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A new study suggests that the fate of the Gulf Stream may be determined by a “tug of war” between two types of melting of the Greenland ice sheet.
Through the past Ice AgeBetween 16,800 and 60,000 years ago, massive icebergs washed off the coast of North America, dramatically weakening important ocean currents.
Researchers have found that even though icebergs are calving from the Greenland ice sheet at a rate just as high today as they have in the past, runoff from Greenland’s coast could help halt the breakup. They published their findings in the journal Nature on May 30. Science.
“There’s a tug-of-war between more effective but slowing ice runoff and less effective accelerating runoff,” the lead author says. Zhou Yuxin“It’s these two effects that we’re primarily concerned about,” said Dr. Schneider, a postdoctoral researcher at the University of California, Santa Barbara, told Live Science.
The Atlantic Meridional Overturning (AMOC), which includes the Gulf Stream, controls the climate by bringing nutrients, oxygen, and heat to the tropical oceans in the north and cold waters to the south. Two stable states can existThere is a powerful, fast system that we currently rely on, and a much slower, weaker system.
Climate change is slowing this flow as the Greenland ice sheet melts and freshwater flows in, making the water less dense and saline. The flow is slowing and, Heading for collapse.
The melting of icebergs from the Laurentide Ice Sheet, which covered much of North America during the last glacial period, is known as the Heinrich event. While today the melting of these icebergs is caused by climate change, during the Last Glacial Maximum it was likely caused by a combination of ocean heat and the weight of the ice that had accumulated on the ice sheet.
This resulted in icebergs sliding into the ocean and freshwater draining off the shelf, resulting in a dramatic weakening of the AMOC over hundreds of years.
To assess whether an iceberg that broke off from Greenland could trigger the current AMOC disruption, Zhou and his colleagues studied layers of sediment deposited by past Heinrich events. The researchers focused on thorium-230, a type of radioactive element that is produced at a constant rate in ocean waters, meaning its concentration is diluted to a detectable level by fresh meltwater from the iceberg.
Related: The Gulf Stream stopped providing nutrients during the last ice age, and the same thing may be happening now.
By comparing their results with projections of ice runoff from Greenland, the researchers found that modern climate change (since the Industrial Revolution) corresponds to a “moderate” Heinrich event.
But there’s an important difference between then and now: During the last ice age, the AMOC began slowing down even before icebergs started calving, and today, the AMOC is in a “fairly healthy state” without any significant slowdown, even though it’s more volatile than previously thought, Zhou said.
Zhou said the changes we’re seeing are driven by the relationship between iceberg break-up and the melting of freshwater flowing directly from the ice shelves. Icebergs are the most important driver of this slowdown, with run-off playing a secondary role. But while melting causes the slowdown, it also slows iceberg formation, and their interactions create a tug-of-war that will determine the future of the AMOC.
But if the AMOC is thought to be starting from a stronger position today than it was in ancient times, Zhou said, there may be reason for cautious optimism.
“This will become more and more clear as more studies are published,” Zhou said, “but I think our study shows that it’s unlikely that the AMOC will weaken significantly in the near future, up until 2100.”
But the researchers point out that the study doesn’t take into account other effects of warming oceans or the Arctic, meaning other climate scientists have warned against uncritically applying its assumptions to the modern day.
“The big question is that how the AMOC behaved (and responded to meltwater) during the last glacial period was likely very different from today.” David Thornally“If you have a huge ice sheet in North America, and meltwater is draining into the ocean in different places, the ocean-atmosphere climate system will behave differently,” the professor of ocean and climate sciences at University College London told Live Science.
Thornalee said these differences, combined with recent research suggesting the AMOC may already be weakening and approaching a tipping point, could be conclusive evidence.
“There are many things that still need to be worked out before we can be confident about the future behavior of the AMOC, including how good our models are and how easily the modern AMOC can become destabilized. [and] “There may be unexpected surprises, for better or worse,” Thornalee said, “but there is good reason to be concerned about the AMOC and we should apply the precautionary principle. We really don’t want to see first-hand the climate impacts of AMOC breakdown. This is just one of many climate impacts that we should do all we can to avoid.”
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