Japan is a small country with a very high population density, dominated by large cities like Tokyo.
Most of the GHCN stations there are urban, and show a strong summer warming trend since the 1950’s.
There was a large jump in summer temperatures at the very hot summer of 1994.
I did some analysis of this trend, and noticed that stations away from Tokyo tend to show little of no summer warming since 1994, but stations in and near Tokyo show a lot of warming.
Away from Tokyo, cooling or little warming.
Tokyo – warming.
It looks to me like all of the summer warming in Japan since at least 1994 is due to urban heat island effects – as the population grows, the number of cars grow, the use of air conditioning grows, the amount of asphalt grows, etc. This is quite likely the same problem with many other countries.
The only large temperature record I trust is the US, and that one is massively tampered with to comply with the global warming scam.