The Carbon Bathtub: MIT professor makes a clear climate analogy

There's a brief but interesting article in the December issue of National Geographic that describes a simple analogy for looking at CO2 emissions. In The Carbon Bathtub, writer Robert Kunzig reports on a concept that John Sherman uses with his grad students at MIT: a bathtub filling up with CO2.

Imagine the faucet represents the CO2 emissions we are putting into the atmosphere - currently, around 9.1 billion (that's with a "b") metric tons per year, most of that coming from fossil fuels. The drain represents the approximate 5 billion metric tons of CO2 being absorbed - 30% absorbed by plants and soils, 25% by oceans, 1% by sediments and rocks, and the rest remains in the atmosphere adding to the rise in temperature.

Because we have been putting more into the tub than it can drain out, the tub has been steadily filling up. Currently, the tub is at the 385 ppm level, heading towards the 450ppm level by mid-century. The 350 ppm that has been recommended by some scientific groups still represents 745 billion metric tons of carbon! So, how do we drain the tub and how long will it take?

Unfortunately, with the bathtub analogy, it will take a long time. Even if we were to stop all CO2 emissions tomorrow, the drain works slowly - plant and soil absorption can get maxed out, ocean absorption is slow and rocks and sediments are even slower. It's been estimated that to go from 450 ppm to 350ppm could take centuries.

Discouraging? Well, perhaps, but it can also be viewed as a reason for saying "no time like the present" for making some significant changes. The CO2 emissions issue is not a light switch - flip it and it all goes away. It is our responsibility to future generations to act now and act decisively.

MIT's John Sherman will be at the international Copenhagen Climate Conference in December to help diplomats better understand the problem by illustrating the bathtub analogy (in much greater detail than I have done here). Hopefully, he will have an impact least a potential sensible climate treaty goes down the drain.

Read the article online.