This periodicity appears to be largely controlled by variations in the tilt of the Earth's axis and to changes in the shape of the Earth's orbit around the sun.
(This periodicity; appears; ) (This periodicity; to be largely controlled; by variations in the tilt of the Earth's axis and to changes in the shape of the Earth's orbit around the sun)
This periodicity This periodicity
appears to be largely controlled
by variations in the tilt of the Earth's axis and to changes in the shape of the Earth's orbit around the sun