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Is There a Human Influence on Extreme Temperatures in China?

Wen, Q.H., Zhang, X., Xu, Y. and Wang, B. 2013. Detecting human influence on extreme temperatures in China. Geophysical Research Letters 40: 1-6, doi:10.1002/grl.50285.
This paper (Wen et al., 2013) compares observed and model-simulated spatiotemporal patterns of changes in extreme temperatures during 1961-2007 using an optimal detection method. Four extreme indices (annual maximum daily maximum and minimum temperatures, and also annual minimum daily maximum and minimum temperatures) were studied using model simulation with a second-generation Earth System Model of the Canadian Climate Center (CanESM2). Four external climate forcing mechanisms (solar irradiance, greenhouse gases, anthropogenic aerosols and land-use change) together with combined natural forcing (solar irradiance and volcanic aerosols) mechanisms were considered and combined effects of all natural plus anthropogenic forcing mechanisms were investigated. An optimal fingerprint method based on generalized linear regression was also used in this study to detect the impact of various forcing mechanisms and their combinations on extreme temperature patterns over China. So what did the authors find?

According to the authors, the study identified a "clearly detectable anthropogenic influence on extreme temperatures over China." For warm extremes, greenhouse gas forcing may have contributed about 89% of observed warming of the warmest day-time temperature and 95% of observed warming of the warmest night-time temperature. The study also found that the impact of land-use change in China is contributing to the warmest day-temperature increases.

The main motivation of the study appears to be to "identify human influence (in particular increasing concentration of atmospheric CO2) on extreme temperatures." What is sorely missing in this comprehensive modeling study, however, is a standard synoptic climatology analysis of extreme temperatures to find out if large-scale circulation patterns and their changes could explain these extreme temperatures. Yet, such a study has already been reported in the literature (Ding et al., 2010), which analyzed changes in hot days (HD, defined when max temp is > 35°C) and heat waves (HW, defined when max temp is > 35°C for three consecutive days) in China using temperature data over the same period (1961-2007). The Ding et al. study (which uses 512 stations in China) finds a remarkable increase in HDs in all regions after the 1990s, but this increase seems to be linked to changes in large-scale circulation patterns. In addition, the frequency of HDs was high during the 1960s and 1970s, low in the 1980s and high afterwards. Also the number of days of "trace and light rain" decreased significantly in recent decades and this led to an increased number of HDs in most regions of China.

Another study (Zhang et al., 2008) analyzes extreme maximum and minimum temperatures at 66 sites in the Yellow River Basin. This study found a significant upward trend in the frequency and intensity of "high temperature events" in the west and north part of the Yellow River, but trends in most stations in the middle and lower Yellow River basin were not significant. The warming in the Yellow River basin was mainly due to significantly increasing winter temperatures, while summer maximum temperatures increased only slightly. Increasing summer droughts in the west part of the Yellow River seems to be the main contributor to the significant upward trend of extreme hot events. Yet, once again, large-scale circulation patterns and associated changes (rain OR no rain situations) were able to explain changes in high temperature events without having to invoke any link to anthropogenic warming. Interestingly (or unfortunately?) none of the above two comprehensive studies were referred to or cited in the Wen et al. study.

In summary the conclusion by Wen et al. that "human influence is detectable on extreme temperatures in China" appears to be based on model-driven analysis that does not represent important observed realities, that large-scale circulation changes have led to fewer rain/cloud days, and subsequently, to extreme warm temperatures. These large-scale circulation changes are governed by several parameters few, if any of which, include anthropogenic (CO2-induced) warming. As such, the conclusion that 'increasing extreme temperatures over China are linked primarily to anthropogenic warming" is not well substantiated.

Additional References
Ding, T., Qian, W. and Yan, Z. 2010. Changes in hot days and heat waves in China during 1961-2007. International Journal of Climatology 30: 1452-1462.

Zhang, Q., Xu, C.-Y., Zhang, Z., Ren, G. and Chen, Y.D. 2008. Climate change or variability? The case of Yellow River as indicated by extreme maximum and minimum air temperatures during 1960-2004. Theoretical & Applied Climatology 93: 35-43.

Archived 15 May 2013