Rise of the heat

soaring mercury Over half of the world's population now lives in urban areas; hence, it is important to understand how the climate and its extremes

Rise of the heat

Sheetal Rathod is a 35-year-old fashion designer, born and brought up in Mumbai. She knows the city like the back of her hand. However, lately, she cannot make much sense of the weather in the metropolis, which she believes is ‘freak and extreme’. “I clearly remember walking daily to school at noon, as I had afternoon school. It used to be hot in the summer months, but I never felt uneasy walking in the afternoon. However, in the last five to ten years, summer season is unbearably hot and I fear getting a heat stroke.

Mumbai’s weather has definitely changed, though I don’t know the reason behind it,” says Rathod. A recent study titled, ‘Changes in Observed Climate Extremes in Global
Urban Areas’ published in Environmental Research Letters, the scientific journal of the Institute of Physics, throws some light on Rathod’s predicament.

This joint research, conducted by  Indian and American scientists, has found that “Urban areas have experienced significant increases in the number of heat waves during the period 1973–2012, while the frequency of cold waves has declined. Almost half of the urban areas experienced significant increases in the number of extreme hot days, while almost two-thirds showed significant increases in the frequency of extreme hot nights.” Of the five years with largest number of heat waves, four were in recent times – 2009, 2010, 2011, and 2012.

Cities are getting hot
Cities are viewed as engines of growth with more than half the global population
residing in urban spaces. In the past, a number of research studies have documented changes in weather conditions across different areas; however, none of these studies have specifically looked at the urban areas. They mostly covered regions, countries, or continents.

In the present study, researchers analysed observed changes in climate
extremes in global urban areas during the last four decades using daily data from selected Global Summary of the Day (GSOD) stations of the National Climatic Data Center (USA), which is the world’s largest active archive of weather data.

To begin with, they identified all the urban areas globally (about 650) with population greater than 2,50,000. This list was then refined based on the area’s proximity to a GSOD station and the availability of complete weather records. Finally, a list of 217 stations (including some Indian cities, such as Mumbai, New Delhi, Kolkata etc) with complete records for the period 1973-2012 was zeroed in on.

Once the data was obtained for the 217 urban areas, the researchers identified
extremes for temperature, precipitation (any form of water – liquid or solid — falling from the sky) and wind, and calculated heat waves, cold waves, as well as individual extreme hot days and nights. Heat waves were defined as periods where the daily maximum temperature was hotter than 99 per cent of days for the period 1973–2012, for a consecutive period of six or more days.

The results showed that there were statistically significant increases in the number of heat waves per urban area during the last four decades. Results also showed a general decline in cold waves, and around 60 per cent of urban areas experiencing a significant decline in extreme windy days.

Around 17 per cent of urban areas experienced a significant increase in daily precipitation extremes, and around 10 per cent experienced a significant increase in the annual maximum precipitation.

Simply put, extreme precipitation means very heavy downpour or wet storm.
“Over half of the world’s population now lives in urban areas; hence, it is important to understand how the climate and climate extremes, in particular, are changing in these areas. Urban areas make up a relatively small part of the global land area; however, they are the centre of wealth, so damage to urban infrastructure could
result in potentially large economic losses,” says Vimal Mishra, professor of civil engineering at IIT Gandhinagar, who is the lead author of the study.

Mishra and co-researchers also did some comparative analysis of increase in temperature and heat waves in urban and non-urban areas and found that “stations located in non-urban areas showed somewhat lower increase in the number of hot days than those located in urban areas”. This could be due to the urban heat island effect, they claim.

Urban heat island is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities.

Growing evidence
There is a growing evidence of rise in temperature in various Indian cities. In 2009, the regional India Meteorological Department (IMD) office at Mumbai analysed 100 years’ weather data, from 1901 onwards, and found a rise of 1.62°C in the average maximum temperature of metropolis (see graph). Even a degree rise in temperature is linked to a surge in various vector-borne diseases.

The same year, Pune-based researchers released their study of long-term temperature trends at four cities of India – New Delhi, Kolkata, Mumbai, and Chennai – and concluded that annual mean temperature in all the four cities has increased. More recently, in 2013, a research study on rising temperature in Dehradun, Uttarakhand has found that “During 1967–2007, annual maximum, annual minimum and annual mean temperatures increased about 0.43°C, 0.38°C and 0.49°C, respectively”.

Another comparative temperature study of three south Indian metropolitan cities – Bengaluru, Hyderabad and Chennai – shows that the temperature of Bengaluru city has increased much more than the other two cities. Researchers of
the University of Mysuru, who conducted this study, claim that built-up land and barren areas have the highest temperature when compared with other land uses, while water bodies and vegetation have the minimum temperature .

Adapt or perish
According to Mishra, the results of his joint study have important implications for policy makers. Increase in heat wave can translate into increased mortality in urban areas. Similarly, increased warming in urban areas will lead to a high energy demand for cooling (air-conditioning). Increase in precipitation means more urban flooding.

Scientists are already pointing towards the huge economic cost of climate change on Indian cities (see table). There is a need to enhance the density of climate stations in urban and surrounding non-urban areas to provide the baseline data that will be essential for climate change adaptation and decision-making.  

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