- The uplift of the Sierra Nevada altered atmospheric circulation, and Southern California lost its summer rainfall.
Basically, when the West was flat, the airflow was zonal, meaning along latitude lines. With a solid 2 mile high barrier in place, it is easier for the westerly air currents to "go around" the Sierra Nevada then to continue zonal flow.
We still get winter rain because the whole atmospheric circulation pattern in the Northern Hemisphere moves south in the winter, and the Jet Stream occasionally finds its way over Southern California.
- The climate of the Earth has been cooling since about 50 million years ago, and become really cold, with glaciers, in the last 2 million years.
