Changes in the amount of electromagnetic energy reaching the Earth from the Sun, known as solar irradiance, may significantly affect climate change.  As this figure shows, changes in the solar activity which are indicated by changes in sunspot number correspond to changes in the total solar irradiance.  Variations in solar irradiance of a few tenths of a percent could cause a global surface air temperature change of 0.1° C.  Therefore, it is necessary to monitor solar irradiance.
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The figure shows measurements by NASA’s Nimbus 7/Earth Radiation Budget (ERB) instrument, Solar Maximum Mission (SMM)/Active Cavity Radiometer Irradiance Monitor (ACRIM) and the Earth Radiation Budget Satellite (ERBS) and NOAA-9 satellite/Earth Radiation Budget Experiment (ERBE) instruments.