Shifts in Earth's orbit triggered ancient megadroughts

For years, scientists have known that North America experienced extended droughts thousands of years ago. These events caused massive forest die-offs and reshaped entire ecosystems. But until now, the exact reason behind the ancient dry spells remained unclear.

An international research team has found a strong link between megadroughts and a subtle but powerful force – changes in Earth’s orbit. The study was focused on a warm period in Earth’s history called the Holocene, which began after the last ice age and continues today.

Reconstructing regional climate shifts

The researchers used fossilized pollen grains, collected from sites across North America, to track how moisture levels changed over the past 11,000 years. A machine-learning algorithm helped them reconstruct regional climate shifts during this time.

What the team discovered was striking. Moisture conditions in many areas were lower than modern levels for thousands of years, and the timing of the droughts varied depending on location.

Thousands-of-years long drought

Study lead author J. Sakari Salonen is an Academy of Finland research fellow in the Department of Geosciences and Geography at the University of Helsinki.

“The dry conditions first kick off 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,” explained Salonen.

“In the mid-continent, where we have modern prairie region, we see most severe drought at about 7,000 years ago, so thousands of years later than at the Atlantic coast.”

By this time, said Salonen, 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.

Exploring the cause of ancient droughts

Jack Williams from the University of Wisconsin-Madison noted that there’s a common misperception that water is always available, particularly in the eastern United States.

“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,” said Williams.

To test the cause of these ancient droughts, the scientists ran climate simulations using high-resolution models. These simulations matched the pollen-based reconstructions and helped explain what triggered the widespread dryness.

What the simulations revealed

According to the researchers, two main factors were at play. In the early Holocene, a high-pressure system over the remnants of the ice sheet pushed moisture away from large parts of the continent. This created dry conditions, especially in the east.

As the ice sheet melted, another factor came into play – rising temperatures. Warmer summers dried out the soil faster than rainfall could replenish it.

Frederik Schenk is an atmospheric physicist at Stockholm University and visiting scientist at the University of Helsinki.

“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,” said Schenk.

“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.”

Greenhouse gases and modern droughts

The researchers point out that while the causes of past and future droughts differ, the consequences may look familiar.

Back then, it was a gradual shift in Earth’s orbit that warmed summers and triggered megadroughts. Today, rapid greenhouse gas emissions are driving a much faster version of the same trend.

“If the climate projections for this century hold true, we’re looking at a speed-run repeat of what happened, for natural causes, ten thousand years ago,” said Salonen.

Despite projected increases in precipitation, much of North America is expected to dry out by the end of the century. That’s because warmer air causes more evaporation, which can lead to dry soils even when rainfall increases.

Earth’s orbit and megadroughts

Earth’s orbit constantly changes over long timescales. These changes, called Milankovic cycles, affect the planet’s tilt and distance from the Sun. They occur over tens of thousands of years and have shaped Earth’s climate history.

About 20,000 years ago, Earth’s orbit limited sunlight in the Northern Hemisphere, helping massive ice sheets form. By 10,000 years ago, those ice sheets had mostly melted, ushering in the Holocene – a warm interglacial period. Between 10,000 and 4,000 years ago, Earth was warmer than it is today.

This recent study shows that the same orbital conditions that warmed Earth also set the stage for centuries-long droughts in North America. And while Earth has been slowly cooling since then, the sharp warming caused by human activity in the last century has thrown us off the natural cycle.

The scientists warn that understanding what happened in the past could be crucial for preparing for what comes next.

Their work offers a stark reminder: even slow, natural changes in Earth’s orbit were enough to trigger enormous shifts in climate. With much faster changes underway, the effects could be just as severe – only now, they’re happening in real time.

The full study was published in the journal Nature Communications.

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