However, these main suspects are probably working together:

1. Variation in solar heat output.
2. Precession of the Earth's axis.
3. Nutation of the Earth's axis.
4. Movements of the Earth's crustal plates that change atmospheric and oceanic flow.
5. Volcanic emissions.

Also, there are these minor suspects:
6. Deforestation and desertification.
7. Overfishing.
8. Fossil fuel combustion.
9. Particulate and gasseous emissions from non-fuel sources.

Unfortunately, the first five are totally beyond human control, now or ever, and are likely to be far more important than the others. The last four are probably insignificant compared to the first.

Remember, the Earth's climate is NORMALLY tropical and subtropical almost everywhere. The last million years has been an ice age, but this is unusual. Our current situation is purely happenstance, it is NOT the normal or regular state of the Earth's climate. In the long run, ocean levels will rise some 100 meters, flooding nearly all of some US states and quite a few foreign countries. It is unlikely that we can do a damn thing to prevent this from happening. However, nobody knows anything FOR SURE. Blaming ANYTHING or ANYBODY at this time is completely silly.