How does climate change contribute to extreme heat? Is it causing more severe and frequent heatwaves?
Every month since June 2023 has seen record-breaking temperatures and the speed at which extreme heat events are being recorded is unprecedented.
Climate change is making heatwaves longer, more extreme and more frequent. Burning fossil fuels releases greenhouse gas emissions – like carbon dioxide – that trap more heat in the atmosphere. The more emissions are produced, the more heat gets trapped, leading to more global warming and increasingly extreme weather events.
Of the 152 extreme-weather attribution studies carried out by scientists around the world over the past 20 years, 93 per cent concluded that human-induced climate change made extreme heat events more likely or more severe.
The June 2024 heatwave in the US, Mexico and Central America was made 35 times more likely and 1.4°C warmer because of climate change.
Heatwaves can trigger a domino effect of impacts. Extreme heat can lead to droughts, wildfires, poor air quality, water and food insecurity, and power shortages, devastating communities worldwide.
It is estimated that wildfires in Canada in 2023 burned 7.8 million hectares of land and released 3 billion tonnes of carbon dioxide, generating air pollution that reached as far as Europe and China. The released carbon dioxide can contribute to future heatwaves and create a dangerous feedback loop.
Heatwaves and flooding are often connected. Extreme heat can increase the likelihood of subsequent flooding. Warmer air can hold more moisture and the ground, which has become hard and dry because of the heat, is less able to absorb rainfall. In 2022, an early and prolonged heatwave in India and Pakistan contributed to devastating floods across Pakistan and parts of India and Bangladesh.
The impacts of extreme heat are also unequal. Developing countries and lower-income communities are disproportionately vulnerable to heat-related sickness or death, due to factors such as location or a lack of access to air conditioning. A 2022 study found that lower-income populations face a 40 per cent higher risk of heatwave exposure compared to those with higher incomes. This inequality is set to worsen as temperatures increase.
What is a heatwave? What is the difference between a heatwave and a heat dome?
A heatwave is an extended period of abnormally warm weather. It is defined specifically in relation to a location and the historical average temperature of that location. When that above average heat goes on for at least two days, it is designated as a heatwave.
A heat dome occurs when hot air becomes trapped under an area of high pressure. Like putting a lid on a pot, this high-pressure system creates a ’dome’ of hot air that cannot escape, causing temperatures to soar. A higher-pressure system also means that there is little or no cloud cover, so the radiation from the sun intensifies the heat further.
The June 2024 heatwave in the US, Mexico and Central America was a heat dome and made temperatures 11°C to 16°C hotter than normal in parts of the US.
What are the impacts of extreme heat on areas like human health, food security and the economy?
Heatwaves cause a complex mix of direct, indirect and so-called cascading impacts.
Between 2000–19 there were approximately 489,000 heat-related deaths each year. Excess deaths during extreme heat events are mainly cardiovascular. But heat stress can also exacerbate other underlying illnesses, increase the risk of accidents and transmission of some infectious diseases, and increase medical emergencies of heatstroke. Wildfire smoke can also cause or worsen heart and lung diseases, even leading to increased risk of dementia.
The way heat affects populations is very varied and unequal. Death rates are highest for elderly people, children, those with vulnerable conditions, and low-income communities.
When it comes to food security, heatwaves introduce problems across the value chain, from production through to access to food. There are negative impacts on food production because extreme heat reduces crop productivity and agricultural labour capacity. Heatwaves can also worsen drought conditions which are a major driver of food insecurity.
Here too, impacts are unequal. For example, issues with production often lead to food price increases. Reduced ability to purchase food leads to lower dietary diversity – which disproportionately affect people with lower disposable income.
Water supply and quality are also compromised during heatwaves, a time when more water is needed for drinking, cooling or irrigating crops.
Heatwaves also impact on labour and people’s ability to be productive. In 2019, an estimated 302 billion working hours were lost due to higher temperatures, 52 per cent more than in 2000. To put this into context, COVID-19 resulted in 580 billion lost working hours.
In 2022, the economic cost of this was forecasted to be 4.1 per cent GDP lost for Africa as a region, mainly from losses in the agricultural sector. 70 per cent of the global workforce is facing injury or death from extreme heat. In Africa, 93 per cent of the workforce is at risk.
Other economic impacts include disruption to infrastructure and energy grids. Where there is installed air conditioning capacity, a surge in electricity demand during an extreme heat event can lead to energy shortages and blackouts.
Heatwaves also put physical pressure on energy infrastructure and can cause pieces like transformers and cable insulation to overheat. These blackouts can set off a cascade of negative impacts, such as transport disruption, manufacturing stoppages and citizen safety.
Transport can also be disrupted by extreme heat causing metal railway lines to expand, as seen in the UK and elsewhere, in turn disrupting the flow of goods and services.
These different heatwave impacts can also combine and create a cascading chain of risks that trigger so-called tipping points of abrupt changes. For example, wildfires in boreal forests could contribute to loss of trees at a scale large enough to transform them from areas that absorb carbon dioxide from the atmosphere into areas that release more carbon dioxide than they absorb.
