After the mid-1990s, the global surface temperature presented a significant warming trend. According to the World Meteorological Organization, the global mean surface temperature of the period from 2011 to 2015 has increased by 0.57° over the period from 1961 to 1990. This warming trend provides favorable background for occurrence of hot summers, and exacerbates extreme heat events. Based on statistics, the casualties caused by heat events from 2001 to 2010 have increased by 23 times relative to those from 1991 to 2000.