Probably the sun, orbit and tilt. Ocean currents are also a factor moving heat around. Considering the earth's land masses are moving with plate tectonics the ocean currents are changing too beyond their known cycles. Nothing is static regardless of what the AGW crowd claims. We have an infantile understanding of the sun at this point and our recorded history of it activities is a blink of an eye in geological time terms. There are only two significant energy sources (that I can think of off hand), the sun and the earths core. Everything else is tiny noise.
Weathermen usually mention "El Nino" but then conclude we don't know what, how, etc.
Sun spots, cycles, deep ocean temps and currents, and dozens of factors to learn about.
I would enjoy a simple lay explanation about the "models," variables, track record for predictions, etc.