IPCC Assessment in depth, Part 6: What risks does climate change pose to the world’s cities?

The threat that climate change poses to human well-being and the health of the planet is “unequivocal”, says the latest report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC). In this in-depth Q&A series, Carbon Brief unpacks the key findings of the report and the developments since the IPCC’s last assessment. (Read previous section here)

By Carbon Brief

Urban areas are home to more than half of the world’s population – and are constantly growing. Between 2015-20, urban populations grew by more than 397 million people – with more than 90% of growth situated in “less developed regions”, the report finds.

The report projects that by 2050, an additional 2.5 billion people could be living in urban areas. It adds that roughly 90% of this increase is expected to be in Asia and Africa – with India, China and Nigeria alone responsible for 35% of the growth.

Since the AR5 report, “unplanned and informal” cities – particularly in Asia and Africa – have expanded rapidly. More than half of the urban population in sub-Saharan Africa and roughly one-third of South Asia’s urban population were living in informal housing in 2018, according to the AR6 report.

It goes on to explain that cities are often hotspots for climate extremes, such as heatwaves and flooding – due to a combination of their location, population density and concentration of buildings and infrastructure.

For example, it says that most of the population exposed to heatwaves in the coming decades will live in urban regions – in large part because the urban heat island effect can add 2C to local warming. Cities in mid-latitudes are particularly at risk from rising temperatures – and could be “subject to twice the levels of heat stress compared to their rural surroundings under all RCP scenarios by 2050”. The report adds:

“Depending on the RCP, between half (RCP2.6) to three-quarters (RCP8.5) of human population could be exposed to periods of life-threatening climatic conditions arising from coupled impacts of extreme heat and humidity by 2100.”

Unsurprisingly, exposure to heat islands is uneven – with low-income communities, children, elderly people, disabled people and ethnic minorities more exposed. For example, the report highlights research from South Africa, which shows that “housing occupied by poor communities regularly experience indoor temperature fluctuations that are between 4C and 5C warmer compared to outdoor temperatures”.

The figure below shows the distribution of the global population exposed to hyperthermia – abnormally high body temperature – from extreme heat for a) the present day; b) in 2050; and c) in 2100. Future projections are shown for a range of RCP scenarios. Red shading indicates the highest exposure, while yellow indicates the lowest exposure. The top 15 urban areas by population size are named on the map.

The report also highlights that urban expansion and changing rainfall patterns could cause nearly one third of all “major cities” worldwide to exhaust their current water resources by 2050.

Meanwhile, it finds that flooding is a key issue for cities, which have a high density of people and infrastructure, and have large areas of “impermeable surfaces” that prevent rainfall from easily soaking into the ground. It adds this many cities are located near the sea, increasing the risk of coastal flooding:

“Much of the world’s population, economic activities and critical infrastructure are concentrated near the sea (high confidence), with nearly 11% of the global population, or 896 million people, already living on low-lying coasts directly exposed to interacting climate- and non-climate coastal hazards.”

The report has high confidence that climate change will cause sea levels to rise and increase the severity and frequency of tropical cyclone storm surges, driving higher levels of risk coastal flooding.

By 2050, more than a billion people located in low-lying cities and settlements will be at risk from “coastal-specific climate hazards”, the report says with high confidence. However, it notes that the amount of damage caused will be dependent on future warming levels and socioeconomic scenarios.

Between $7-14tn of coastal infrastructure assets will be exposed by 2100, according to the report – affecting sectors including transportation, housing, power production and “information technology”.

For example, the report notes that even if warming is limited to 2C above pre-industrial temperatures, the number of airports at risk of storm surge flooding could rise from 269 to 338. It adds that the airports affected are disproportionately busy and currently account for up to 20% of the world’s passenger routes.

Population growth will also be a key driver of increasing flood risk – as the report is virtually certain that “economic development is disproportionately concentrated in and around coastal cities and settlements”.

One projection, for example, says that urban land will expand by 0.6-1.3m km2 between 2015-50. Even without accounting for the impacts of climate change, this means that the urban land exposed to floods and droughts will increase by more than 2.5 times between 2000 and 2030.

The figure below shows risks to people, land and infrastructure from a 1-in-100 year coastal flood event – where yellow indicates no sea level rise and purple indicates two metres of additional sea level rise, compared to 2020 levels.

The size of the circle indicates the magnitude of the impact. The top left quadrant of each circle shows the number of people affected, top right shows the number of flights disrupted, bottom left shows the kilometres of coastline affected, and bottom right indicates whether sea level rise will have a positive or negative impact on wetlands.

There is medium evidence that coastal regions with fast-growing populations in Africa, southeast Asia and small islands will be particularly at risk of flooding over the 21st century.

In an FAQ explaining why cities are especially vulnerable to the impacts of climate change, the report also points to a failure in governance, which allows the gap between rich and poor communities to grow:

“Demographic change, social and economic pressures and governance failures that drive inequality and marginality mean that increasing numbers of people who live in towns and cities are exposed to flooding, temperature extremes and water or food insecurity. This leads to an adaptation gap, where rich neighbourhoods can afford strategies to reduce vulnerability while poorer communities are unable to do the same.

“Although this would be so even without a changing climate, climate change increases the variability and extremes of weather, exposing more people, businesses and buildings to floods and other events. The combination of rising vulnerability and increasing exposure translates to a growth in the number of people and properties at risk from climate change in cities worldwide.”

The report also discusses the issue of air pollution in cities – noting that 95% of the global population currently live in regions where fine particulate matter (PM2.5) floating in the air exceeds the World Health Organisation guidelines.

It says that future emissions of air pollutants will broadly decline by 2050, as societies “become wealthier and more willing to invest in air pollution controls”. However, it adds:

“Whereas cities in East Asia and South Asia currently have large exposure to anthropogenic air pollution, African cities may emerge by 2050 as the most polluted because of growing populations and demand for energy, increased urbanisation, and relatively weak regulations to control emissions.”

Next: What does the IPCC report say about public health, conflict and migration?

All sections:

1. What is the IPCC report?
2. How is climate change affecting land ecosystems?
3. What impact is global warming having on marine life?
4. How is climate change affecting the world’s water?
5. What does the IPCC report say about impacts on food and agriculture?
6. What risks does climate change pose to the world’s cities?
7. What does the report say about public health, conflict and migration?
8. How does climate change affect poverty and progress towards the sustainable development goals?
9. How is the world adapting to climate change?
10. What the latest IPCC report says about carbon fee and dividend

Authors:

Aruna Chandrasekhar holds an MSc in environmental change and management from the University of Oxford. She previously worked as a freelance journalist for a variety of publications, including the New York Times, Guardian, Scroll and Caravan.

Daisy Dunne holds a BSc in biology from the University of Bristol and a science journalism MA from City, University of London. She was The Independent’s climate correspondent from November 2020 to 2021. Prior to this, she was Carbon Brief’s science writer from 2017 to 2020.

Josh Gabbatiss covers climate and energy policy. Josh holds a BSc in zoology from the University of Bristol. He previously worked as science correspondent at The Independent and prior to that was a freelance journalist for a variety of scientific publications including New Scientist and BBC Earth.