Ocean currents in Atlantic are driven by a large-scale, north-south pattern called the Gulf Stream System. Photo by NASA/Goddard Space Flight Center Scientific Visualization Studio
Feb. 25 (UPI) -- It's been over a millennium since the Gulf Stream System was as weak as it is today.
According to a new study, published Thursday in the journal Nature Geoscience, the Atlantic Meridional Overturning Circulation, or Gulf Stream System, has slowed significantly over the last several decades.
Authors of the new study warn the slowdown could disrupt weather patterns across North America and Europe.
"The Gulf Stream System works like a giant conveyor belt, carrying warm surface water from the equator up north, and sending cold, low-salinity deep water back down south," study co-author Stefan Rahmstorf said in a press release.
"It moves nearly 20 million cubic meters of water per second, almost a hundred times the Amazon flow," said Rahmstorf, a professor of ocean physics at Potsdam Institute for Climate Impact Research.
Scientists suspect the Gulf Stream's slowdown is being driven -- or at least accelerated -- by human-caused global warming. The speed of the conveyer belt has declined 15 percent since the middle of the 20th century.
Dozens of studies have looked at the Gulf Stream's most recent deceleration, but the authors of the latest study wanted to compile a more comprehensive portrait of the historic slowdown.
Researchers used proxy data from ocean sediments and ice cores to estimate historic Gulf Stream flow rates.
"For the first time, we have combined a range of previous studies and found they provide a consistent picture of the AMOC evolution over the past 1,600 years," Rahmstorf said.
"The study results suggest that it has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century," Rahmstorf said.
Scientists estimated historic flow rates for the Gulf Stream System by analyzing water mass properties and deep-sea sediment grain sizes, data captured in marine sediment samples and ice cores.
While any single proxy datapoint is likely to be imprecise and prone to uncertainty, researchers were able to produce a more reliable portrait by compiling multiple proxy datasets across a large timescale.
"Assuming that the processes measured in proxy records reflect changes in AMOC, they provide a consistent picture, despite the different locations and time scales represented in the data," said study co-author Niamh Cahill.
"The AMOC has weakened unprecedentedly in over 1000 years," said Cahill, an assistant professor of mathematics and statistics at Maynooth University in Ireland.
For climate scientists, the Gulf Stream System's slowdown isn't a surprise.
Numerous climate models have predicted that as the levels of greenhouse gas in Earth's atmosphere increase and global temperatures rise, the Atlantic Meridional Overturning Circulation would hit the brakes.
The Gulf Stream System's convection is driven by global differences in water temperature and density.
As water moves toward the Arctic and cools, it becomes denser and begins to sink. The cool, dense water eventually starts flowing back toward the equator, rising as it warms.
Because global warming is more pronounced in the Arctic, temperature gradients in the atmosphere and ocean are getting weaker.
Scientists suggest melting glaciers and increased rainfall have increased the supply of fresh water in the Arctic, decreasing water density. As result, water flowing in from the south isn't cooling and sinking as quickly.
The latest findings also confirmed a link between the Gulf Stream slowdown and the growth of a blob of cold water in the northern Atlantic, which has limited the flow of heat through the region.
Scientists suggest the Gulf Stream slowdown is already altering a variety of climate patterns.
"The northward surface flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast," said co-author Levke Caesar.
"This is due to Earth's rotation that diverts moving objects such as currents to the right in the northern hemisphere and to the left in the southern hemisphere. As the current slows down, this effect weakens and more water can pile up at the U.S. east coast, leading to an enhanced sea level rise," said Caesar, a scientist with the Irish Climate Analysis and Research Unit at Maynooth University.
Models have linked the Gulf Stream slowdown with an increased risk of extreme heat waves and summertime droughts in North America. Some simulations suggest the slowdown could alter the path of major weather systems, causing extreme winter storms to stall over Europe.
"If we continue to drive global warming, the Gulf Stream System will weaken further -- by 34 to 45 percent by 2100 according to the latest generation of climate models," said Rahmstorf.