Changingspatio-temporaltrends of heat wave and severe heat wave events over India: An emerging health hazard

Heat wave (HW) and severe heat wave (SHW) events are the manifestations of extreme temperature causing an array of impacts on health, ecosystem, and economy. Since the mid-20th century, an increasing trend in the characteristics of heat waves has been observed over India causing an increased rate in human mortality. Our study aimed to analyse monthly, seasonal, and decadal variations along with long-term trends of HW and SHW events for pre-monsoon (March-May) and early summer monsoon (June-July) season during 1951-2016. HW and SHW events were identified using revised criteria given by the India Meteorological Department (IMD) using daily gridded maximum temperature data at 0.5 degrees x 0.5 degrees resolution from IMD. The study found a Spatio-temporal shift in the occurrence of HW events with a significantly increasing trend in three prominent heat wave prone regions that is northwestern, central, and south-central India, the highest being in West Madhya Pradesh (0.80 events/year), while a significantly decreasing trend was observed over an eastern region that is Gangetic West Bengal (-0.13events/year). SHW events showed a southward expansion and a spatial surge during the decades of 2001-2010 and 2010-2016. Tri-decadal comparative assessment shows a decadal increase of around 12 HW and 5 SHW events post the 1980s. State-wise Pearson's correlation between HW/SHW events and observed mortality reveals that the eastern coastal states that are Odisha and Andhra Pradesh show a significant positive correlation of 0.62 and 0.73, respectively. This significantly increasing trend in HW and SHW events may pose a grave risk to human health, predominantly on the vulnerable sections of the society. Heat waves need to be recognized as a potential health risk and demand further study, robust preparedness, and policy intervention.