The invaluable pseudonymous Tamino has a brilliant explanation of the causes of the “global cooling” trend in the mid-20th century. There’s nothing new, except the clarity of the writing. So if you’ve ever been stumped by a skeptic who suggests that anthropogenic climate change theorists can’t explain why the planet cooled for the three decades following the Second World War, bookmark this post.
Just a tease:
… the 1940-1975 time period experienced anthropogenic global cooling. This cooling was from the same root cause as volcanic cooling, namely aerosols (mostly sulfate aerosols) in the atmosphere. Whereas volcanic eruptions are natural, and often inject aerosols into the stratosphere (the upper layer of earth’s atmosphere), aerosols from industrial activity are man-made and almost entirely in the troposphere (the lower layer of earth’s atmosphere, where most of our weather takes place).
I will add a few small things:
First, there has been some musing about the accuracy and consistency of the temperature records before, during and after the Second World War, due to changes in how measurements were made. But it is unlikely that any real disparities can fully account for the cooling trends.
Second, Tamino doesn’t mention the causes for the changes in aerosol production. The unprecedented post-war industrial boom explains much of the increase, and the introduction of the Clean Air Act in the U.S. in 1970 (and similar legislation elsewhere in the following years) goes a long way toward explaining the decline in aerosol levels.
Third, there remains a cooling effect from continuing aerosol production. Regulatory efforts around the world — think low-sulfur diesel — will remove more aerosols in the coming decades, thus reducing the cooling effect from fuel emissions and in all likelihood unmasking more of the warming trend from the heat-trapping effects from those same emissions.