Extreme weather

• 1. Introduction

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  Extreme weather killed more than 5,000 people directly during a single American heat wave in 1936. That figure comes from one of the deadliest summers on record in the United States, and it sits alongside another number that tells a different story: in the 1920s, natural disasters claimed roughly 5.4 million lives worldwide in a single decade. By the 2010s, that number had fallen to around 400,000, even as Earth's human population had quadrupled and average temperatures had climbed 1.3 degrees Celsius. How can extreme weather become simultaneously more costly, more frequent, and yet less deadly? And what exactly counts as extreme? This documentary looks at the different types of extreme weather, the forces behind them, the human choices that make them worse, and the economic reckoning that follows when they arrive.

• 2. What Makes Weather Extreme

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  The IPCC Sixth Assessment Report sets a precise threshold: an extreme weather event is one that is as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations. In plain terms, a weather event is extreme when it falls at the outer edge of what a location has historically recorded. But rarity alone does not define the category. Weather can also be called extreme when it causes substantial disruption to a community or ecosystem, even if it is not statistically unprecedented in that location.

  The World Meteorological Organization draws a related but distinct line. It reserves the term severe weather for any weather condition that poses risks to life, property, or requires authorities to intervene. Severe weather is therefore a particular subset of the broader extreme category. This distinction matters for researchers, because definitions that vary across disciplines change what gets counted and therefore what conclusions get drawn.

  The main types of extreme weather recognised across the field include heat waves, cold waves, droughts, heavy precipitation events, tropical cyclones, and tornadoes. Each carries its own measurement criteria and produces its own pattern of harm, meaning no single framework captures them all.

• 3. The Hidden Threat of Heat

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  Heat waves are periods of abnormally high temperatures and elevated heat index values. Unlike hurricanes or tornadoes, they leave no visible trail of destruction, which is part of why they remain one of the less recognised forms of extreme weather despite their lethality. The European heat waves of summer 2003 are estimated to have caused 30,000 excess deaths through heat stress and air pollution combined. A separate calculation for the UK alone found that extra pollution during the hot summer of 2006 cost approximately 460 lives.

  The physiological cascade begins in plants as well as people. When temperatures climb beyond a threshold, plants close their leaf pores, called stomata, to conserve water. That mechanism cuts off the plants' ability to absorb pollution and ozone, leaving more of both suspended in the air that people breathe, and mortality rates in human populations rise as a result.

  Dried soils also become more vulnerable to erosion, shrinking the land available for agriculture. Bodies of water can evaporate to a degree that devastates marine habitats and strips the nutrition from the water that remains. Over 200 cities across the United States have registered new record high temperatures, and Australia's worst heat wave, in 1938-39, killed 438 people; its second worst came in 1896. The urban heat island effect compounds all of this by pushing overnight temperatures higher than surrounding rural areas.

• 4. Cold That Kills and Freezes

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  The U.S. National Weather Service defines a cold wave as a rapid fall in temperature within a 24-hour period requiring substantially increased protection for agriculture, industry, commerce, and social activities. The precise threshold depends on the geographical region and the time of year, because what constitutes dangerous cold in one place may be unremarkable in another.

  Beyond frostbite and hypothermia in people and livestock, cold waves produce a set of second-order hazards that are easy to overlook. Poorly insulated water pipes freeze and can rupture as the expanding ice splits them, causing property damage. Water mains may break and become unreliable at the same moment that firefighters need them most, making fires, paradoxically, more dangerous during extreme cold than in milder conditions.

  The Year Without a Summer in 1816 stands as one of history's starkest examples of cold-driven crop failure. It was one of several years during the 1810s when volcanic eruptions reduced incoming sunlight and triggered freakish summer cold snaps that killed crops before they could be harvested, producing famines. A 2023 study found that weak extreme cold events are significantly decreasing in frequency across the northern hemisphere with global warming, but that strong cold events show no significant trend overall, and are actually increasing in Siberia and Canada. The JRC PESETA IV project, which concluded in 2020, found that overall climate change will lead to a decline in the intensity and frequency of extreme cold spells on balance.

• 5. Causes, Attribution, and Climate Change

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  A 2016 report from the National Academies of Sciences, Engineering, and Medicine called for improved shared practices in attribution research and stronger links between research outcomes and weather forecasting. Attribution research is the field that attempts to explain the causes behind specific extreme events, and it has shifted considerably in recent decades. Early work focused on predicting individual events; contemporary research focuses on tracing the causal threads behind broader trends.

