The Sierra Nevada snowpack, which accounts for more than one third of California’s water supply, has fallen to its lowest level since the Middle Ages, according to a new study released on September 14. Measured on April 1, when it generally has reached its greatest depths, the snowpack was found to be barely 5 percent of the 1950-2000 average, making it the smallest in 500 years.

This conclusion was a result of a study of 1,500 ancient blue oak trees in the Central Valley. Tree ring patterns reflect annual rainfall and temperatures of a region. Since the oaks run parallel to the Sierra Nevada, scientists say they serve as a good proxy for the snowpack level of the mountain range at any given year. The results, which were confirmed 10 times, showed that the low snowpack level is indeed unprecedented over 500 years.

Scientists had already mapped out thousands of years of California’s rainfall, putting the current drought into historical context. But the same hadn’t yet been done for the Sierra Nevada snowpack levels.

Although rainfall is obviously instrumental, snow is the indicator of how much water will be available later in the season. While water rushes downhill as soon as it falls, snow remains stationary through winter, before slowly releasing water as temperatures rise. Snowmelt feeds rivers and streams, which serves the water needs of the Californian population, supports fish and other wildlife, and replenishes soil moisture and reservoirs.

As rain is scarce during California’s spring and summer, Sierra Nevada’s snowpack is a critical source of water. The problem is: California’s last winter reached record-breaking temperatures that resulted in more precipitation falling as rain instead of snow. With less snowpack, the region’s agriculture sector, drinking water supplies and hydroelectric power, which were already strained, are now under even more stress.

According to the authors of the study, things are only going to get worse. “We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures,” said Valerie Trouet, who led the study.

California has been under a severe drought for four years, leaving more than 120 of the state’s reservoirs less than a fifth full. The lack of moisture has allowed rampant wildfires to destroy more than 100,000 acres, hundreds of homes, and businesses.

However, California used to be one of the most blessed locations on Earth. Why the drought now? Why the wildfires? Will California survive? To find out the answers to these questions and more, request our free booklet Why ‘Natural’ Disasters?