A few parts per million


There’s a persistent rumor that CO2 concentrations are too small to have a noticeable radiative effect on the atmosphere. (It appears here, for example, though mixed with so much other claptrap that it’s hard to wrap your mind around the whole argument – which would probably cause your head to explode due to an excess of self-contradiction anyway.)

To fool the innumerate, one must simply state that CO2 constitutes only about 390 parts per million, or .039%, of the atmosphere. Wow, that’s a really small number! How could it possibly matter? To be really sneaky, you can exploit stock-flow misperceptions by talking only about the annual increment (~2 ppm) rather than the total, which makes things look another 100x smaller (apparently a part of the calculation in Joe Bastardi’s width of a human hair vs. a 1km bridge span).

Anyway, my kids and I got curious about this, so we decided to put 390ppm of food coloring in a glass of water. Our precision in shaving dye pellets wasn’t very good, so we actually ended up with about 450ppm. You can see the result above. It’s very obviously blue, in spite of the tiny dye concentration. We think this is a conservative visual example, because a lot of the tablet mass was apparently a fizzy filler, and the atmosphere is 1000 times less dense than water, but effectively 100,000 times thicker than this glass. However, we don’t know much about the molecular weight or radiative properties of the dye.

This doesn’t prove much about the atmosphere, but it does neatly disprove the notion that an effect is automatically small, just because the numbers involved sound small. If you still doubt this, try ingesting a few nanograms of the toxin infused into the period at the end of this sentence.

2 thoughts on “A few parts per million”

  1. Fantastic illustration Tom. The “debate” over global warming continues to perplex me. I don’t think anyone is saying that the science is 100% certain, but rather that the risk is easily great enough that action is absolutely necessary.

    What is more is that the action to reduce emissions just so happens to have tremendous economic and social benefits. I think folks are starting to recognize this as an underlying stalling point for productive discussion. As I mentioned to John Sterman after his talk, even if you can convince everyone of global warming there will still be tremendous pushback because it will require significant change that currently is undefined.

    What is needed is a clear and concise description of what that changed world will look like, what its benefits are, and how we get there. In John’s longer talk that you posted he keyed into this a little more and during our chat he said that they were doing some work here as well.

    I’m really looking forward to hearing more because I think it’s time to put the climate debate to bed. Debates like this are like software development, you’ll never have 100%. So just declare that we’ve won and that we already know how much we need to reduce emissions. Now we have to win the debate on how we do just that.

    1. I was sitting by a doctor on a plane the other day. We got talking about smoking – his experience is that even patients who fully understand that it’ll kill them still won’t quit. The approach that seems to work is to talk about immediate benefits: not smelling like smoke, walking up stairs without huffing & puffing, whiter teeth, etc. There are a lot of similar arguments for climate policy, but it’s really discouraging that zero thought for the future is the norm.

      I think there are two classes of people who remain unconvinced: hardcore skeptics – very few with a real understanding of the science and a lot who are just bonkers or don’t care about truth, and casual skeptics who don’t have the inclination, time or skill to separate the science wheat from the pseudoscience chaff. The hardcore skeptics and paid disinformers prey on the casual skeptics.

      I’ve been thinking that a simple model, showing how all the different lines of evidence for nontrivial climate sensitivity fit together, and how measurement problems affect the answer, might immunize some of the more sophisticated casual skeptics against the BS they get from places like Fox News and the WSJ editorial page. I doubt that’s sufficient to solve the problem, but it might be a useful step.

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