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Market

Climate Change and the Stock Market

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No, I am not going to say how climate change impacts the stock market or even more far out how the stock market may impact climate change.  The point here is that the two phenomena share a statistical property that makes both controversial and difficult to interpret.

To get started, take a look at the chart below.  It plots the return on the market minus the return on 30-day Treasury bills over the 101-year period from 1927 to 2018.  The data are taken from Prof. Ken French’s website.  Basic analysis of risk and return predicts that the average return on the market would be significantly greater than the average return on Treasury bills – and it is.  But try to see that from the chart.  The problem is that the underlying relation, that the average return on the market exceeds that on 30-day Treasury bills by more than 8% per year, is dwarfed by the variability of the market returns.  It takes careful analysis and more than forty years of data to identify a statistically significant difference between the two returns.  Trying to draw conclusions from short-run variation in stock prices is a hopeless endeavor.

The same is true of climate change.  Serious long-run statistical analysis leads to the conclusion that human industrial activity has led to, and continues to lead to, a slow inexorable increase in average global temperatures.  This is analogous to the long-run difference between the return on the market and the return on Treasury bills.  But there is immense short-run and location dependent variation in weather.  The California drought, for example, may be due in some part to climate change, but most of it is probably due to random weather variation.  California had prolonged droughts well before the industrial revolution.  What makes interpreting the market or the climate so difficult and controversial is that we humans have short lives and shorter attention spans.  We want to understand things in terms of what is happening now, or at least in terms of what happened recently.  Unfortunately, in the case of the stock market and climate change that is not possible.  The only way to understand either is careful statistical analysis of long-run data.

Explaining Stock Market Moves After the Fact

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Whenever the market moves sharply, the media feel compelled to offer an explanation.  Sometimes this is reasonable.  For instance, a sudden increase in reported inflation is likely to be bad news for the market.  But much of the time, there is no logical explanation for why prices have moved.  Steve Ross, the seminal financial theorist, said it best before his untimely death in 2017.  Professor Ross put it this way,

It is one thing not to be able to predict what asset returns will be since they will depend on news, and news, by definition, is information that has yet to be revealed.  It is another, though, to observe the movement of prices and not know why they moved after the fact.  I am particularly troubled that contemporaneous news seems to explain so little of the contemporaneous prices.

By contemporaneous news, Professor Ross had in mind the revelation of actual value relevant information such as announcement of last month’s inflation rate, not ex-post speculation or what biologist Stephen Jay Gould called “just so stories.”  Examples of such just so stories include things like rising fear of inflation and panic selling.

When most of a company’s value is attributable to growth options the problem is compounded.  Market values rest not on what a company is doing today, but what it could do in the future.  As we have noted repeatedly in this blog, dreams of the future can evaporate for no apparent reason.  The last six weeks have been a good reminder of that.  Why they evaporated at this particular juncture is anybody’s guess.