Over the last 100 years, America has added huge amounts of pavement and buildings which keep nighttime temperatures up several degrees.
Despite our best efforts to create warming, temperatures have remained flat – which is no use for fundraising. So we progressively reduce older temperatures, in order to compensate for the fact that they are no use politically until we do.
I’ve read that the white paint on them had discolored and turned darker which increased the heat inside the boxes they used to house the thermometers. Seems like a logical reason to me.
I would agree with the adjustments if they were of the opposite sign to account for the UHI
(and it’s not like they don’t know this. As long as what they produce has to please Tom Karl, it will be the reverse of what they know to be true)