Relevance: GS-1: Important Geophysical Phenomena

Key Phrases: Urban heat island, Heatwave, Climate Change, NASA’s Ecosystem Spaceborne Thermal Radiometer Experiment (Ecostress), Global Warming

Why in News?

• Several parts of the country are reeling under heatwave conditions. Cities, especially, are a lot hotter than rural areas. This is due to a phenomenon called an “urban heat island”.

What is an urban heat island?

• An urban heat island is a local and temporary phenomenon experienced when certain pockets within a city experience a higher heat load than surrounding or neighbouring areas on the same day.
  • For example, a greener locality like Pashan in Pune often records cooler temperatures than urban areas like Shivajinagar, Chinchwad, or Magarpatta.
• The variations are mainly due to heat remaining trapped within locations that often resemble concrete jungles.
• The temperature variation can range between 3 to 5 degrees Celsius.

Why are cities hotter than rural areas?

• Lack of Vegetations: Rural areas have relatively larger green cover in the form of plantations, farmlands, forests, and trees compared to urban spaces. This green cover plays a major role in regulating heat in its surroundings.
  • Transpiration is a natural way of heat regulation. This is the scientific process of roots absorbing water from the soil, storing it in the leaves and stems of plants, before processing it and releasing it in the form of water vapour.
  • On the contrary, urban areas lack sufficient green cover or gardens and are often developed with highrise buildings, roads, parking spaces, pavements, and transit routes for public transport. As a result, heat regulation is either completely absent or man-made.
• Dark Surfaces: Black or any dark coloured object absorbs all wavelengths of light and converts them to heat, while white reflects it.
  • Cities usually have buildings constructed with glass, bricks, cement, and concrete — all of which are dark-coloured materials, meaning they attract and absorb higher heat content. Thus, forms temporary islands within cities where the heat remains trapped.
• Thermal Mass: Buildings contain a lot of thermal mass, which means they store a lot of heat during the day and are slow to release the heat overnight.
• Climate Change: The more extreme heatwaves in urban areas, mainly in northern regions, are a factor that contributes to urban heat island formation. Urban heat islands also exacerbate the changes in the climate, so the problem feeds on itself.
• Increased Use of Air Conditioner: Mechanical air conditioning exhausts heat into the environment around the building, directly adding to the problem. As a result, the outside environment is warmed, leading to increasing atmospheric temperature.
• Urban Canopy: In urban areas, there are multilayer buildings. The heat reflected by a building is trapped by the nearby taller buildings, which is known as the urban canopy. UHI is exacerbated by the formation of the urban canopy.
• Wind Blocking: Due to the presence of densely situated buildings, the velocity of wind is reduced, and it lessens the cooling effect by convection. So, the trapped heat intensifies the effect.
• Air Pollutants: Exhaust gases from vehicles, and industrial pollutants released in the atmosphere, trap solar radiation, causing an increase in temperature, and the microclimate effect becomes stronger.

What has NASA said about urban heat islands in India?

• NASA recently pointed out heat islands in urban parts of Delhi, where temperatures were far higher than nearby agricultural lands.
• NASA’s Ecosystem Spaceborne Thermal Radiometer Experiment (Ecostress) captured an image covering an area of about 12,350 square kilometers, which showed a large red patch around Delhi and smaller red patches around neighbouring cities of Sonipat, Panipat, Jind, and Bhiwani.

• These red patches, implying higher temperatures, were the heat islands, while the rural areas around the cities witnessed lower temperatures.
• Ecostress is an instrument with a radiometer that was sent onto the International Space Station in 2018. It can measure temperatures on the ground, as opposed to the air temperature.
  • Ground temperatures are likely to be higher during the day than air temperatures and almost the same during the night.
  • Primarily, Ecostress is tasked with measuring the temperature of plants and understanding their water requirements and the impact of the climate on them.

Impacts of Urban Heat Island:

• Thermal Ratio: It may result in a doubling of the urban to the rural thermal ratio in the following decades.
• Increased Energy Consumption: It may increase the overall energy consumption for cooling (i.e., refrigeration and air-conditioning), hence increasing the energy production by power plants, which leads to higher emissions of heat-trapping greenhouse gases.
  • Increased energy demand means more costs to citizens and governments, which in large metropolitan areas may induce significant economic impacts.
• Elevated Greenhouse Gas Emissions and Air Pollution: UHIs promote high air temperatures that contribute to the formation of ozone precursors, which combined photochemically produce ground-level ozone.
• Discomfort and Danger to Human Health: A direct relationship has been found between UHI intensity peaks and heat-related illness and fatalities, due to the incidence of thermal discomfort on the human cardiovascular and respiratory systems.
  • Heatstroke, heat exhaustion, heat syncope, and heat cramps are some of the main stress events, while a wide number of diseases may become worse, particularly in the elderly and children.
  • Respiratory and lung diseases have been shown to be related to high ozone levels induced by heat events.
• Secondary Impacts on Weather and Climate: The anomalous warmth of the city creates relatively low air pressures that cause cooler, rural air to converge on the urban centre, thus forcing warm air to ascend (i.e., convection), which at higher altitudes condensates and precipitates.
  • Studies carried out in several cities have shown that urban-induced precipitation and thunderstorm events are mainly initiated by the UHI.
  • Reductions in snowfall frequencies and intensities, as well as reductions in the diurnal and seasonal range of freezing temperatures.
• Disturbance to the ecosystem: High temperatures may produce physiological and phenological disturbances in ornamental plants and urban forests.
  • Adverse heat can also significantly reduce the availability of food, shelter, and water.

Solutions to Urban Heat Island:

• Use Light Coloured Material for construction: The use of light-coloured concrete and white roofs has been found to be effective in reflecting up to 50% more light and in cutting down the ambient temperature.
• Green Roofs and Vegetation Cover: Green roofs absorb heat and act as insulators for your home, reducing the energy needed to provide cooling and heating.
• Planting Trees in Cities: The practice of tree planting within and around cities is an incredible way of reflecting solar radiation while at the same time decreasing the urban heat island effect. Trees provide shade, absorb carbon dioxide, release oxygen and fresh air, and provide a cooling effect.
• Use energy-efficient appliances and equipment: Using efficient appliances and equipment in your home can help to lighten the load on the electric grid during heat waves.
• Implementation and Sensitization of Heat Reduction Policies and Rules: The state implementation of environmental policies such as the Air Pollution Act, Low carbon fuel standards, use of renewable energy, and clean car rule standards can impressively regulate the anthropogenic inducers of urban heat island effect.

Conclusion:

• With fewer emissions, the level of greenhouse gases in the atmosphere can be reduced, thus decreasing the effects of climate change and global warming.
• Education and outreach can also be done to ensure communities are aware of the economic and social benefits of planting trees and eco-roofing.

Source: Indian Express

Mains Question:

Q. What is an Urban Heat Island? What are the causes, effects, and solutions to Urban Heat Island?

Q. Urban planning is the best mitigating strategy for “urban heat island “. Explain.