Total solar irradiance has been measured since 1978 during solar cycles 21-23. The researchers found that solar activity cycles cause small changes in total solar irradiance, with a peak-to-peak variation of about 0.1%. They also observed a trend of increasing total solar irradiance by 0.04% per decade. The upcoming solar activity minimum for cycle 24 is approaching, with some sunspot magnetic polarity reversals already detected.