Changes in Hot Days and Heat Waves in China: 1961-2007

This recent (August 2010) study by Ding et al. (2010) obtains climatology of hot days (HD) and heat waves (HW) in China using daily maximum and minimum temperature records at 512 stations during 1961-2007. The HD and HW are defined in two ways: one in which the daily maximum temperature (DMT) exceeds 35C (a criterion applied in nationwide meteorological agencies in China) and the other in a relative sense, DMT > the 90th percentile threshold of a local daily temperature distribution around the day. The study period (1961-2007) is of importance in the present debate on global warming and climate change, as most of the human-activity induced warming is claimed to have occurred during this period and in particular during the recent 30 year-period from 1977 till 2005 or about.

Among some of the important conclusions from this study are:
1. The HD and HW have increased significantly during the study period in most of China, especially over the southeastern coast and northern China, but decreased significantly at some stations in the lower reaches of the Yellow River. The HD occurred most frequently (more than 5 days per year) in monsoonal eastern China and northwest China.
2. Over most of China except northwestern region, the frequency of HD was high during the 1960s and 1970s, low in the 1980s and high afterwards, with strong inter-annual variations. In eastern China, low frequencies of HD occurred from the 1980s to early 1990s.
3. The changes in HD and HW were closely related to occurrence of rainy/cloudy days (extreme hot days rarely occurred on a cloudy or rainy day) and also atmospheric circulation patterns (regional and large-scale) at the inter-annual and inter-decadal scales.

These and few other conclusions of this study need to be examined in the context of present perception and notion that 'a warmer future climate would bring more heat waves and more hot days'. For China (and for other regions of the world as well), there is a considerable inter-annual as well as inter-decadal variation in the number of HD and HW. The decades 1960s and 1970s witnessed higher frequency of HD and HW in China when the global mean temperature was declining. Also large-scale circulation patterns and associated changes in rainy/cloudy days can impact frequency of HD and HW.

This study also refers to several other recent studies on temperature analysis in China. Of particular interest here is a study by Zhang et al. (2008, Theoretical & Applied Climatology 93: 35-43) which analyzes maximum and minimum temperature data of 66 stations located in the Yellow River basin in central China for the period 1960-2004. Zhang et al. obtain significant upward trend of high temperature events for stations in west and north part of the Yellow River basin, while stations in the middle and lower Yellow River basin do not show significant upward trend. Further for most stations of the Yellow River basin, the maximum summer temperature shows NO significant upward or downward trend during the period of study.

In summary, the frequency of HDs and HWs in China has large spatial as well as temporal variability, due possibly to changes in regional atmospheric flow patterns and also changes in local weather pattern like cloud cover, rain etc. Future changes in HD and HW in China and also elsewhere will most certainly depend upon many local and regional features (cloud cover, rain/no rain) and atmospheric flow patterns and NOT on 'human-activity induced' warming alone. A couple of recent papers (Diffenbaugh and Ashfaq 2010; Clark et al. 2010) use regional/global climate models to simulate increasing frequency of heat waves in the US and elsewhere in a warmer future world. In view of the discussion above, such model projections (of increasing heat waves in future) must be interpreted with utmost caution (and lot of grain of salt!).

Additional References
Clarke, R.T., Murphy, J.M. and Brown, S.J. 2010. Do global warming targets limit heatwave risk? Geophysical Research Letters 37 L17703 doi:10.1029/2010GL043898.

Diffenbaugh, N.S. and Ashfaq, M. 2010. Intensification of hot extremes in the United States. Geophysical Research Letters 37 L15701 doi:10.1029/2010GL043888.