evidence: Each of the research teams that produce large-scale temperature estimates has developed methods for dealing with the potential biases and sources of error such as those described in the preceding paragraph. For example, NASA GISS uses a linear interpolation procedure to "fill in" missing data and temperatures in areas between observing stations, and data from urban stations (which are identified based on either population density data or "nightlight" levels observed by satellite) are adjusted so their long-term trends match those of neighboring rural stations. The University of East Anglia instead corrects the station-level data first and then uses a simple averaging procedure to combine the data. These procedures have been developed over several decades (e.g., Hansen and Lebedev, 1987) and are constantly reevaluated to identify and correct for additional sources of error. It was recently determined, for example, that a change in the way that certain ship-based temperatures were treated introduced a spurious signature into the mid-20th-century temperature record including an abrupt drop of ~0.5°F (0.3°C) in 1945 (Thompson et al., 2008). [...] Many lines of evidence support the conclusion that most of the observed warming over at least the last several decades is due to human activities: [...] Detailed observations of temperatures, GHG increases, and other climate forcing factors from an array of instruments, including Earth-orbiting satellites, reveal an unambiguous correspondence between human-induced GHG increases and planetary warming over at least the past three decades, in addition to substantial year-to-year natural climate variability (Hegerl et al., 2007).