Most of India experienced extended extreme heat—with peaks up to 47 degrees Celsius in some areas—from mid-May through early June, resulting in a reported 2,500 deaths. Additionally, over 1,000 deaths are now being reported from a heat wave in neighboring Pakistan.

This extreme weather raises important questions about climate change and resilience: How hot was it? What factors contributed to the high death toll? How did this year compare to previous years? Who were the most vulnerable populations? And, most importantly, what lessons can be learned to help reduce the health impacts of future heat waves, in light of increasing weather extremes, poverty and other environmental pressures?

Extreme heat has severe impacts on the human body. Depending on age and humidity level, prolonged activity in temperatures above 40 degrees C can lead to heat-related hazards like exhaustion or heat stroke. Media reports cited a temperature of 47 degrees C in the northern Indian city of Bamrauli and peaks above 45 degrees C for “days on end” in New Delhi. A heat index, which accounts for heat and humidity, of 65 degrees C was reported in eastern India. While these temperatures were undoubtedly hot (4 to 6 degrees C above normal), it was also the duration of the heat wave that appears to stand out, lasting up to two weeks in some places.

Temperatures at New Delhi’s airport, shown by the gray bars, were 4 to 6 degrees C above historical May maximum temperatures (red line) for 14 days straight. (Source:

Focusing on New Delhi given the availability of monitoring data, we pulled some data from India’s Central Pollution Control Board (CPCB) to confirm temperatures measured in the city, ranging from 22 to 43 degrees C at this particular monitoring station for the period shown.

Even more enlightening in terms of the link to health impacts is an exploration of the patterns of elevated temperatures within the city center or heat island. Previous studies have shown mixed results on whether an urban heat island exists in New Delhi—some showing a heat island effect of up to 3.5 degrees C compared to surrounding areas, and others not identifying such a feature. Satellite-based measurements of the temperature of the land surface in New Delhi during the heat wave show a pronounced (several degrees Celsius) urban heat island at night. The heat island contributed to a lack of nighttime respite from the heat in urban centers, which likely increased the death toll.

Hourly data from the Punjabi Bagh station in New Delhi show consistently high peak temperatures from May 18-31 and increasingly higher nighttime temperatures, particularly from May 21-26. (Source: India Central Pollution Control Board)

Air pollution in India, both indoor and outdoor, is an additional stressor on health, responsible for an estimated 1.6 million deaths per year. The weather patterns across India during this heat wave (some areas experiencing high humidity, others experiencing dry, dusty winds) were conducive to increased air pollution, including ozone and fine particulate matter (PM2.5), also likely adding to the death toll.

Satellite-based measurements of nighttime land surface temperatures in New Delhi show an urban heat island of up to 5 degrees Celsius in the city center, indicating less nighttime cooling with implications for health. (Source: NASA MODIS Aqua)

Ground- and satellite-based monitoring provides a sense of the levels of PM2.5 in New Delhi during the heat wave. Data from CPCB for two monitors in New Delhi show 24-hour average PM2.5 concentrations between 50 and 235 micrograms per cubic meter during the heat wave. For reference, the World Health Organization (WHO) has established PM2.5 guidelines of 25 micrograms per cubic meter for the 24-hour mean and 10 micrograms per cubic meter for the annual mean. The CPCB measurements align with satellite measurements of light scattering that are used as a surrogate for PM2.5 concentrations, which suggest a high concentration of PM2.5 in New Delhi from May 27 to 29. Similarly, a CPCB monitor of ozone in New Delhi showed elevated levels from May 27 to June 4, with an 8-hour mean as high as 267 micrograms per cubic meter on May 27, compared to the WHO guideline of 100 micrograms per cubic meter for an 8-hour mean.

The populations that were reported to perish in highest numbers were the very poor, elderly, outdoor laborers and homeless, likely with pre-existing health problems and a lack of access to relief. In India, extreme poverty is especially visible through the prevalence of slums in urban centers, serving as home to millions in New Delhi, with limited electricity and potable water. Informal workers lack protection and are forced to work in the hottest conditions. In climate terms, these are sensitive populations. Impacts to infrastructure (e.g., electricity shortages, buckling roads) also impact sensitive populations relying on associated services such as from hospitals.

Ground-based measurements of fine particular matter show unhealthy concentrations within many parts of the New Delhi for the entire period shown, but especially on May 28. (Source: India Central Pollution Control Board)

Higher adaptive capacity would allow the most vulnerable populations to take measures to protect themselves, although the adaptive capacity of impoverished populations can be limited. At a community level, local meteorological services predicted this heat wave and communicated warnings to the public. Efforts were made to deliver water to those most in need; electricity shortages, although not necessarily worse than normal, were also experienced by some populations. The degree to which the targeted assistance reached the most sensitive populations is unclear. Media reported that many workers were unable to avoid outdoor work as advised, and that others did not believe the severity of the forecast.

The death toll of 2,500 indicates that there is a need to assess heat waves, the contribution of other stressors such as air pollution, and find ways to build resilience. Heat waves in India are projected to become longer, more frequent, and more intense, due to climate change. The question then becomes where and how to focus additional climate adaptation resources to have the greatest positive impacts.

Satellite measurements of light extinction and scattering are used to estimate fine particle concentration. Here, the New Delhi area is circled which shows high Aerosol Optical Depth values (unitless) for May 27-29, 2015. (Source: NASA Terra MODIS, via Giovanni Interactive Visualization and Analysis)

Exposure can be reduced through urban greening to reduce the urban heat island, implementing protection for informal workers, and controlling other stressors such as air pollution. Adaptive capacity can be boosted through more effective cooling centers with reliable electricity and cool roof construction. A project in the western Indian city of Ahmedabad tested out some of these approaches, focused on developing “Heat Health Action Plans” and training citizens and healthcare workers on heat stress. An example of a win-win solution would be to outfit cooling centers with rooftop solar panels, providing reliable electricity during typically sunny heat waves without increasing fossil fuel-based emissions that contribute to climate change and harm human health.

In the long term, reducing the impacts of stronger heat waves on urban populations requires strengthening social protection systems and implementing policies to ensure electricity reliability, for example, through distributed renewable energy. In the short term, boosting resilience to heat waves requires clear communication of vulnerability and impact assessments to policymakers who can assess high impact options, with the most vulnerable populations in mind. Heat waves such as this one demand focused attention on understanding and leveraging climate and socioeconomic information to make urban populations more resilient to such tragedies.