Double the number of dangerous days
Over the past decade, several deadly heatwaves have swept through many cities, with this summer seeing widespread heat waves across the globe. Researchers predict that such extreme heat events will become more common as greenhouse gas emissions continue. A study conducted by researchers from the University of Washington in the United States and Harvard University in the United States examined the global impact of these dangerous heat waves by the end of the century, based on different emissions scenarios. The findings were published in the journal Nature Climate and Environment.
Record-breaking heat in recent summers will occur far more often in regions like North America and Europe. Lucas Vargas Zeppetello, who led the research as a doctoral student at UW and is now a postdoctoral fellow at Harvard, explains that by 2100 outdoor work could become challenging for more than half the year even if emissions begin to fall.
Heat waves driven by climate change threaten large areas of habitability around the world. The authors project, for example, a sixteenfold rise in dangerous heat wave events in Chicago.
Extremely high temperatures also pose public health risks, contributing to heat-related illnesses and chronic conditions.
Vargas Zeppetello notes that even less extreme heat events have shown deadly potential in recent years, and unprecedented events are likely to occur without adequate adaptation. The discussion highlights how a combination of temperature and humidity, the heat index, affects the human body. The US National Weather Service defines a danger heat index as 39.4 degrees Celsius and extremely hazardous at 51 degrees Celsius, indicating unsafe conditions for people at any duration.
These standards were originally created for indoor workers in places like boiler rooms and were not meant for outdoor conditions, but they are increasingly relevant today, according to Vargas Zeppetello.
Impact on global temperatures and risk of dangerous days
The study suggests that even if nations meet the Paris Agreement goals, warming at 2 degrees Celsius would lead to three to ten times more dangerous days by 2100 in the United States, Western Europe, China, and Japan. In tropical regions, dangerous days could double, occupying about half of the year.
The researchers emphasize a range of possible outcomes for 2100. They point out that the emissions choices being made now will shape the living conditions of future generations.
Rather than following four fixed emission pathways used in IPCC reports, the authors apply a statistical approach that blends historical data with estimates of population trends, economic growth, and carbon intensity. Carbon intensity measures the amount of carbon emitted for each dollar of economic activity. This method allows the creation of a realistic set of century-long scenarios.
Adrian Raftery, co-author and professor of statistics and sociology at UW, notes that the approach offers plausible ranges for carbon emissions and future temperatures that are rooted in historical data.
The authors translate rising carbon dioxide levels into global temperature trajectories and examine how these changes would alter monthly weather patterns across the planet.
Watered-down gains, heavier risk for the tropics
The most probable outcome suggests a warming of about three degrees Celsius by the end of the century. While this difference might seem small, avoiding it would bring substantial benefits for populations in tropical regions, where heat stress is most acute.
Estimates indicate only a very small chance of limiting global warming to 1.5 degrees Celsius by 2100 in line with the updated Paris targets. The more likely scenario points to roughly two degrees Celsius by 2050.
The tropics could experience dangerously high heat on many days, while mid-latitude areas could endure increased heat stress. Under the worst-case scenario, without emissions control, extremely dangerous conditions may become common near the equator, particularly in India and parts of sub-Saharan Africa, making outdoor activity hazardous for extended periods.
The researchers stress that while these scenarios are alarming, they also highlight opportunities to prevent them. They urge aggressive efforts to cut emissions and to strengthen adaptive measures to protect populations, especially vulnerable groups such as the elderly and low-income outdoor workers.
The study underscores the need for both reducing future greenhouse gas emissions and boosting resilience in communities around the world. Without adaptation, heat-related health challenges could rise sharply.
Reference work: Nature Climate and Environment, a study by a collaboration between the University of Washington and Harvard University.
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