Confusion over causation

One of the trickiest concepts in earth science has got to be causation.  We like to write things like “earthquakes caused by fracking” or “volcanoes are caused by subduction” or things like that. But it can get a lot more confusing; this is true in spades when we start talking about situations where “causation” is equivalent to “liable for.”

Take “arc volcanoes are caused by subduction” as a starting point.  What does it mean to cause something?  Well, one aspect is that if we remove that feature, then we don’t get the result: if we don’t have subduction, we don’t get an arc volcano.  Another is that if this is the sole cause, wherever that process is taking place, there should be that result: if subduction is the sole cause of arc volcanoes, then we should always find arc volcanoes where there is subduction. In this case, plate boundaries lacking subduction usually don’t have arc volcanoes (how you define an arc volcano is why the answer is a bit of a weaselly one).  But we know of subduction zones lacking arc volcanoes.  So there is something else that is a cause. In this case, it would appear to be the presence of asthenosphere above the subducting slab.

We can get into some more unclear situations rather easily.

Let’s take a step farther out.  Can we say “mantle convection causes plate tectonics“? Certainly mantle convection does not have to produce plate tectonics: we can look at Io and Venus as examples of bodies almost certainly having a convecting mantle that seem to lack plate tectonics, so we know there is something more going on.  But if we turned off mantle convection, would we get plate tectonics?  Probably not.

How about the reverse?  Can we say “plate tectonics causes mantle convection?” Although you will find lots of folks who say no, an excellent argument can be made that mantle convection is largely being driven by the descent of the cold upper boundary layer (subducting slab) and that in the absence of such behavior (as in ice-covered ponds), the vigor of convection would be much less.

The problem here is that we are really talking about two facets of the same process; they don’t so much cause one another as accompany one another.  This is also why we often get tangled in saying that subduction causes plate motions: you can reproduce plate motions in a lot of ways (GPE distribution of plates was used once and worked well; slab suction–where more than the weight of the slab is at play–has been used, etc).  A lot of this can border on fairies dancing on the head of a pin type stuff.

But in the American legal system, causation is critical. Consider “the earthquake caused the building to collapse.” This would sound as though there was no fault in the building; its collapse was caused by the earthquake.  But of course the construction of the building might have been insufficient: “the building’s poor construction caused the building to collapse in the earthquake.” You can see trouble brewing here a mile away.

We can get even farther into the weeds, though, as the case above is one where liability might reside in the adherence to building codes and the like. How about “the disposal well caused the earthquake [that damaged my house]?” For most of us, the increase in seismicity means around many such wells [not all by any means] would mean that, without the well, there would be no earthquakes: the operation of the well was the deciding factor.  But most such earthquakes are the product of a shear failure that usually reflects stresses in the region: in other words, there are other factors in play (which is why only some wells seem to cause earthquakes).  This gets even more complex when you consider that such stresses might be slowly increasing over time (we don’t really know) and so that earthquake maybe would have occurred naturally at some point anyways; in other words, if there had been no tectonic stresses, would there have been an earthquake?  Probably not.  Do we know when a natural earthquake would have occurred? No. Did the well cause an earthquake where none would occur, or make an earthquake occur a bit sooner than otherwise expected, or maybe have the misfortune to come along just as the well was drilled?  We might be able to assign a probability to each of those options, but choosing one? Hard to prove.

The grimmest application of causation might well be in weather disasters.  Did global warming cause the California drought?  Would we have gotten the drought had global warming not occurred? Opinions of scientists are mixed; there have been natural droughts, but models of climate change suggest that they might increase in severity on a warmer Earth. Almost certainly the best we can do is to say that there might be an increased percentage in drought years by XX% and so say that a particular drought owed some fraction of its existence or severity to global warming.  We all would like to know if superstorm Sandy was caused by global warming or not, but the problem is that these transient phenomena are embedded in a complex system with many things going on.

John Muir famously said that “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” Although there are lots of ways to interpret this, it does suggest that a cause might also be an effect.  Simplifying things to the point where you say “X caused Y,” while emotionally satisfying, might be too simplistic.


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