The 40°C threshold is hit earlier and lasts longer

40°C is an important threshold for summer in India. With global warming and deepening climate crisis, what is happening to this threshold on India’s temperature map? An HT analysis of the India Meteorological Department’s (IMD) gridded database shows that the 40°C heat threshold now arrives earlier in most places and lasts longer , although there is no significant change in its geographical spread. Here is a detailed analysis of the data.

Faster arrival: Over 90% area that breaches 40°C threshold does so earlier now

For 91.2% of the area that crosses the 40°C threshold, the first day of the breach in the decade 2015-2024 was earlier than during 1981-2010, the period which IMD averages to find the “normal” temperature of a place. For 56.7% of the area that crosses the threshold, the breach is at least a week earlier. This includes Delhi and neighbouring districts – IMD’s gridded data does not allow one to isolate Delhi alone – where the 40°C threshold now arrives 10 days earlier than in the past. Are there places where the threshold is breached later than it was in 1981-2000? This area is small: only 6.5% , and the delay in 5.5% of the area is under a week.

The analysis above should be read with two caveats. Around a quarter of the country’s area - northeastern states, hilly states and Union territories in the north, and coastal areas in peninsular India – does not cross the threshold at all. This does not mean that they are not becoming warmer. Moreover, even lower-than-40°C maximum temperatures in coastal areas can be far more harmful for the human body because coastal areas tend to more humid. Another thing to keep in mind while reading these numbers is that the dataset used here gives average maximum temperature for grids. Each grid is a box bound by a latitude and longitude one degree apart or roughly a 111km x 111km square. This is a large enough area for an isolated breach of the threshold within a grid to be offset by somewhat lower temperatures in the rest of the grid.

Later withdrawal: The last breach of 40°C threshold is also getting extended in most parts

It is not just that the arrival of 40°C+ temperatures is now earlier. The departure of that threshold is also now later than in the past. To be sure, delayed departure is relatively less widespread than early arrival. Only 52% of the area that breaches the threshold sees a late departure of such breaches, with the length of the delays a week or longer in only 17% of the area. There is almost no change in the last instance of the breach in 13% of the area, and the departure is sooner now in 34.5% of area. To be sure, the departure is earlier by less than a week in most places where this happens.

Longer stay: Days of 40°C+ have increased in 80% of area that breaches the threshold

While the area that now breaches the 40°C threshold at least once has not increased significantly between the two periods being compared here, its incidence has become more frequent. The total number of days on which the breach happens has increased in 80% of the area where it happens at all. HT showed this in an analysis published in April 2023 using data for the full year up to 2022. Updating this up to 2024 does not change the broad trend. The area days of the breach -- the number of days for which each grid breached 40°C multiplied by their area -- has also steadily increased every decade.