New Evidence Points to Possible North Atlantic Changes – Or Does it?

March 3, 2021

Significant piece in the New York Times (which is graphically rich, so do go to the link) summarizing what we know about AMOC, the Atlantic Meriodonal Overturning Circulation. The current, which most people think of as “the Gulf Stream”, is getting a lot of attention due to some observations suggesting a slowing, which could have global consequences.

New York Times:

Now, a spate of studies, including one published last week, suggests this northern portion of the Gulf Stream and the deep ocean currents it’s connected to may be slowing. Pushing the bounds of oceanography, scientists have slung necklace-like sensor arrays across the Atlantic to better understand the complex network of currents that the Gulf Stream belongs to, not only at the surface, but hundreds of feet deep.

“We’re all wishing it’s not true,” Peter de Menocal, a paleoceanographer and president and director of the Woods Hole Oceanographic Institution, said of the changing ocean currents. “Because if that happens, it’s just a monstrous change.”

The consequences could include faster sea level rise along parts of the Eastern United States and parts of Europe, stronger hurricanes barreling into the Southeastern United States, and perhaps most ominously, reduced rainfall across the Sahel, a semi-arid swath of land running the width of Africa that is already a geopolitical tinderbox.

Colleagues studying ice cores from the Greenland ice sheet were seeing evidence of strange climatic “flickers” in the past. As Earth warmed from the deep freeze of the last ice age, which peaked around 22,000 years ago, temperatures would rise, then abruptly fall, then rise again just as swiftly. Dr. Broecker theorized this was caused by stops and starts in what he called the ocean’s “great conveyor belt”— the AMOC.

The clearest example began about 12,800 years ago. Glaciers that had once covered much of North America and Europe had retreated considerably, and the world was almost out of the deep freeze. But then, in just a few decades, Greenland and Western Europe plunged back into cold. Temperatures fell by around 10 degrees Celsius, or 18 degrees Fahrenheit, in parts of Greenland. Arctic-like conditions returned to parts of Europe.

The cold snap lasted perhaps 1,300 years—before reversing even more abruptly than it began. Scientists have observed the sudden changes in the pollen deposited at the bottom of European lakes and in changes in ocean sediments near Bermuda.

This forced a paradigm shift in how scientists thought about climate change. Earlier, they had tended to imagine creeping shifts occurring over many millennia. But by the late 1990s, they accepted that abrupt transitions, tipping points, could occur.

This didn’t bode well for humanity’s warming of the atmosphere. Dr. Broecker, who died in 2019, famously warned: “The climate system is an angry beast and we are poking it with sticks.”

Scientists also emphasize that the ultimate consequences of that weakening remain unclear. That’s in part because the world is in such uncharted territory. In the past, Europe became drastically cooler when the current shut down, but today any cooling might ultimately be muted or possibly canceled out by continued global heating.

But if past is prologue, a drastically altered AMOC could certainly shift rainfall patterns, scientists said, making parts of Europe and Northern Africa drier, and areas in the Southern hemisphere wetter. Changing ocean currents might affect marine ecosystems that people rely on for food and livelihood.

A changing Gulf Stream could also accelerate sea-level rise along parts of the Atlantic coast of the United States. In 2009 and 2010, when the stream inexplicably weakened by 30 percent, the Northeast saw seas rise at a rate unprecedented in the entire roughly 100-year record of tide gauges.

And if water in the tropical and subtropical Atlantic becomes warmer because that heat is no longer shunted north, the expanding reservoir of energy could strengthen hurricanes, something that scientists at the National Oceanography Centre in the United Kingdom argue is already happening. Hurricanes derive their energy from heat in the water.

Finally, in a perverse twist, a shutdown of the AMOC could exacerbate global heating. The ocean absorbs nearly one-third of human carbon dioxide emissions. But the sinking of salty, dense water—the overturning portion of the AMOC—is critical to that absorption. So, if the AMOC stops or greatly slows, and that water stops sinking, the accumulation of heat-trapping gases in the atmosphere could accelerate.

