Urban Heat Island Effects Amplify Climate Change Impact

The year 2023 was the hottest since global records began in 1850. The average temperature in 2023 was recorded at 1.18°C (2.12°F) above the 20th-century average of 13.9°C (57.0°F), surpassing the previous record set in 2016 by 0.15°C (0.27°F).

This alarming trend is further underscored by the fact that the ten warmest years in the 174-year record occurred in the last decade, between 2014–2023.

Across the globe, countries are grappling with unprecedented heatwaves, droughts, and wildfires, all with just 1.1°C of global warming compared to the pre-industrial era. In this rapidly warming world, cities face a significantly higher risk of extreme heat than rural areas.

Urban areas currently house around 55% of the world’s population, and this number is projected to rise to 68% by 2050,  pointing to the urgent need for climate adaptation measures.

The urban heat island (UHI) effect exacerbates the temperature increases caused by climate change. Structures made of concrete, steel, and glass trap heat. Urban density, paved surfaces, waste heat from factories, vehicles, and structures, and the lack of vegetation also contribute to UHI.

The image below, sourced from the Inside Climate News, visually represents how the urban heat island effect (UHI) amplifies the impact of climate change. This phenomenon, which causes cities to be significantly hotter than their rural surroundings, is a crucial factor in the increasing threats of extreme heat in urban areas.

The UHI effect puts cities’ temperatures up to 7°F hotter than in rural areas.

Chapter 6 of the IPCC Climate Change 2022: Impacts, Adaptation and Vulnerability report serves as a reminder of the need for cities to adapt and build resilience to climate change hazards. With 2.5 billion people projected to live in cities by 2050 and 90% of this increase expected in Asia and Africa, the number of people highly exposed to climate risks is growing. This highlights the urgent need for cities to adapt and build resilience to climate change hazards such as sea level rise, storms, cyclones, intense precipitation, and heat waves.

Extreme heat not only leads to fatalities, particularly among vulnerable groups such as the elderly, young children, and those with underlying health conditions, but also has far-reaching effects. It can impair concentration and learning, affecting educational outcomes, especially in South Asia and Africa.

Moreover, high temperatures can reduce work productivity by up to 20% and strain infrastructure, leading to significant economic losses. In the US alone, extreme heat costs the economy $100 billion annually, a figure that is projected to rise to $500 billion by 2050 if temperatures continue to increase. These figures point out the urgent need for measures to address the effects of extreme heat and the resulting financial burdens it is causing societies.

An analysis by WRI of the IPCC report notes that for every tiny increment of warming, the number of hot days, cities, and exposed populations will increase. For 1.5°C of warming, only 67 cities will experience 150 or more days a year of temperatures over 35°C; under 3°C, it rises to 197, and more than half of these urban areas are in India. The modelling does not even include the UHI effect, which means that more cities and residents will be affected by intense heat.

Less developed and lower-income regions and resource-trapped cities will most suffer the effects of intense heat. These cities are in South Asia, Sub-Saharan Africa, Latin America, and the Caribbean. It is also the urban poor and the billions living in the slums and informal settlements already suffering from poor air quality and cheap building materials like metal roofs and without cooling systems that will be most affected.

The IPPC report offers many climate adaptation pathways for cities to adapt to climate change effects. The first is to identify the urban adaptation gaps. The report notes that critical climate adaptation gaps for all hazards exist worldwide and hinder adaptation. Many cities cannot identify social vulnerabilities and communities and lack planning for protection and access to funding arrangements. These gaps can be closed through improved local decision-making supported by science, technology, and local knowledge to boost buy-in adaptation solutions.

Climate adaptation through social infrastructure, nature-based solutions, and grey infrastructure.

The report notes that prioritising investment to reduce climate risk for low-income and marginalised residents and targeting informal settlements produce the most significant gains as these approaches can advance environmental justice.

Participatory planning for infrastructure provision and risk management to address climate change and underlying drivers of risk in informal and underserviced neighbourhoods, the inclusion of Indigenous knowledge and local knowledge, communication, and efforts to build local leadership, heightens public awareness, inclusive action and enhance the well-being of the most vulnerable communities.

Despite increasing knowledge about the benefits of nature-based solutions for adaptation, studies show that they are still under-recognised and under-invested in urban planning and development.

Nature-based strategies, including street trees, green roofs, green walls, and other urban vegetation, can reduce heat and extreme heat by cooling private and public spaces. Shading and evapotranspiration are the primary mechanisms for vegetation-induced urban cooling. Outdoor green spaces and parks may also slightly reduce indoor heat hazards. Besides lowering the temperature, NBS may also contribute to lower energy costs by reducing extra demand for conventional cooling sources (e.g., air conditioning), especially during peak demand periods.

‘Grey’ or physical infrastructure is a priority for climate adaptation because its performance is sensitive to climate (particularly extreme events), and decisions on design and renovation have long-lasting implications and are hard to reverse.

Avoiding longer-term impacts on society, the economy, and the environment will require future investment and retrofitting of existing infrastructure, which will be undertaken in the context of the risks of climate change.