By 1975, scientists had figured out that UHI was warming temperatures by 2-4 degrees
Science News, Vol. 107
March 1, 1975
Already man-made climate changes are clearly evident around cities. Buildings and pavement tend to store more heat than vegetation in the surrounding countryside, hence temperatures can range some 4.0 degrees higher in the summer and 2.0 degrees higher in the winter.
The NOAA UHI adjustment for 1975 is -0.09 degrees, less than 3% of the actual effect.