This is the kind of story that makes me skeptical about the risk of anthropogenic global warming. I'm sure there are a lot of explanations for why such adjustments are "necessary", but I wasn't born last night.
The US accounts for 6.62% of the land area on Earth, but accounts for 39% of the data in the GHCN network. Overall, from 1880 to the present, approximately 99% of the temperature data in the USHCN homogenized output has been estimated (differs from the original raw data). Approximately 92% of the temperature data in the USHCN TOB output has been estimated. The GHCN adjustment models estimate approximately 92% of the US temperatures, but those estimates do not match either the USHCN TOB or homogenized estimates. ...
It should also be noted, that the U.S. Climate Reference Network, designed from the start to be free of the need for ANY adjustment of data, does not show any trend, as I highlighted in June 2015 in this article: Despite attempts to erase it globally, "the pause" still exists in pristine US surface temperature data
(HT: Power Line.)