California Governor Jerry Brown declared a drought state of emergency on January 17, 2014, after the driest year in the state's history.
California remains in a state of emergency, and is poised to meet—or break—dryness records in 2015.
While Californians have since followed Brown's mandate to restrict water usage by 25 percent, decreased water flow in the state's major rivers and groundwater has threatened agriculture, drinking water sources, and the survival of already endangered animal and plant species.
These circumstances have resulted in a fierce debate, in which politicians and the media have argued back and forth about whether or not the drought is due to anthropogenic—human- caused—climate change.
However, scientists at Columbia have found that the question of whether or not anthropogenic climate change has led to California's drought does not have a simple answer. According to the researchers, the drought can neither be attributed solely to anthropogenic climate change, nor dissociated entirely from it.
Ben Cook, a research scientist at the Lamont-Doherty Earth Observatory who studies climate records spanning thousands of years before humans began influencing it, said that he thinks the question itself is misstated.
"Whenever some extreme weather climate event happens, whether it's a hurricane or a flood or a drought, there's always the question of 'Did climate change cause this?' In some ways, that's the wrong question," Cook said. "The right question is, 'Was there an anthropogenic influence that made this event more likely or more intense, and how large of an influence was that?'"
The distinction between the two questions is subtle but critical. Rather than treating natural variability—climate change caused by natural factors and not humans—and anthropogenic climate change as mutually exclusive causes of the drought, Cook said that he treats them as necessarily interconnected.
Cook is part of a team at Lamont-Doherty combining the expertise of climate modelers, bioclimatologists, and climate dynamicists. The team's most recent paper, published in August's issue of Geophysical Research Letters, concluded that drought in 2014 was mostly due to natural variability, with only 5 to 18 percent of it due to anthropogenic climate change.
By stitching together information from various methods of climate measurement—historical records of temperature and precipitation, natural records of climate such as ice cores and tree rings, and computer simulations of past and future conditions—the researchers hope to draw a nuanced picture of the drought.
Without the inclusion of multiple forms of measurement, the researchers would have insufficient data.
The historical record of California's weather, for example, only starts in 1895. By also using tree rings dating as far back as A.D. 900, researchers have been able to expand their data set by hundreds of years.
Tree rings, in particular, can provide researchers with data to reconstruct soil moisture and local temperature during past droughts.
"Tree rings really provide us with something unique, because they are these independent gauges of soil moisture, where they don't require any calculations," co-author and Lamont-Doherty assistant research professor A. Park Williams said.
For Williams, this direct measurement makes tree rings uniquely trustworthy and the most reliable method of telling how much water was in the soil in any given year.
"By using all these approaches together, hopefully we're optimizing how much we can learn, and each approach can kind of compensate for the weaknesses of the others," Williams said.
But Williams warned against hasty conclusions made from paleorecord data.
He cited a Los Angeles Times article from October, which he said misconstrued Columbia researchers' findings and concluded that anthropogenic warming was "not a major cause."
"That is a very misinformed way to simplify the issue," Williams said.
Although natural variability has caused longer droughts in the past, Williams said that this doesn't rule out the role anthropogenic climate change plays in the current drought. Anthropogenic climate change is not only responsible for the severity of the current drought, but it will also increase the likelihood that droughts as long as 35 years recur in the future, according to Williams.
Moreover, Williams said that the 5 to 18 percent of the drought attributed to anthropogenic climate change is a significant factor.
"The lesson that we are trying to tell is I think a very simple one—but is nonetheless very difficult for politicians to get—that natural variability causes droughts, and what we are doing is making the droughts worse," Williams said.
Cook's climate models have predicted that if we do not reduce our current level of greenhouse gas emissions, the western United States, including California, will get drier than it has ever been during the last millennium.
"At the end of the day with climate change, there's still going to be a lot of water in the West," Cook said. "It's not going to turn into this desert hellscape, but it is going to require us to change the ways that we both value water and the way that we use water."