EPA gets the bathtub

Eli Rabett has been posting the comment/response section of the EPA endangerment finding. For the most part the comments are a quagmire of tinfoil-hat pseudoscience; I’m astonished that the EPA could find some real scientists who could stomach wading through and debunking it all – an important but thankless job.

Today’s installment tackles the atmospheric half life of CO2:

A common analogy used for CO2 concentrations is water in a bathtub. If the drain and the spigot are both large and perfectly balanced, then the time than any individual water molecule spends in the bathtub is short. But if a cup of water is added to the bathtub, the change in volume in the bathtub will persist even when all the water molecules originally from that cup have flowed out the drain. This is not a perfect analogy: in the case of CO2, there are several linked bathtubs, and the increased pressure of water in one bathtub from an extra cup will actually lead to a small increase in flow through the drain, so eventually the cup of water will be spread throughout the bathtubs leading to a small increase in each, but the point remains that the “residence time” of a molecule of water will be very different from the “adjustment time” of the bathtub as a whole.

Having tested a lot of low-order carbon cycle models, including I think all possible linear variants up to 3rd order, I agree with EPA – anyone who claims that the effective half life or time constant of CO2 uptake is 10 or 20 or even 50 years is bonkers.