By Donella Meadows

–August 23, 1990–

I have been getting impolite letters from California about bathtubs. You wouldn’t think that would be a subject to cause wrath in readers, but there seems to be something touchy about Californians when it comes to bathtubs.

The whole affair started a few weeks ago when I attempted to educate John Sununu about the greenhouse effect. I pointed out that life on earth emits 110 billion tons of carbon a year through respiration and decomposition, and that green plants absorb 110 billion tons of carbon through photosynthesis. The human economy upsets that balance by emitting 5 or 6 billion extra tons a year — which Sununu had labeled trivial. That’s where I brought in the bathtub analogy.

Here’s what I said: “Picture a huge bathtub half full of water. Now imagine 110 gallons a minute pouring in through the faucet and 110 gallons a minute pouring out through the drain. Water flows through the tub like crazy, but the level stays constant. Now turn the input flow up by five gallons a minute. What happens? The water in the tub starts to rise. It will keep going up as long as that extra five gallons is flowing in — until it spills over and makes a big mess.”

That all seemed pretty straightforward to me, until the responses came back from California: “A simple experiment at home would have convinced Meadows that this is not so. Instead the water level rises to a new height and stays there, and the same is true if the amount is increased by 10 gallons or more…. That Meadows … does not know basic science, has apparently no instinct for it, .. and makes fun of the president’s adviser in a syndicated column is astounding. Is hers indicative of the level of understanding of science which many environmentalists have?”

Another more florid correspondent added, “You had better stick to the soft sciences…. It is a pity that you, with you Bambi biology and your Sunday Supplement Science are lending sustenance to those who feel that females are incapable of the mental rigors required of practitioners in the hard sciences and engineering.”

Well, let’s ignore the tone of those letters and get to the substance. Any time 115 units of something is pouring into a reservoir and 110 is pouring out, that reservoir is going to go on filling. The only way it can rise to a new height and stay there is if there is some FEEDBACK, so that the rising level in the reservoir either closes the faucet down to 110 or opens the drain so that 115 units can flow out.

In the case of bathtubs there actually is such feedback, as observed way back in 1643 by Torricelli, the inventor of the barometer. A greater water depth will increase the pressure and the flow out the drain, somewhat, up to a limit. For a small excess flow, depending on the configuration of the bathtub, the water level may indeed rise to a new equilibrium. For a large excess flow — well, I do not recommend trying that simple experiment at home.

We are actually concerned about atmospheres, of course, not bathtubs. In the case of carbon dioxide there is also a small feedback to the drain, which I mentioned in my communication to Sununu. The rising partial pressure of carbon dioxide in the atmosphere causes more of it to dissolve in the ocean. That safety valve is removing about half of the excess carbon dioxide human beings are emitting.

We are also CLOSING one carbon dioxide drain as we cut, pave, desertify, and poison the world’s green plant cover. And we’re TURNING UP the faucet, as our fossil fuel and wood burning increase.

Whatever the theoretical effect of Torricelli’s principle in bathtubs, we don’t have to argue about what is happening to carbon dioxide in the atmosphere. It is measurable and measured. It has gone up from 270 parts per million a century ago to 355 parts per million now, and it’s rising exponentially — faster and faster. Whatever natural corrective feedbacks exist, they are insufficient. We have unbalanced an atmospheric gas that was once in balance. No one is sure what the consequences will be. So much for bathtubs and the atmosphere. There remains the question of why this arcane topic got everyone so mad.

Sununu made me and many others mad, because he dismissed a problem that seriously concerns us, and he did it with a scientific inaccuracy, which he must have known was flippant nonsense. Feeling that he had violated something important to me, I wrote a disrespectful column, using an analogy that I knew and pointed out was a simplification. Feeling that I had violated something important to them (maybe the dignity of a high government official, maybe Torricelli’s principle), some readers cranked up the disrespect level one more notch and fired back at me.

This is a childish way of fighting. Pointing out that Sununu started it makes it even more childish. I hereby declare a unilateral withdrawal from ad hominem arguments, even if the former governor from New Hampshire goes on making them nearly irresistable. The real issues here, the underlying fears that trigger our anger, are that on the one hand a bunch of environmental alarmists will force unnecessary changes in our fossil-fuel-powered way of doing things, and that on the other hand a bunch of technological conservatives with heavy stakes in fossil-fuel industries will drive the climate and the earth’s ecosystems into overheated chaos.

Those are serious concerns, worthy of continued and intense public discussion — and worthy of the greatest scientific clarity and personal respect we are capable of summoning, whatever side we’re on.

Copyright Sustainability Institute 1990