Human released greenhouse gas pollution changes the climatic system through a variety of mechanisms. Trade winds and jet streams change their patterns of movement, and the distribution of moisture in the air changes, with precipitation either lacking more than usual or being more abundant than usual. The patterns of movement of major air masses and the increased bifurcation of air masses into more wet than usual and more dry than usual can result in long periods where region experiences excess precipitation or a lack of precipitation. When the latter happens, there can be a drought.
Increasingly, the California drought is being seen as an effect of climate change. Air masses that should have contributed precipitation in the form of mountain snow, which in turn feed the western ground water system, have been kept away. Increased temperature has increased evaporation. Other factors related to climate change have contributed. The result is an historic drought over California that shows at present no sign of stopping any time soon. There was hope that last winter there would be additional precipitation, and there was some, but not enough.
A paper just out in Geophysical Research Letters uses modeling and historic data to confirm that the current California drought is very likely an effect of climate change. The paper is “Temperature Impacts on the Water Year 2014 Drought in California“, by Shraddhanand Shukla, Mohammad Safeeq, Amir Aghkouchak, Kaiyu Guan, and Chris Funk. Here is the abstract, which is pretty self explanatory and understandable:
California is experiencing one of the worst droughts on record. Here we use a hydrological model and risk assessment framework to understand the influence of temperature on the water year (WY) 2014 drought in California and examine the probability that this drought would have been less severe if temperatures resembled the historical climatology. Our results indicate that temperature played an important role in exacerbating the WY 2014 drought severity. We found that if WY 2014 temperatures resembled the 1916-2012 climatology, there would have been at least an 86% chance that winter snow water equivalent and spring- summer soil moisture and runoff deficits would have been less severe than the observed conditions. We also report that the temperature forecast skill in California for the important seasons of winter and spring is negligible, beyond a lead-time of one month, which we postulate might hinder skillful drought prediction in California.
The caption for the graphic above is: “Percentiles of potential evapotranspiration (ETo) during WY 2014 with respect to 1979 to 2012 climatology.”
I find the ancillary finding of the lack of skill of temperature forecasts in California. One would expect low skill in forecast models that are designed under a given climatology, when that climatology shifts as it seems to have done.