Total Solar Irradiance

The climate on earth is determined by the incoming solar energy.  This incoming solar energy is quantified by the Total Solar Irradiance (TSI), which is defined as the incoming solar power per unit surface perpendicular to the sun-earth direction, at the mean sun-earth distance of 1 Astronomical Unit.  The TSI is measured from space, and can be extended backwards in time using long-term ground observations such as the Sunspot Number.