our planet. All this data is
illustrated in the graph. The graph looks weird because of the large range of CO2 level, but this was necessary to illustrate the following. The insert is made up of only Vostok ice-core data. Notice that as long as the CO2 level remains below 400 ppm, there is a strong correlation between CO2 level and global temperature (R2 . = 0.93868). This is illustrated by the straight line drawn through the points. Once, however, the level surpasses 400 ppm, the correlation appears to fall apart. For example, note that a temperature of 72˚F occurred at four CO2 levels. A temperature of 63˚F was found at three levels, one occurring at 160 ppm. This is data, known as Phanerozoic Time data, describes the behavior of atmospheric CO2 over 600 million years of Earth’s history. It is clear that once the level of CO2 surpasses 400 ppm, it’s ability to act as a greenhouse gas on planet Earth ceases to exist, and that other forces are controlling planet temperature. The most logical explanation of this is that the 15 micron band of infrared radiation that gives CO2 the ability to act as a greenhouse gas, which we know is limited based on the temperature of the planet, has been, for all practical purposes, fully absorbed once the level of CO2 reaches a level approaching 400 ppm. This is consistent with the fact that Earth’s temperature has not increased in the past 15 years, while CO2 levels continue upward. In fact, if you look at the graph (Vostok data), it appears that average global temperature should be around 70˚F at today’s level of 400 ppm. It is nowhere near that value.
It is important to point out that on a warmer planet (e.g., Venus), since there is more radiation at 15 microns because the planet is warmer, it would require a higher level of CO2 before essentially complete absorption (saturation) has occurred. Using the light-shade analogy, a brighter sunlight would require more shades to blot out the light coming through a window. It appears, therefore, that CO2 sensitivity is only important as long as doubling the concentration of CO2 doesn’t put the concentration over 400 ppm.
Data for graph was constructed from Vostok Ice-Core Data and from data published by Scotese, C. R., http//www.scotese.com/climate.ht, and Berner, R. A. and Kothavala, “Geocarb III: A Revised Model of Atmospheric CO2 Over Phanerozoic Time,” Amer. Journal of Science, 301, 182-204 (2001).