New measurements by NASA's Goddard Institute for Space Studies indicate that 2012 was the ninth warmest year since 1880, and that the past decade or so has seen some of the warmest years in the last 132 years.

One way to illustrate changes in global atmospheric temperatures is by looking at how far temperatures stray from "normal", or a baseline. For the following map, NASA picked a baseline period using temperatures between 1951 and 1980, and compared temperature global temperature readings from 2012.

NASA's Earth Observatory blog explains:

The average temperature in 2012 was about 14.6 degrees Celsius (58.3 degrees Fahrenheit), which is 0.55°C (1.0°F) warmer than the mid-20th century base period. The average global temperature has increased 0.8°C (1.4°F) since 1880, and most of that change has occurred in the past four decades.

The long-term trend is clearly represented by plotting temperature anomalies on the line graph, this time from 1880 onward. Again, from NASA:

The line plot above shows yearly temperature anomalies from 1880 to 2011 as recorded by NASA GISS, the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center, the Japanese Meteorological Agency, and the Met Office Hadley Centre in the United Kingdom. All four institutions tally temperature data from stations around the world and make independent judgments about whether the year was warm or cool compared to other years.

Even with natural variations due to seasons and other events, we see that average atmospheric temperatures are increasing from decade to decade, which agrees with results from climate models and simulations. And as expected, temperatures have increased as greenhouse gas (carbon dioxide, methane, etc) emissions have increased from the U.S. Industrial Revolution in the late 19th Century onwards.