The AMOC ocean current could collapse. If it does, it could change Chicago's weather for 1,000 years.

In the 2004 blockbuster film "The Day After Tomorrow," global warming shuts down a major ocean current, leading to abrupt climate change and catastrophic impacts around the world. It seemed far-fetched then, but studies today suggest the same ocean current may be on the verge of collapsing. If it does, it could change Chicago's weather for 1,000 years.

Northwestern University PhD student Aidan Burdick is part of a specialized research team in the department of Earth and Environmental Science. He spends his days in a state-of-the art laboratory, sifting through mud to look for fossilized flies.

In those ancient flies, he sees a thermometer.

"By far the biggest control on where these different species live is the air temperature," Burdick explained.

And buried in the ancient climate record are clues about something much bigger.

"We were pretty sure we were underestimating the rate at which ice is melting," said Dr. Erin Pettit, professor of geophysics and geology at Oregon State University.

Pettit is part of a global team of researchers who spent six weeks in Greenland over the summer measuring glacier melt. Her project used underwater robots to drill little microphones onto the glacier face to listen.

"The process of melting glacier ice is really noisy, and so we can actually use the sounds to get an idea of how fast the ice is melting," she explained.

And that will give scientists an idea of how likely it is that the Atlantic Meridional Overturning Circulation, or AMOC, ocean current will collapse.

An AMOC collapse could have catastrophic ripple effect

AMOC is a massive conveyer belt of heat in the ocean, flowing from Florida to Europe, driven by small differences in saltiness across the ocean. As fresh water melts from Greenland ice and pours into the North Atlantic, it's disrupting a delicate balance and slowing the current down.

"For the first 30 years of my career—  I always considered the risk of crossing that AMOC tipping point a few percent. Now in the light of research in the last 5 years or so, I've actually had to revise that to 50/50," said Dr. Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research.

Rahmsdtorf is a world-renowned expert on the possibilities of an AMOC collapse and what it would mean for society.

"The impacts would be so massive, that it's very hard to imagine that we could sensibly adapt," he said.

His projection shows a huge change in rainfall patterns around the world, with some areas getting much drier. That could change where people can grow food, leading to mass migration and even political instability.

He also said temperatures in northwest Europe and parts of North America would cool down significantly, especially in the winter, and seal levels would rise a foot and a half on the East Coast, leading to more frequent coastal flooding major cities.

While Rahmstorf's research shows that an eventual collapse is no longer a low-probability event, not all scientists agree on his timeline. The latest international climate change report finds "moderate confidence" that the current won't abruptly collapse this century. But Rahmstorf said that's missing the point.

"Even if the full shutdown isn't reached before 2100, the models show that it may well already be less than half as strong as it used to be," he said.

That in and of itself would already lead to major global impacts. And none of these projections even take into account ice melting from Greenland, which is speeding up a potential shutdown.

Rahmstorf and 43 other climate scientists signed an open letter Nordic leaders, insisting the risk has been underestimated in previous research and urging leaders to prepare for, and try to prevent, what he called a catastrophe.

"The only way to really reduce the risk of crossing that tipping point is to get out of fossil fuels as fast as we can," Rehmstorf said.

But that's no small task, as fossil fuels currently account for at least 65% of global energy consumption.

Impacts on Chicago weather

To understand how an AMOC collapse could affect Chicago weather, we have to go back to Burdick's tiny flies. They were alive the last time this ocean current weakened 12,000 years ago.

By determining which species of fly lived where back then, Burdick can tell temperatures in Chicago were colder and spruce trees, more typical of colder northern climates, thrived here.

Research shows that winter temperatures then were as much as 10 degrees colder than they are now, making Chicago's winters as cold as Minnesota's.

"We think the event lasted about a thousand years," Burdick said.

While global warming could leader to a millennium of colder Chicago winters, researchers are careful to note that if this ocean current collapses, it wouldn't cancel out global warming. While our part of the world could cool off, other parts could warm even faster.

"When you're looking at these climate changes in the past and thinking about their impacts on our modern society, it's certainly concerning," Burdick said.  "Most climate scientists could not look you in the eye and say they didn't have some level of nervousness or apprehension about what could happen in the future."

Observations from the Greenland ice melt expedition are expected to be incorporated into climate models in the coming years, which will help scientists better predict the odds that this ocean current could shut down.