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Heatwaves: how hot can it get?
- Global Heat Extremes: The world is currently experiencing record-shattering heat, with India recording its hottest March in over a century and England reaching temperatures hotter than the Caribbean and Western Sahara.
- Mortality Trends: Hospital admissions are rising globally, and heat-related deaths in South Asia exceeded 110,000 annually between 2000 and 2019.
- In the US, heat causes more fatalities than tornadoes, hurricanes, or floods combined.
- The 2003 European heat wave resulted in approximately 70,000 deaths.
- The 2003 heat wave was identified as being roughly twice as likely due to climate change.
- The 2020 Siberian heat wave (38°C) was made 600 times more likely by climate change.
- Defining Heatwaves: The term "heatwave" is context-dependent, varying by region and historical norms.
- UK definition: Three consecutive days with temperatures between 25°C and 28°C.
- New York definition: Three consecutive days of 32°C or above.
- Indian plains definition: Temperatures exceeding 40°C and 4.5°C above typical levels for two days.
- Meteorological Mechanisms:
- Anticyclones: Large, slow-moving high-pressure systems (clockwise in the Northern Hemisphere) cause air to sink and compress, heating it and preventing convective cooling.
- Jet Stream Blocking: Amplified meanders in the polar front jet stream create "blocking" patterns that trap high-pressure systems in place for weeks or months.
- Heat Domes: A combination of sinking air and soil moisture depletion creates self-perpetuating "heat domes" that trap heat; 2021 Pacific Northwest temperatures reached a record 49.6°C.
- Wet Bulb Temperatures: A critical metric for human survival where high humidity prevents evaporative cooling.
- Temperatures above 35°C are lethal to healthy humans within approximately six hours as the body overheats from the inside out.
- While no location has sustained this level yet, several stations in South Asia, the Persian Gulf, and North America have recorded temperatures near or above 31°C.
- Future scenarios predict unlivable wet bulb conditions could become common in specific regions.
- Secondary Mortality Factors:
- Heat stress exacerbates respiratory and circulatory issues, particularly for the elderly and those with underlying conditions.
- "Tropical nights" (temperatures staying above 20°C) are increasing; disturbed sleep during these nights is correlated with higher mortality rates.
- Climate Change Attribution:
- Global warming has pushed average atmospheric temperatures up by at least 1°C from pre-industrial levels, significantly shifting the probability curve for extreme high temperatures.
- While the warming trend guarantees more frequent and intense heatwaves, scientists lack consensus on whether atmospheric circulation patterns (like jet stream blocking) will increase or decrease in frequency.
- Adaptation and Policy:
- Success Stories: France implemented a post-2003 heat wave strategy involving regular welfare checks on vulnerable populations and the use of cooled public spaces (e.g., school gyms), which significantly reduced subsequent mortality.
- High-Risk Areas: Regions with no historical precedent for extreme heat, such as Afghanistan, Central Asia, and parts of Central America, are identified as highly susceptible to future lethality.
- Future Requirements: Governments (national, city, and local) must prioritize adaptation policies, while the public requires education on personal and community risk mitigation.
- Long-term Outlook: Even with stabilized greenhouse gas levels, the planet's higher baseline temperature will ensure a continued prevalence of severe heatwaves.