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Climate scientists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages.
Today's global temperature is typically measured by how it compares to one of these past long-term periods. For example, the average annual temperature for the globe between 1951 and 1980 was around 57.2 degrees Fahrenheit (14 degrees Celsius). In 2015, the hottest year on record, the temperature was about 1.8 degrees F (1 degree C) warmer than the 1951–1980 base period.
That calculation comes from NASA and NOAA. Other agencies may come up with a slightly different number because there are several techniques for calculating a global average, depending on how one accounts for temperatures above the data-sparse oceans and other poorly sampled regions.
Since there is no universally accepted definition for Earth’s average temperature, several different groups around the world use slightly different methods for tracking the global average over time, including:
The important point is that the trends that emerge from year to year and decade to decade are remarkably similar—more so than the averages themselves. This is why global warming is usually described in terms of anomalies (variations above and below the average for a baseline set of years) rather than in absolute temperature.