Abstract
Abstract. On 19 May 2016 the afternoon temperature reached 51.0 °C in Phalodi in the northwest of India – a new record for the highest observed maximum temperature in India. The previous year, a widely reported very lethal heat wave occurred in the southeast, in Andhra Pradesh and Telangana, killing thousands of people. In both cases it was widely assumed that the probability and severity of heat waves in India are increasing due to global warming, as they do in other parts of the world. However, we do not find positive trends in the highest maximum temperature of the year in most of India since the 1970s (except spurious trends due to missing data). Decadal variability cannot explain this, but both increased air pollution with aerosols blocking sunlight and increased irrigation leading to evaporative cooling have counteracted the effect of greenhouse gases up to now. Current climate models do not represent these processes well and hence cannot be used to attribute heat waves in this area. The health effects of heat are often described better by a combination of temperature and humidity, such as a heat index or wet bulb temperature. Due to the increase in humidity from irrigation and higher sea surface temperatures (SSTs), these indices have increased over the last decades even when extreme temperatures have not. The extreme air pollution also exacerbates the health impacts of heat. From these factors it follows that, from a health impact point of view, the severity of heat waves has increased in India. For the next decades we expect the trend due to global warming to continue but the surface cooling effect of aerosols to diminish as air quality controls are implemented. The expansion of irrigation will likely continue, though at a slower pace, mitigating this trend somewhat. Humidity will probably continue to rise. The combination will result in a strong rise in the temperature of heat waves. The high humidity will make health effects worse, whereas decreased air pollution would decrease the impacts.
Highlights
In India, the highest temperatures occur before the monsoon starts, typically in May or the beginning of June
We mainly focus on the area of the 2016 record heat wave and mention other regions, notably the location of the 2015 heat wave in Andhra Pradesh and Telangana
An all-time Indian temperature record was recorded in Phalodi, Rajasthan, India, on 19 May 2016
Summary
In India, the highest temperatures occur before the monsoon starts, typically in May or the beginning of June. As we were writing this article, Wehner et al (2016) published an investigation of the anthropogenic influence on the May 2015 Andhra Pradesh and Telangana and June 2015 Karachi heat waves using the 1- and 5-day mean daily maximum of temperature and heat index As society is adapted to the weather of that location, we show the anomalies relative to a long-term (1981–2010) mean of TXx. The second definition we use is the monthly maximum of the daily maximum of the wet bulb temperature (Sullivan and Sanders, 1974) as a measure that combines heat and humidity and indicates how well the body can dissipate heat through perspiration. At the end we synthesise our findings into a qualitative overview of anthropogenic forcings on the heat waves
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