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Weathermen are noticing that 25 of their observation stations from north, northwest and central India registered major increases in heat waves in the first decade of this century, and five stations – mainly from northwest India – showed an increase in severe heat waves

Data shows that many areas in India have experienced heat waves extending to eight or more days on an average per season. (Image by Bert Kaufmann)

Data shows that many areas in India have experienced heat waves extending to eight or more days on an average per season. (Image by Bert Kaufmann)

Long-term data from 103 weather stations analysed by the India Meteorological Department (IMD) in Pune have confirmed that heat waves are recurring more frequently and with greater intensity. Over the past half-century, from 1961 to 2010, these have increased by a third.

The most recent decade studied, 2001-2010, which is the warmest for India as well as the rest of the world, showed a significant rise in peak temperatures. From 580 such days per year measured by all these weather stations in the previous decade from 1991-2000, the number rose to 670 days per year.

There is no universal definition of a “heat wave”. In India, it is said to occur when the maximum temperature exceeds 40⁰C in the plains and 30⁰C in the hills. Further, the departure from the seasonal average should be 5-6⁰C, while a severe heat wave is 7⁰C or more above average. In temperate countries, the ceiling is much lower. In the US, for example, a heat wave occurs when temperatures exceed 32.2⁰C (90⁰F) for three consecutive days.

Over the past half-century, the IMD has also observed that 25 stations from north, northwest and central India registered major increases and five stations – mainly from northwest India, where the Rajasthan desert is located – showed an increase in severe heat waves.

It found that many areas of the country – north, northwest,central and Orissa/Andhra – have experienced heat waves extending to eight or more days on an average per season. Severe heat waves were mainly experienced over the north, northwest and central parts of the country.

Many areas of west Rajasthan, Punjab, Haryana, east Rajasthan, Madhya Pradesh, Chhattisgarh, Vidarbha, western Uttarakhand, east Uttar Pradesh, western parts of Jharkhand and Bihar, Gangetic West Bengal, northern parts of Orissa, Telangana, coastal Andhra Pradesh, eastern parts of Rayalaseema and north Tamil Nadu on an average have experienced eight or more heat wave days.

India’s biggest “hot-spot”, as measured by IMD stations, is Nellore in Andhra Pradesh. It suffered from as many as 30 heat wave days in 1964and 35 in 1996, the biggest numbers recorded by any station. It figures most frequently among locations in India – as many as 18 years out of 39 – which have registered more than 15 heat wave days in a year over the half-century.

According to the IMD, the mean land temperature “anomaly” for the country – deviation from the average – for 111 years ending 2011 was 0.59⁰C. However, in 2010 alone, this went up to 0.93⁰C, which was the highest of any year in this period.

According to a paper by D.S. Pai and his colleagues at the IMD and Regional Meteorology Centre in Chennai, “Associated with this warming trend, severe heat wave events that killed thousands of people have occurred in the country.” Data was obtained from media reports and IMD’s annual reports on weather disaster events.

In six days in May 2002, 1,000 people were killed in characteristically intense heat in Andhra Pradesh and 200 others died the same month elsewhere in the country. In the following year, Andhra saw the deaths of 1,000 persons, while 250 lost their lives in the summer of 2010 in north India.

As the scientists state, “The recent decade (2001-2010) registered the highest number of deaths due to heat wave events compared to previous three decades.” What is more, they assert that the hot years following “El Niño” – the global phenomenon associated with a band of warm ocean water temperatures that periodically develops off the Pacific coast of South America and can affect the monsoon, as it has this year – have accentuated mortality in the country.

Raghu Murtugudde of the Earth System Science Interdisciplinary Centre of the University of Maryland told that “El Niño peaks in December-January-February and our heat waves are usually during March-April-May when there is no rain and heat is building up. It does not end till the summer of the following year.”

In a paper written with scientists from the Centre for Climate Change Research at the Indian Institute of Tropical Meteorology in Pune, Murtugudde points out that global warming may account for bigger heat events worldwide than El Niños. They select 2009 as an example where the IMD reported a severe drought, with a shortfall of 22% in the monsoon rains, and a 48% deficit in June itself. An intense heat wave that month claimed a hundred lives.

This coincided with Mexico’s worst drought in 70 years and 17 southern US states were similarly affected. Britain experienced its worst heat in three years, while Australia recorded its hottest August, 2.47⁰C above average.

Pai and his colleagues assert that “the huge increase in the heat wave days during the last decade is mainly caused by the increase in the events associated with the three El Niño years (2002, 2004 and 2009) of the decade.” They cite the deaths in the country following El Niño years in 2003, 2005 and 2010 as corroboration of this trend.

However, Murtugudde told, “I would say the link to El Niño would not be the most important factor for the heat wave trends if any trend does exist. An important factor about heat waves is that as you warm the temperatures, say due to global warming, the extremes tend to increase more in percentage.

“First of all, there is no trend in El Niño activities over the last 50 years. Between 1950 to about 1975, there were weaker El Niños with more La Niñas [the cooling counterpart of El Niño as part of the broaderEl NiñoSouthern Oscillation (ENSO)climate pattern] and during 1976-1998 there were a few more and strong El Niños with fewer La Niñas.

“Since 1998, we have had basically cold temperatures in the East Pacific with warming occurring mostly near the dateline during 2002, 2004 and 2009. These unusual conditions occurred during 2009, which is also happening this year.

“The fact that heat waves occur in the year following El Niño is not a big mystery. The El Niño warming peaks in December-January-February and does not end till March-April-May of the following year but this can be confused with the regular spring warming that happens every year in the eastern Pacific.”

He believes that the key pieces of information missing here are:

  • Global warming itself is clearer in the tropics and is more obvious  in the summer months and for India, it will be during March-April-May since the monsoon can cool the temperatures during June-July-August
  • The Indian Ocean itself is warming pretty rapidly so winds blowing from the ocean are warmer now
  • Urbanization and deforestation over the last 50 years are likely contributing to things like the urban heat island effect which can be very severe – up to 3⁰C or much more.
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