Climate observations - The instrumental record

A survey is given of the available instrumental data for monitoring and ana lysis of climatic variations. We focus on temperature measurements, both ov er land and ocean, at the surface and aloft. Over land, the older observations were subject to exposure changes which ma y not have been fully compensated. The effects of urbanization have been la rgely avoided in studies of climatic change over the last 150 years. There are few records for pre-1850 outside Europe and eastern North America, and the global network shows a recent decline. Over the ocean, sea surface temp erature (SST) has been measured using buckets, engine intakes, hull sensors , buoys, and satellites. Many of these data have been effectively homogeniz ed, but new challenges arise as observing systems evolve. Available SST and marine air temperature datasets begin in the 1850s. The data are concentra ted in shipping lanes especially before 1900, and very sparse during the wo rld wars, but additional historical data are being digitized. The radiosonde record is short (similar to 40 years) and has major gaps ove r the oceans, tropics and Southern Hemisphere. Instrumental heterogeneities are beginning to be assessed and removed using physical and statistical te chniques. The MSU record is complete but only began in 1979, and is not hig hly resolved in the vertical: major biases, mainly affecting the lower-trop ospheric retrieval, have been reduced as a result of recent analyses. Advanced interpolation or data-assimilation techniques are being applied to these data, but the results must be interpreted with care.