As urban areas have grown, the island effect has increased temperatures. Of course there has been no correction for this effect.
Urban Heat Island Effects Have Not Yet Been Removed from Official GHCN Warming Trends – Watts Up With That?
The greater a station’s population growth, the greater the observed warming trend. This is pretty convincing evidence that the raw GHCN data has substantial UHI effects impacting the computed trends (probably no surprise here). Note the UHI temperature trend averages 66% of the raw temperature trends.
The regression line fit to the data intercepting zero shows that those stations with no population growth have, on average, no warming trend.