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The GHCN US temperature record shows a large divergence between temperatures taken at airports, and those taken at the rest of the stations. This is most likely due to the introduction of pavement.
The first airplane flight was in 1903, so we can probably assume that most of the airport stations were very rural at the time – thus the much lower temperatures. As asphalt and pavement was added, the temperatures rose.
Next, look at the systematic divergence between the two data sets. What happened after 1990???
h/t to Michael D Smith for the suggestion.
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