Growing up in the 1990s in India meant having seen an ad for a glucose-based drink on television in which the Sun literally sucks the life-force out of children with a giant straw as they are playing. This ad has started to hit closer to reality. India has increasingly been in the grip of more frequent and intense heat waves, with outdoor workers especially struggling with the rising mercury.

A recent study published in Science Advances showed that it wasn’t just India: the whole world is grappling with slower and longer heat waves.

Temperature and circulation

Heat waves have a terrible impact on human and animal life, with increased risk of wildfires, damaged crops, and worse health. Analysing temperatures around the world from 1979 to 2020, Wei Zhang, a climate scientist at Utah State University, and his colleagues studied how they have changed over time.

On average, they found, heat waves have slowed down nearly 8 km/day each decade and lasted longer by about four days — the effects being particularly drastic in North America and Eurasia. Heat waves have also increased in frequency, from about 75 events averaged over 1979-1983 to about 98 over 2016-2020.

“In thinking about heat waves and how they would change in the future, there are two pieces of the puzzle that climate scientists think about,” Rachel White, an atmospheric scientist at the University of British Columbia, said. “One of them is thermodynamics: it’s just about the temperature. As temperatures are getting warmer, heat waves are going to get warmer. The second piece is the dynamics: the atmospheric circulation patterns that cause heat waves.”

There are still some open questions around how those might change in a warming world.”

The heat moves

Previous studies have mostly focused on how frequent heat waves are or how hot it gets during one. In this study, the researchers classified contiguous heatwaves as events with extremely high temperatures, covering more than a million square kilometres, and lasting for longer than three days. They then tracked the movement of these huge masses of hot air over space and time, studying how far and how fast they were moving – one of the first groups of scientists to do so.

Instead of just focusing on the frequency and the intensity of heatwaves, the study also checked how fast they were propagating and how long they lasted. By looking at how heat waves move over time and space, Dr. White believes the study has bridged the gap between the thermodynamic and dynamic pieces of the heat waves puzzle a little more than before.

“This study is looking at heatwaves like an object that can move and can travel and propagate, which you would miss if you were just looking at one point,” she said. “If you just look at one point, you can be like, ‘oh, the heat wave lasted for 5 days’. But the object itself lasted for longer, it just moved. That’s what they are doing here, tracking them as they are moving, which is cool.”

The guiding hand falters

But what could be causing them to move so sluggish? The scientists analysed the upper atmosphere’s air circulation patterns, to see how the moving air could affect these big blobs of heat. They found that over the years, the jet stream — a fast, narrow current of air that flows from west to east high up in the troposphere — has become weaker.

The jet stream guides atmospheric waves, waves that are caused by the earth’s rotation and which influence the earth’s surface temperature. As the jet stream weakens, these waves also move more slowly, leading to more persistent weather events, and more spells of high and slow-moving heat.

To check if human activity had played a role in this outcome, the researchers ran simulations with temperature data from 1979 to 2020, but included scenarios with and without human greenhouse gas emissions. They found that though natural climate variability and natural events also influenced how heat waves had changed, human activity and greenhouse gas emissions have played a dominant role in rendering the slower-moving and longer-lasting heat.

Dr. White said the next steps would be to further tease apart the role of atmospheric air circulation patterns in contributing to heat-wave dynamics on the ground. Country-specific changes in heat waves over time would also be some of the missing pieces of the puzzle she would like to see. “I think there’s just a lot that can be done with this dataset, now that they have created it,” she said.

Heat waves are changing

Dr. Zhang does plan to delve deeper into regional differences as part of the group’s next steps, while also working on climate adaptation strategies. “Given that heat waves have such a huge impact on human health and the environment, we need to think about climate adaptation,” he said.

In densely populated urban areas, some strategies to better mitigate changes in heat waves would be to plant more trees and increase green infrastructure – an undertaking Dr. Zhang has himself been involved in. Together with Tree Utah, an NGO, he has been engaging people in planting and taking care of trees. He has also been teaching a class on Climate Adaptation Science at Utah State University, where he helps students learn and apply climate adaptation strategies, with projects like working with farmers on alternative crops.

“This paper is another form of evidence that climate change is altering these extreme weather events,” Dr. Zhang said. Adding to the already long list of studies, like how the intensity of hurricanes has increased or how there is extreme precipitation, this study, in Dr. Zhang’s words, “is another signal of how climate change could influence our daily lives, our health, our environment — by changing the behaviour of heat waves.”

Rohini Subrahmanyam is a freelance journalist.

A man covers his head with cloth to protect himself during a heat wave in Bhubaneswar, April 24, 2024.
| Photo Credit: Biswaranjan Rout/The Hindu

The complexities of the ways in which global warming manifests in local weather continue to underscore the need to model globally but predict locally.

