Monday, May 26, 2003


Mike Kerrigan has written in to challenge my claim that the Clean Air Act was responsible for the elimination of London’s smog. I knew somebody would challenge that. Julian Simon’s point that pollution was declining in London before the Act was passed is well-known both among libertarians and among opponents of the Greenies. I mostly agree with Simon but I am myself a research statistician and I know how carefully you have to treat trend data. You have to read it in conjunction with ALL the other data on the question if you are to arrive at reasonable conclusions. What Simon overlooks in this is that trends in biological data commonly reach an asymptote (i.e. eventually level out) -- as the trend to cleaner air in London now in fact has. It is precisely the way Greenies ignore this and rely on straight-line extrapolations that normally unglues them. In the London case, the air pollution was indeed declining up to the 1950s -- mainly because of London’s de-industrialization and population decline -- but it seems clear that it is only the compulsory phasing out of domestic coal fires that has allowed the pollution to continue to decline to its present low level.

And if we look at Lomborg’s graph (p.4, PDF file) there DOES in fact seem to be a levelling out followed by sharper decline in both smoke and sulphur dioxide levels in London during the 1950’s. The Act was passed in 1956.


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