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 2012, the global temperature was about 1.03°F (0.57°C) above the long-term average for the 20th century, according to NOAA's National Climatic Data Center. That number made 2012 the 10th warmest year on record within a database going back to 1880. But among years with La Niña events under way (which typically cool the climate), 2012 was the third warmest on record.
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.