New research led by the University of East Anglia (UEA) has uncovered a sobering reality for urban populations: many tropical and subtropical cities are set to warm significantly faster than previously expected. As the world approaches the 2°C global warming benchmark, these urban centres face a dual threat from both rising global temperatures and the intensifying urban heat island (UHI) effect.
Using a combination of state-of-the-art climate projections and machine learning models, researchers found that monsoon regions are particularly vulnerable. Specifically, cities in India, China, and Western Africa are expected to see the most dramatic shifts.
Cities traditionally remain warmer than their rural surroundings because of the urban heat island phenomenon. This occurs due to factors such as regional climate, dense infrastructure, and a lack of vegetation. However, the study—published in the Proceedings of the National Academy of Sciences (PNAS)—shows that climate change is actively amplifying these heat islands.
Which Cities Are Most at Risk?
The study examined 104 medium-sized cities with populations between 300,000 and one million. In 81 per cent of these cities, daytime land surface temperatures will warm more than the surrounding hinterlands.
The data highlights several specific cities experiencing extreme “additional” warming:
- Significant Increases: Jalandhar (India), Fuyang (China), and Kirkuk (Iraq) are projected to experience 0.7-0.8°C of additional warming compared to their rural counterparts.
- Extreme Increases: Asyut (Egypt), Patiala (India), and Shangqui (China) face even grimmer projections, with additional warming of 1.5-2°C. This represents an increase of up to 100 per cent more than their surrounding rural areas.
- Negligible Changes: Conversely, some cities show minimal differences. Marrakech (Morocco) is one such city. Campo Grande (Brazil) also demonstrates little to no difference between urban and rural warming rates.
The Danger of Underestimating Urban Heat
Lead author Dr Sarah Berk noted that global climate models are vital. However, they often struggle to capture temperature trends in smaller or medium-sized cities. This research bridges that gap. It reveals that many medium-sized cities represent a massive proportion of global urban areas. These cities are warming at rates that far exceed rural averages.
Prof Manoj Joshi, a co-author from UEA, warned that current projections likely underestimate the level of urban heat stress. For instance, while some hinterlands are projected to warm by 1.5-2°C, several cities in northern India and North-East China could actually see temperatures rise by as much as 3°C.
Q&A: Understanding the Urban Warming Crisis
Why are medium-sized cities the focus of this study?
Medium-sized cities (populations 300,000 to one million) are more than 2.5 times as common as megacities. Despite their prevalence, they are often overlooked in global climate models that typically only capture data for the largest urban areas.
What role does machine learning play in this research?
The researchers combined machine learning with state-of-the-art projections to simulate how urban heat islands react under a 2°C warming scenario. This allowed them to predict temperature changes at a much more granular level than traditional models.
Which regions will feel the most impact?
The study indicates that monsoon regions will experience the highest levels of amplified urban warming. This is especially true for regions in Western Africa, India, and China.
Frequently Asked Questions (FAQ)
Q: What is an Urban Heat Island (UHI)?
An Urban Heat Island is a phenomenon where urban areas are significantly warmer than their rural surroundings. This is caused by human activities, heat-retaining building materials, and a lack of natural cooling from vegetation.
Q: How does this affect human health?
Increased urban warming heightens the risk of heat-related health issues. Because the cities studied are already in the warmer parts of the world, additional temperature spikes significantly increase vulnerability for urban populations.
Q: When will we see these temperature increases?
The projections are based on a 2°C global warming benchmark, which scientists believe is likely to be reached during the second half of this century.
Q: Who supported this research?
The study was supported by the Natural Environment Research Council and the ARIES Doctoral Training Partnership, involving researchers from UEA and the Karlsruhe Institute of Technology.
