The causes behind these ancient droughts have long puzzled scientists. In a new paper published in Nature Communications, researchers from the University of Helsinki, along with colleagues from the U.S., Germany, and Sweden, explored the timing and distribution of these droughts across a broad region stretching from the Rocky Mountains to the Atlantic Ocean.
"Evidence from the last 11,000 years shows that North American water supplies have undergone dramatic changes, at times experiencing droughts as severe as the 1930s Dust Bowl for centuries to millennia," explains Bryan Shuman, a co-author from the University of Wyoming. "Understanding the causes and consequences of these droughts can help improve our ability to anticipate future changes."
The team analyzed fossil pollen data from numerous North American sites, using machine learning algorithms to reconstruct moisture levels over the Holocene. Led by J. Sakari Salonen, an Academy of Finland research fellow at the University of Helsinki, the study revealed that much of the continent experienced long periods of drier conditions than today.
However, the severity and timing of these droughts varied across regions. Salonen notes that the earliest intense droughts occurred around 11,000 years ago in the northeastern U.S. and nearby Canadian regions, areas that are typically wetter today. In contrast, the mid-continent, including the modern prairie regions, faced its most severe drought roughly 7,000 years ago, well after the Atlantic coast had begun to recover its moisture levels.
Jack Williams, a contributor from the University of Wisconsin-Madison, highlights the significance of understanding these regional patterns. "In the eastern U.S., there's often an assumption that water is always plentiful. But past evidence shows that prolonged droughts can dramatically alter fire regimes and forest composition, even in wetter regions."
The team also compared their reconstructions with advanced climate simulations, which helped uncover the mechanisms behind these ancient dry spells. According to Frederik Schenk, an atmospheric physicist at Stockholm University, these simulations revealed that early Holocene droughts were influenced by high-pressure systems associated with lingering ice sheets, which shifted moisture patterns. As the ice sheets melted, rising summer temperatures further intensified drying across the continent.
Interestingly, these findings have modern implications. The study suggests that as current climate change progresses, North America could again face extensive drying, despite potentially increased rainfall, due to higher evaporation rates driven by warming temperatures. This scenario mirrors the natural shifts that occurred in the early Holocene, though the underlying causes are different, now driven primarily by the rapid rise in atmospheric greenhouse gases.
"If modern climate projections hold true, we may be heading for a rapid, human-driven replay of the natural drought cycles from ten thousand years ago," Salonen warns.
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