NCDC Frequently Asked Questions
What is the difference between the gridded dataset and the index values?
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The land and ocean gridded dataset is a large file (~24 mb) that contains monthly temperature anomalies across the globe on a 5 deg x 5 deg grid. The anomalies are calculated with respect to the 1971-2000 base period. Gridded data is available for every month from January 1880 to the most recent month available. You can use it to examine anomalies in different regions of the earth on a month-by-month basis. The index values are an average of the gridded values (see the FAQ on calculating the global temperature anomalies); however, the anomalies are provided with respect to the 20th century (1901-2000) average. They are most useful for tracking the big-picture evolution of temperatures across larger parts of the planet, up to and including the entire global surface temperature.