Current Atlantic Meridional Overturning Circulation weakest in last millennium by L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill and S. Rahmstorf, Feb 25, 2021, Nature Geoscience The paper is a tart succinct read.
The Atlantic Meridional Overturning Circulation (AMOC)— one of Earth’s major ocean circulation systems—redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about ad 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades. …
Atlantic Ocean circulation at weakest in a millennium, say scientists, Decline in system underpinning Gulf Stream could lead to more extreme weather in Europe and higher sea levels on US east coast by Fiona Harvey Environment correspondent, 26 Feb 2021, The Guardian
The Atlantic Ocean circulation that underpins the Gulf Stream, the weather system that brings warm and mild weather to Europe, is at its weakest in more than a millennium, and climate breakdown is the probable cause, according to new data.
Further weakening of the Atlantic Meridional Overturning Circulation (AMOC) could result in more storms battering the UK, more intense winters and an increase in damaging heatwaves and droughts across Europe.
Scientists predict that the AMOC will weaken further if global heating continues, and could reduce by about 34% to 45% by the end of this century, which could bring us close to a “tipping point” at which the system could become irrevocably unstable. A weakened Gulf Stream would also raise sea levels on the Atlantic coast of the US, with potentially disastrous consequences.
Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research, who co-authored the study published on Thursday in Nature Geoscience, told the Guardian that a weakening AMOC would increase the number and severity of storms hitting Britain, and bring more heatwaves to Europe.
He said the circulation had already slowed by about 15%, and the impacts were being seen. “In 20 to 30 years it is likely to weaken further, and that will inevitably influence our weather, so we would see an increase in storms and heatwaves in Europe, and sea level rises on the east coast of the US,” he said.
Rahmstorf and scientists from Maynooth University in Ireland and University College London in the UK concluded that the current weakening had not been seen over at least the last 1,000 years, after studying sediments, Greenland ice cores and other proxy data that revealed past weather patterns over that time. The AMOC has only been measured directly since 2004.
The AMOC is one of the world’s biggest ocean circulation systems, carrying warm surface water from the Gulf of Mexico towards the north Atlantic, where it cools and becomes saltier until it sinks north of Iceland, which in turn pulls more warm water from the Caribbean. This circulation is accompanied by winds that also help to bring mild and wet weather to Ireland, the UK and other parts of western Europe.
Scientists have long predicted a weakening of the AMOC as a result of global heating, and have raised concerns that it could collapse altogether. The new study found that any such point was likely to be decades away, but that continued high greenhouse gas emissions would bring it closer.
Rahmstorf said: “We risk triggering [a tipping point] in this century, and the circulation would spin down within the next century. It is extremely unlikely that we have already triggered it, but if we do not stop global warming, it is increasingly likely that we will trigger it.
“The consequences of this are so massive that even a 10% chance of triggering a breakdown would be an unacceptable risk.”
Research in 2018 also showed a weakening of the AMOC, but the paper in Nature Geoscience says this was unprecedented over the last millennium, a clear indication that human actions are to blame. Scientists have previously said a weakening of the Gulf Stream could cause freezing winters in western Europe and unprecedented changes across the Atlantic.
The AMOC is a large part of the Gulf Stream, often described as the “conveyor belt” that brings warm water from the equator. But the bigger weather system would not break down entirely if the ocean circulation became unstable, because winds also play a key role. The circulation has broken down before, in different circumstances, for instance at the end of the last ice age.
The Gulf Stream is separate from the jet stream that has helped to bring extreme weather to the northern hemisphere in recent weeks, though like the jet stream it is also affected by the rising temperatures in the Arctic. Normally, the very cold temperatures over the Arctic create a polar vortex that keeps a steady jet stream of air currents keeping that cold air in place. But higher temperatures over the Arctic have resulted in a weak and wandering jet stream, which has helped cold weather to spread much further south in some cases, while bringing warmer weather further north in others, contributing to the extremes in weather seen in the UK, Europe and the US in recent weeks.
Similarly, the Gulf Stream is affected by the melting of Arctic ice, which dumps large quantities of cold water to the south of Greenland, disrupting the flow of the AMOC. The impacts of variations in the Gulf Stream are seen over much longer periods than variations in the jet stream, but will also bring more extreme weather as the climate warms.
As well as causing more extreme weather across Europe and the east coast of the US, the weakening of the AMOC could have severe consequences for Atlantic marine ecosystems, disrupting fish populations and other marine life.
Andrew Meijers, the deputy science leader of polar oceans at British Antarctic Survey, who was not involved in the study, said: “The AMOC has a profound influence on global climate, particularly in North America and Europe, so this evidence of an ongoing weakening of the circulation is critical new evidence for the interpretation of future projections of regional and global climate.
“The AMOC is frequently modelled as having a tipping point below some circulation strength, a point at which the relatively stable overturning circulation becomes unstable or even collapses. The ongoing weakening of the overturning means we risk finding that point, which would have profound and likely irreversible impacts on the climate.”
Karsten Haustein, of the Climate Services Center in Germany, also independent of the study, said the US could be at risk of stronger hurricanes as a result of the Gulf Stream’s weakening.
“While the AMOC won’t collapse any time soon, the authors warn that the current could become unstable by the end of this century if warming continues unabated,” he said. “It has already been increasing the risk for stronger hurricanes at the US east coast due to warmer ocean waters, as well as potentially altering circulation patterns over western Europe.”
Dr Levke Caesar, of Maynooth University in Ireland, and the lead author of the paper, said sea level rises on the east coast of the US were another potential consequence. “The northward surface flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast. 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,” she said. “As the current slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise.”
