Hotter Himachal

From April 1 to June 20 this year, Himachal Pradesh experienced 28 heatwave days—the highest-ever on record for the three-month period. Shimla recorded its highest temperature for the season at 30.6°C in mid-June, according to data released by the India Meteorological Department (IMD), while Una, Bilaspur, Hamirpur and Neri all saw temperatures soar over 40°C in May and June.

The impact has been wide-ranging for the fragile mountain state, from diminished tourism to widespread forest fires. During the three months till June, the state recorded over 2,700 forest fires, which is the highest since 2007, as per its Forest Protection and Fire Control division. The flames destroyed some 23,000 hectares. All this, as the state was still reeling from a warm and dry winter that led to depletion of waterbodies and poor horticultural production (see ‘Where is the snow’, Down To Earth, 1-15 February, 2024).

While 2024 may have set some temperature records, heatwaves in Himachal Pradesh are, in fact, on a rising trend for the past four decades, according to a study by researchers with Dr YS Parmar University of Horticulture and Forestry, Nauni, Solan. Using the daily maximum temperature data for 1984-2023, recorded at the University’s agromet laboratory, the researchers find that more heatwaves and severe heatwaves are being seen in the winter-spring months than in the summer. According to IMD, a heatwave occurs if the maximum temperature of a station exceeds by 4.5-6.4°C, or in absolute terms, crosses 45°C. A severe heatwave, on the other hand, occurs when temperatures exceed 6.4°C above normal, or cross 47°C.

From 1984 to 2023, Himachal Pradesh experienced 669 heatwaves (see ‘Warm through the year’). While February topped the list with 110 heatwaves, January saw 106 events and March 105 events. In contrast, September saw the least number of heatwaves (5) during the period studied, followed by November (10) and August (11). All months, except June-August, showed an increasing trend in the number of heatwaves.

This indicates that the winter temperature in the state is increasing at a higher rate compared to the summer, notes the analysis. Moreover, it has been observed that the minimum temperatures are rising at higher rates than the maximum levels. A similar trend is seen for severe heatwaves: February saw 46 events during the period studied, followed by March (36) and January (26). April, May, June and December also saw some severe heatwave events, but none were observed in August-November.

The researchers also note a fluctuation in the temperature data, particularly during the spring months. The most heatwaves, 16, occurred in April 2004, followed by 14 in March 2021. Most severe heatwaves also occurred in 2004, with 12 in March, but the second-most events, 11, occurred in February 2006. Such drastic variation indicates how temperatures are getting unpredictable in the state. Further analysis shows that between 1984 and 2023, the mean number of heatwaves (including severe heatwaves) was 16.72. Some 19 years recorded more events than this mean and 21 years less than this. The numbers fluctuate here too—2004 tops with 49 heatwaves, followed by 1999 (with 42 events) and 2022 (with 38 events). The rate of increase for heatwaves and severe heatwaves was 0.21 and 0.14 per year, respectively.

If the decadal distribution is considered, during the first decade (1984-93), heatwaves occurred every month except November. But in the subsequent decades, they occurred in two peaks over the summer and winter months, shifting from the former to the latter. In other words, through the decades, heatwaves increased in December-March and decreased in March-May. If such a disturbing shift in heatwave patterns over Himachal Pradesh continues, the impacts that are already being seen could aggravate and become year-round in the state.

MS Jangra is principal scientist and Priyanka Jattan is research scholar at the department of environmental science, Dr YS Parmar University of Horticulture and Forestry, Nauni, Solan, Himachal Pradesh. The authors have led the analysis on heatwaves in Himachal Pradesh

This was first published in the 1-16 August, 2024 Print edition of Down To Earth