Solar Variation
The Sun is the source of a large percentage of the heat energy input to the climate system.
Lesser amounts of energy is provided by the gravitational pull of the Moon (manifested as tidal power), and geothermal energy.
The energy output of the Sun, which is converted to heat at the Earth's surface, is an integral part of the Earth's climate.
Over the coming millenia, the Sun will continue to brighten and produce a correspondingly higher energy output; as it continues through what is known as its "main sequence", and the Earth's atmosphere will be affected accordingly.
On more contemporary time scales, there are also a variety of forms of solar variation, including the 11-year solar cycle and longer-term modulations. However, the 11-year sunspot cycle does not appear to manifest itself clearly in the climatological data.
Solar intensity variations are considered to have been influential in triggering the Little Ice Age, and for some of the warming observed from 1900 to 1950. The cyclical nature of the sun's energy output is not yet fully understood; it differs from the very slow change that is happening within the sun as it ages and evolves, with some studies pointing toward solar radiation increases from cyclical sunspot activity affecting Global Warming.