1. The Sun's radiation output based on the sunspot cycle. The more sunspots, the higher the Earth's temperature.
2. The level of the shielding effect based on volcanic ash output.
The latter is of interest as I type this due to the eruption of the Taal volcano in the Philippines. It appears that this could turn into a MAJOR eruption that has the potential to equal Mount Pinatubo's eruption in 1991. If that happens, we could have climatic effects on Earth lasting a decade.
Volcanic activity and low solar activity seem to be correlated at some level. Some have suggested anyway. It’s going to be a combination of factors that cause the sudden drop in temperatures.