There are two kinds of external forcings of the Earth system. One is the Sun. The other is the volcanic effects. Energy infux from the Sun is the driving force of weather and climate. The changes of the solar irradiance will have a significant impact on the Earth envionment.
Volcanoes inject large amounts of sulfur compounds, chlorine and water into the atmosphere.
The frequency of the major volcanic eruptions could also have a major impact on how the future climate evolves.