THEN THERE ARE THOSE CONSEQUENCES that fall in the category of “global weirding.”

Scientists at the U.K.’s National Oceanography Centre have somewhat counterintuitively linked the cold blob in the North Atlantic with summer heat waves in Europe. In 2015 and 2018, the jet stream, a river of wind that moves from west to east over temperate latitudes in the northern hemisphere, made an unusual detour to the south around the cold blob. The wrinkle in atmospheric flow brought hotter-than-usual air into Europe, they contend, breaking temperature records.

“That was not predicted,” said Joel Hirschi, principal scientist at the centre and senior author of the research. It highlights how current seasonal forecasting models are unable to predict these warm summers. And it underscores the paradox that, far from ushering in a frigid future for, say, Paris, a cooler North Atlantic might actually make France’s summers more like Morocco’s.

Even so, Dr. Hirschi takes a wait-and-see stance on whether the AMOC is actually slowing. “I have great respect for what Dr. Rahmstorf is doing. And it may well be spot on in the end,” he says. “But I’m afraid the data, the really robust data, is not there.”

Susan Lozier, a physical oceanographer and dean at the College of Sciences at Georgia Tech, also has her doubts about whether the AMOC is currently slowing. At issue, she says, is how scientists infer changes in the AMOC. We can directly measure many aspects of the ocean, such as temperature (it’s warming), oxygen levels (they’re declining), even how stratified it has become (more so). “There are very strong signals in the ocean of climate change,” she said.

But most studies on the AMOC don’t measure the “conveyor belt” directly. Instead, they use proxies to infer that the overturning has changed.

Such inference can be problematic when considering changes that occur over short time frames, says Dr. Lozier, because the changes observed could have other causes. Consider that cold blob in the North Atlantic, she said. Dr. Rahmstorf and others see it as evidence of a weakening Gulf Stream, but Dr. Lozier notes that shifts in wind patterns or how storms move over the ocean could also underlie the phenomenon. “There are other ways to explain it,” she said. “A lot of our conceptual understanding of AMOC is in isolation of other things going on in the ocean.”

Direct measurement of the AMOC only began relatively recently. A line of sensors between the Bahamas and the Canary Islands, called Rapid, was installed in 2004. A second sensor array, spanning the North Atlantic from Canada to Greenland to Scotland and called Osnap, went live in 2014. (Dr. Lozier is the international project lead for Osnap.)

Neither project has operated long enough to produce clear trends, in Dr. Lozier’s view. What they have shown, though, is lots of natural variability. In 2009 and 2010, for example, the AMOC weakened — “people were like, ‘Oh my God, this is happening,’” she said — only to pick right back up again over the following years.

They’ve also revealed a system of currents that’s far more complex than once envisioned.

Dr. Broecker’s old schematics of the AMOC posit a neat warm current flowing north along the western edge of the Atlantic and an equally neat cold current flowing back south below it. In fact, says Dr. Lozier, that deeper current is not confined to the western edge of the Atlantic, but rather flows southward via a number of “rivers” that are filled with eddies. The network of deep ocean currents is much more complicated than once envisioned, in other words, and figuring out how buoyant meltwater from Greenland might affect the formation of cold deepwater has become more complicated as well.

This is the place scientists currently find themselves in. They suspect the AMOC can work like a climate switch. They’re watching it closely. Some argue that it’s already changing, others that it’s too soon to tell.

“There’s no consensus on whether it has slowed to date, or if it’s currently slowing,” said Dr. Lozier. “But there is a consensus that if we continue to warm the atmosphere, it will slow.”

5 Responses to “New Evidence Points to Possible North Atlantic Changes – Or Does it?”

  1. redskylite Says:

    A brilliant bit of reporting from the NYT handsomely presented with rich visuals (and free from annoying pay wall reminders. The RAPID AMOC monitoring system in place and maintained since 2004, and at least the ocean current is being monitored and hopefully will continue as we sail into the future. We are forewarned of the abrupt and catastrophic changes found by paleoclimatology in the past and know what we have to do to prevent an early repeat.

