To document air temperature, scientists measure land surface temperatures a short distance above the ground at stations around the world. These researchers standardize the measurements by accounting for elevation, latitude, time of observation, and type of instrument, and then integrate the information to form a long-term record at a particular location.
Scientists combine measurements of land surface temperatures and sea surface temperatures to calculate the global average temperature. They report this average as the difference from a historical base period. For example, NASA compares the global average temperature each year to a base temperature of roughly 57.2° F (14° C)—an average derived from several decades.
Three major research centers regularly calculate the global average temperature. Although each center uses a slightly different technique, all the results show the same two trends. The first is that each of the last three decades has been hotter than the one before. The second is that the long-term average global temperature is rising.