World / Wed, 15 May 2024 The Indian Express

Explained: The criteria for heatwaves, how climate change is affecting it

A new study has said that the abnormally high temperatures were made about 45 times more likely by climate change. AdvertisementThis is the third straight year that heatwaves in the early part of summer in India have been attributed to climate change. Attribution science is a relatively new field of study that is becoming increasingly important in measuring the impacts of climate change. Climate change is a global phenomenon, and scientists have been extremely cautious about blaming any individual weather event on climate change. But there is plenty of evidence now to suggest that heatwaves are becoming more frequent, intense, and prolonged because of climate change.

The extreme heat in parts of eastern and southern India this April has been attributed to climate change. A new study has said that the abnormally high temperatures were made about 45 times more likely by climate change.

In other words, if there was no climate change, it would be extremely unlikely that such abnormally high temperatures would be recorded at that time.

The research is by World Weather Attribution, an international group of researchers that tries to ascertain whether a particular extreme weather event was caused by climate change.

This is the third straight year that heatwaves in the early part of summer in India have been attributed to climate change. The same researchers had earlier shown that the excessive heat in March-April of 2022 and April of 2023 was also most likely caused by climate change.

Attribution science is a relatively new field of study that is becoming increasingly important in measuring the impacts of climate change. Climate change is a global phenomenon, and scientists have been extremely cautious about blaming any individual weather event on climate change. However, new tools and methodologies developed in the last two decades have made it possible to say whether a particular event was made more or less likely by climate change.

Heatwaves in India

Heatwaves are not defined by high temperatures. They are defined by abnormalities in temperature. For example, a place that normally sees a temperature of 40 degree Celsius during summer is not said to be experiencing a heatwave even if the temperature rises to 42 or 43 degrees. On the other hand, another location would be said to be facing a heatwave even at 35 degrees if its normal temperature during that time is 27 or 28 degrees (See box).

Heatwave days across states in recent years. Heatwave days across states in recent years.

Heatwave criteria. Heatwave criteria.

Heatwaves are quite common in northern, central, and eastern India during summer. But there is plenty of evidence now to suggest that heatwaves are becoming more frequent, intense, and prolonged because of climate change.

Last year, several parts of the country experienced heatwave conditions in February, technically a winter month for India. Maximum temperatures were 5 to 11 degrees higher than normal, easily meeting the criteria for heatwaves. It put the India Meteorological Department (IMD) in a fix because heatwaves are meant to be declared only in the April-July period.

The mean temperature for February for the country as a whole last year was 1.36 degree Celsius higher than normal, making it the second hottest February in India ever. The year 2023 also ended up as the second warmest ever for India.

The heatwave outlook for this year was more grim. Heatwaves at the beginning of summer were likely to be more prolonged, lasting for as long as 10 to 20 days at a stretch in some places, instead of the usual 4 to 8 days. True to prediction, an 18-day heatwave was recorded in Odisha in April, the second longest spell for the state ever, according to an analysis by Climate Central, a US-based climate research organisation.

Gangetic West Bengal recorded the highest number of heatwave days for any month in the last 15 years, the analysis said. Not surprisingly, eastern India recorded its warmest April ever.

On Tuesday, the IMD said a fresh spell of heatwaves was likely to start over northwest India from Thursday.

Impact of heatwaves

Prolonged exposure to heat can result in dehydration and cardiovascular and respiratory diseases, and exacerbate existing weaknesses, even leading to sudden deaths. Data on illnesses and deaths caused by excessive heat is not maintained well in India.

Efforts to collect and collate these data began only about a decade ago. But reliable figures are still not available, and there are wide divergences among numbers reported by various agencies such as the IMD, National Disaster Management Authority (NDMA), the Integrated Disease Surveillance Programme or the National Crime Records Bureau (NCRB).

For instance, the Health Ministry, in reply to a Parliament question last year, said it had information about just 33 heat-related deaths in 2022. But the NCRB, which counts heat-related mortality among accidental deaths caused by forces of nature, reported 730 deaths for 2022. In that same reply, the Health Ministry reported 264 heat-related deaths in the first six months of 2023.

Data collected and maintained by IMD and NDMA showed a sharp decline in the number of heat-related deaths ever since state governments and district administrations began implementing heat action plans. It was evidence for the success of early warnings and heat action plans, but this dataset has been showing a reversal of trends in the last couple of years. This could be because of better reporting or an increase in the severity of heatwaves.

Mitigation of heatwaves

All the 23 states known to be vulnerable to heatwaves now have heat action plans to manage the adverse impacts. Simple measures like the provision of cool drinking water at public places, free distribution of oral rehydration solutions, closure of schools and colleges during peak hours, and providing access to parks and other shaded places have brought relief to large numbers of people and prevented illnesses and deaths in several cities.

However, much more needs to be done, especially because heatwaves getting more prolonged and severe. Local administrations must mandatorily regulate activities even in unorganised sectors like construction. This would ensure that non-essential activities, especially those carried out in the open, are stopped during the worst time. Like schools and colleges, office timings can also be tweaked.

All organised outdoor activities, including sports, would need to be strictly regulated. Some experts lament the lack of adequate funding for the heat action plans.