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Extreme weather is, by definition, rare on our planet. Ferocious storms, searing heatwaves, and biting cold snaps illustrate what the climate is capable of at its worst. However, since Earth’s climate is rapidly warming, predominantly due to fossil fuel burning, the range of possible weather conditions, including extremes, is changing.

Scientists define “climate” as the distribution of possible weather events observed over a length of time, such as the range of temperatures, rainfall totals, or hours of sunshine. From this, they construct statistical measures, such as the average (or normal) temperature. Weather varies on several timescales—from seconds to decades—so the longer the period over which the climate is analyzed, the more accurately these analyses capture the infinite range of possible configurations of the atmosphere.

Typically, meteorologists and climate scientists use a 30-year period to represent the climate, which is updated every 10 years. The most recent climate period is 1991–2020. The difference between each successive 30-year climate period serves as a very literal record of climate change.