Temperature rising trends at Japanese cities during the last hundred years and their relationships with population, population increasing rates and daily temperature ranges.

Abstract

Long-term temperature trends at 60 stations in Japan were evaluated by using monthly data during 1891-1992. The polynomial trends of minimum and maximum temperature were obtained with a least-squares method which included discontinuous changes due to site changes. The minimum temperature (Tmin) is found to show a rising trend with an average rate of about 2-5°C/(100 years) at large cities and about 1°C/(100 years) at medium-sized ones. The rising rate (T↑min) tends to be higher after World War II than before, but some cities in Hokkaido show exceptionally high rising trends of Tmin in the early part of the analysis period. Changes in maximum temperature (Tmax) are smaller, although a weak rising trend before World War II is found for many cities. The dependence of the temperature rising rate on the city size (population of cities or towns) was examined. It is found that T↑min is positively correlated with population parameters with only slight differences in correlation coefficients according to their functional forms (the logarithm and low powers). There is also a weak correlation between the rising rate of maximum temperature (T↑max) and the population parameters. A similar analysis was made for the increasing rate of the population parameters. There is a positive correlation between T↑min and the net increasing rate of the population adjusted to the present area of the city. On the other hand, there is little correlation between T↑min with the nominal increasing rate of population without adjustment for changes in the area of the city. Analysis was also made about the relationship between T↑min and the daily temperature range (R), which was regarded as a measure of the intensity of nighttime cooling. It is shown that T↑min has a positive correlation of 0.3-0.5 with R in old times, such as that in the 1910's, but is hardly correlated with R in recent years.