Rising concentrations of carbon dioxide — or CO2 — in the atmosphere tend to boost temperatures at Earth’s surface. But CO2 increases could eventually shut down the flow of a major ocean current, a new climate study concludes. Without this moving water, wintertime temperatures could plummet in parts of Europe by 7 degrees Celsius (13 degrees Fahrenheit). A stalling of this ocean current also could change rainfall patterns across the globe.
This is similar to the scenario that played out in the 2004 disaster movie, The Day After Tomorrow. Fortunately, it’s not nearly as extreme as the dire freezing that the movie’s characters had to live through.
The current in the Atlantic ferries warm water northward from the Southern Hemisphere. These waters keep winter temperatures mild in Northern Europe and affect rainfall. Previously, computer models of climate didn’t predict that a shutdown in this current would occur.
They do now.
Wei Liu is a climate scientist at Yale University in New Haven, Conn. He and his colleagues noticed a problem in climate models. Freshwater is important for understanding climate change. But in the old models, freshwater was moving the wrong way between the Atlantic Ocean and the Southern Ocean (located around Antarctica). It flowed in the opposite direction from what scientists see in nature. Liu and his colleagues fixed this problem.
Then they set up an extreme test. They examined how growing CO2 levels might affect the North Atlantic current. For these tests, they had a computer double CO2 levels in the atmosphere. Within 300 years, the computer predicted, the Atlantic current would shut down.
Such an extremely fast rise in carbon dioxide is unrealistic. But it is an important test, the scientists say. The ocean current isn’t stable after all, they concluded January 4 in Science Advances. Liu says they now need a more realistic test “to predict what the future will look like.”
Predicting the behavior of the current is hard. Good predictions depend on having long-term data on actual Atlantic currents. In fact, notes Gerald Meehl, only some 20 years of such data exist for the Atlantic current. A climate scientist, Meehl works at the National Center for Atmospheric Research in Boulder, Colo.
A global conveyor belt
This Atlantic current is also called the Atlantic Meridional Overturning Circulation, or AMOC. It works like a colossal conveyor belt. It carries warm water along the Atlantic Ocean surface, moving from south to north. Near Greenland, the current makes a U-turn. Then the cold water sinks and flows south again. These two halves form a loop. This loop keeps northwestern Europe warm. The current also drives rainfall across the tropical Atlantic.
Climate change has been warming waters in the North Atlantic. Those waters now are less dense than they used to be. They are therefore less likely to sink. And that could threaten to slow the AMOC down.
The ocean’s saltiness also plays an important role in the operation of this conveyor belt. In earlier computer models, freshwater flowed from the Southern Ocean into the Atlantic Ocean. As Earth’s climate warmed, the flow of that freshwater slowed and the Atlantic became saltier. This salt-laden water is denser, like cold water, and should speed the AMOC back up.
But ocean observations show that freshwater flows differently. It flows into the Southern Ocean from the Atlantic Ocean, not the other way around. For his team’s new study, Liu corrected the direction of the flow of freshwater here.
Then the researchers doubled the carbon dioxide in the atmosphere compared to what levels had been back in 1990. Now the computer predicted that the North Atlantic would warm and the AMOC would slow. Less warm water moved northward. And as the AMOC slowed, less freshwater flowed from the Atlantic into the Southern Ocean. This left the Atlantic less salty. Over time, the AMOC slowed and weakened. Eventually, that conveyor belt of water just stopped moving.
That was a prediction by the computer. If that also happened in real life, countries such as England and Iceland would become colder. Earth’s growing levels of carbon dioxide normally would warm the atmosphere. But the cooling from a stalled current would overwhelm that effect. It would cancel greenhouse warming.
The researchers also tried the simulation without correcting for the true direction of the freshwater flow. Now the current wasn’t disrupted any longer.
Shutting down the conveyor belt over a 300-year period wasn’t even the worst that might happen, the researchers note. A warming climate has been melting ice in Greenland. That also can boost freshwater flow into the Atlantic Ocean, helping to slow down the AMOC. But in their new study, Liu’s team didn’t study this factor.
Antarctica A continent mostly covered in ice, which sits in the southernmost part of the world.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
climate The weather conditions prevailing in an area in general or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
concentration (in chemistry) A measurement of how much of one substance has been dissolved into another.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
Greenhouse effect The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants as greenhouse gases. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically apart of North America, just east of Northern Canada, Greenland has been politically more linked to Europe. Indeed, Vikings arrived in Greenland around the 10th century, and for a time the island was a colony of Denmark. In June 2009, Greenland became an independent nation. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), it averages the fewest people per square kilometer of its surface area.
Iceland A largely arctic nation in the North Atlantic, sitting between Greenland and the western edge of Northern Europe. Its volcanic island was settled between the late 800s and 1100 by immigrants from Norway and Celtic lands (ones governed by the Scots and Irish). It is currently home to roughly a third of a million people.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
scenario An imagined situation of how events or conditions might play out.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might vary in response to various situations or over time.
Journal: W. Liu et al. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances. Published online January 4, 2017. doi: 10.1126/sciadv.1601666.