The city, particularly its periphery, has been gradually warming up, leading to some areas being 8° Celsius warmer than the coolest areas, shows a study.
Researchers from National Institute of Technology-Karnataka, Surathkal, used satellite imagery to tabulate changes in Land Use and Land Cover as well as Land Surface Temperatures (LST) between 1989 and 2017. The study was published in Environmental Monitoring and Assessment in April this year.
Nithya R. Govind and H. Ramesh, from NITK’s Department of Applied Mechanics & Hydraulics, report that the urban area has increased from 4% to 43% between 1989 and 2017 while vegetation has correspondingly decreased from 28% to 16%. They found that barren land dropped from 55% to 40% in Bengaluru Urban district.
The consequence is that Bengaluru experiences urban heat effect – in which concrete traps heat in pockets that warm up faster than other patches. Previous studies had shown that in the past four decades, the temperature in the city had increased by 2°Celsius.
An analysis of the thermal infra-red band of maps revealed major differences between temperatures in some parts of the city.
In 1989, 53% of the area experienced temperatures in the range of 32-38° Celsius while 45% experienced temperatures in the range of 26-32° Celsius. In 2001, nearly three-quarters of the city experienced temperatures between 38° and 44° Celsius while 16% experienced in the 32-38° Celsius range. By 2017, 68% of the area was experiencing 38-44° Celsius while 27% experienced 32-38° Celsius.
“By 2001, a series of droughts had removed soil moisture and enabled more heating of the land surface. In 2017, a series of rains had raised soil moisture levels. But the trend is strong. There is increasing surface temperatures in the city, with even cooler areas experience higher temperatures than three decades ago,” said Dr. Ramesh, associate professor, NITK.
The average increase in LST was 7° Celsius. Researchers found a strong overlap between land use changes and LST. Between 1980 and 2017, the LST of urban and barren land saw the highest increase of 8° Celsius; vegetation by 7° Celsius; and water by 6° Celsius.
On an average, nearly a quarter of the city experiences temperatures higher than the average while just 14% experiences temperatures lower than the average. Most of the cooler areas are in the city centre, which has gardens and waterbodies.
“Urbanisation has seen the use of materials that emit heat, unlike parks and vegetation which absorb heat. In the centre of the city, there is a distribution of heat between places that are emitting heat (like Shivajinagar) and nearby parks (for instance, Cubbon Park). However, in outer areas there is no sink to take in the heat,” said Dr. Ramesh, who intends to continue to research to see if this differential heating affects rainfall.
