Ice age clues point to more extreme weather patterns in our future

The study, published in Nature, combines data from ancient shells of marine organisms with advanced climate modeling to shed light on how El Niño patterns might change in a warming world.

El Niño is a climate phenomenon characterized by the irregular but periodic warming of sea surface temperatures in the central and eastern Pacific Ocean. This leads to disruption of global weather patterns and causes extreme events like droughts, floods and heat waves.

"El Niño is a formidable force of nature -- it induces droughts, floods and wildfires, disrupting marine and terrestrial ecosystems across the planet, with pervasive societal impacts across numerous sectors, from agriculture to the aviation industry," said Kaustubh Thirumalai, the study's co-lead author and an assistant professor in the U of A Department of Geosciences.

El Niño events occur approximately every two to seven years, and anticipating how these events might change in the future is a major challenge for climate scientists.

"There are several state-of-the-art climate models out there, and they suggest different El Niño responses to ongoing and future human-caused warming," Thirumalai said. "Some say El Niño variations will increase, others say it will decrease -- it is a complex, multifaceted phenomenon. So, addressing what might happen to El Niño is a key priority for climate science."

To address this uncertainty, the research team -- which included collaborators from the U of A, University of Colorado Boulder, University of Texas, Middlebury College and Woods Hole Oceanographic Institution -- turned to the past. They focused on the Last Glacial Maximum -- a period about 20,000 years ago when there were ice sheets over much of North America and Europe.

The researchers used the Community Earth System Model -- developed to simulate the Earth's climate system and predict future climate scenarios -- to simulate climate conditions from the Last Glacial Maximum to the present day. This model is a collaborative project primarily led by the National Center for Atmospheric Research, with contributions from numerous institutions. The modeling portion of the study was conducted by co-lead author Pedro DiNezo at the University of Colorado Boulder.

To validate this model, Thirumalai and his team compared the model's results with data from the remains of tiny marine organisms called foraminifera. They are found in ocean samples extracted from the seabed that contain layers of sediments deposited over thousands to millions of years.

"These beautiful, microscopic creatures, which float in the upper ocean, build shells that lock in the ocean temperature when they were alive," Thirumalai said.

As foraminifera grow, they secrete shells using materials from the surrounding seawater. The chemical composition of these shells changes based on the water temperature. This enables the preservation of a snapshot of ocean conditions at the time the shell formed.

When foraminifera die after a few weeks of life, their shells sink to the ocean floor and become part of the sediment. By analyzing shells from different layers of sediment, scientists can reconstruct ocean temperatures from thousands of years ago and compare them to the model simulations of past climates.

The team analyzed individual foraminiferal shells, allowing them to capture seasonal temperature variations that would otherwise be impossible to detect.

"We zoom in to a tiny section of the sediment core and analyze multiple individual shells from the same layer. This gives us a range of Pacific Ocean temperatures within a short time period, which we can compare between the ice age and today," Thirumalai said.

The study found that El Niño variability was significantly lower during the Last Glacial Maximum compared to the present day, and that future extreme El Niño events could become more prevalent as the planet warms. This could lead to more intense and frequent weather disruptions worldwide. Importantly, these findings suggest a common mechanism of extreme El Niño variations under both ice age and future conditions, allowing the researchers to validate the climate model's prediction.

"This gives us more confidence in the model's projections for the future," Thirumalai said. "If it can accurately simulate past climate changes, it's more likely to give us reliable predictions about future changes in the El Niño system."