WEST LAFAYETTE, Ind. — As Earth’s climate continues to warm, the urban heat island effect raises concerns that city-dwellers will suffer more heat stress than their rural counterparts. However, new research suggests that some cities actually experience a cooling effect.
More than 60 percent of urban areas in India experience a day-time cooling effect, according to the study, which was published in Scientific Reports. The cooling effect has been observed in the past, but this paper is the first to directly identify a cause: lack of moisture and vegetation in non-urban areas surrounding the city.
More than 60 percent of urban areas in India experience a day-time cooling effect, according to the study, which was published in Scientific Reports. The cooling effect has been observed in the past, but this paper is the first to directly identify a cause: lack of moisture and vegetation in non-urban areas surrounding the city.
“When the areas around cities are running low on water and they aren’t being irrigated, they turn into hot, dry, barren fields,” said Matthew Huber, a professor of earth, atmospheric and planetary sciences at Purdue University. “When that happens, there’s actually more water available to evaporate in the cities than the surrounding countryside. It’s like the cities are sweating.”
More commonly, cities are warmer than their rural surroundings, known as the heat island effect. As cities develop, they lose vegetation and surfaces are paved or covered with buildings. With less shade and moisture, they aren’t able to keep themselves as cool.
Huber and his collaborators obtained temperature data from 89 cities in India and then used a climate model to determine the effects of irrigation. By turning irrigation “on and off” in the model, they found that both urban heating and cooling effects are largely controlled by agriculture and moisture availability from irrigation.
Season and region also affect whether an urban area will experience a heating or cooling effect. The study showed that during the pre-monsoon season, a majority of urban areas were cooler than their rural surroundings during the day. During the post-monsoon season, they were warmer. Urban areas in the Gangetic Plain, northwestern India, and southern tip of the west coast were warmer during both seasons.
Despite day-time cooling in some of India’s urban centers, nearly all of them experienced warming at night. The effects of night-time warming were especially intense in the semi-arid western region of India.
....
Dominant control of agriculture and irrigation on urban heat island in India
Rahul Kumar, Vimal Mishra, Jonathan Buzan, Rohini Kumar, Drew Shindell & Matthew Huber
Scientific Reports 7, Article number: 14054(2017)
doi:10.1038/s41598-017-14213-2
As is true in many regions, India experiences surface Urban Heat Island (UHI) effect that is well understood, but the causes of the more recently discovered Urban Cool Island (UCI) effect remain poorly constrained. This raises questions about our fundamental understanding of the drivers of rural-urban environmental gradients and hinders development of effective strategies for mitigation and adaptation to projected heat stress increases in rapidly urbanizing India. Here we show that more than 60% of Indian urban areas are observed to experience a day-time UCI. We use satellite observations and the Community Land Model (CLM) to identify the impact of irrigation and prove for the first time that UCI is caused by lack of vegetation and moisture in non-urban areas relative to cities. In contrast, urban areas in extensively irrigated landscapes generally experience the expected positive UHI effect. At night, UHI warming intensifies, occurring across a majority (90%) of India’s urban areas. The magnitude of rural-urban temperature contrasts is largely controlled by agriculture and moisture availability from irrigation, but further analysis of model results indicate an important role for atmospheric aerosols. Thus both land-use decisions and aerosols are important factors governing, modulating, and even reversing the expected urban-rural temperature gradients.