Ancient Weather Patterns Teach Us About Drought


By Julianne Glaser

California is now entering its fourth year of drought. The U.S. Drought Monitor reports more than two-thirds of California is in “extreme” drought, with more than 40 percent of the state in “exceptional” drought, the most extreme category. Worse yet, the area received only five percent of normal snow pack precipitation from the Sierra Nevada Mountains this winter, which provides most of the area’s water.

According to the Institute of the Environment at the University of Arizona, California’s drought is the worst since weather records began in the late 1800s. Researchers contend that though this drought is unprecedented in modern times, analysis of ancient data indicates droughts that lasted for decades in past centuries.

Nature ’s Rain Gauges Reveal Weather History in the West

A dendrochronological study by Daniel Griffin and Kevin Anchukaitis published in Geophysical Research Letters confirms recorded weather data and indicates that the recent drought is the worst in 1200 years. Researchers compared width of tree rings to determine wet years (wide rings) versus dry years (narrow rings) to create a timeline before climatological data was available.

“We were genuinely surprised at the result,” said Griffin. “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.”

What ’s Causing the Dry Weather?

Through climate models and weather archives, scientists determined that the prime cause of this drought is increased sea surface temperature in a portion of the Pacific Ocean. Unusually warm sea surface temperature changed atmospheric circulation and blocked winter storms from reaching the west coast. The result: dryer conditions in the southwestern part of North America.

A study sponsored by the National Oceanic and Atmospheric Administration and led by Professor Richard Seager of Columbia University, found that natural weather patterns and not man-made global warming are responsible for the current California drought.

“It’s important to note that California’s drought, while extreme, is not an uncommon occurrence for the state. In fact, multi-year droughts appear regularly in the state’s climate record, and it’s a safe bet that a similar event will happen again,” noted Seager.

Some outside climate scientists criticized the conclusions claiming the report did not take into account how record warmth worsened the drought.

Extension Questions

  • Why are blue oak trees used by scientists to study climate data?
  • What other data can scientists learn from tree rings?
  • What are America’s “dust bowl years”? When did they occur, what were the causes and impacts?