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A new study from the University of Helsinki has provided a compelling new explanation for the devastating droughts that took place in North America thousands of years ago. This period, known as the Holocene, covers the time of a generally warm climate following the last ice age. These exceptionally long-lasting droughts had drastic impacts on forest dieback and ecosystem transformations; understanding their causes is essential to improving societal resilience to future climate variations.

The reasons for these long droughts, however, have remained obscure. In the new study in Nature Communications, University of Helsinki researchers, joined by co-authors from U.S., German and Swedish research institutions, analyze the development and causes of the drought in eastern North America, in a broad region spanning from the Rocky Mountains to the Atlantic Ocean.

"Evidence from the last 11,000 years highlights that North American water supplies have changed dramatically in the past, at times extending drought as severe as the 1930s Dust Bowl drought for centuries to millennia. Understanding the causes and consequences of these droughts can help improve our ability to anticipate future change," explains Bryan Shuman, co-author of the study from University of Wyoming.

In the new study, the researchers used datasets of fossil pollen grains, which have been recovered from numerous locations in North America over the past several decades. Using a computer algorithm based on machine learning, the researchers were able to reconstruct the variation of moisture levels over the Holocene based on the fossilized pollen.

The study was led by J. Sakari Salonen, an Academy of Finland research fellow at the Department of Geosciences and Geography, at the University of Helsinki. According to Salonen, the study confirms that the moisture conditions were continuously below modern levels for thousands of years.

Salonen notes, however, that different parts of North America experienced different drought histories. "The dry conditions first kick in in the northeastern U.S. and the nearby regions of Canada, which is today one of the wetter regions of North America. Here, we see peak drought already at 11,000 years ago," says Salonen.

The drought, then, shifts west. Salonen explains, "In the mid-continent, where we have the modern prairie region, we see the most severe drought at about 7,000 years ago, so thousands of years later than at the Atlantic coast. By this time, the Atlantic coast was already getting wetter, and it's like the entire climate anomaly causing the thousands-of-years-long drought started migrating westward."

"Particularly in the eastern U.S., there's often a perception today that water is always available. However, studies of the past show that these long droughts can trigger major shifts in fire regime and tree die-offs, even in the east. So it's critical to understand the spatial patterns of these droughts and their causes," says Jack Williams from the University of Wisconsin–Madison, who also contributed to the study.

The researchers compared their Holocene climate reconstructions to numerical climate simulations which they ran for the same time periods. These supercomputer experiments allowed the scientists to explore what could have caused the long droughts.

Frederik Schenk, atmospheric physicist at Stockholm University and visiting scientist at the University of Helsinki, says that the new generation of climate models, performed at 2–4 times greater resolution than before, proved decisive in unraveling the causes of the drought.

The causes revealed by the climate simulations, says Schenk, are two-fold. "The simulations show that in the early Holocene, the high-pressure system at the ice sheet, remaining in the northern part of the continent, steered moisture transport, which explains the geographic shift in dry conditions," says Schenk. After the ice sheet melted, drought set in across the entire region, as summers got increasingly warm and dry.

The Holocene drought has interesting parallels with the future climate that is expected to take hold in North America over the next decades, as anthropogenic climate change progresses. By the end of this century, most of North America is projected to become increasingly dry, despite gradually increasing precipitation. This is because the increase in evaporation, due to the warming climate, is predicted to dry out surface soils even as rainfall increases.

The researchers note that this is exactly what they see in their simulations of the Holocene drought. "Overall, a warmer climate will be wetter. But this doesn't apply to all regions. As seen in our results, there are clear regional limits of how much rain can be delivered to balance the warming-driven evaporation. For the strong summer warming in the past, it just didn't add up, giving way to millennia-long droughts," explains Schenk.

However, the underlying causes of the past and future drought are different. In the early Holocene, drying was caused by a slow change in Earth's orbit, which caused summers to get warmer, finally pushing much of North America into drought. Now, the ongoing warming and the predicted drier conditions are due to the rapid increase in greenhouse gases in the atmosphere.

"If the climate projections for this century hold true, we're looking at a speed-run repeat of what happened, for natural causes, 10,000 years ago," says Salonen.