What are governments doing about heatwaves and severe weather events?
A number of governments have implemented heat action plans, designed to mitigate and adapt to the risks of extreme heat at city or national level. These plans outline the process for issuing extreme heat warnings, coordinating interagency responses, preparing government and health professionals, and initiating community outreach programmes, especially for vulnerable communities and outdoor workers.
Early warning systems play a crucial role by sending out key messages via radio, text messages, TV campaigns or internet ads to inform the public about the coming heat and survival strategies, like staying hydrated and indoors.
Heat action plans are moving from a focus on responding to heatwaves once they occur (reactive measures) to preventing or reducing impacts before they happen (mitigative measures). However, governments often lack the capacity to collect and analyse extensive climatological data, which is crucial for developing a more granular understanding of the impact of heatwaves.
Regional cooperation and information-sharing between countries is increasing and is particularly important when it comes to sharing weather and climate-related data.
Appointing chief heat officers, a role first introduced in Miami in 2021, has become a key strategy. A chief heat officer focuses on urban heat mitigation measures, protection of homes and public education.
There is now also a global heat officer who has piloted categorizing heatwaves by their threat level and naming them, similar to hurricanes. This has been shown to help people understand the importance of the warnings and make the danger more apparent. The first named heatwave, Zoe, occurred in Seville in 2023.
Legislative initiatives are also emerging. Spain has announced laws mandating that employers adjust working conditions during periods of extreme heat, including reducing or modifying the hours of the workday. It is an innovative step, not many countries have a designated threshold for when work needs to stop.
A significant challenge for governments is determining responsibility for implementation, as these are multifaceted challenges that do not necessarily sit within one layer of government. The challenge is heightened if national and city-level governance are of different political leanings.
Another important aspect is finance. There is widespread need and demand for funding heat resilience measures, particularly in the Global South. Despite over $700 million pledged to the loss and damage fund at COP28 – a dedicated fund to compensate countries hit hard by climate disasters – climate finance is not yet reaching those most vulnerable.
Ultimately, the long-term solution is to reduce emissions to reduce the intensification of heatwaves. While these measures can address some of the direct impacts of extreme heat, the indirect threats and tipping points require more attention and resources to build resilience.
There is also a lot of opportunity and potential in how governments respond to extreme heat: it is not only a risk and a challenge. Adapting sectors such as transport, construction and agriculture can create millions of green jobs, fostering economic growth.
How can cities become climate resilient?
In cities, heatwaves are exacerbated by the urban heat island effect due to a concentration of surfaces that absorb and and retain heat, combined with often poor climate planning. It is expected that the number of cities exposed to extreme temperatures will nearly triple over the next decades.
By 2050, around 70 per cent of the population will live in cities. This will be partly due to the continued expansion of big metropolitan areas, but also a rapid increase in what are currently second tier cities. Rapidly urbanizing cities in developing countries are more vulnerable to climate change.
While this brings challenges, it is also a chance to steer urban development in a way that is more resilient to extreme heat.
There are a growing number of initiatives to help cities across the world tackle heatwaves, including C40 and its Cool Cities Network, which shares guidance and case studies. The Resilient Cities Network enables city officials to learn from each other on different policy and strategy areas and share experiences on common heat-related challenges.
Cities need to build both social and physical resilience to extreme heat. On the social side, vulnerability mapping works to get detailed information about urban heat, where vulnerable people live and how social protection support or emergency communication needs to be targeted to reach them.
On the physical side, a key action is integrating more natural ways of cooling into urban spaces, including more green and blue infrastructure like urban forests, street trees, more permeable surfaces and green roofs.
Cities like Paris in France and Medellin in Colombia have developed ‘cool islands’ and ‘green corridors’ to give residents protection when walking or to escape hot dwellings.
But any project like this has to be carefully planned so the trees survive. In Madrid, a forest planted around the city’s edge to help with cooling has been calling a ‘tree cemetery’ because so many died from drought.
Another key action is adapting buildings. Many cities must address people’s heat exposure in homes and dwellings that are in informal settlements. In Ahmedabad in India, the city and non-government organizations have together developed climate adaptation solutions, including painting the roofs of 17,000 homes white to lower indoor temperatures and support women living in informal communities.
Architecture such as narrow alleyways that maximise shadows and internal courtyards, common in Gulf countries, can help to cool urban areas.
How will extreme heat change the world?
Heatwaves are not merely weather phenomena but existential threats that will touch every part of our societies. Beyond the more obvious direct and indirect impacts on areas like human health, food security and the economy, there are a whole range of further interconnected effects.
Migration and security are important areas. Extreme heat is going to cause populations to move internally within a country and across borders. Violent crime and political instability will increase. In 2021 a heatwave in Iraq cut out all the power in Baghdad and the southern provinces, which led to massive protests and the resignation of the electricity minister. Events like these could become more frequent.