Global Surface Temperature Anomalies
When it comes to tracking surface temperature trends, scientists don't use just one dataset. Instead, there are several groups of scientists who keep track of global average surface temperatures, each relying on their own datasets and methods. Two teams are located in the United States, one at NASA's Goddard Institute of Space Studies (GISS) in New York, and the other at the National Oceanic and Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC). Another key group works in the U.K., and a team based out of the Japanese Meteorological Agency also helps keep tabs of global average surface temperature trends. Each group uses different methods to analyze the data, which results in slight discrepancies between the measurements of one group versus another.
However, as this graphic — produced by NASA with data assistance from Climate Central — shows that all of the groups' data exhibits the same trend of warming temperatures during the past several decades. The time series show annual values (January through December means) of globally averaged temperature anomalies from 1880 to 2010. The green line shows values from NOAA/NCDC's data; the red line shows NASA/GISS data; the blue line shows the UK Hadley Center/CRU data; and the purple line shows data from the Japanese Meteorological Agency.
All four datasets have been constructed including both land and sea surface observations. The time series have been centered around the common reference period 1961-1990, i.e., each value is the difference between that year's temperature and the average temperature from the corresponding dataset over the 30-year period 1961-1990. In order to measure the agreement among the four data sources, the difference between the corresponding values of each dataset has been computed for each year.
This graphic demonstrates that over the long-term, the surface temperature records are all consistent in showing a sharp uptick in global temperatures beginning during the latter half of the 20th century, and continuing on through the present day. The close agreement among the datasets helps give scientists greater confidence in concluding that temperatures are increasing, due in part to the burning of fossil fuels.