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The frequency of heat waves and their duration over the main heat-prone areas of the country rose by around 2.5 days in the last 30 years due to global warming, a latest paper by the India Meteorological Department (IMD) showed.
On the other hand, the paper added that the frequency and duration of cold waves have been declining due to higher minimum temperatures.
The paper titled, ‘Meteorological Monograph: Heat and Cold Waves in India; Processes and Predictability,’ said that on an average, heat wave areas experience two heat waves during the season (March-June). They last between five and seven days.
“However, the frequency of heat waves and their maximum duration are increasing, which is due to global warming,” it said. The paper was released earlier this week.
The Met department declares a heat wave when the maximum temperature is above 40 degrees Celsius and 4.5 degrees Celsius above normal. A severe heat wave is declared when the temperature is above 40 degrees Celsius and 6.5 degrees Celsius above normal.
Heat waves usually occur during March-June in central and north-western India (heat wave zone) and in the coastal areas of Andhra Pradesh and Odisha. In this region, the frequency of heat waves is slightly lower than in northern India.
The paper also said that heat waves could even spread to southern India, where no heat waves are currently reported.
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Heat waves and cold waves are extreme temperature events that occur due to anomalies in the atmospheric circulation on a planetary scale and are supported by local factors.
“It is well known that global warming triggers extreme temperature events like heat waves all over the world,” it said.
The study called upon the IMD to start systematic research into the health implications of rising temperatures and humidity.
“We need to establish evidence-based thresholds to develop and activate different responses to heat and cold waves,” it said.
The paper said that a cold wave is said to occur when the normal minimum temperature is 10 degrees Celsius or more and at least 5 degrees Celsius below normal.
If the normal minimum temperature at stations is below 10 degrees Celsius, the deviation should be at least 3 degrees Celsius to be called a cold wave.
Cold waves are generally observed over central and north-western India during the winter season from December to February.
Previous observations suggest that the frequency and duration of cold waves over India are decreasing, possibly due to the increase in minimum temperatures.
There are two types of cold waves in India. One is associated with La Nina, with central and northwest India affected by cold waves.
The second type occurs during the El Nino phase, but such cold waves are generally confined to the extreme northern parts of India.
The paper said that on a short-to-medium term scale, heat and cold waves are predictable up to 5-7 days in advance.
Predicting heat waves on longer-time scales (at least up to two weeks) and on seasonal time scales is possible.
“With unchecked global warming, the probability of compound extremes, such as the simultaneous occurrence of droughts and heat waves, is also likely to increase. Until we have studied the causes in depth, it is difficult to attribute a heat wave to human influence,” the Met paper said.
