Since November 1978 a complete set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 23 years. From measurements made by different space-based radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI is compiled with an overall precision of order 0.05 Wm–2 and a secular trend uncertainty of ±3 ppm/year. This time series is compared with an empirical model of irradiance variability based on sunspot darkening and brightening due to faculae and network. From this comparison the model is calibrated and used to estimate possible changes of TSI in the past, using historical proxies of solar activity. For this purpose, stellar observations provide information about the possible range of solar variability over the last millennium when changes of Earth's climate are well documented. Together, the paleo solar and climate data enable a discussion of the extent of global climate change that can be explained by a variable Sun.