Rising night-time heat an urgent health hazard

Why in the News?

India’s heat crisis is increasingly becoming a night-time public health emergency, as evidence shows that night temperatures are rising faster than daytime temperatures. This reduces the body’s ability to recover from daytime heat. The concern is significant because mortality sharply increases when night temperatures remain above 28-30°C, while existing heat action plans remain largely focused on daytime heatwaves.

Why Are Rising Night-Time Temperatures Emerging as a Major Public Health Threat?

1. Physiological Recovery: Cooler nights allow the human body to recover from daytime heat. Persistently warm nights prevent recovery, resulting in prolonged heat exposure and cumulative stress.
2. Sustained Heat Burden: Continuous exposure transforms heat stress from a daytime phenomenon into a prolonged condition, increasing health risks without adequate relief.
3. Vulnerable Populations: Low-income groups living in densely packed houses without natural ventilation or cooling systems face disproportionate exposure.
4. Public Health Blind Spot: Heat action plans largely focus on daytime heatwaves, while night-time thermal stress remains under-recognised.
5. Extreme Night Heat: Climate Trends data across 200 Indian cities (1986-2018) found that in cities such as Delhi, minimum night temperatures frequently exceeded 32°C and sometimes crossed 35°C. This indicates that nights are increasingly failing to provide thermal relief.

How Are Night-Time Temperatures Rising Faster Than Daytime Temperatures in India?

1. Urban Heat Retention: Concrete, asphalt, and built surfaces absorb heat during the day and slowly release it at night, preventing cooling.
2. Declining Green Cover: Reduced vegetation lowers evapotranspiration, weakening natural night-time cooling.
3. Urban Heat Island Effect: Dense urban settlements trap heat and restrict airflow, keeping cities warmer after sunset.
4. Anthropogenic Heat Emissions: Air conditioners, vehicles, industries, and energy use release residual heat into urban environments.
5. Climate Change: Rising baseline temperatures are increasing both daytime and night-time heat, with warmer nights showing faster escalation in several regions.

What Trends Indicate the Rise of Night-Time Temperatures in India?

1. Long-Term Trend: A Climate Trends analysis using IMD data found that night-time temperatures increased faster than daytime temperatures between 1986 and 2015.
2. Temperature Rise: Mean annual temperatures increased by ~0.63°C, while coldest night temperatures increased by ~0.4°C, indicating warming even during recovery periods.
3. Future Projection: By the 2070s, night temperatures during the warmest day may rise by 4.7°C, alongside a 5.5°C rise in daytime maximum temperatures.
4. Regional Variation: Metropolitan cities are projected to witness stronger warming due to urbanisation and dense built-up surfaces.

Why Does Urbanisation Intensify Night-Time Heat Exposure?

1. Urban Heat Island Effect: Urban surfaces such as concrete, roads, bricks, and metal infrastructure absorb heat during the day and radiate it at night, preventing cooling.
2. Loss of Green Spaces: Reduced vegetation lowers natural cooling and evapotranspiration, increasing retained heat.
3. Water Body Degradation: Shrinking lakes and wetlands reduce local cooling capacity.
4. Built Environment: Dense construction blocks airflow and traps heat in residential clusters.
5. Air Conditioner Heat Emissions: Cooling devices release waste heat outdoors, increasing ambient night-time temperatures in urban neighbourhoods.

What Evidence Links Night-Time Heat with Mortality Risks?

1. Ahmedabad Case Study: The Indian Institute of Public Health, Gandhinagar analysed mortality data in Ahmedabad and found a strong correlation between night-time heat and all-cause mortality.
2. Critical Threshold: Mortality rises sharply when maximum night-time temperature exceeds 28°C.
3. Mortality Spike: If night-time temperature remains below 28°C, all-cause mortality averages around 145 deaths/day. When temperatures rise above 30°C, mortality increases to approximately 265 deaths/day.
4. Significance: Findings indicate that night temperatures may be as important as daytime heat in determining heat-related deaths.

Why Are Existing Heat Action Plans Inadequate in Addressing Night-Time Heat?

1. Daytime Bias: Most heat action plans focus on extreme daytime temperature warnings, overlooking night-time risks.
2. Intermittent Heatwave Focus: Current interventions primarily target short-duration heatwaves rather than persistent elevated temperatures throughout summer.
3. Housing Deficit: Existing policies inadequately address thermal discomfort in informal settlements and overcrowded housing.
4. Limited Preparedness: Long-term urban planning for cooling remains insufficient despite recurring summer heat extremes.

What Immediate and Long-Term Measures Can Reduce Night-Time Heat Stress?

Immediate Measures

1. Passive Cooling: Reflective coatings, whitewashed roofs, and cool roofs reduce heat absorption and indoor temperatures.
2. Ventilation Enhancement: Natural ventilation and airflow management improve indoor cooling in cramped households.
3. Community Awareness: Public advisories on hydration, cooling practices, and vulnerable population protection reduce exposure risks.

Long-Term Measures

1. Urban Greening: Expanding green spaces and tree cover improves cooling through evapotranspiration.
2. Blue Infrastructure: Restoration of urban lakes and water bodies moderates local temperature rise.
3. Climate-Responsive Urban Design: Heat-resilient housing, ventilation corridors, and reflective materials reduce heat retention.
4. Inclusive Heat Governance: Heat Action Plans must incorporate night-time temperature indicators and vulnerable settlements.

Conclusion

India’s heat crisis can no longer be assessed through daytime temperatures alone. Recognising night-time heat as a major climate-health risk is essential for building effective Heat Action Plans, resilient cities, and equitable protection for vulnerable populations.

PYQ Relevance

[UPSC 2017] “Climate change” is a global problem. How India will be affected by climate change? How Himalayan and coastal states of India will be affected by climate change?

Linkage: The PYQ examines impacts of climate change on ecosystems, economy, disasters, and human systems including health. The article provides a specific case study of climate change impact in India, rising night-time heat causing increased mortality and urban heat stress.