Global average temperature since 1880. This graph from NOAA shows the annual trend in average global air temperature in degrees Celsius, through December 2012. For each year, the range of uncertainty is indicated by the gray vertical bars. The blue line tracks the changes in the trend over time. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

Averaged over all land and ocean surfaces, temperatures have warmed roughly 1.33°F (0.74ºC) over the last century, according to the Intergovernmental Panel on Climate Change (see page 2 of the IPCC's Climate Change 2007: Synthesis Report Summary for Policymakers (PDF)). More than half of this warming—about 0.72°F (0.4°C)—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 1.26° F or 0.7º C since 1979), especially over the Northern Hemisphere.

The graph above clearly shows the variability of global temperature over various time intervals (such as year to year or between decades) as well as the long-term increase since 1880.

There are slight differences in global records between groups at NCDC, NASA, and the University of East Anglia. Each group calculates global temperature year by year, using slightly different techniques. However, analyses from all three groups point to the decade between 2000 and 2009 as the hottest since modern records began more than a century ago. Temperatures in the 2010s have been running slightly warmer still.

U.S. average temperature since 1895. The blue line is the 12-month average for each year. The green line is a calculation of the trend during each decade. The red line shows the trend over the last century of records. The gray line is flat because it is a simple average for the entire period. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

What about the United States?

The year 2012 was the warmest on record for the contiguous United States, according to the National Climatic Data Center (NCDC). As shown in the graph at right, 2012 was substantially warmer—a full degree Fahrenheit (0.6°C)—than any other year since national records began in 1895. The U.S. warming rate of about 1.3°F (0.72°C) per century (red line in the graph at bottom right) is roughly comparable to the global rate of warming (see above).

Although the U.S. racked up several cooler years from 2008 to 2010, the decade as a whole (2000–2009) was the nation's warmest on record, with an average temperature of 54.0°F. In contrast, the 1990s averaged 53.6°F, and the 1930s averaged 53.4°F.