The 1990 IPCC report showed US daily maximum temperatures plummeting at 1.3C per century since 1930.
The report also showed US daily minimum temperatures flat since 1930.
From this, they should have been able to approximate how much UHI was occurring, by subtracting the maximum temperature anomaly from the minimum temperature anomaly. This is because UHI primarily affects minimum temperatures. Had they done this they would have seen that UHI is causing a 1.2C/century increase in nighttime temperatures.
But instead of doing the right thing and adjusting recent nighttime temperatures downwards (thus ending their global warming scam) they came up with a set of adjustments which cool older temperatures, and warm recent temperatures.