Using Radiative Signatures to Diagnose the Cause of Warming Associated with the Californian Drought

California recently experienced among the worst droughts of the last century, with unprecedented precipitation deficits and record high temperatures. The dry conditions caused severe water shortages in one of the economically most important agricultural regions of the US, particularly in the Central Valley. It has been hypothesized that anthropogenic warming is increasing the likelihood of such extreme droughts in California, or more specifically, that these drought conditions are a consequence of warmer temperatures from the enhanced greenhouse effect. Process studies suggest, however, that increased temperatures during droughts are a consequence of reduced evaporative cooling. Here we use surface radiation components from NASA's Clouds and Earth's Radiant Energy Systems (CERES), climatic data and direct flux tower measurements to investigate the cause of warming associated with the recent Californian Drought. Based on radiative signatures and surface energy balance we show that the warmer temperatures were not associated with an enhanced greenhouse effect by anthropogenic warming. The radiative signature showed decreased longwave downward radiation during the water years 2013–2014 compared to the decadal mean of 2001–2012. Instead, increased solar downward radiation in combination with reduced evaporative cooling from water deficits enhanced surface temperatures and sensible heat transfer to the atmosphere. We conclude that the drought was not associated with warming by increased longwave downward radiation, and that there is no simple relation between warmer surface temperatures and drought.