This information paper examines different ways of measuring atmospheric temperatures. It looks at the record of surface measurements made during the past century or so and compares satellite temperature recordings with those from ground instruments. The meteorological standard for surface air temperature was chosen in the 19th century as the ambient air temperature in a shaded location 1.5 metres from the ground. Errors in temperature records may be random, such as operator error or an instrument fault. Errors may also be systematic, such as changes to the instrument housing, instrument errors and changes to the surrounding area.
Researchers seek to remove errors from temperature records before looking for long-term trends.
It is well known that urbanisation, with heat-absorbing structures and materials such as concrete, can raise local temperatures. Typically, researchers eliminate from their records the largest urban centres and remove the localised urban warming trend from measurements made at remaining urban stations. The localised urban warming trend is removed either by applying a correction factor or by removing the station completely from the record. Similar efforts are made to remove systematic biases from marine records. Figure 1 shows global temperature changes from 1860-1999.