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Science Summary: Heat wave in Phalodi, India, 19 May 2016

Published: 28th March 2017 10:51Last Updated: 29th March 2017 14:47

Introduction

On Thursday 19 May 2016, India experienced an all-time record high temperature for any calendar day. The high temperature reached 51°C in the city of Phalodi in the Jodhpur district of the state of Rajasthan. By some accounts it was the third-highest temperature ever documented globally. It was so hot that many residents of this city of about 50,000 simply remained indoors. Those who did venture outside in Gujarat’s Valsad found their sandals sticking to molten roads.

Temperatures were high across much of Rajasthan on that day, with a majority of stations recording maximum temperatures above 46°C. The state capital of Jaipur saw its hottest day in the past 11 years, with a maximum temperature of 46.5°C, while Delhi, India’s capital, reached 46.8°C.

In order to be able to act to reduce disaster risk and plan for the impacts of climate change, decision-makers need to understand the extent to which extreme events such as this are the result of human emissions of greenhouse gases in the atmosphere and related global warming. In response to this need a scientific team comprising Climate Central, the Royal Netherlands Meteorological Institute (KNMI), the University of Oxford and the Indian Institute of Technology Delhi (IITD) – as part of the World Weather Attribution (WWA) partnership, which also includes the Red Cross/Red Crescent Climate Centre and the University of Melbourne – undertook an analysis of whether human-induced climate change had contributed to the heat wave event – to inform decision-makers whether such  heat waves are more likely to happen in the future.

This summary* presents an overview of what is currently known about the changing nature of high temperatures in India. Included is an analysis of the highest daily maximum temperature of the year for Rajasthan, the region that set the recent record. For comparison, the same analysis is provided for the location of the 2015 heat wave in Andhra Pradesh. Using both observational data and models, these analyses aim to offer a better understanding of the role of human-caused climate change as well as other factors, notably aerosols and irrigation. 

*Excerpts from this summary are provided below. For more detail see the full text - download available from the right-hand column.

Key messages

  • Consistent with human-caused climate change, annual mean temperatures across India are increasing.
  • Heat waves in a relatively small area of India are becoming more frequent and more intense, but this is not true for most of the country.
  • On 19 May 2016, the city of Phalodi in Rajasthan set an all-time record for any calendar day, hitting 51°C.
  • This analysis used peer-reviewed methods to see if climate change is affecting the risk of record heat like that on 19 May 2016 in north-western India, and like that of a similar one-day heat event in Andhra Pradesh in May 2015.
  • The analysis did not find that human-induced climate change played a role in these individual heat waves. This runs counter to studies done on similar extreme heat events in other parts of the world.
  • The lack of a detectable climate change trend may be due to the masking effect of aerosols on warming, and on irrigation use.

Methods and Tools

Recent advances in the science of extreme event attribution now make it possible for scientists rapidly to provide an objective, quantitative initial estimate of the relative contribution of global warming to specific classes of extreme weather (heat waves, cold waves, drought and heavy rainfall). Using peer-reviewed methods, these analyses provide estimates of the return-time period of the event both today and in the past – before there was a strong human influence on the climate system. The ratio of these is a measure of the extent to which climate change affected the likelihood of the event. Overall, extreme event attribution can provide valuable information to decision-makers faced with difficult and critical questions about changing risks and underpin adaptation strategies at a more local level.

To assess the potential link between the highest daily maximum temperature of the year in north-western India or Andhra Pradesh and human-caused greenhouse gases in the atmosphere, the World Weather Attribution team  conducted independent assessments using three peer-reviewed approaches: statistical analyses of the historical temperature record; the temperature trends in global coupled climate models; and the results of thousands of simulations of possible weather with a global climate model. Applying multiple methods provides scientists with a means to assess confidence in the results. 

Conclusions

The analysis found that human-induced climate change increased the risk of 51°C maximum temperatures to be exceeded in the grid box around Phalodi in 2015 from a 1 in 20–30-year event to a 1 in 7–10-year event, so a small increase in probability. At the same time, an event of this magnitude is expected to occur only every 40–49 years in a year of the same overall warming as 2015, but without the specific sea surface temperatures that were observed. The influence of the sea surface temperatures, and thus natural inter-annual variability on the likelihood of an event like this to occur, is therefore higher than the influence of human-induced warming.

In conclusion, the two approaches that are trusted give an increase in probability of heat waves in both regions, but the increase is not statistically significant and may be due to a lack of data homogeneities (trends in missing data) and remaining model inaccuracies. This is in contrast to other parts of the world, such as Europe, where the role of human-induced climate change in heat waves is very pronounced. The question is therefore why the increase is so much less, and in some regions absent. Two factors can counteract the warming effect of greenhouse gases: the cooling effects of aerosols and of irrigation. 

Role of Aerosols

Increasing concentrations of aerosols in the atmosphere block sunlight and so cause a cooling effect on the ground. There is qualitative evidence that this effect has been operating in India over the past few decades.

Based on the qualitative analysis of trends in aerosol optical depth and highest daily maximum temperature of the year, it is likely that aerosols are partly offseting the surface warming trend over India – particularly when it comes to maximum temperatures in the pre-monsoon season. Optical depth over the period 2003–2015 is decreasing (clearer skies) to the north and increasing (more pollution) to the south. There is general agreement between the decreasing trend in optical depth and increasing trend in maximum temperature. This hypothesis is supported by other research.  

High temperatures with polluted air kill many more people than high heat alone. This means that the impacts of heat waves may well have become worse, even if the maximum temperatures have not shown strong trends. 

Role of Irrigation

Another effect that can counteract heating is increased irrigation. The extra water availability means that incoming solar energy is used more for evaporation and less for raising temperatures. There is evidence that increased irrigation has decreased dry-season maximum temperatures in many regions of the world. In India, relative humidity has increased significantly in the northern and central regions, corresponding with regions that are said to have more irrigation. This trend probably also contributes to the lower or absent trends (observed in the analysis) in pre-monsoon maximum temperatures.

Just as with air pollution, increased humidity also exacerbates the health effects of heat waves. Even if the maximum temperatures do not increase much, the combination of high temperatures with increasing humidity makes the impacts of heat waves more dangerous. 

Vulnerability and exposure

Excessive heat can have a devastating impact on human health, resulting in heat cramps, exhaustion, and life-threatening heat strokes where children and the aged are most vulnerable. It can also aggravate pre-existing pulmonary conditions, cardiac conditions, kidney disorders and psychiatric illness.

High air pollution in India exacerbates many of these problems and for health impacts, heat combined with humidity can also be more dangerous than dry heat. High humidity slows down the evaporation of sweat from the skin, preventing the body from cooling properly. This can be particularly dangerous for anyone who works outdoors. 

The groups most vulnerable to heat extremes are typically the elderly, the very young, the mentally ill and those with certain pre-existing health problems; however, there are gaps in the current understanding of vulnerable groups in the Indian social and economic context. Overall, heat-health impacts in India are probably underestimated due to a lack of reporting and of systematic public record-keeping. 

Further Resources

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10 years of weADAPTweADAPT has been sharing knowledge on climate adaptation for the last 10 years.

Thanks for being part of the community! We're constantly trying to improve this platform and we'd be really grateful if you could take a minute to let us know your opinion.

Have your say