Evaluation of background and urban warming trends based on centennial temperature data in Japan

Abstract

   Monthly temperature data for the period 1916-2010 in Japan were analyzed to quantitatively evaluate background (non-urban) and urban warming trends. In addition to data available from electronic devices, published data books were digitized in order to acquire adequate data at non-urban sites from the early to middle 20th century. To homogenize the temperature time series at each station, adjustments were made to eliminate bias arising from changes in observation times and schedules, and data at non-urban stations recorded with different observation systems were combined using principal component analysis and spatial interpolation. The results indicate that the warming trend of background (non-urban) daily mean temperature was 0.88°C/century averaged over the country, with a larger trend for minimum temperature (1.21°C/century) than maximum temperature (0.67°C/century). Trends were found to be higher at stations with higher population densities in the surrounding area. The anomaly relative to the non-urban trend was found to exceed 1°C/century at highly urbanized stations with population densities of 3000 km^-2 or more, but statistically significant albeit small anomalous warming was also detected at stations in sparsely urbanized areas with population densities of 300 to 1000 km^-2.