The effects of heat on human health are still poorly understood

On July 21, 2024, two-thirds of the global population experienced the uncomfortable effects of extreme heat. According to experts from two meteorological organizations, the 1.3 °C increase in global temperatures since the pre-industrial era—recorded in 2024—has added an average of 41 days where heat has harmed human health and ecosystems. 

The study, entitled When Risks Become Reality: Extreme Weather in 2024, was released on December 27. During an embargoed webinar held a few days prior, researchers from World Weather Attribution (WWA) and Climate Central presented the major findings of their work, offering a year-end review of 2024. 

According to Julie Arrighi, director of the Red Cross and Red Crescent Climate Centre, the impact of heatwaves remains poorly understood. In many countries, public health authorities are unable to track deaths linked to extreme heat or drought or assess its full impact on human health. 

“Mortality during heatwaves doesn’t always peak on the hottest days, because the most vulnerable people with pre-existing conditions are often the first to suffer and die from the heat,” said Arrighi during the webinar. 

Friederike Otto, lead researcher at WWA and climate science professor at Imperial College London, emphasized the need to better alert populations during extreme heat events. Why? Because the effects of heatwaves and droughts are among the most severe consequences of climate change for human populations. 

The study highlighted an April 2024 heatwave in Mali, where a hospital reported an increased number of deaths as temperatures approached 50 °C. “Reported by local media, the announcement was a rare example of healthcare professionals raising the alarm about the dangers of extreme heat in real-time.” 

Similar reports came from doctors in Mexico, Morocco, and Saudi Arabia during heatwaves in those regions. “Informing local journalists when emergency departments are overwhelmed is a simple way to alert the public that extreme heat can be deadly,” the study explains. 

The report also provides links to WWA’s various analyses measuring how climate change affects the intensity and frequency of extreme weather events. 

Drought 

In early 2024, southern Africa experienced significant agricultural losses due to a lack of rainfall. Famine affected more than 20 million people across Zimbabwe, Zambia, Malawi, Angola, Mozambique, and Botswana. 

These countries are accustomed to extended droughts during December to February when El Niño is dominant. However, rainfall anomalies—exacerbated by climate warming—are particularly devastating in regions reliant on subsistence farming, the study notes. 

Since the third quarter of 2023, the Amazon River basin in South America has also suffered from severe drought. By January 2024, the river’s flow reached its lowest levels in 120 years. Approximately 30 million people living in Brazil, Peru, Colombia, Venezuela, Ecuador, and Bolivia have had their daily lives disrupted by drought and extreme heat. Hydroelectric power generation, a key source of energy in these countries, has often been interrupted due to water shortages. 

The Northern Hemisphere has not been spared either. In Italy, the island regions of Sicily and Sardinia have faced an unusual drought since the summer of 2023. Local authorities have had to ration potable water, including for agricultural irrigation. Crop production and livestock farming have been heavily affected, and tourism—an essential economic activity for both islands—has also slowed significantly. 

Wildfires 

Drought and heatwaves have also made wildfires increasingly difficult to control. Once again, 2024 was a very active year for wildfires, particularly in the Americas. In February, wildfires in Chile caused 132 deaths and injured more than 1,000 people. In Canada and the United States, wildfire activity remained significant, as it was in 2023. 

Canada experienced its second-worst wildfire season on record, surpassed only by 2023, when Quebec, Alberta, and British Columbia were particularly affected. Studies have also examined the impact of wildfires on air quality and the rising number of deaths linked to inhaling fine particulate matter. 

To understand how climate change influenced high temperatures in 2024, scientists identified “dangerous heat days” by calculating local temperature thresholds for the hottest 10 per cent of days between 1991 and 2020. These days, which exceed the 90th percentile of historical temperatures, are associated with increased health risks. Researchers then compared the number of days exceeding this threshold to a scenario without climate change. 

Climate Central developed an index called the Climate Shift Index (CSI), which measures the likelihood of a temperature anomaly compared to pre-industrial levels. When the CSI exceeds 2, the probability of the extreme event occurring is twice as high as it was about 200 years ago. 

On July 21, 2024, the CSI exceeded 2 in regions home to more than 5.3 billion people. This was the day with the highest average global temperature, a record that was broken the following day. 

The study also examines the impact of other extreme climate events, including heavy rainfall, floods, tropical storms, and cold spells. In such cases, prevention measures and early warning systems can help save lives, as demonstrated in Central Europe in September. 

In 2024, 219 extreme events exceeded critical thresholds worldwide, with 29 of these studied separately by WWA researchers to quantify how much climate change contributed to their severity. 

“We know exactly what needs to be done to stop things from getting worse : stop burning fossil fuels. The top resolution for 2025 must be transitioning away from fossil fuels, which will make the world a safer and more stable place,” said Friederike Otto.