Temperature anomaly

In atmospheric sciences, the average temperature is commonly calculated over a period of at least 30 years over a homogeneous geographic region, or globally over the entire planet.

[1] Anomalies provide a frame of reference that allows more meaningful comparisons between locations and more accurate calculations of temperature trends.

[1] Using different base periods does not change the shape of time series charts or affect portrayal of the trends within them.

[1] Different meteorological organizations have used respective base periods for global mean surface temperature datasets, such as 1951–1980 (NASA GISS[2] and Berkeley Earth[3]), 1961–1990 (HadCRUT U.K.[4]), 1901–2000 (NCDC/NOAA[5]), and 1991–2020 (Japan Met[6]).

The standard deviation—symbolized by a lower case sigma, σ—quantifies the degree of variation of a dataset's values (see coloured bands in chart at right).

Various global surface temperature datasets originally had different reference periods, but for meaningful comparison have been adjusted to have the same "0 °C" reference temperature. Without such an adjustment, the traces would be vertically offset from each other. Here, the "0 °C" value is determined based the average for 1850-1900—considered to be the "pre-industrial" temperature—and does not indicate an absolute measured temperature of "0 °C".
Though northern America has warmed more than its tropics, the tropics have more clearly departed from normal historical variability (coloured bands: 1σ, 2σ standard deviations). [ 7 ] The two charts have the same reference period.