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article imageSlow down of Atlantic Ocean circulation is bad news for everyone

By Karen Graham     Apr 12, 2018 in Science
The Atlantic Ocean circulation that carries warmth into the Northern Hemisphere’s high latitudes is slowing down because of climate change - and is at its weakest point in the past 1,600 years.
New research led by University College London (UCL) and the Woods Hole Oceanographic Institution (WHOI) provides compelling evidence that the Atlantic Ocean's circulation - called Atlantic meridional overturning circulation (AMOC) - has been slowing down.
So, what's the big deal about this news? The AMOC is vital to regulating global climate. It is a constantly-moving system, often called the Global Ocean Conveyor Belt, that brings warm salty water from the Equator via the Gulf Stream up toward the North Atlantic.
In the North Atlantic, this current releases its heat into the atmosphere and warms Europe. Then, the cooler water sinks back down to the deep ocean and makes its way all the way to Antarctica where the Great conveyor Belt again circulates that water back to the Gulf Stream.
The great ocean conveyor belt
The great ocean conveyor belt
Intergovernmental Panel on Climate Change
The AMOC has declined in strength by 15 percent since the mid-20th century to a “new record low,” the scientists conclude in a peer-reviewed study published in the journal Nature on Wednesday.
"Our study provides the first comprehensive analysis of ocean-based sediment records, demonstrating that this weakening of the Atlantic's overturning began near the end of the Little Ice Age, a centuries-long cold period that lasted until about 1850," said Dr. Delia Oppo, a senior scientist with WHOI and co-author of the study, reports Science Daily.
Lead author Dr. David Thornalley, a senior lecturer at University College London and WHOI adjunct, believes that as North America began to warm after the Little Ice Age, freshwater disrupted the AMOC, causing a slowing of the current.
However, as the climate has warmed, Arctic sea ice, ice sheets and glaciers surrounding the Arctic began to melt, causing a huge gush of freshwater to push into the North Atlantic, exacerbating the slowdown of the AMOC.
The influx of fresh water diluted the seawater, making it too light to sink to the depths of the ocean, disrupting the circulation. The research team reached this conclusion after examining sediment cores, measuring the size of sediment grains deposited by the deep-sea currents.
On May 2  2001  the Moderate-resolution Imaging Spectroradiometer (MODIS) obtained this spectacular ...
On May 2, 2001, the Moderate-resolution Imaging Spectroradiometer (MODIS) obtained this spectacular image of the Atlantic Ocean's Gulf Stream. The false colors in the image represent "brightness temperature" observed at the top of the atmosphere. The brightness temperature values represent heat radiation from a combination of the sea surface and overlying moist atmosphere.
The larger the grains, the stronger the currents. They then used a number of methods to make palaeoclimate reconstructions of near-surface ocean temperatures in regions where the temperature is influenced by AMOC strength. "Combined, these approaches suggest that the AMOC has weakened over the past 150 years by approximately 15 to 20 percent," says Thornalley.
Co-author Dr. Jon Robson, a senior research scientist from the University of Reading suggests the findings suggest a gap in current climate models. "North Atlantic circulation is much more variable than previously thought," he said, "and it's important to figure out why the models underestimate the AMOC decreases we've observed."
A second study confirms slowdown of circulation
A second study published in the journal Nature on April 11, led by Levke Ceasar and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, used climate model data and past sea-surface temperatures to reveal that AMOC has been weakening more rapidly since 1950 due to climate change.
Melting sea ice in the Arctic produces fresh water  slowing the circulation of denser salt water and...
Melting sea ice in the Arctic produces fresh water, slowing the circulation of denser salt water and thereby slowing warming currents
The big takeaway with the two studies is the complementary evidence that the present-day AMOC is exceptionally weak, and they provide both longer-term historical perspective as well as a detailed insight into recent changes.
"What is common to the two periods of AMOC weakening -- the end of the Little Ice Age and recent decades -- is that they were both times of warming and melting," said Thornalley. "Warming and melting are predicted to continue in the future due to continued carbon dioxide emissions."
As the Washington Post points out, not only is this research critical to understanding the sharp increase in cold weather in the northern latitudes in Europe and North America, but it is also critical to the fisheries off the U.S. Atlantic coast. New England's economy has seen the cod fishery collapse while Lobster fisheries have moved further north.
This is “something that climate models have predicted for a long time, but we weren’t sure it was really happening. I think it is happening,” said one of the second study’s authors, Stefan Rahmstorf, “And I think it’s bad news.”
More about atlantic ocean current, Climate change, AMOC system, deep convection, Weather
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