The visible causes depend on the timescale of the climate data being
studied: 

DECADES, CENTURIES, AND TIMESCALES TO A FEW THOUSAND YEARS

Variable solar irradiance means that the Earth receives changing
amounts of energy from the Sun 

Variable solar eruptivity means that the solar wind and magnetic field
also change, and this means that there is a varying amount of
protection for the Earth from cosmic rays, which cause cloud formation
and so affect climate 

TENS AND HUNDREDS OF THOUSANDS OF YEARS TO MILLIONS OF YEARS

The Milankovitch Cycles, which are variations in the Earth's axial
tilt, precession which is the planet's wobble on its rotation axis,
and the eccentricity of the Earth's orbit around the Sun 
accretion of interstellar dust onto the Sun as the Solar System orbits
the galactic nucleus 

SPORADIC FORCINGS THAT CAN HAPPEN AT ANY TIME

volcanic eruptions 
space impacts 
nearby supernovae