Urban Greening & Cool Roofs Help Cities Address Extreme Heat

As climate change fuels rising temperatures worldwide, low-cost adaptation strategies can help cities stay cool.

Trees along the road in Delhi, India (Source: Wikimedia Commons).

Co-authored by Benjamin Steiger, Caroline Hoffman, Kathleen Chen (recent MPH graduates of Columbia University Mailman School of Public Health), Sophie Kimball (MBA student at Columbia University Mailman School of Public Health and Columbia Business School), and Prima Madan ( cooling and efficiency expert working as a consultant at NRDC).

As temperatures rose to record highs in the US Pacific Northwest lately the question of how to deal with a world that is hot and getting hotter has become more and more urgent. Extreme heat is of particular concern in urban areas because of the density of buildings and roads, which absorb solar radiation and re-emit heat through the urban heat island (UHI) effect. Being able to experience these heat islands daytime temperatures up to 4˚C warmer than rural areas and people living in such areas may suffer from increased risk of heat-related morbidity and mortality during extreme heat, especially for vulnerable populations.

Urgent need for cheap climate adaptation

Local solutions are urgently needed to tackle the rising temperatures that put people at risk. Adaptive land cover techniques can act as low-cost measures that help communities address many associated climate and health risks. Two well-documented, low-cost strategies are urban greening, which increases the quantity and quality of green spaces in cities, and cool roofs, which improve the ability of roofs to reflect incoming solar radiation rather than retain heat.

Although these adaptive land cover strategies have been implemented around the world for years, they are the subject of increasing scientific research for their benefits to environmental conditions (including local temperatures and air quality), human health (including potential reduction of heat-related diseases) and reduced energy demand for cooling. in buildings. Over the past six months, our research team has conducted an extensive literature review to investigate the environmental, health and economic benefits of adaptive land cover interventions, focusing on India, a country strongly vulnerable to climate change. We recently presented highlights of our findings during a session at the 17th International Conference on Urban Health.

Mute the urban heat island

Urban greening interventions can help to counteract the harmful health effects of the urban heat island effect on people. Urban green space reduces air temperature on a micro scale through shade, and can also change the thermal energy balance of the environment and the cool local environment. Researchers in Bangalore, India, found that cooling can attenuate heat up to 347 m from the boundary of a green space. In some contexts, urban greening can also aid in the deposition and diffusion of air pollutantsreducing some of the health risks exacerbated in urban heat islands.

Overall, both modeling and experimental studies demonstrate the potential for cool roofs (including roofs with solar reflective coatings and green roofs) to lower outdoor and indoor air temperatures. Cool roofs modify the properties of a roof to provide a net cooling benefit by increasing solar reflectance and thermal emission on a roof (coated with a cooling paint, membrane or solar reflective tiles) or by evaporation on a vegetation covered green roof. For example, a 2016 experimental study in Nagpur, India, showed that schoolrooms with white cool roof paint on their roofs average 1.5-2.1 °C cooler and up to 3.3-4.3°C cooler over a 10-week observation period than dark gray roofs. In addition, a 2019 modeling study estimated that increasing the total green roof space in Mumbai, India to achieve 50% green roof coverage was associated with a reduction of 2-2.2°C.

Addressing health risks from rising temperatures

Exposure to extreme heat can impair the body’s ability to thermoregulate, increasing the risk of heatstroke and the worsening of underlying respiratory, cardiovascular and other chronic diseases. By lowering the temperature of the ambient and indoor air, urban greening and cool roofs can dampen exposure to extreme heat and reduce the risk of heat-related death. Studies have shown that increased access to and use of green space is positively associated with individual health benefits, including: mental health improvements and reduce the prevalence of chronic disease. In addition, exposure to increased canopy cover has been associated with: lower incidence of self-reported fair to poor general health and less psychological distress. Populations most at risk for heat-related morbidity, including lower socioeconomic groups, children and the elderly, may greater improvement in general health through greater exposure to greenery in the city compared to other populations, which has implications for achieving health equity.

Reducing the demand for energy

By providing shade to adjacent buildings and by reducing a building’s absorbed solar radiation, street trees and cool roofs, respectively, can reduce the demand for energy to cool buildings. This route results in significant energy-related cost savings and potentially reduced pollution emissions from power plants. For example the Chicago Urban Forest Climate Project Chicago showed that a 10% increase in street tree cover can reduce annual energy costs by 5-10%. Whitewashing existing black roofs can: reduce energy consumption for cooling by 14-26%, while a study of 760 m2 cool roofs in India indicated that energy savings offset total implementation and maintenance costs in less than 3.3 years in warm to temperate climates.

Extend Deployment

Given the many benefits of adaptive land cover in the face of growing climate threats, many cities are expanding their efforts in cool roofs and urban greening programs. In response to growing health risks from rising temperatures, including the deadly heat wave of 2010 that led to a surplus of more than 1,300 deaths, Ahmedabad, India, launched South Asia’s first heat action plan. That plan includes provisions to implement cool roofs on hundreds of buildings in low-income communities.

In the US, the OneNYC Subscription to address the climate crisis in New York City, has committed to adding more street trees in the most heat-sensitive areas, while the NYC CoolRoofs program offers cool roof installations for free for a range of building types, including low-income homes and schools. Given the current and increasing health threat posed by extreme heat and the strength of the available evidence, it is imperative that similar investments in urban greening and cool roofs be made in cities around the world.

Climate change is not a distant threat, it is here and now, and it is getting worse every day that we continue to rely on polluting fossil fuels instead of cleaner, healthier, renewable energy sources such as solar and wind. This summer’s scorching heat episodes are a vivid example of the significant and growing risks of the climate crisis to our health. In the face of rising temperatures, investment and careful planning to expand adaptive land cover can help people cope with the heat and help cities protect their most vulnerable inhabitants.

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