Adventures in Cause and Effect, Part II

Whenever the public sees a change they are concerned about, they ask why. They want to know the cause. For example, an observation is made that the amount of ozone in the atmosphere near Antarctica is decreasing. At face value, this doesn’t sound alarming, but then someone does some calculations that indicate that decreases in ozone have also been observed in New Zealand and Australia, which means a greater potential for skin cancer for humans who live there. Now the public is concerned. What’s the cause?

The problem with identifying causes in environmental issues like this is that we usually don’t have comprehensive data and the environment is, like the stock market example mentioned in part I, a very complex beast. It may be that there is no identifiable cause either because the observation simply reflects natural variability or because our data are insufficient to determine a cause. Unfortunately, the “no identification” answer is not an acceptable one to society. We have to identify a cause even if there isn’t one that we can find.

I’ll get back to the “ozone problem” in part III because it’s an interesting one that illustrates what is probably the best we can do with not particularly good data. But first, I’d like to point out some examples where identifying cause and effect in environmental problems is a slam dunk and where it isn’t so easy.

The first example I’d like to point out is one that I worked on for a few strange years, sinking islands in the California Delta. It’s a case where cause and effect identification is a slam dunk.

The Delta is a key area for water transfer from northern to southern California. If the Delta water would ever turn brackish, southern California's water supply would be in deep trouble. What keeps the Delta water fresh are a series of islands that essentially keep San Francisco Bay water from migrating inland.

Unfortunately, those islands have been sinking for decades. Some are now as much as 20 feet below sea level. The only reason they haven’t flooded already is that dams surround their peripheries. The Delta is a mess.

Maintaining those dams costs money. And the federal and state governments, until recently, weren’t keen on providing that money (since Hurricae Katrina, they’ve decided that dam maintenance in the Delta is a good idea). If you’re a private landowner you might want to try to find someone with deep pockets to pay for dam maintenance. You have sinking land. Maybe you can find someone to sue for causing that land to sink.

What is the cause of the sinking land? At face value, the cause is a simple one. The islands have soil that is very rich in carbon. About a century ago, the islands were drained for agriculture, which exposed the carbon rich soils to the atmosphere for the first time. When you do that the solid carbon oxidizes away to form CO2, gas. The islands are sinking literally because the soil is turning into gas.

If that’s the case, the landowners are responsible for their own problem. They drained the land. They made the islands sink. But they wanted to blame someone else. So they drummed up another potential cause: oil companies.

The lands of the Delta contain one of the richest natural gas fields in California. These fields started to be pumped many decades ago. When you pump out gas, you lower the pressure of the gas in the subsurface. It’s like letting air out of a balloon. And land sinks as a result. It can sink a tremendous amount. In some areas in the country, land has sunk as much as 20 feet due to pumping of oil and gas.

Now you can see where this is going. The landowners have dollar signs in their eyes. They might be able to sue the oil companies for causing their land to sink. Maybe they’re responsible for most of their land sinking. Maybe they can get them to pay for dam maintenance.

I was asked to determine if it was possible that natural gas pumping was a significant cause for the islands sinking. I looked at the declines in pressure in the natural gas fields. I used some basic theory and made some back of the envelope calculations. The simple answer was no way. It wasn’t possible that natural gas pumping was a cause for much of the sinking. I handed in my report.

It turned out that my report wasn’t enough to satisfy the State of California. I thought it was a slam dunk, but they didn’t think so. Maybe my theory was wrong. I told them that the theory I used had been accepted by the scientific community for decades. It didn’t matter. It was theory. They wanted absolute proof.

This is not a typical state of affairs in the cause and effect game. Usually an expert comes up with a best guess based on his knowledge base, and government accepts that expert judgment. I had to scratch my head more to come up with better proof.

So I did scratch my head. And I came up with an idea to measure how much natural gas had caused land to sink. I told the State of California my measurements – which I thought were a waste of time – would cost about $150K to make. They said fine. I was surprised as hell that they wanted to throw this much money away. But I did the work.

I made my measurements. They showed that natural gas pumping had caused the land to sink by at most four inches. That was it. Four inches out of 20 feet of sinking might have been caused by natural gas. The State was happy. The landowners weren’t.

In this case, I had theory and measurement to show that the islands were sinking due to causes other than natural gas pumping. Later on, I was able to show that the amount of sinking was completely consistent with the amount of carbon in the soil and the rate at which carbon can oxidize into CO2 gas. I had proved that the landowners were responsible for their own mess. No one disputed the results.

Most environmental problems aren’t this easy. The data are vague. The systems are much more complex. The theory isn’t particularly well established. And that’s when both scientists and society can get into trouble. I’ll talk about that more next time.