## 4 Discussion
### The Spatiotemporal Changes of Temperature Trends  
Due to the physical mechanism and thermal modifications of the urban boundary layer, this causes the city center to be warmer than the surrounding rural areas. This phenomenon as termed the UHI effect [90; 91; 92]. Figure 12 shows the time series of temperature in Shanghai from 1960 to 2010 based on in situ observations. It is evident that mean annual temperature, mean summer temperature, and mean winter temperature recorded at stations located in the Ur, OUr and OS regions generally exhibit an upward trend from 1960 to 2010. From the 1960s to the early 1980s, the temperature and change trends among those three regions are relatively synchronized. However, from the mid-1980s, the observed annual average temperature differences gradually become more prominent due to rapid urbanization. The temperature of Ur becomes gradually higher than measurements from the gauges distributed in OUr and OS. As previously indicated, associated with the expansion of the built-up area, the central area has a relatively high degree of built-up area percentage, which inevitably increases the impervious surface cover with its high heat capacity. Therefore, the quantitative relationship betweenthe temperature and the changing rate of the built-up in the Ur, OUr and OS regions during 1979-2010 is further discussed.  
The spatial pattern of mean annual temperature indicates that areas with an upward change trend are mainly distributed over the Ur and OUr regions (Figure 13a). Urbanization and LULC changes may contribute to the spatial distribution difference of the evolution trends of temperature in Shanghai. The largest rise in mean annual temperature occurs in the southwest of OUr, where the change rate of built-up area percentage is the highest. However, it is noted that most of the Chongming Island and northern OUr region had an upward trend with respect to the extreme high temperature and days of extreme high temperature from 1979 to 2010 (Figure 13b,c).  
Figure 14 shows the relationship between temperature and the change rate of the built-up area in the Ur, OUr and OS regions. Although the warming trend for mean annual temperature was observed in Ur, the increase in the built-up area percentage has a poor linear fit for a change in temperature. In contrast, the temperature warming trends for OUr and OS are positively related to the growth of the built-up area (correlation coefficient >0.5). Many previous studies have concluded that land use change caused by urbanization is the primary driving factor for the increase in temperature. [PERSON] and [PERSON] [87] estimated that there was a 0.27 \({}^{\circ}\)C mean surface warming per century due to land-use changes, which is at least twice as high as previous estimates based only on urbanization. However, the days of extreme temperature only show lightly positive relationship with the growth of the built-up area in OS (correlation coefficient >0.3).  
Figure 12: Time series of temperature in Shanghai from 1960 to 2010: (**a**) mean annual temperature; (**b**) mean summer temperature; and (**c**) mean winter temperature.  
Figure 13: Spatial distribution of the trends for: (**a**) mean annual temperature; (**b**) extreme high temperature; and (**c**) days of extreme high temperature from 1979 to 2010.