Extreme Weather Science: How Climate Change is Rewriting Weather Records

The question that follows every major extreme weather event — did climate change cause this? — has driven the development of one of the most practically important new fields in climate science: extreme event attribution. Attribution science uses climate models, statistical analysis, and physical reasoning to quantify how climate change has altered the probability and intensity of specific extreme weather events. The 2021 Pacific Northwest heat dome, which killed over 1,000 people in Canada and the US and pushed temperatures above 49°C in British Columbia, was found by rapid attribution analysis to have been "virtually impossible" without climate change. The 2022 Pakistan floods that submerged one-third of the country were made significantly more likely and more intense by anthropogenic climate change. These findings have transformed both public understanding and legal accountability for climate change damages.

Heatwaves — The Deadliest Extreme

Heatwaves — prolonged periods of abnormally high temperatures — are the deadliest form of extreme weather in most countries. Climate change intensifies heatwaves through three mechanisms. First, it shifts the temperature distribution upward, making extreme events more frequent: what was a 1-in-50-year extreme may become a 1-in-5-year event at +2°C. Second, it increases the absolute intensity of heat extremes — the Pacific Northwest heat dome was approximately 2°C more intense than models suggest would have been possible without climate change. Third, it potentially increases the duration of extreme heat by altering the atmospheric patterns — particularly the weakening of the polar jet stream associated with Arctic warming — that cause heat waves to stall and persist over affected regions. The 2022 European heatwave killed approximately 60,000 people — the deadliest natural disaster in European recorded history.

Flooding — The Clausius-Clapeyron Effect

The physics of climate change's effect on flooding is determined by the Clausius-Clapeyron equation: a warmer atmosphere holds more water vapour — 7% more per degree of warming — and when this water precipitates, it does so more intensely. This relationship has been confirmed by both observations and climate models: the most intense rainfall events are intensifying at approximately 7% per degree of warming in most regions, broadly consistent with Clausius-Clapeyron scaling. The implications for flooding are significant: the same storm system produces heavier rainfall in a warmer world, overwhelming drainage systems designed for the historical precipitation regime. Urban flooding is particularly vulnerable — storm drains designed for historical 1-in-100-year events face those events with increasing frequency as climate change shifts the precipitation distribution toward heavier extremes.

Tropical Cyclones — Intensifying in a Warmer World

The relationship between climate change and tropical cyclones (hurricanes and typhoons) is one of the most intensively studied questions in climate attribution science. The overall frequency of tropical cyclones — the total number of storms forming each year — is projected to remain stable or possibly decrease under warming, because the atmospheric dynamics that drive cyclogenesis are influenced by multiple factors that partly offset each other. However, the proportion of cyclones that reach the highest intensities (Category 4 and 5) is projected to increase substantially — a change driven by the warmer sea surface temperatures and increased atmospheric moisture that fuel intense convection. Observational evidence is broadly consistent with this projection: the proportion of all cyclones reaching Category 3 or higher intensity increased by approximately 25% between 1979 and 2017, and the poleward migration of peak cyclone intensity — as warming extends the zone of sea surface temperatures warm enough to sustain intensification — has been documented in both hemispheres.

Heat Dome Events — When Blocking Creates Disasters

The 2021 Pacific Northwest heat dome — which pushed temperatures above 49°C in Lytton, British Columbia, and killed over 1,400 people in Canada and the western United States — illustrated a mechanism of extreme heat creation that is becoming more frequent with climate change. Heat domes form when a high-pressure system stalls over a region, trapping and compressing air beneath it. As air descends under high pressure, it warms adiabatically, creating extreme surface temperatures. The stalling of such blocking patterns — which prevents the normal west-to-east progression of weather systems — appears to be becoming more common as the Arctic warms faster than the tropics, weakening the temperature gradient that drives the jet stream and making it more prone to large-amplitude, slow-moving waves. When these waves create stationary high-pressure ridges over populated areas during summer, the result is heat dome events of extraordinary intensity.

The attribution science applied to the Pacific Northwest heat dome found that the event was "virtually impossible" without climate change — meaning that the combination of temperature anomaly (approximately 5°C above historical records) and geographic extent could not have occurred in the pre-industrial climate. This conclusion was reached by comparing climate model simulations with and without anthropogenic forcing, finding that the probability of such an event in the historical climate was vanishingly small. The event also demonstrated a property of climate change that attribution science has consistently found: extreme events do not scale linearly with average warming. As the tail of the temperature distribution extends into previously uncharted territory, events occur that have no historical precedent — "the new abnormal" — whose probability changes dramatically with relatively small increases in mean temperature.