The Carlisle Mosquito Online

Friday, February 9, 2007


Carlisle Comments: Overflowing bathtubs and global warming

What does the average person's attitude towards global warming have to do with the overflowing of a bathtub that is simultaneously filling and draining? Well, a lot if you listen to Professor John Sterman, the MIT Jay Forrester Professor of Management, and the director of the System Dynamics group at the Sloan School of management.
On Tuesday, January 23, 11 of us met in front of the First Religious Society to car pool to Lexington for a scrumptious Chinese buffet followed by an engrossing two-and-a-half hour talk by Sterman about what it is in our cognitive faculties that makes Americans so complacent about global warming. In a room at Carey Memorial library holding 120 there was standing room only.

Sterman advanced a compelling argument that the complacency comes from a lack of proper understanding of how to think about graphs and the dynamics of something so simple as a bathtub that is simultaneously being filled and emptied. He gave numerous examples of how his own bright, technically savvy, graduate students flunk tests of the dynamics of "stocks and flows," of cumulative effect. (For more information about "stocks and flows," examples of incorrect public thinking, and the application of "bathtub dynamics" to global warming, visit:

The concept all boiled down to one critical application ­ how to think about the effect that publicly mandated CO2 emission control would have on CO2 accumulation in the atmosphere and therefore on global warming. The global rate of CO2 emission (the input water flow to the bathtub) minus the simultaneous global absorption rate of CO2 (the outflow rate of an unstoppered bathtub) can result in a bathtub that empties, or that stabilizes, or that overflows.

According to Sterman, the present worldwide rate of human-caused CO2 emission is twice that of the ability of the world to absorb the CO2. Assuming that the CO2 absorption rate stays the same (in fact, it is predicted to decline), it is easy to see that the goal of decreasing emissions by 20 percent by 2020, which many proposals have as a target, will not significantly slow the increase of CO2 in the atmosphere. To keep the CO2 level where it is now (and to keep the planet from a runaway heating trend), we would immediately have to decrease emissions by roughly 50 percent or more, according to Sterman.

In summary, stabilizing the world's climate requires that the global CO2 emission rate be cut to the point where it equals the global CO2 absorption rate. Unfortunately, a great deal of increased CO2 accumulation and a great deal of global warming will have occurred before this can be realistically accomplished.

When I was a three-year-old and our family was returning by boat from a public health assignment that my father had in Germany, I was told that I was so seasick that I pleaded with my mother to take the stopper out of the bathtub (the ocean). I didn't know that the bathtub concept would come back 75 years later to haunt me.

New climate-change report
Since I wrote the above article, the Intergovernmental Panel on Climate Change (IPCC) released its first installment on a newly revised assessment of the state of climate change science, the first major revision in several years. In the words of the Union of Concerned Scientists (UCS), "Today's findings [February 2, 2007] conclude that it is 'unequivocal' that the Earth's climate is warming; that current atmospheric concentration of CO2 far exceeds the natural range of the last 650,000 years; and that it is 'very likely' (>90%) that human heat-trapping emissions are the cause." Other language in the report suggests to me that now even greater cuts in human-caused CO2 emission than Sterman has discussed will be needed to keep the earth's climate from going out of control.

2007 The Carlisle Mosquito