The fate of the Gulf Stream will be decided by a tug-of-war between two types of melting of the Greenland ice sheet, a new study suggests.
During the last Ice AgeBetween 16,800 and 60,000 years ago, large icebergs broke up off the coast of North America, dramatically weakening vital ocean currents.
Now researchers have found that despite today’s high rates of iceberg break-off from the Greenland ice sheet, as in some of these events in the past, the flow of runoff water from Greenland’s coasts could potentially halt this disruption. The researchers published their findings on May 30 in the journal Science.
“There is a tug-of-war between the more effective but slowing ice runoff and the less effective, accelerating runoff,” said the lead author. Yuxin Zhoua postdoctoral fellow at the University of California, Santa Barbara, told Live Science. “Those are the two influences we’re most concerned about.”
The Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream, determines the climate by transporting nutrients, oxygen and heat in tropical waters northwards and cold water southwards. The current can exist in two stable states: a stronger, faster one, which we rely on today, and another that is much slower and weaker.
Climate change is slowing this flow by feeding freshwater from Greenland’s melting ice, making the water less dense and less salty. This has led to a growing number of studies suggesting that the Flow slows down and could even towards collapse.
Heinrich events are the outflow of icebergs from the Laurentide Ice Sheet, which covered most of North America during the last ice age. The current cause of this melting is climate change, but during the last glacial maximum it was probably a mixture of warming ocean and the weight of ice accumulating on the sheet.
This caused icebergs to slide into the sea and freshwater to flow down from the shelf, leading to a dramatic weakening of the AMOC within a few centuries.
To assess whether iceberg calving in Greenland today could trigger a breakdown of the AMOC, Zhou and his colleagues examined layers of sediment deposited by previous Heinrich events. The researchers focused on thorium-230, a form of the radioactive element that is produced at a constant rate in seawater, meaning its concentrations are detectably diluted by fresh meltwater from icebergs.
Related: The Gulf Stream stopped transporting nutrients during the last ice age – and the same could happen now
By comparing their results with projected ice runoff from Greenland, the researchers found that modern climate change (since the Industrial Revolution) is comparable to a medium-range Heinrich event.
But there are key differences between then and now. During the last ice age, the AMOC had already begun to slow before icebergs began to calve. And although the AMOC has more fluctuations than previously thought, it is currently in a “fairly healthy state” with no significant slowdown, Zhou said.
Zhou said the changes we see today are driven by the relationship between iceberg calving and freshwater melting directly from the shelf. Icebergs are the main factor in this slowdown, while runoff plays a minor role. But while melting causes some slowdown, it also slows iceberg production, creating a tug-of-war whose interplay will determine the future of the AMOC.
However, assuming that the AMOC is starting from a stronger position today than in the past, this could be a reason for cautious optimism, Zhou said.
“This will become increasingly clear in the future with new studies,” Zhou said. “But I think that in the near future before 2100, the AMOC is unlikely to weaken much, according to our study.”
However, the researchers pointed out that their study does not take into account other effects of ocean and Arctic warming, meaning that other climate researchers warn against uncritically extrapolating the study’s assumptions to the present.
“A big problem is that the behavior of the AMOC (and its response to meltwater) during the last ice age is probably very different from today’s behavior,” David ThornhillProfessor of marine and climate sciences at University College London, told Live Science. “The ocean-atmosphere-climate system behaves differently when there are massive ice sheets in North America and meltwater enters the ocean at different locations.”
These differences could prove crucial when combined with recent studies suggesting that the AMOC has already weakened and may even be approaching a tipping point, Thornalley said.
“We still have a lot to find out to be sure about the future behavior of the AMOC: how good our models are; how easily the modern AMOC can be destabilized; (and) there could be unexpected surprises, both good and bad,” Thornalley said. “But there are enough reasons to be concerned about the AMOC, and we should apply the precautionary principle – we really don’t want to experience the climate impacts of an AMOC breakdown firsthand. It’s just one of many climate impacts that we should avoid at all costs.”
