Abstract/Summary

The Sixth Assessment Report of the Intergovernmental Panel on Climate Change stated that ”it is unequivocal that human influence has warmed the atmosphere, ocean and land”. One key piece of evidence for this is the global average of the instrumental record of surface temperature. A change in surface temperature of 1.5˚C relative to a reference “pre-industrial” period is the measure internationally agreed to mark the transition to a climate where dangerous impacts become common. This chapter will discuss the observational evidence that ensures a reliable record of the changing temperature, some of the challenges that have been overcome, and some that remain in improving that record and more fully understanding its uncertainty.