The waning phase of the strong El Niño of 2023 brings the expected warm temperatures across the globe — while cooler temperatures spread from Pakistan across India to West Bengal during March. This band remained cool throughout 2023 even as record temperatures made relentless headlines.

What do the heat waves have to do with global warming?

Global warming also creates unique features locally that modulate heat waves on top of cool background temperatures. Heat waves over India have been of special concern this season because of the general elections. Some persistent circulation patterns have been creating heat waves and this pattern should serve as another focal point for improving predictions.

It was apparent in March that the anticyclonic circulations over the North Indian Ocean were the drivers of unusual rainfall over Odisha. An anticyclone has winds moving in a clockwise direction, with air sinking down in the middle of it. As this air hits the ground, it is compressed and warmed and can create a high pressure heat dome. An anticyclonic circulation could also explain the historic Dubai floods of April 17.

And these anticyclones exist over the North Indian Ocean and the Indian subcontinent even now.

What links anticyclones to heat?

The persistence of the anticyclones is not unusual in and of itself. During the pre-monsoon season, the upper-level Indian Easterly Jet (IEJ) begins to take shape in the upper atmosphere, at around the 10 degrees N latitude, across the Arabian Sea, peninsular India, and the Bay of Bengal. A strong westerly jet exists to the north around 30 degrees N, and the two together can generate an anticyclonic pattern over the Indian Ocean and the Indian subcontinent.

An easterly jet refers to strong winds coming from the east while westerly jets come from the west. These are natural seasonal features. The westerly jet is pushed north during the monsoon season and the IEJ dominates the Indian subcontinent. During the pre-monsoon season, a strong anticyclone can bring dry and hot weather over many parts of India while a weak anticyclone produces milder weather.

The key question then is whether the anticyclone is strong this year and if that is related to global warming and, thus, the heat waves.

How are heat waves amplified?

The pre-monsoon season is India’s summer and heat waves are to be expected. The focus is always on predicting them accurately and providing early warnings to save lives. The background drivers of the duration, intensity, and frequency of heat waves are helpful to identify the hotspots of heat waves at the timescales relevant to the evolution of the weather and the climate.

The record warming of 2023 has so far not been fully explained since it was much warmer than what we expected just from the superposition of El Niño on global warming. But the impact of the El Niño during its pre-monsoon demise on the IEJ tends to produce a stronger and more persistent anticyclone and thus longer lasting and more intense heat waves.

So, the heat wave season this year is consistent with the warmer temperatures due to the El Niño itself as well as the ‘steroids’ being added by the unexplained warming of 2023.

This background state of cool seasonal temperatures but a strong and persistent anticyclone is important. It can help the India Meteorological Department ensure predictions are done with accurate background conditions and build the early warnings accordingly.

What are the stages of early warnings?

Returning to the local manifestation of global warming: accurate early-warning systems take a three-step approach called the ‘ready-set-go’ system, under the so-called ‘Subseasonal-to-Seasonal Predictions’ project of the World Climate Research Program under the World Meteorological Organisation. India is part of this project, has invested heavily in S2S predictions, and has made impressive progress in improving the accuracy of predictions.

Preparing the system and guiding the National Disaster Management Agency (NDMA) requires this three-step approach to function efficiently and effectively. Considering there are more than 1.2 million polling stations for the general elections this year, the optimal use of resources to prepare for, mitigate, and recover from extreme events requires location-specific information at each step.

The ‘ready’ step provides a seasonal outlook — where the background state, or the external factors (such as global warming and the El Niño), are used to maximise the accuracy of longer-lead forecasts. The ‘ready’ step allows the NDMA, its local agencies, and all local governments to ready their disaster response systems.

The subseasonal predictions refer to the extended range of weeks two to four, which contribute to the ‘set’ step. Resource allocations and identifying potential hotspots to move resources including personnel ensure disaster-preparedness is set to go.

The ‘go’ step is based on short- (days 1-3) and medium- (days 3-10) range forecasts. At this step, everything hits the road to manage a disaster, including rescue efforts, hydration centres, heat shelters, etc.

How’re preparedness and recovery faring?

All evidence suggests India’s prediction system and early warning systems continue to improve and the NDMA has worked these details well into its ‘ready-set-go’ system.

The remaining challenges are to build resilience for the future by better predicting the trajectory of the weather at every location over India. This is a significant challenge but budding efforts for predictions at 10-year timescales have shown promise.

The coordination from national to neighbourhood levels and early-warnings from days to a decade are taking shape. Governments, their departments, and the people at large need to be trained and engaged with to make this a sustained success. India’s dream of sustained economic development depends on this.

Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.