13 May 2019
By Joshua Rapp Learn
Cyclical variations in wind and sea surface temperatures in the Pacific Ocean may have contributed to a drought that played an important role in the outcome of the U.S. Civil War, according to a new study.
The new research used tree ring data to reconstruct the influence of El Niño and La Niña conditions on droughts across North America for the past 350 years, including during the American Civil War.
The Civil War drought – one of the worst to afflict the U.S. in centuries – occurred in the mid-1850s to the mid-1860s. That drought is infamous for its effects in the U.S. Southwest and parts of the Great Plains, where it led to the near extinction of the American bison and played an important role in changing the course of the Civil War by causing food and water shortages, slowing the advance of part of the Confederate army in 1862.
The drought effects extended far north of the core southwestern area usually impacted by La Niña, spreading into the Great Plains.
“It may very well be that [La Niña] played a significant role in the evolution of the sustained drought during the early 1860s,” said Max Torbenson, a geosciences PhD candidate at the University of Arkansas and the lead author of the new study in the AGU journal Paleoceanography and Paleoclimatology.
The El Niño/Southern Oscillation (ENSO) is a term for the cyclical variation in winds and sea surface temperatures that occurs in the tropical eastern Pacific Ocean. This includes the warm phase, called El Niño, and the cool phase, called La Niña, each lasting a few months and recurring every few years.
“These two phases affect the direction of storm tracks from the Pacific, and in turn influence how much rain falls, especially over the Southwest,” Torbenson said.
The magnitude of El Niño and La Niña conditions vary as well. A body of previous research has shown the stronger La Niña periods can cause severe droughts in the U.S. Southwest and Mexico, such as the one that afflicted Texas, New Mexico and parts of northern Mexico in 2011.
Researchers previously had only about 70 years of records that show how ENSO affected climate in parts of the U.S. Torbenson and his co-authors wanted to see whether they could push the record of how ENSO affects the extent of droughts to back before 1950.
To do that they tapped into the International Tree-Ring Data Bank, a public database of information gleaned from tree ring samples all around the world. Tree rings reveal past climate conditions by the thickness of a year’s growth: a thick tree ring means a year of abundant rain while a series of thin ones in a row point to a multi-year drought. Because of the strong relationship between ENSO and winter rainfall, the rings can also tell the story of past La Niña and El Niño conditions.
The researchers focused in on tree-ring chronologies from parts of northern Mexico, Texas and New Mexico, where the ENSO effects are felt the strongest, and produced estimates of ENSO variability back to 1675. These estimates were then compared to drought reconstructions based on the local tree rings of other parts of the U.S. stored in the North American Drought Atlas and broken into a grid system across the country.
Their results indicate that ENSO influence on drought has waxed and waned in areas far beyond the core southwestern U.S. and northern Mexico region. One notable signal they detected was the Civil War drought. During the mid-1800s, significant correlations between the ENSO estimates and drought reconstructions reached further east than at any other time, and included impacts over the Great Plains and even the confluence of the Mississippi and Ohio Rivers. The Civil War drought coincided with one of the most persistent La Niña periods in the estimates.
Torbenson said this long-term examination of the relationship between ENSO and droughts in the region could be a tool for predicting future drought conditions and for water management, especially in areas outside of the core ENSO region.
“There appears to be some pattern that could be helpful moving forward knowing when ENSO influences rainfall in certain areas,” such as eastern Texas and the Great Plains, he said. But the research itself is also compelling, he said, as it reveals the way the climate affected a critical period in U.S. history.
“I definitely think it’s something that makes us imagine the hardships of the past,” he said.
Joshua Rapp Learn is a freelance writer. Follow him on Twitter: @JoshuaLearn1