What does this mean for us?

Urban Heat Island (UHI)

Urban Heat Island (UHI) is a phenomenon where urban areas experience significantly higher temperatures than their rural surroundings due to human activities. This temperature disparity arises from the concentration of buildings, roads, and other infrastructure that absorb and retain heat. UHI matters greatly to urban planners and public health experts because it exacerbates heatwaves, increases energy consumption, and poses serious health risks, particularly to vulnerable populations.

El Niño and La Niña Impact on UHI

Global climate changes driven by El Niño and La Niña can amplify the UHI effect by:

1. Temperature Variations: El Niño leads to warmer temperatures, intensifying UHI, while La Niña brings cooler temperatures, which can mitigate UHI but also introduce extreme weather events.
2. Altered Precipitation Patterns: El Niño reduces rainfall, worsening UHI by reducing cooling from evaporation, whereas La Niña increases rainfall, providing some relief but adding humidity.
3. Storm Frequency and Intensity: El Niño increases storm frequency, disrupting UHI temporarily but potentially causing long-term heat retention through infrastructure damage.

Land Surface Temperature as an Indicator of UHI

Land Surface Temperature (LST) is a critical indicator of UHI because it directly measures the heat emitted from the earth’s surface, which is often higher in urban areas due to materials like asphalt and concrete that absorb and retain heat. LST helps in mapping UHI intensity and spatial distribution, guiding urban planners in implementing cooling strategies such as green roofs, urban forests, and reflective materials.

Satellite Assessment of UHI

Urban planners and scientists use various satellites and sensors to assess UHI. Some key satellites and their sensors include:

• Landsat series: Provides detailed images of LST and land cover changes.
• MODIS aboard NASA’s Terra and Aqua satellites: Offers frequent and broad-scale observations of LST.
• Sentinel-2: Provides high-resolution imagery useful for urban area studies.
• VIIRS (Visible Infrared Imaging Radiometer Suite): Provides LST data with better spatial resolution and coverage.

Limitations of Satellite Data

While satellite data are invaluable for assessing UHI, they come with limitations:

• Resolution and Scale: Some satellites provide data at resolutions that may not capture fine-scale urban variability.
• Temporal Coverage: Certain satellites may not pass over the same location frequently enough to capture rapid changes in urban temperatures.
• Surface vs. Air Temperature: Satellites measure land surface temperature, which can differ from the air temperature experienced by humans. This can lead to discrepancies in assessing actual heat exposure.
• Atmospheric Interference: Factors such as cloud cover can obstruct accurate temperature measurements from satellites.

The case of Pune City in Maharashtra

Pune, like many rapidly urbanizing cities, experiences the UHI effect, where urban areas are significantly warmer than their rural counterparts. The image below shows the LST of Pune district and Pune city in 2023. It is evident that LST is higher in the urbanized parts of the district, along highways, and in the city areas, while it is lower in rural and hilly regions. The built-up area in Pune city has increased from 116.6 km² in 1990 to 166.9 km² in 2019, reflecting a 43.1% rise over three decades. The overall mean LST in Pune shows an increasing trend during the summer season (5.8%) and a decrease in the winter season (12.4%) from 1990 to 2019 (Gohain et al., 2020). We can see that LST in Pune is lower near big water bodies such as dams and rivers; see the image below.

The impacts of the UHI effect in Pune include:

Health Risks

• Vulnerable Populations: Children, older adults, and those with pre-existing health conditions face higher risks of heat stress.
• Heat-Related Illnesses: UHIs contribute to respiratory issues, heat cramps, exhaustion, non-fatal heat strokes, and increased mortality rates.

Energy Consumption

• Increased Demand: Higher temperatures boost electricity usage, especially for cooling, raising overall and peak energy demands.
• Power Outages: Extreme heat can overload electrical systems, leading to controlled brownouts or blackouts.

Environmental and Economic Impact

• Air and Water Quality: Elevated temperatures increase air pollution and can harm aquatic ecosystems.
• Human Comfort and Productivity: Heat stress reduces comfort and can negatively impact worker productivity and economic output.

Mitigating the effects

Understanding the interaction between global climate phenomena like El Niño and La Niña and the Urban Heat Island effect is crucial for developing resilient urban environments. While satellite data provides essential insights into UHI patterns, addressing its limitations through complementary ground-based observations and advanced modeling techniques is essential for comprehensive urban heat management. Urban planners and public health experts must leverage these insights to mitigate UHI impacts, ensuring healthier and more sustainable urban living conditions.

To combat the UHI effect, the city of Pune can implement several strategies such as- Planting more trees, creating parks, and installing green roofs/walls to cool the environment; using reflective materials for roofs and permeable pavements to reduce heat absorption; encouraging energy-efficient cooling systems and renewable energy sources to reduce grid strain; developing heat action plans and community programs to educate and prepare residents for heatwaves; and updating building codes to include green infrastructure and providing incentives for adopting UHI mitigation measures.