TAMPA, Fla. (WFLA) — The drought in the western United States has now reached unprecedented territory. That’s because the past 22 years have been the driest such years since at least 800 AD, according to a new study.

The team of scientists who conducted the research found five other megadroughts over the past 1,200 years.

You might ask how they know this, being that traditional weather records only go back a century in this region? The answer is tree rings.

Scientists can use the tree rings of ancient trees to determine the how healthy they were growing during a specific time in history. To put in simplified terms, if the tree rings are thin, then the tree was likely stressed due to lack of water. If the tree rings are thin for decades at a time, then the region was likely in an extended drought.

Tree rings can be used to determine the climate of the time by examining the thickness of rings.

The study’s co-author, Dr. Jason Smerdon of Lamont-Doherty Earth Observatory of Columbia University, says the droughts were caused by an unlucky string of La Niña events in the Pacific Ocean. You can view parts of the interview with Smerdon in the video story above.

In a La Niña, cooler-than-normal ocean waters persist in the Tropical Pacific Ocean, which leads to a weaker-than-normal subtropical jet stream. This often causes the storm track to shift away from the southwest and southern U.S., leading to less rain and drought. That drought can be found coast to coast from California to Florida.

But Smerdon says this current drought in the west has been made worse by human-caused climate change. The team of researchers found that climate change made the drought 42% more intense, turning what would have been an otherwise moderate drought into the driest period in over a millennium.

Current western drought severity on the left, compared to a hypothetical of the current drought without climate change on the right. The extra redness on the left illustrates how much worse human-caused climate change is making the current drought.

But the main cause of the dryness is not lack of rain, it is the warmer temperatures associated with climate change. When the air is warmer, it has more energy and capacity to evaporate moisture. This dries out the vegetation and soil.

As a result, the west is suffering through dangerously-low reservoir levels, longer and more intense wildfire seasons and farmers and ranchers that are struggling to make ends meet.

The researchers say past megadroughts sometimes lasted a few decades, which means this current drought could last to, or beyond, 2030.