Wednesday, April 16, 2008

President Bush and greenhouse gases

I haven't been a political blogger, and I'm not about to start now. Yet the news of the past few days does offer ways to illustrate systems concepts I've mentioned before, and so I thought I'd point out what I hope is obvious to all here.

For but one example, take US President Bush's goal of having greenhouse gas (GHG) emissions stop growing by 2025, which is stirring up comment world-wide.

In system dynamics terms, GHG emissions (largely CO2) are a flow, and the amount of CO2 in the atmosphere is a stock. If you recall what I've written before on stocks and flows, you'll see that stopping the increase of a flow does not mean that the stock will decrease; it simply means that it will increase less rapidly.

In other words, even if we do meet this goal, things may well continue to get worse well after 2025, but they will at least get worse less rapidly after then.

I want to show you a little model that demonstrates that behavior, but, to publish it here, I'd like to get the numbers at least close to right, and that would take a bit of research time I don't have tonight. Let me try an analogy, instead; those of you who studied and remember the calculus can probably make a more elegant argument, and those who do system dynamics models can create one on your own in a few minutes (if you have the needed parameters, let me know, or post a pointer to your model).

In the real world, we are emitting CO2 into the atmosphere by breathing, burning fossil fuels, and the like. That stock of CO2 in the atmosphere is growing and threatening climate havoc.

Some of that CO2 is taken out of the atmosphere each year through the action of photosynthesis and perhaps other mechanisms.

According to the science I read, we have too much CO2 in the atmosphere at present, and our global CO2 emissions per year, already above what the environment can naturally purge, are increasing. If that weren't the case, there would be little reason for President Bush's call to action.

Let's look at an analogous situation. For example, let's say you have a bathtub that's three-fourths full of water. The drain is open, but it's partially clogged, and so it's draining slowly.

In addition, the faucet is turned on, putting more water in the tub. It so happens that the water is currently coming into the tub faster than the partially-stopped drain can take it out, so the water level is rising, causing fears for the well-being of the bathroom floor.

The person controlling the faucet is opening the faucet as we speak, letting water come into the tub at an ever faster rate. That person, realizing the risk to the floor, promises to stop opening the faucet anymore in about 15 minutes.

What do you think will happen to the floor?

Even with the rough data I supplied, I hope you can see that the water will rise increasingly rapidly for the next 15 minutes. If the person takes their hand off the faucet in 15 minutes, the water will continue to rise until it overflows the tub (assuming it doesn't overflow sooner). The only way to save the floor is to reduce the flow of water from the faucet to below the flow of water out of the drain before the tub overflows. Even if they started reducing the flow of water out of the faucet now, the water in the tub would still rise until the inflow was less than the outflow.

Of course, this is a silly little example; the real world of GHG emissions is much more complex. Yet the general principle of stocks and flows holds: as long as the inflow exceeds the outflow, the stock will rise.

I'm not about to use this short, informal essay to argue for or against specific GHG or climate proposals or to try to balance climate stability against economic stability. I am suggesting that we all remember the lesson of stocks and flows when we are thinking about or evaluating policies such as these.

PS: Thanks to colleague Wayne Wakeland for, in a totally different situation, reminding me of the effectiveness of simple bathtub models (and I hope it worked here!).

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Blogger Drew said...

Yeah, Bill, well said!!! You are right, don't need a model to make these clear points.


17 April, 2008 11:34  
Blogger Bill Harris said...

Drew, thanks for your comment. Others stopping by might be interested in a related model that Drew did some time ago. I had seen it then and forgotten it when I started this essay, but colleague Jim Pittman reminded me of it, and it does fit quite well with this posting.

17 April, 2008 22:24  

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