    This subject surfaces to the media and is hot news from time to time, but monitoring is steady and ongoing and it is well worth a visit to the “RAPID” project from time to time to see the latest news.

    Quote – Numerical models suggest that the MOC is likely to weaken by about 30% in the coming century as a consequence of greenhouse gas emissions. Furthermore, paleoclimate records suggest that during the last Ice Age the MOC has undergone abrupt rearrangements that were responsible for a cooling of European climate of between 5-10C.

  2. grindupbaker Says:

    See at 16:48 at Stefan points out that the issue is that oceanographers aren’t certain of where the AMOC’s present ~17.5 Sv is on that stability diagram so they aren’t certain that the 0.80 Sv of freshwater forcing is the amount that will reach the “critical desalinization point” and stop the AMOC (following which it doesn’t restart until Hell freezes over because it’s a bi-stable mechanism, once broken it must go much farther back the other other way, much colder & much saltier then now to get it going again). 0.80 Sv of freshwater forcing is 25,000 billion tonnes / year of fresh water from rain + snow + melted ice off Greenland. The Arctic Ocean and very high north Atlantic freshwater budget is:
    billion tonnes / year
    5,700 Arctic ocean precipitation in excess of its evaporation, plus
    4,300 Asia & N. America river discharges into the Arctic ocean, plus
    520 Regular historical water+ice off Greenland, matching its precipitation
    270 Extra water+ice off Greenland because its losing mass.
    10,790 Total
    So when that 10,790 billion tonnes / year total of fresh water reaches 25,000 billion tonnes / year by whatever combination of reasons (such as the 4% more for all the rain/snow fall per 1 degree of global warming) that’s the WG1 climate scientists best estimate of when the AMOC will be brought to a full stop (permanently as far as humans of the next few centuries are concerned).
    The AMOC moves 550,000 billion tonnes / year of water north in the Atlantic near the surface & south in the Atlantic at depths like 800-1800 metres (just rough from an old memory). The north surface part delivers 1,200,000 gigawatts of heating power from the tropics up to the north Atlantic, across Greenland & western Europe and to the edge of the Atlantic Ocean. It is estimated by simple calculation that that region gets warmed a special extra 5 degrees on annual average (Stefan Rahmstorf’s envelope calculation, I calculated 6 degrees in my head) by the AMOC warm water heat. Note that the entire mid-high latitudes also get separately warmed by warm winds from south and that is 4,800,000 gigawatts of heating power (4.0x as much as AMOC warming) with a slight, less likely, chance that it’s 6,000,000 gigawatts of heating power (5.0x as much as AMOC warming) because Kevin Trenberth wasn’t clear about that in his talk about global power surfeits & deficits.

    • grindupbaker Says:

      And Scandanavia of course. Earth absorbs 122,500,000 gigawatts of heating power from the Sun. 2,200,000 gigawatts of heating power is shifted from ocean surface to land (ocean surface is warmer than land).

  3. grindupbaker Says:

    “As Earth warmed from the deep freeze of the last ice age, which peaked around 22,000 years ago, temperatures would rise, then abruptly fall, then rise again just as swiftly”. Yet again Greenman obfuscates and confuses global warming with regional climate changes on the subjects of D-O oscillations and Younger-Dryas. One has to read the detail below and figure it out. This is low quality and Greenman should stop doing it, as I advised him a couple years ago. The coal/oil shills do this rubbish relentlessly with the Greenland-only proxy data with “Earth has warmed many degrees in a few years in the past. Was it our SUVs then ?” and Greenman should stop compiling lazy confusing rubbish like the coal/oil shills like he did in quote above and some prior times. Just stop it already.

    • grindupbaker Says:

      I missed the bit where that was just a direct quote from the NYT and not Greenman’s penning. I still object to parroting journalistic stuff that’s unclear like that example (and which is endemic from what I’ve read here & there) but oh well whatever.

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