The Escalating Threat of Extreme Heat

I. Executive Summary This briefing summarizes the critical insights from "When Will Extreme Heat Become Unlivable?", focusing on the escalating danger of extreme heat, particularly "wet-bulb events." Historically rare, these humid heat waves are projected to become increasingly common and lethal with global warming. The sources highlight the physiological mechanisms that make humid heat so deadly, the unexpectedly low thresholds for human survivability, and the limitations of current adaptive measures like air conditioning. The overarching message is a call to action for rapid decarbonization and comprehensive urban planning to mitigate a future where large parts of the Earth become unlivable for humans. II. Key Themes and Most Important Ideas/Facts A. The Distinction and Growing Threat of Humid (Wet-Bulb) Heat Waves Dry vs. Humid Heat: The briefing distinguishes between "dry heat waves," typically caused by "large high pressure systems that trap hot air near the surface," (e.g., 2003 European heat wave, 2021 Pacific Northwest heat dome) and "extreme humid heat waves, often referred to as wet-bulb events." The Scariest Byproduct of Climate Change: Wet-bulb events are described as "one of the scariest byproducts of climate change" due to their unprecedented danger. While "relatively rare historically," a new study projects that "by two degrees Celsius of global warming, many of the most populated places on Earth are likely to experience them." Mechanism of Humid Heat: As temperatures rise due to climate change, "we can hold a lot more moisture in the atmosphere," leading to increased humidity, especially in locations with a moisture source. B. The Physiological Danger of Wet-Bulb Temperatures Impaired Sweating Mechanism: Humans primarily cool down by sweating. However, "when the air is humid, sweat can't evaporate as easily, so we don't cool off as well." This inefficiency makes humid heat "so muggy and uncomfortable for us and why it can be so dangerous." Wet-Bulb Temperature Defined: Wet-bulb temperature "combines heat and humidity into a single number," serving as a critical indicator for dangerous or lethal conditions. It mimics human skin's evaporative cooling: "Water evaporates off the wet sock while sweat evaporates off our wet skin. In both cases, evaporation carries away heat into the surrounding environment." The higher the humidity, "the more difficult and less efficient that evaporation is to take place," diminishing the body's ability to cool itself. "Limit of Compensability" and Uncompensable Thresholds: Scientists define the "limit of compensability" as the point where the human body's core temperature begins to increase uncontrollably, despite its efforts to regulate through sweat. New research shows that "uncompensable wet-bulb temperatures range from about 19 to 32 degrees Celsius," which is "a lot lower than we previously believed." Contextualizing Wet-Bulb Temperatures: A 19-degree Celsius wet-bulb temperature "does not equal 19 degrees on a regular thermometer," but is "actually more like 46 degrees Celsius with low humidity or 32 degrees Celsius with higher humidity." This threshold varies based on "activity level, exact air temperature, and humidity levels and age." Vulnerability of Older Adults: "Older adults thermo regulatory responses are worse, they sweat less efficiently, they also have higher risks of comorbidities." C. Projected Impacts and Geographic Vulnerabilities Global Warming Scenarios:2 Degrees Celsius Warming (potentially within two decades): "About a third of Earth's land area" could cross uncompensable wet-bulb thresholds for older adults. This includes "many of our most populous places on earth. Most of India, most of the populated area of China. This is most of the eastern portion of the U.S." Paris, for example, would experience deadly heat waves like the 2003 event (which killed "up to 3,400 people in Paris alone") "once every few years rather than once a century." 4 Degrees Celsius Warming (higher end projection): "Over 60% of Earth's landmass could see these dangerous wet-bulb temperatures." Real-World Examples:The 2015 Karachi, Pakistan heat event, where "temperatures up to 45 degrees Celsius combined with oppressive humidity, power outages and water shortages," resulted in "at least 1200 people died in just 10 days." This event "passed the uncompensable wet-bulb threshold for older adults for something like eight to 10% of the time." At 2 degrees Celsius warming, the same event in Karachi would cross the "uncompensable threshold for younger adults as well." At 4 degrees Celsius warming, models predict the Karachi event would "cross the uncompensable threshold for older adults for about 90% of the heat waves duration." Heat as the Deadliest Disaster: Heat is already "the deadliest weather disaster in the U.S. in many parts of the world." D. Limitations of Adaptation Measures, particularly Air Conditioning AC as the Primary Defense: The only way to survive these extreme conditions when core body temperature rises uncontrollably is "by cooling yourself or your environment." "Air conditioning is really the major adaptation measure and protection measure that we have." Ineffectiveness of Other Measures: "It doesn't matter how breezy it is, it doesn't matter how much shade you're under. It doesn't matter how much water you're drinking." Flaws of AC:Cost and Accessibility: AC is "expensive" and "energy intensive," making it unaffordable for many, especially in vulnerable regions. Exacerbating Urban Heat Island Effect: AC "doesn't eliminate heat, it just moves it." Cooling indoors transfers heat outside, "which warms the surrounding air" and "can worsen the urban heat island effect." Grid Overload and Outages: Power grids are not designed for universal, simultaneous AC use. "During heat waves, widespread outages become more likely," posing immediate risk to people if AC is their only protection. "If AC units were our only way of protecting people as soon as the power system fails...that's going to put people immediately at risk." Compounding Hazards: The risk is amplified by "compounding hazards like a heat wave after a hurricane" that wipes out power. Fueling Global Warming: AC currently accounts for "about 3% of global greenhouse gas emissions," and with a projected "nearly triple" increase in AC units worldwide by 2050, this could "positively feed back on global temperature making the temperature even higher." E. Solutions and Path Forward Clean Energy Transition: The "powerful argument for transitioning as quickly as possible to clean energy" is highlighted as the only way to meet growing AC demand without accelerating climate change. Beyond AC: Other crucial measures include: Rethinking city design: "installing white or green roofs and planting trees." Building more "cooling centers." Upgrading "electrical grids to be more resilient." Prioritizing "protecting the most vulnerable during heat waves." Choice and Urgency: While "we are headed into uncharted territory," the briefing concludes with a message of agency: "it's also true that we still have time to choose how bad it gets." III. Conclusion The threat of extreme heat, particularly wet-bulb events, is a critical and escalating consequence of climate change. These events pose a direct threat to human survivability due to their ability to overwhelm the body's natural cooling mechanisms at surprisingly low thresholds. While air conditioning offers a temporary solution, its inherent limitations in terms of cost, environmental impact, and grid reliability necessitate a rapid and comprehensive shift towards clean energy and resilient urban planning. Failure to act will lead to widespread unlivable conditions across the most populous regions of the Earth, transforming historically rare events into frequent, devastating occurrences.