NCDC Frequently Asked Questions
Why do some of the products use different reference periods?
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The maps show temperature anomalies relative to the 1971-2000 base period. This period is used because it has the widest distribution for historical data, which enables more resolution (detail) for comparing region-by-region effects. For the global-scale averages (global land and ocean, land-only, ocean-only, and hemispheric time series), the reference period is adjusted to the 20th Century average for conceptual simplicity (the period is more familiar to more people, and establishes a longer-term average). The adjustment does not change the shape of the time series or affect the trends within it.