1] Changes in Solar Output
The only source of energy for running the Earth's climate system is the Sun. As seen in Figure 1 below, solar activity has fluctuated with an overall increase within the 600 years from 1400 to 2000. The amount of solar energy received by the Earth does not remain constant. The changes in solar output have had influences on the total energy stored in the Earth's system, thereby causing the Earth's climate to change periodically.
Figure 1: Variations in solar activity over 6 centuries, based on observations of sunspots and beryllium(Be) isotopes.
2] Variations in the Earth's Orbital Characteristics
The Earth travels around the Sun on a certain orbital. However, the orbital is not a standard circle and the Sun is not in the centre of the orbital. The relative position between the Earth and the Sun changes all the time, while the Earth travels along the orbital. As a result, distribution and abundance of sunlight reaching the Earth's surface changes. In particular, when the Earth is closer to the Sun in January, the energy received from the Sun will be greater. Therefore, variations in the Earth's Orbital Characteristics can also cause climate change.