Anthropogenic climate disruption (ACD) has progressed to a point where it is, literally, changing one of the most important ocean circulatory currents in the world.
In a paper recently published in the journal Geophysical Research Letters, NASA researchers confirmed that the circulation of the Atlantic meridional overturning circulation (AMOC) is slowing down. In 2009 and 2010 that shifting had already been linked to a sudden and extreme five-inch sea level rise on the East Coast.
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While most people have likely never heard of the AMOC, it plays a critical role in both global climate patterns and sea level rise along the Eastern Seaboard of the US – and it is being changed for the worse due to our carbon dioxide emissions.
Before we can fully comprehend the ramifications of what is happening with this ocean current system, a basic understanding of the AMOC is in order.
“When the AMOC is changed, it has significant impacts on the weather in Europe, along with other places.”
The AMOC is a flow of warm, salty water that starts in the tropics and runs northward into the high latitudes, where the air is much colder and extracts heat from it. The water releases its heat to warm Europe. The water then cools, which causes it to become denser and sink. The cold water then cycles back southward, and the circulation continues. The AMOC keeps the United Kingdom warmer than it typically would be, given the relatively high latitude of the country. It also plays a role in determining sea level along the eastern United States.
But the AMOC is receiving massive influxes of cold water from the Greenland ice sheet, which is now melting in dramatic fashion, according to NASA.
Dr. Thomas Delworth, a research scientist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, notes the direct effect of changes in ocean circulation on weather.
“When the AMOC is changed, it has significant impacts on the weather in Europe, along with other places,” Delworth told Truthout.
One of those impacts that we are already seeing play out dramatically is sea level rise along the United States’ East Coast.
“Recent research links sea level changes to changes in the AMOC,” Dr. Gerard McCarthy, a senior research scientist with the National Oceanography Centre in the UK, told Truthout. “So, [cities from] Atlantic City up to New York City and Boston [are] seeing sharp sea level rises due to the overturning circulation declining. We are seeing this now, absolutely.”
McCarthy works on RAPID, a study led by the University of Arizona that highlights the role of ocean circulation changes that cause extreme sea level jumps along the northeastern coast of the United States. Earlier in 2014, McCarthy’s work showed that a dramatic sea level jump of 128 millimeters (five inches) along this coast was directly linked to an “unprecedented” drop in the strength of the AMOC during 2009 and 2010.
Delworth is seeing the same thing.
“When you weaken the AMOC, you increase sea level along the northeastern US coastline,” Delworth said. “You get more rapid sea level rise. So a substantial decline [of the AMOC] over the coming decades adds to sea level rise that is already happening. So weakening it complicates life along the East Coast of the US.”
Both Delworth and McCarthy think the AMOC will continue to weaken in the long-term future, due to ACD impacts. The question is only, how much?
Delworth sees the rapidly melting Greenland ice sheet as a critical factor in determining the extent to which the AMOC will weaken. This is due, in large part, to the fact that the faster the ice sheet melts, the further the AMOC will weaken.
More precise scientific instrumentation to measure the AMOC was only put in place in 2004, so there have only been 11 years of data collected, and Delworth hopes more measuring capabilities will be available in the near future.
But even with the data collected thus far, Delworth says there is enough to have elicited this warning: “It could be less stable than we think.”
The AMOC is known as the “global conveyor belt” because it brings warm water and temperatures to the eastern US and Europe, and recycles cold water from northern latitudes back down to the tropics. In this way, it functions as a weather regulator.
But now that ACD is altering the natural processes that impact the AMOC, the entire weather system it regulates could begin to break down. When this happens, Europe’s weather will begin to turn dramatically colder, and will become beset by more extreme storms, both in power and frequency.
“A significant change in the strength of the AMOC would alter winds, temperatures and precipitation patterns around the globe.”
As noted, the portion of the AMOC disruption that is already occurring is largely due to Greenland dumping massive amounts of cold water into the Northern Atlantic. This fresh water from Greenland is lighter than saltwater, so it stays on the surface, preventing saltwater from sinking, which begins to inhibit the current, slowing it. In the long term, the current could be shut down altogether, which would lead to dramatic shifts in global weather patterns, including the possibility of bringing much of Northern Europe back into an ice age.
“On the long timescales, we expect that circulation to weaken,” Delworth explained. “A prime driver of the circulation is that in the tropics the water is less dense and warmer and lighter and piles up higher. In the north, it is denser and sinks, so as we warm the planet, we reduce that density contrast between the tropics and high latitudes.”
Delworth said he expects to see these trends continuing in the coming decades. Additionally, he predicts a continuation of the trend we are seeing today of warmer waters from the tropics being pushed further north, hence contributing further to the melting of Greenland, as a recent study shows is already happening.
Julienne Stroeve, a senior research scientist at the National Snow and Ice Data Center in Boulder, Colorado, told Truthout that scientists “believe this freshwater influx could have profound impacts on the circulation of ocean water worldwide. Ocean circulation, in turn, is a major driver for weather patterns, both in the Arctic and around the world.”
With an ongoing weakening of the AMOC, Delworth expects us to “see more likelihood of tropical storms and increasing risk of drought in North America.”
According to Delworth, in monsoon-dependent regions stretching from Africa to India, the changing AMOC will “move the tropical rain belts further north, so the Sahel of Africa will see the mean rainfall decrease as those rain belts move further north as the North Atlantic warms, while a colder North Atlantic pushes the rain belts further south.”
McCarthy expects similar impacts.
“A significant change in the strength of the AMOC would alter winds, temperatures and precipitation patterns around the globe, with potentially strong local effects along the east coast of the United States and the west coast of northern European countries,” he said.
Precedent for Large-Scale Changes
Climate predictions generally rely on state-of-the-art models used by the Intergovernmental Panel on Climate Change – and the models are clear on the future of ocean circulation.
“Pretty much universally, these models predict that the AMOC is going to be changed as we move forward,” McCarthy said. “Freshwater input by the high Arctic is impacting it. So a large input of freshwater from Greenland as it melts is having knock-on implications.”
He added that historically, as ice sheets have retreated, as we are now seeing in Greenland, “they have halted the current and caused ice ages.”
Stroeve said that current climate models show the AMOC slowing, but not stopping completely, during the 21st century, emphasizing that the predicted changes are nonetheless significant.
Large-scale changes in ocean currents like the AMOC “are not unprecedented,” she added. “About 12,000 years ago, scientists believe a massive increase in freshwater influx from melting Arctic ice caused the AMOC to shut down entirely, contributing to a major shift of earth’s climate.”
McCarthy expects that we will see “a slow decline of the AMOC,” and that North America will be “one of the main areas where the overturning circulation impacts the weather, but over multidecadal timescales.”
It is challenging to detect long-term slowdowns in the AMOC due to natural variability and the fact that there are only 11 years of actual measurements of its circulation, as Delworth noted.
“Over that 11-year period we have measured a decline, but don’t know yet if it’s due to variability or climate change,” he said.
When it comes to how fast the AMOC is slowed due to ACD impacts, time will tell. But given the ever-increasing intensity of these impacts – and the fact that the melting of Greenland only continues to speed up – it seems likely the future will bring continued dramatic global changes in ocean circulation.