Thermal stress over north-western India: Climatology, trends and extremes

Abstract

Climate change, driven by both natural and anthropogenic activities (i.e. industrial and vehicular emissions, deforestation etc.), is a worldwide concern due to its adverse impacts on various systems. It is causing a significant rise in the global average temperature, resulting in the increase of frequency of extreme events such as heat waves. These extremely hot weather conditions severely impact human society's productivity by lowering performance and increasing heat-related illnesses such as cardiovascular disease and heat stroke. This leads to an increase in both morbidity and mortality. Therefore, it is crucial to examine the effects of heat stress on human life. When our bodies overheat, they try to cool down by using a variety of thermoregulation mechanisms, including sweating, which is not solely dependent on air temperature. Humidity also plays a role, as the evaporation process is influenced by the moisture content of the air around us. As a result, researchers all over the world are focusing more on thermal indices that contain a variety of other parameters (such as humidity, insolation etc.) in addition to air temperature (dry).The Universal Thermal Comfort Index (UTCI) is one such index that we utilised to investigate the level of discomfort due to heat stress in the Indian subcontinent for the summer season (April - July) over the period 1981- 2019. We particularly focussed on NW Indian region because this region showed the highest UTCI during summer season. Also, the temporal trend analysis for the past 39 years showed that UTCI value over the NW India (+0.024 Kelvin year- 1 ) is increasing at a faster pace than the entire region (+0.017 Kelvin year- 1 ). Soil moisture was found to be lowest across this region which highlights the semi-arid nature of the climate over this area. These pre-existing dry conditions play a major role in increasing surface temperature and related surface radiative fluxes during the summer season. Homogeneous increasing trends for UTCI, surface temperature, and relative humidity were observed over our study region. From composite analysis, the soil temperature was observed to be higher on extreme thermal stress days than on normal days, resulting in a higher SSHF and subsequent more warming of the surroundings. Because of the pre-existing dry conditions, soil moisture and latent heat flux were found to be particularly low over NW India (hotspot region). Strong synoptic winds coming from Arabian Sea during extreme thermal stress days are the primary source of moisture over the region, which could be causing the high thermal stress in combination with high air temperatures. This research is crucial because it can be an important input in the development of various policies and initiatives to mitigate and adapt to future heat stress events.