Scientists see stronger evidence of slowing Atlantic Ocean circulation, an ‘Achilles’ heel’ of the climate, The Atlantic meridional overturning circulation, a system of currents, is weaker than it has been in 1,000 years by Chris Mooney and Andrew Freedman, Feb. 25, 2021, Washington Post
A growing body of evidence suggests that a massive change is underway in the sensitive circulation system of the Atlantic Ocean, a group of scientists said Thursday.
The Atlantic meridional overturning circulation (AMOC), a system of currents that includes the Florida Current and the Gulf Stream, is now “in its weakest state in over a millennium,” these experts say. This has implications for everything from the climate of Europe to the rates of sea-level rise along the U.S. East Coast.
Although evidence of the system’s weakening has been published before, the new research cites 11 sources of “proxy” evidence of the circulation’s strength, including clues hidden in seafloor mud as well as patterns of ocean temperatures. The enormous flow has been directly measured only since 2004, too short a period to definitively establish a trend, which makes these indirect measures critical for understanding its behavior.
The new research applies a statistical analysis to show that those measures are in sync and that nine out of 11 show a clear trend.
Prior research had suggested that the AMOC was at its weakest point in a millennium or more, and suggested a roughly 15 percent weakening since about 1950. But when it comes to the latest evidence, “I think it just makes this conclusion considerably stronger,” said Stefan Rahmstorf, an author of the research and an oceanographer with the Potsdam Institute for Climate Impact Research in Germany.
The study was published in Nature Geoscience by scientists from the Potsdam Institute, Ireland’s Maynooth University and University College London.
The AMOC is driven by two vital components of ocean water: temperature and salt. In the North Atlantic, warm, salty water flows northward off the U.S. coastline, carrying heat from the tropics. But as it reaches the middle latitudes, it cools, and around Greenland, the cooling and the saltiness create enough density that the water begins to sink deep beneath the surface.
The water then swings back southward and travels all the way to the Southern Hemisphere, submerged, where it makes its way to the Antarctic as part of a global system of ocean currents. The entire system is known as the ocean’s thermohaline circulation (“thermo” meaning heat and “haline,” salt), and it plays many critical roles in the climate. It is also referred to as the global ocean conveyor belt, because it redistributes heat worldwide.
In the North Atlantic, most important is the transport of heat northward, which has a moderating effect on Europe’s climate in particular. But the circulation can be weakened by making northern water more fresh and less salty, and therefore less dense. That’s what climate change — through a combination of more rain and snow, more melting of Arctic sea ice, and huge freshwater pulses from Greenland — is thought to be doing.
The AMOC has captured the popular imagination because of its depiction in the lurid sci-fi film “The Day After Tomorrow,” in which various disastrous events occur after a sudden halting of the current. Nothing nearly so dire is underway, and scientists say that although a shutdown is possible in the future as climate change continues, steady weakening is the more likely course in the near future.
The late climate scientist Wallace S. Broecker wrote in 1997 that the AMOC is the “Achilles’ heel” of the climate system, citing evidence that it has switched on and off repeatedly over the course of Earth’s history, with the power to flip warming periods to intense cold in the Northern Hemisphere.
Scientists do not expect anything so severe in our future, especially because greenhouse gases will continue to cause offsetting warming. However, they note that even the modest slowing of 15 percent has been accompanied by odd temperature patterns in the ocean and the significant upending of certain key fisheries, such as lobster and cod off the coast of New England.
In particular, a recurrent “cold blob” has been observed in the ocean to the south of Greenland — a large region that is bucking the overall global warming trend and instead showing a marked cooling pattern. Scientists think this is evidence that less warm water is reaching this region than previously, and that it may also be a result of runoff from the melting ice sheet.
At the same time, warm water has lingered instead off the coast of the northeastern United States, where the Gulf of Maine is showing some of the fastest-warming ocean water anywhere in the world.
Michael Mann, a climate scientist at Penn State who co-wrote a major 2015 study with Rahmstorf showing a slowing AMOC, said the cold blob has been “remarkable” in its staying power.
Mann said that the cold blob’s presence since 2015 has helped convince him that the ocean current’s slowdown is a robust finding. “The remarkable persistence of the cold blob since the publication of our article has convinced me, ‘Yeah, it’s real.’ ”
At the same time, though, the latest study does rely on “proxy evidence,” rather than direct measurements of the circulation.
To give an example, one of the proxy-based studies, published last year, uses sediment collected from the seafloor to the south of Iceland, and the microscopic organisms it contains, to detect a shifting of water types over time. It therefore infers more warm water arriving in recent decades as opposed to colder conditions over hundreds of previous years.
The current study’s conclusions are, by necessity, only as good as the proxies are. And the complexity of different currents in the Atlantic, as well as different definitions of the AMOC, can call into question what the proxies are actually measuring, said Marilena Oltmanns, an oceanographer at the National Oceanography Center in Britain.
Still, Oltmanns noted, “I think it is commendable that the authors used so many different proxies. So, even if only one of these proxies is linked to the AMOC, it should be sufficient to justify their conclusion.”
Rahmstorf understands the concern but thinks the big picture nevertheless presents a major shift.
“The proxies from different regions are measuring different aspects of a complex 3-D AMOC circulation,” he said. “To me, the key is that all these pieces of the AMOC puzzle fit together so nicely in the proxy data.”
Tragic that so many are anti-truth and anti-science, avoid reading (and thinking), and vote for and worship:
etc. etc. etc.