The recent report of the Intergovernmental Panel on Climate Change (IPCC, 2007) has stated that the warming in the climate system is unequivocal, with an increase in global mean temperature of 0.74 °C during 1906–2005. The amount of warming varies over the globe. Urban areas generally have a warm bias relative to the surrounding rural region, termed the urban heat island effect. Thus the trend calculated from met station data may have a warming bias due to increasing urbanisation (Kukla et al., 1986; Ren et al., 2008; Parker et al., 2010). Accurate estimation of the urban influence on the long-term trend is a challenging task, particularly in densely populated, highly developed countries such as Japan, as it is difficult to find stations that are not influenced by urbanisation. Several US-based studies (i.e., Peterson, 2003; Peterson and Owen, 2005; Parker, 2006; Stone, 2007) found only a small urban bias in their analysis, but significant urban bias (∼0.11 °C/decade) was reported in north and northeast China, mostly after the 1950s (Jones et al., 2008; Ren et al., 2008). A series of studies over Japan (Fujibe, 1995, 2009, 2011 and references therein; Kato, 1996; JMA, 2009) based on temperature and population data focused on how to effectively detect and quantitatively estimate the urban bias in the trends of the last few decades to centennial scale. Although none of the studies in Japan looked for evidence of residual urban bias in the stations that were classified as rural, it was found that the warming rate across Japan varied widely, from 0.35 to 2.95 °C/century (Schaefer and Domroes, 2009) during 1901–2000. The average trend for all 60 Japanese stations is 1.41 °C during 1900–1999. This value is higher than many of the estimates of mean average warming that have been made for other countries: for example, the United States (∼0.4 °C/century, Knappenberger et al., 2001), United Kingdom (0.64–0.77 °C during 1861–2000, http://www.metoffice.gov.uk/climate/uk/about/UK_climate_trends.pdf), Spanish Mediterranean (0.9 °C for the period 1870–1996, Quereda Sala et al., 2000), and rapidly developing countries like India (∼0.7 °C/century, Dash et al., 2007) and North China (1.16 °C during 1961–2000, Ren et al., 2008).
Urban bias on the country scale is normally estimated through the difference in regionally averaged warming between urban and rural sites, so it is important that the urban and rural stations are classified correctly. There is, however, no robust method which selects rural stations with a high level of confidence to eliminate urban influence completely. For monitoring background warming in Japan, the Japan Meteorological Agency (JMA) selected 17 rural stations, hereafter denoted by 17_JMA, that were considered to have not been highly influenced by urbanisation (JMA, 2009). The average warming of these stations was 1.13 °C during 1898–2009, still relatively high compared to global warming and it has been suspected that these 17 stations, some of which have population over 100 000, were not entirely free from the influence of urbanisation (Fujibe, 2009). In this study, we consider alternative methods for choosing rural stations and compare the chosen stations to 17_JMA. We also present possibly the first independent attempt to objectively quantify the extent of the residual urban bias in the rural set of 17_JMA. This estimate is formed by analysing the current generation climate model simulations which contributed to the World Climate Research Programme's coupled model intercomparison project phase 3 (CMIP3) together with observations of warming of the neighbouring ocean region, which is explained in more detail in Section 4. In addition, using this ensemble as a baseline, an estimate is made of the urban contamination in the highly populated metropolitan cities and rapidly growing suburban cities.