No abstract

Urban heat island is among the most evident aspects of human impacts on the earth system. Here we assess the diurnal and seasonal variation of surface urban heat island intensity (SUHII) defined as the surface temperature difference between urban area and suburban area measured from the MODIS. Differences in SUHII are analyzed across 419 global big cities, and we assess several potential biophysical and socio-economic driving factors. Across the big cities, we show that the average annual daytime SUHII (1.5 ± 1.2 °C) is higher than the annual nighttime SUHII (1.1 ± 0.5 °C) (P < 0.001). But no correlation is found between daytime and nighttime SUHII across big cities (P = 0.84), suggesting different driving mechanisms between day and night. The distribution of nighttime SUHII correlates positively with the difference in albedo and nighttime light between urban area and suburban area, while the distribution of daytime SUHII correlates negatively across cities with the difference of vegetation cover and activity between urban and suburban areas. Our results emphasize the key role of vegetation feedbacks in attenuating SUHII of big cities during the day, in particular during the growing season, further highlighting that increasing urban vegetation cover could be one effective way to mitigate the urban heat island effect.

The relationship between vegetation phenology and climate is a crucial topic in global change research because it indicates dynamic responses of terrestrial ecosystems to climate changes. In this study, we investigate the possible impact of recent climate changes on growing season duration in the temperate vegetation of China, using the advanced very high resolution radiometer (AVHRR)/normalized difference vegetation index (NDVI) biweekly time-series data collected from January 1982 to December 1999 and concurrent mean temperature and precipitation data. The results show that over the study period, the growing season duration has lengthened by 1.16 days yr À1 in temperate region of China. The green-up of vegetation has advanced in spring by 0.79 days yr À1 and the dormancy delayed in autumn by 0.37 days yr À1 . The dates of onset for phenological events are most significantly related with the mean temperature during the preceding 2-3 months. A warming in the early spring (March to early May) by 1 1C could cause an earlier onset of green-up of 7.5 days, whereas the same increase of mean temperature during autumn (mid-August through early October) could lead to a delay of 3.8 days in vegetation dormancy. Variations in precipitation also influenced the duration of growing season, but such influence differed among vegetation types and phenological phases.

China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where rapid urbanization has occurred. Our estimated warming of mean surface temperature of 0.05°C per decade attributable to urbanization is much larger than previous estimates for other periods and locations. The spatial pattern and magnitude of our estimate are consistent with those of urbanization characterized by changes in the percentage of urban population and in satellite-measured greenness. Land-use changes from urbanization, creating an urban heat island (UHI), have been suspected as partially being responsible for the observed warming over land during the last few decades because of (i) the observed decrease in the diurnal temperature range (DTR) resulting from a larger increase or a smaller decrease in minimum temperature relative to maximum temperature and (ii) a lower rate of warming observed over the past 20 years in the lower troposphere compared with the surface (1). The area-weighted average warming effect of UHI over land during the 20th century has been estimated to be Ͻ0.06°C per century (1-4) globally and approximately 0.06ϳ0.15°C per century (5, 6) in the U.S. based on differences in temperature trends between rural and urban stations. A much larger estimate of 0.27°C per century in the U.S. has been reported recently (7) by comparing trends in observed and reanalysis surface temperatures over the period from 1950 to 1999.China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. From 1978 to 2000, China's gross domestic product grew at an average annual rate of 9.5%, compared with 2.5% for developed countries and 5% for developing countries; the number of small towns soared from 2,176 to 20,312, nearly double that of the world average during this period; the number of cities increased from 190 to 663; and the proportion of urban population rose from 18% to 39% (see the Peopledaily article at http:͞͞english. peopledaily.com.cn͞200111͞27͞eng2001112785410.shtml and the State Family Planning Commission of China web site at www.sfpc.gov.cn͞EN͞enews20030320-1.htm). In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where most of China's urbanization has occurred. Data and MethodsThe UHI effect has been estimated by comparing observed temperatures in urban stations with those in their surrounding rural stations, but such results largely depend on how rural versus urban stations are classified and whether the data are homogeneous (7-9). Population data often are used to identify a station as urban and rural, but such information generally is out-of-date, and thus satellite measurements of night lights have been substituted re...