Irrigation in India is enhancing humidity, which is in turn increasing ‘moist heat stress’ experienced by up to 46 million people, according to a study published in Nature Geoscience. Heat stress occurs when the human body cannot cool itself, and can result from high environmental temperatures alone (dry heat stress) or from high temperatures with humidity (moist heat stress) — the latter is more closely linked to human mortality in India.
Ongoing human-induced climate change is expected to intensify temperature extremes in India that could exacerbate heat stress, especially among those working outdoors. In addition, India’s irrigation area has doubled in recent decades. Cooling due to water evaporating from irrigation systems might partially counteract dry heat stress, although associated changes in humidity that could affect moist heat stress are poorly understood.
Vimal Mishra and colleagues analysed the influence of irrigation on both dry and moist heat stress using a variety of in situ and satellite observations, together with state-of-the-art numerical simulations. They found that although irrigation causes land surface cooling, it also leads to substantially higher surface humidity by reducing the height of the lowest part of the atmosphere, known as the planetary boundary layer. As a result, irrigation mitigates dry heat stress, but enhances moist heat stress.
The authors conclude that recent intensification of irrigation practices in India has enhanced moist heat stress, and the associated risks to human health, across the local region and neighbouring countries of Pakistan and Afghanistan. Similar impacts from irrigation are also expected in other regions with similar semi-arid and monsoonal climates.
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