Cities in northern plains used to cool down during night, but nowadays they are trapping this heat overnight sending their citizens into a sleepless frenzy. Let's take Delhi for example
The joint 2017 analysis by India Meteorological Department and Indian Institute of Technology-Madras found Delhi’s heat index to have registered a higher growth rate compared to the national average. Delhi’s heat index has increased by 0.6 degrees Celsius per decade in summers and 0.55°Celsius per decade during monsoons.
Delhi’s summers and monsoons are hotter by 3.6°C and 3.3°C on the heat index compared to the 1950s. An analysis by the Centre for Science and Environment, a New Delhi-based non-profit, of weather data for 2010-2017 reveals that average heat index of both the seasons has witnessed a steady upward trend.
Heat index is said to be in the danger band when in the range 41-54°C. During such periods, it causes cramps and exhaustion, and there is a possibility of heat strokes with continued physical activity. Heat index of Delhi has consistently been in the danger band during the summer (March-June) and monsoon (July-September) seasons since 2016 (See table: Average heat index during summer and monsoon).
A dramatic increase in the number of days on which the heat index of Delhi crossed into the extreme danger band — conditions when heat strokes are imminent—has been noted.The most severe heat wave ever recorded in India was in 2016 and it is reflected in the Delhi data as well with the heat index of the city shooting above 54°C mark on 51 days in that year.
Overall, it has been noted that Delhi is not only getting hotter in general but the intensity of the heat conditions is also becoming more severe (See: Number of extreme danger heat index days).
A few studies have documented urban heat island effect within the city. A 2013 study found significant variations in night-time land surface temperatures. The Central Business District of Delhi (Connaught Place, a high density built-up area), and commercial and industrial areas display typical heat island condition with temperatures upto 40°C higher than the suburbs.
The study attributed this increase in surface temperature at city-level to the cumulative impact of human activities, and changes in land-use pattern and vegetation density. Another study found an intra-city ambient temperature difference of upto 3°C during the spring season.
It also concluded that this difference is capable of raising electricity demand by 1,856 GWh over the base electricity requirement of the city with a corresponding increase in CO2 emissions by 1.52 million tonne.
The health costs of the heat island effect are massive. A study of excess mortalities in Asian cities due to urban heat island effects suggests that mortality increases by 5.8 per cent per 1°C temperature rise over a threshold of 29°C in Delhi.
This rate is only 1.8 per cent for Hong Kong owing to better infrastructure to shield its citizens from thermal stress and higher per capita income. This is indicative of disproportionate impact of heat stress on economically vulnerable sections of the society as they have inadequate means to shield themselves. The reality is that this section makes up a significant majority of the population in Indian cities.
These health effects are bound to worsen. A study by the National Institute of Urban Affairs (NIUA) found that the contribution of urban heat island effect will intensify the impact of climate change-induced extreme heat and heat stress in Delhi. The study estimates that the number of heat wave days in Delhi is 2.3 times that of the adjacent rural areas.
This difference increases to 7.1 times in short-term and 13.8 times in long-term projections. Overall, the frequency of heat waves for urban areas is expected to increase from 0.8 each summer to 2.1 and 5.1 in short- and long-term projections respectively.
The intensity of heat waves in urban areas would increase from current 40°C to 45°C in short-term projection to 49°C in the future. If we add the temperature increases due to urban heat island effect (3–120C), we get temperatures of 52–610C, which will make certain parts of the city well nigh inhabitable.
At present, ACs are the most effective (and resource-intensive) means to cool indoor spaces to survive the urban hearth. However, rampant use of ACs is problematic as it adds fuel to the outdoor fire, making cities hotter.
The release of waste heat from ACs into the ambient environment exacerbates urban heat island effect in the immediate surroundings. A study in Tokyo found that waste heat from air conditioners alone caused a temperature rise of 1–2°C or more on weekdays in the office areas in Tokyo. The magnitude of urban heat island effect on weekends and holidays was found to be lesser due to abatement in the use of ACs.
Another study done in Phoenix, US found that waste heat released from AC systems increased the mean air temperature by more than 1°C at night, inducing increased demand of cooling at night. A detailed analysis on select districts of Paris found that local temperature variations resulting from heat island effect are proportional to the waste heat rejected locally by ACs.
This can be seen in Delhi as well. Historically, the city is known to have cooler nights with cool breezes blowing, helping the denizens to sleep comfortably even during peak summers. But nights are getting increasingly warmer.
A CSE study found that the average daily minimum temperature in May 2018 did not drop below 29°C. In fact, ambient temperature was consistently above 30°C beyond midnight in the city. These uncomfortable sleeping conditions have been driving night-time electricity demand. An analysis of the hourly peak demand in the city corroborates the observation that thermal comfort at night is an issue in the city (See: Typical hourly electricity demand pattern in May).
Air conditioning is a key parameter of health problems due to heat waves because, on the one hand, it reduces mortality but, on the other hand, depending on the heat management, it can increase street temperature, thereby increasing the heat stress on people who don’t have access to an AC.
Further, traditional building design and urban form of Delhi (and most Indian cities) is of a low, close and dense network with shaded alleyways where people could keep cool during summers. The introduction of ACs in such an urban form ends up heating the entire neighbourhood.
On the other hand, the rapid constructions of high-rises and decreasing green spaces that embody new India are even worse off as they don’t have any passive means to keep cool and are, therefore, captive users of ACs, shooting out millions of mini-heat jets into the urban air shed, creating undue physical and economic stress in the city and reducing the overall quality of life.
(This is an excerpt from the upcoming CSE report A Midsummer Nightmare, which decodes the link between comfort, space cooling and energy consumption in a climate-stressed world)
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