Instrumental temperature record

The instrumental temperature record is a record of temperatures within Earth's climate based on direct, instrument-based measurements of air temperature and ocean temperature. Instrumental temperature records are distinguished from indirect reconstructions using climate proxy data such as from tree rings and ocean sediments. Instrument-based data are collected from thousands of meteorological stations, buoys and ships around the globe. Whilst many heavily-populated areas have a high density of measurements, observations are more widely spread in sparsely populated areas such as polar regions and deserts, as well as over many parts of Africa and South America. Measurements were historically made using mercury or alcohol thermometers which were read manually, but are increasingly made using electronic sensors which transmit data automatically. Records of global average surface temperature are usually presented as anomalies rather than as absolute temperatures. A temperature anomaly is measured against a reference value (also called baseline period or long-term average). For example, a commonly used baseline period is the time period 1951-1980. The longest-running temperature record is the Central England temperature data series, which starts in 1659. The longest-running quasi-global records start in 1850. Temperatures are also measured in the upper atmosphere using a variety of methods, including radiosondes launched using weather balloons, a variety of satellites, and aircraft. Satellites are used extensively to monitor temperatures in the upper atmosphere but to date have generally not been used to assess temperature change at the surface. In recent decades, global surface temperature datasets have been supplemented by extensive sampling of ocean temperatures at various depths, allowing estimates of ocean heat content. The record shows a rising trend in global average surface temperatures (i.e. global warming) driven by human-induced emissions of greenhouse gases. The global average and combined land and ocean surface temperature show a warming of 1.