These maps, developed by NASA's Goddard Institute for Space Studies (GISS), depict how much various regions of the world have warmed or cooled when compared with a base period of 1951-1980. They show temperature anomalies, or changes, not absolute temperature.
The objective is to produce a key data record describing ongoing climate change and a basis for evaluating simulations of global changes in response to changes of atmospheric carbon dioxide, aerosols, and solar activity.
The temperatures we experience locally and in short periods can fluctuate significantly due to predictable cyclical events (night and day, summer and winter) and hard-to-predict wind and precipitation patterns. But the global temperature mainly depends on how much energy the planet receives from the Sun and how much it radiates back into space − quantities that change very little. The amount of energy radiated by the Earth depends significantly on the chemical composition of the atmosphere, particularly the amount of heat-trapping greenhouse gases.
To conduct its analysis, GISS gathers publicly available data from 6,300 land-based meteorological stations around the world; ship-based and satellite observations of sea surface temperature; and Antarctic research station measurements. These data sets are loaded into a computer analysis program − available for public download − that calculates temperature change relative to the average temperature for the same time period (month or year) during 1951-1980.
The period of 1951-1980 was chosen largely because the U.S. National Weather Service uses a three-decade period to define "normal" or average temperature. The NASA GISS analysis effort began around 1980, so the most recent 30 years at the time was 1951-1980. It is also a period when many of today's adults grew up, so it is a common reference that many people can remember.
Shades of red and orange indicate areas where the average monthly temperatures are warmer than they were in that area during the base period from 1951-1980. Shades of blue show cooling compared to the base period. For example, if New England is red in a March of a given year, then it is warmer than the average March in New England from 1951-1980. If part of Canada is blue in August, then it is cooler than the average for August during the base period. Black areas are where no data are available.