When we think about the impacts of climate change, dramatic images often come to mind, such as devastating floods sweeping through communities, powerful storms destroying infrastructure, and wildfires consuming forests and farmland. But the effects of heatwaves rarely leave visible scars on the landscape, although they are among the deadliest consequences of a warming planet.
For many people, a heatwave may seem like little more than a spell of uncomfortable weather accompanied by sleepless nights. What often goes unnoticed is the significant human toll that emerges in its aftermath. As global temperatures continue to rise, the frequency, duration, and intensity of the events related to extreme heat are expected to increase, placing the lives of billions more people at risk.
The true effects are reflected in rising mortality rates and increased hospital admissions for heatstroke, dehydration, respiratory distress, and cardiovascular complications. These hidden impacts frequently become apparent only after the extreme heat has passed. Scientists estimate that more than one-third of heat-related deaths around the world are already attributable to climate change.
Heat Extremes in Cities: Why Every Degree of Warming Matters
Many climate studies focus on the consequences of a world that is 1.5°C or 2°C warmer than pre-industrial levels – the thresholds that lie at the heart of the Paris Agreement. These temperature limits are no longer distant possibilities, because the global temperatures remained more than 1.5°C above pre-industrial levels for an entire twelve-month period between February 2023 and January 2024.
Scientists now warn that the world is likely to exceed this threshold permanently within the upcoming years, with 2°C of warming potentially following within a few decades if the greenhouse gas emissions remain high. Recognizing this reality, researchers have begun examining the warming scenarios that more closely reflect the current policy trajectories.
Based on the existing emissions pledges, the global temperatures could rise between 1.9°C and 3.1°C above pre-industrial levels by the end of the century, depending on how effectively governments implement their climate commitments.
A future limited to 1.9°C of warming would require ambitious and sustained implementation of current international pledges. In contrast, a 3.1°C world represents a pathway in which existing policies continue with limited additional efforts to reduce emissions.
A Future Defined by Rising Temperatures: Billions Could Face Unprecedented Heat
Researchers recently assessed how many urban residents could be exposed to extreme heat events that are virtually unprecedented under today’s climate conditions. Their findings are alarming under the high-emissions scenario, where approximately four billion people living in cities could experience heat extremes unlike anything currently recorded, occurring as frequently as once every decade.
Under the lower warming scenario of 1.9°C, that number falls dramatically to around one billion people. Hence, every degree of warming avoided could spare roughly 2.5 billion urban residents from exposure to the most severe and dangerous heat extremes in cities.
In practical terms, the research underscores a critical reality that the future is not predetermined. The policy decisions made today will directly influence how many people are exposed to life-threatening heat tomorrow.
The Most Vulnerable Face the Greatest Risks
While the headline figures are concerning, they conceal an even deeper challenge regarding those most vulnerable to heat extremes in cities, because they are often the least equipped to adapt. Young children, older adults, and individuals with pre-existing medical conditions face the highest risks during prolonged heatwaves.
The study found that many of the most exposed populations are concentrated in rapidly expanding urban settlements across South and Southeast Asia. These areas are experiencing both increasing population growth and fast climate vulnerability. Within cities too, the burden of extreme heat frequently falls disproportionately on low-income communities. Limited access to air conditioning, green spaces, quality housing, and healthcare facilities makes adaptation significantly more difficult for people who are already vulnerable.
The Urban Heat Island Effect: Growing Cities, Growing Challenges
At the same time that temperatures are rising, cities themselves are expanding at an unprecedented rate, adding the challenge of urban heat island effect to the heat extremes in cities. In 2018, approximately 55% of the world’s population lived in urban areas. By 2050, that figure is projected to reach 68%, adding billions more people to towns and cities across the world.
This rapid urbanization simply means that more people than ever will be exposed to the combined effects of climate change and urban heat. It also reveals that without effective planning and adaptation strategies, extreme heat could become one of the defining public health challenges of the 21st century. The cities and towns are usually 1–3°C hotter than the surrounding countryside, because they are filled with roads, buildings, and other hard surfaces that absorb and retain heat.
The asphalt, concrete and brick absorb heat from the sun and radiate it slowly and make heat extremes in cities grow significantly than surrounding rural regions, especially during heatwaves. Whereas, some cities can be as much as 7°C hotter, with the effect of urban heat island. This can be dangerous phenomenon, particularly in hot countries.
In very hot conditions, heat extremes in cities can transform already dangerous temperatures into life-threatening conditions. There, the dehydration and heat exhaustion become real risks, and if it gets too hot, it can be lethal.
Why Climate Action Matters
The research delivers a clear and urgent message that the scale of future heat exposure depends largely on the choices humanity makes today. While individuals can contribute by adopting to sustainable lifestyles, the most significant changes must come from governments, industries, and policymakers capable of implementing large-scale reductions in greenhouse gas emissions.
Every fraction of a degree matters here, even the modest reductions in future warming could prevent billions of people from experiencing unprecedented heat extremes and save countless lives by reducing human suffering worldwide.
As heat extremes in cities continue to grow, ambitious climate action is no longer a simple environmental objective or choice – it is a public health necessity. The decisions made today will determine the fate of the future generations – whether they inherit cities that remain livable, resilient, and safe, or urban environments increasingly defined by dangerous and relentless heat.
