- Manmade greenhouse gas emissions.
- Output variations in the gigantic thermonuclear furnace - our sun - around which the earth orbits, which warms and lights our days and energizes directly and indirectly all life on earth.
It may be the case that greenhouse gases are all that prevents a serious cooling spell or ice age. We really have very incomplete knowledge of the total set of interacting factors, their relationships to each other, and the relative strength of each. And we can't let our lack of understanding paralyze us into the immobility climate alarmists advocate. Life goes on.