Solar Variations
The Sun is the source of energy for the Earth’s climate system. Although the Sun’s energy output appears constant from an everyday point of view, small changes over an extended period of time can lead to climate changes. Some scientists suspect that a portion of the warming in the first half of the 20th century was due to an increase in the output of solar energy.
Learning how the Sun changed before modern instruments were available is not easy, but it appears that changes in the output of solar energy have been small over the last million years, and probably even longer.
| 
