Climatologists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages. Between 1961 and 1990, the annual average temperature for the globe was around 57.2°F (14.0°C), according to the World Meteorological Organization.
In addition, we can evaluate climate over longer periods of observation. For example, in 2013, the global temperature was about 1.12°F (0.62°C) above the long-term average for the 20th century, according to NOAA's National Climatic Data Center. That number put 2013 in a tie with 2003 for the 4th warmest year on record in the NOAA database, which goes back to 1880. Among those years with no El Niño events under way (which typically cool the climate), 2013 ranks behind only 2005 for global warmth.
One reason is that there are several different techniques for coming up with 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. A website from NASA's Goddard Institute for Space Studies goes into more detail on the topic of The Elusive Absolute Surface Air Temperature.