Hydro-climatic Change in Japan (1906-2005): Impacts of Global Warming and Urbanization

Hydro-climatic change in Japan from 1906 to 2005 has been analyzed using local climate data from four large metropolitan areas, four cities, and eight rural areas. Mean annual and seasonal air temperature records (Japan Meteorological Agency, JMA) show linear warming trends at all 16 study sites with a strong dependence on population (density). Over the 100 year period investigated, the average warming has been the least (1.06 degrees C) at the rural sites, higher (1.77 degrees C) in the urban areas and cities, and highest (2.70 degrees C) in the large metropolitan areas. The more sparsely populated rural sites had warming trends from 0.73 to 1.24 degrees C per 100 years. In the business district of Tokyo, an average warming of 3.07 degrees C in 100 years was recorded. Warming in Japan has been higher in winter than in summer, and has accelerated significantly since 1981. Average warming at all 16 stations was 3.1 times higher in the recent 25 years (1981-2005) than in the last century (1906-2005). The 1906-2005 average warming at the rural sites (1.06 degrees C) was higher than the global warming reported by the IPCC (0.74 degrees C). Mean annual precipitation has decreased, on average, by 3% (60 mm) number of days with precipitation by 8% (29 days) at the 16 study sites in 100 years (1906-2005), and average daily precipitation intensity has increased by 4%. Annual precipitation amounts have changed the most (7%) in medium sized cities, and the least (2%) at rural study sites; they have also been higher in the warmer south (8%) than in the cooler north (1%) of Japan. Precipitation intensity increases are uncorrelated with air temperatures or their increases. Changes in precipitation from 1906 to 2005 in Japan are more likely caused by global climate change rather than by local urban heat island effects.