The most likely candidate for that climatic variable force that comes to mind is solar variability (because I can think of no other force that can
change or reverse
in a different trend often enough, and quick enough to account for the historical climatic record) and the primary and secondary effects associated with this solar variability which I feel are a significant player
in glacial / inter-glacial cycles, counter climatic trends when taken into consideration with these factors which are, land / ocean arrangements, mean land elevation, mean magnetic
field strength of the earth (magnetic excursions), the mean state of the climate (average global
temperature), the initial state of the earth's climate (how close to interglacial - glacial threshold condition it is) the state of random terrestrial (violent volcanic eruption, or a random
atmospheric circulation / oceanic pattern that feeds upon itself possibly) / extra terrestrial events (super-nova
in vicinity of earth or a random impact) along with Milankovitch Cycles.
The current California drought is bad because for the first time ever, scientists from many different
fields see parallel lines of evidence for the influence of human - induced climate
changes, including the fingerprints of higher
temperatures and
changes in the
atmospheric circulation patterns.
As a researcher
in the
field for more than 30 years, I am not aware of a single peer - reviewed paper or review,
in a quality
atmospheric science journal, that relates the
temperature changes over this period to only natural causes such as
changes in solar activity.