  Climatic phenomena such as El Nino-Southern Oscillation, known as ENSO, and the North Atlantic Oscillation, or NAO, push weather patterns in specific directions in different regions of the world. Record-breaking extreme events of the past two centuries most likely arise when patterns like ENSO or NAO work in the same direction as human-induced warming, amplifying what natural variability would produce on its own.

  A study published in Nature in 2019 used several simulations to examine how melting ice sheets in Greenland and Antarctica could affect overall sea level and sea temperature. Other models have shown that modern temperature rise and the addition of meltwater to the ocean could disrupt the thermohaline circulation, the system responsible for moving seawater and distributing heat around the globe. A collapse of that circulation in the northern hemisphere could push extreme temperatures higher in Europe and produce more frequent storms. There were around 6,681 climate-related events reported during 2000-2019, compared to 3,656 during 1980-1999, a figure that does not include geophysical events like earthquakes or volcanic eruptions. In 2020, the National Oceanic and Atmospheric Administration predicted that over the 21st century, the frequency of tropical storms and Atlantic hurricanes would decline by 25 percent while their maximum intensity would rise by 5 percent.

• 6. Human Choices That Amplify Damage

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  Building homes below sea level or along a floodplain puts residents at increased risk of injury or destruction in any heavy precipitation event, regardless of whether the storm is unprecedented. That choice, multiplied across communities, converts a manageable flood into a catastrophe. Urban planning more broadly amplifies flooding impacts because impervious surfaces such as roads, sidewalks, and roofs prevent storm water from being absorbed by the ground.

  The destruction of wetlands removes a natural reservoir that would otherwise absorb water during storms. Along coastlines, that loss eliminates a buffer zone that limits how far storm surges travel inland during hurricanes or cyclones. Tall structures in cities alter wind movement, pushing warmer air upward and triggering convection that generates thunderstorms. The heat those same structures absorb from the sun, combined with pollution and heat released by vehicles, creates urban heat islands where temperatures rise well above those in surrounding areas.

  This interplay between human choices and atmospheric conditions means that the harm produced by a given extreme weather event is never purely a function of the event's physical intensity. Poor urban planning, wetland destruction, and floodplain development each add to the toll, and each is within the power of communities and governments to change.

• 7. The Toll on Lives, Crops, and Economies

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  On the 23rd of July 2020, Munich Re announced that the 2,900 total global deaths from natural disasters for the first half of 2020 were a record low, and described the figure as much lower than the averages for both the previous 30 years and the previous 10 years. That improvement reflects better early warning systems and disaster response capacity, which is particularly visible in south Asia. A tropical cyclone in Bangladesh in 1991 killed 135,000 people. A 1970 cyclone killed 300,000. Cyclone Amphan, which struck India and Bangladesh in 2020 and was of similar size to both, killed 120 people in total.

  A 2021 study found that 9.4 percent of global deaths between 2000 and 2019, roughly 5 million people annually, could be attributed to extreme temperatures. Cold-related deaths made up 8.5 percent of that total and were decreasing; heat-related deaths made up 0.9 percent and were increasing. A 2023 study published in The Lancet Planetary Health estimated that extreme cold events contributed to over 130,000 excess deaths annually and extreme heat events to over 13,000 excess deaths annually in European urban areas between 2000 and 2019.

  For agriculture, one global study of cereal production between 1964 and 2007 found that drought and extreme heat together reduced national cereal production by 10 percent. Drought affected both harvested area and yields, while extreme heat alone reduced yield without cutting the area under harvest. In economic terms, the IPCC estimated in 2011 that annual losses from weather-related disasters had ranged since 1980 from a few billion to above US$200 billion, with the highest losses occurring in 2005, the year of Hurricane Katrina. A 2023 study placed the cumulative costs of extreme weather caused by climate change at approximately US$2.86 trillion between 2000 and 2019, averaging roughly US$143 billion in losses each year. The World Economic Forum's Global Risks Perception Survey for 2023-2024 found that 66 percent of respondents selected extreme weather as their top risk, the survey having been conducted in the aftermath of the 2023 heat waves.