Introduction
Climate change is often described in terms of global mean temperature — a warming of 1.5°C (2.7°F) or 2°C (3.6°F) above pre-industrial levels. But the impacts of climate change on human society and natural ecosystems are felt primarily through the tails of the weather distribution: the extreme events that occur infrequently but cause disproportionate harm. A warming of 1.5°C (2.7°F) in the global mean temperature corresponds to a much larger shift in the frequency and intensity of extreme heat events, because even a small shift in the mean of a distribution moves the tail dramatically. An event that occurred once every 50 years in a pre-industrial climate may occur once every 10 years at 1.5°C (2.7°F) of warming, and once every 5 years at 2°C (3.6°F).
Attribution science — also called extreme event attribution — is a rapidly developing field that quantifies how much climate change has altered the probability and severity of specific extreme events. Using large ensembles of climate model simulations (comparing thousands of simulations with observed greenhouse gas concentrations to thousands of simulations without anthropogenic forcing), attribution scientists can calculate quantities like: "The probability of a heat wave of this magnitude was increased by a factor of X by climate change" or "Climate change made this rainfall event Y% more intense." The 2021 Pacific Northwest heat dome (which killed ~1,000 people in Oregon, Washington, and British Columbia and caused temperatures of 49.6°C (121°F) in Lytton, British Columbia) was analysed within a week of its occurrence; the attribution study found that this event was "virtually impossible" without climate change — a probability increase of more than 150 times.
Understanding attribution science is important not just for academic understanding but for practical decision-making: it informs insurance markets (which calculate risk premiums based on event probability), infrastructure design standards (which specify the severity of events that must be designed against), and climate litigation (in which attribution studies are increasingly being introduced as evidence of causation).
Key Terms
The scientific discipline that quantifies the contribution of anthropogenic climate change to the probability or magnitude of specific extreme weather events. Uses large model ensembles comparing "factual" world (with observed greenhouse gases) to "counterfactual" world (without anthropogenic forcing). Key outputs: probability ratio (how many times more likely was the event?) and magnitude change (how much more intense?). Pioneered by Pardeep Pall, Myles Allen, Peter Stott and others; published rapidly in near-real-time for major events by World Weather Attribution (WWA) collaboration.
The ratio of the probability of a given extreme event in the current climate to its probability in a pre-industrial or counterfactual climate. PR = 1: no change in probability. PR = 5: five times more likely. PR = 150+: effectively impossible without climate change. The 2021 Pacific Northwest heat dome had PR > 150 (virtually impossible without climate change). The 2022 Pakistan floods had PR ~7–8 (7–8 times more likely with climate change). Computing PR requires large model ensembles because very rare events have high statistical uncertainty from small numbers of occurrences.
A period of abnormally high temperatures (relative to local climatological normal) persisting for at least 2–3 consecutive days. Operational definitions vary by country and agency: typically defined as temperatures exceeding a high percentile (e.g., 90th or 95th) of the local distribution for multiple consecutive days. Heat waves are the deadliest weather phenomenon in many regions — the 2003 European heat wave killed ~70,000 people; the 2010 Russia heat wave killed ~55,000. Humid heat (high wet-bulb temperature) is more lethal than dry heat because it prevents effective evaporative cooling through sweating.
The thermodynamic relationship stating that the water-holding capacity of the atmosphere increases by approximately 7% per degree Celsius of warming. As temperatures increase, the atmosphere can hold more moisture (following the Clausius-Clapeyron equation: dP/dT = L·P/(R·T²), where P is saturation vapour pressure, T is temperature, L is latent heat, R is gas constant). This means that when precipitation occurs, more water falls — making heavy precipitation events more intense. Observed precipitation extremes have increased by ~7% per degree of warming, consistent with the Clausius-Clapeyron relationship, confirming the theoretical prediction.
The temperature measured by a thermometer wrapped in a wet cloth, representing the lowest temperature achievable by evaporative cooling. At high wet-bulb temperatures (> ~35°C (~95°F)), the human body cannot cool itself through sweating even at 100% humidity — core body temperature rises irreversibly, leading to heat stroke and death within hours even for fit young adults at rest in the shade. A 35°C (95°F) wet-bulb temperature corresponds to roughly 46°C (115°F) at 50% humidity or 32°C (90°F) at 100% humidity. Several regions of South Asia and the Persian Gulf have already briefly exceeded the 35°C (95°F) wet-bulb threshold in recent heat events, conditions likely to become more frequent under continued warming.