Trends in temperature, humidity, and summertime extreme heat in China

Studies of global surface temperature trends provide important information about global climate changes. However, trends in local temperatures, and particularly trends in extreme heat events have a more direct effect on human populations. Extreme hot weather is associated with productivity losses and increased energy demanding. Heat waves are often accompanied by episodes of poor air quality, with attendant human health impacts. Human mortality increases have been associated with temperatures in excess of local threshold values in cities around the world (WHO, 1996).

Few studies have reported long-term changes in the frequency of extreme heat events. In the U.S., small increases in mean summertime temperature during the past half century, accompanied by increases in surface humidity (Gaffen and Ross, 1999), have been associated with significant increases in extreme single-day heat stress events and multi-day heat waves (Gaffen and Ross, 1998). We will present a comparable analysis of temperature, humidity, and summertime extreme heat for China, where heat waves can be a more serious concern due to its large population with insufficient supply of energy.

We have analyzed six-hourly surface meteorological observations from 196 stations in China(Kaiser, 1991) to delineate the climatology and variability of temperature, several humidity variables, and apparent temperature ( a measure of heat stress combining both temperature and humidity). Climatological fields and trends were calculated for daytime, nighttime, and the full day. A relatively stable observing system in China during the late half of the century provides temporally homogeneous data records. Station record with discontinuities associated with station relocation have been excluded from the analysis.

Upward trends are observed in both temperature and specific humidity, with stronger trends at night than during the day, consistent with previous studies of maximum and minimum temperature(Karl et al. 1993, Easterling et al. 1997). Increases in summertime temperature and humidity lead to increases in apparent temperature. Defining extremes as events during which local thresholds of apparent temperature are exceeded, we find increases in the frequency of single-day extreme apparent temperature during both nighttime and daytime. The occurrence of three-day heat waves increases nationwide. In addition to trends, aspects of the low-frequency (interannual to inter-decadal) variability of temperature, humidity, and extreme heat will also be presented.