Maybe there isn't "a" root cause.
There have been weather changes for millions of years, most of it a lot more drastic than this, and there are many, many variables capable of influencing it.
The good news is that overall, life will do all right is that after asteroid bombardments, solar variations, ice ages, and periods when the CO2 was twenty times what it is now, things stayed pretty much in the life comfort zone.
People don't think in geological time, and its easy to think of a forest, a desert, or a lake as a permanent feature.