The Obama administration has used the severe drought in California as an example of the unpleasant byproducts of man-made climate change. However, extreme drought is not uncommon in the area-a devastating drought period in 1976 and 1977 has a fair resemblance to the current drought-and there isn't a consensus that what is being experienced is not just a periodic cycle of drought. While that point can be debated, there does seem to be a growing consensus that whether or not climate change is the reason for the drought, it certainly amplifies the effect of the drought.
The recent California drought fits that pattern. In 119 years worth of measurements in California, 2013 was the driest year ever recorded. Combined with higher temperatures than usual, the effect on the important agricultural areas of California is devastating. Not only is there less water falling from the sky, the water that does fall evaporates and is lost more quickly. Northern California received recent heavy rain, partially replenishing the reservoirs, but the reservoirs are still around half the expected capacity for this point in the year.
So should California expect more of the same based on climate changes, or a reversal of fortune with wetter climates in future years for the very same reason? Models are available that can produce either result.
Recent computer projections suggest that in the longer term California should get wetter with an increase in global temperature, because of increased precipitation in the winter months, when the majority of the state's water is accumulated through snowpack in the mountains. The West overall is expected to be drier from dry air arriving from the tropics, but that is not expected to affect the Western slopes of the Sierra mountains and points west, leaving California relatively wet.
However, a ten-year old projection from scientists at the University of California-Santa Cruz has been uncannily accurate in with respect to recent weather-with one disturbing note.
The model starts with an assumption of significant melting of Arctic sea ice, which has already begun on a lesser scale due to increased temperature in the region (and, many scientists believe, due to man-made effects). One of the surprising effects of the model was the development of stubborn high-pressure ridges off of the coast of California, pushing the winds to the north (and in turn, the moisture). Without available atmospheric moisture, California would experience extreme drought.
All of these things have come to pass. Arctic sea ice hit record low levels in 2007 and 2012, high-pressure ridges have blocked moisture for a large part of the last three years, and the California drought is now in its third year. It's possible this is a coincidence, but let's consider the disturbing note mentioned earlier-these conditions were predicted for mid-century.
In the end, the question is always the same: what are the assumptions underlying any climate model, and how closely are the events tracking to this model? It's safe to say Californians are hoping wetter models prevail, but the drier one seems to have a better record in the short-term.