The global climate is controlled by the amount of solar radiation that is received at the surface and the chemistry of the atmosphere. The amount of solar radiation that is received at the surface can vary, due to (a) solar output, (b) the proximity of the Earth to the sun, and (c) the tilt of the Earth on its axis. The proximity of the Earth to the sun changes over the course of 100,000 years or more because the Earth's elliptical orbit changes shape and becomes more or less elliptical in shape. The tilt of the Earth changes over the course of about 41,000 years. In addition to these changes in the Earth's movements are changes in the timing and intensity of the seasons, which are influenced by the combination of the Earth's tilt and the position of the Earth in its elliptical orbit. At present, the Northern Hemisphere's summer (when the Northern Hemisphere is pointed toward the sun) occurs when the Earth is farthest from the sun.
The chemistry of the atmosphere determines how much incoming solar radiation is blocked by clouds and how much thermal radiation, re-radiated from the Earth's surface, is held in by clouds and greenhouse gases.
Solar output also varies depending on sunspot activity, but this has a much smaller effect on climate than other factors mentioned above.
