California has been experiencing a drought since 2012, which is the worst that has been recorded in the last century. But how does it compare to droughts that occurred before human weather records? Though droughts in California aren’t particularly rare, a recent study has found that this drought is the most intense that has occurred in the area over the last 1,200 years. The research was performed by Daniel Griffin and Kevin Anchukaitis of Woods Hole Oceanographic Institution (WHOI), and their paper was published in Geophysical Research Letters.

In order to get a historical record of precipitation in central and southern California, the WHOI team took core samples of blue oak trees in order to examine the tree rings. During times of plentiful rain, the trees took up a great deal of water, leading to wider rings. When drought conditions occurred, the rings were very small and narrow. Luckily, the trees are hardy enough to survive very tough conditions in order to make this analysis possible.

“California’s old blue oaks are as close to nature’s rain gauges as we get,” Griffin said in a press release. “They thrive in some of the driest environments where trees can grow in California.”

The team utilized a climate report released over the summer by the National Oceanic and Atmospheric Administration to create an index of moisture and temperature levels to apply to the tree rings. The data was also compared to several other drought records and tree ring studies pertaining to the region and the continent as a whole. 

Griffin and Anchukaitis found that while the current drought isn’t necessarily exceptional in the lack of precipitation that has fallen in the last two years,  the effect of the arid weather has been exacerbated by unusually high temperatures. While low precipitation is certainly a factor in drought conditions, temperature can make or break its severity.

“We were genuinely surprised at the result,” continued Griffin. “This is California—drought happens. Time and again, the most common result in tree-ring studies is that drought episodes in the past were more extreme than those of more recent eras. This time, however, the result was different.”

The tree rings showed dry periods that would last for decades at a time, with a few years in the middle that were fairly wet. Overall, this is the longest, hottest, and driest period of the last 1,200 years of California’s history. The authors believe that anthropogenic climate change is a driving factor behind the drought and this extreme weather is likely to repeat in the future. Drought conditions can lead to an increased risk of wildfire, and could also lead to increased prices or water rations as residents of California and surrounding states face a perpetual shortage of water.

"But there is no doubt that we are entering a new era where human-wrought changes to the climate system will become important for determining the severity of droughts and their consequences for coupled human and natural systems,” Anchukaitis concluded.