Here’s a question that came in over the summer from a middle school science teacher in Colorado:
In college, I remember something about a “saturation effect,” I think it was called. The idea was that CO2 absorbed only certain wavelengths of long wave radiation and once all that radiation was absorbed, adding more CO2 would not raise the temperature anymore. I haven’t found much about it since. Is there anything to this?
Good question. And I’m not just saying that. I had to look this one up myself! It is true that CO2 and all the other greenhouse gases absorb long-wave radiation at particular wavelengths. CO2 itself absorbs energy over several different wavelengths, too. (See graph below.)
Here’s what I learned: At the very peak of the biggest CO2 absorption band, CO2 DOES keep all of the long-wave radiation from escaping to space. But that’s only for about a micron’s width of wavelength. At all other wavelengths, CO2 is only partially absorbing the heat and there’s plenty of room for more radiation at those wavelengths to be absorbed by more CO2.
So that’s one way of looking at it. But the increased radiation being trapped by more CO2 over the last several decades has actually been measured by satellites. One study compared how much energy was being released from the top of the atmosphere back in 1970 and in 1997. They found that there were slight dips in how much energy was being released to space at the same wavelengths as where the different GHGs absorb heat. That’s one big piece of evidence that more CO2 traps even more heat.
Just to throw one more study on there, Evans (2006) measured how much extra radiation was coming DOWN from extra GHGs in the atmosphere. They found that more energy was being radiated back down to Earth at the same wavelengths where GHGs absorb heat. Here’s a great quote from their article:
“This experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming.”
That make sense? Thanks for letting me learn something!