Warm current that has historically caused dramatic changes in climate is experiencing an unprecedented slowdown and may be less stable than thought - with potentially severe consequences
Damian Carrington The Guardian 11 Apr 18;
The warm Atlantic current linked to severe and abrupt changes in the climate in the past is now at its weakest in at least 1,600 years, new research shows. The findings, based on multiple lines of scientific evidence, throw into question previous predictions that a catastrophic collapse of the Gulf Stream would take centuries to occur.
Such a collapse would see western Europe suffer far more extreme winters, sea levels rise fast on the eastern seaboard of the US and would disrupt vital tropical rains. The new research shows the current is now 15% weaker than around 400AD, an exceptionally large deviation, and that human-caused global warming is responsible for at least a significant part of the weakening.
The current, known as the Atlantic Meridional Overturning Circulation (Amoc), carries warm water northwards towards the north pole. There it cools, becomes denser and sinks, and then flows back southwards. But global warming hampers the cooling of the water, while melting ice in the Arctic, particularly from Greenland, floods the area with less dense freshwater, weakening the Amoc current.
Scientists know that Amoc has slowed since 2004, when instruments were deployed at sea to measure it. But now two new studies have provided comprehensive ocean-based evidence that the weakening is unprecedented in at least 1,600 years, which is as far back as the new research stretches.
“Amoc is a really important part of the Earth’s climate system and it has played an important part in abrupt climate change in the past,” said Dr David Thornalley, from University College London who led one of the new studies. He said current climate models do not replicate the observed slowdown, suggesting that Amoc is less stable that thought.
During the last ice age, some big changes in Amoc led to winter temperatures changing by 5-10C in as short a time as one to three years, with major consequences for the weather over the land masses bordering the Atlantic. “The [current] climate models don’t predict [an Amoc shutdown] is going to happen in the future – the problem is how certain are we it is not going to happen? It is one of these tipping points that is relatively low probability, but high impact.”
The study by Thornalley and colleagues, published in Nature, used cores of sediments from a key site off Cape Hatteras in North Carolina to examine Amoc over the last 1600 years. Larger grains of sediment reflect faster Amoc currents and vice versa.
They also used the shells of tiny marine creatures from sites across the Atlantic to measure a characteristic pattern of temperatures that indicate the strength of Amoc. When it weakens, a large area of ocean around Iceland cools, as less warm water is brought north, and the waters off the east coast of the US get warmer.
The second study, also published in Nature, also used the characteristic pattern of temperatures, but assessed this using thermometer data collected over the last 120 years or so.
Both studies found that Amoc today is about 15% weaker than 1,600 years ago, but there were also differences in their conclusions. The first study found significant Amoc weakening after the end of the little ice age in about 1850, the result of natural climate variability, with further weakening caused later by global warming.
The second study suggests most of the weakening came later, and can be squarely blamed on the burning of fossil fuels. Further research is now being undertaken to understand the reasons for the differences.
However, it is already clear that human-caused climate change will continue to slow Amoc, with potentially severe consequences. “If we do not rapidly stop global warming, we must expect a further long-term slowdown of the Atlantic overturning,” said Alexander Robinson, at the University of Madrid, and one of the team that conducted the second study. He warned: “We are only beginning to understand the consequences of this unprecedented process – but they might be disruptive.”
A 2004 disaster movie, The Day After Tomorrow, envisaged a rapid shutdown of Amoc and a devastating freeze. The basics of the science were portrayed correctly, said Thornalley: “Obviously it was exaggerated – the changes happened in a few days or weeks and were much more extreme. But it is true that in the past this weakening of Amoc happened very rapidly and caused big changes.”
Climate change dials down Atlantic Ocean heating system
Victoria Gill BBC 11 Apr 18;
The circulation system plays a "significant role" in regulating the Earth's climate by distributing heat around the globe.
A significant shift in the system of ocean currents that helps keep parts of Europe warm could send temperatures in the UK lower, scientists have found.
They say the Atlantic Ocean circulation system is weaker now than it has been for more than 1,000 years - and has changed significantly in the past 150.
The study, in the journal Nature, says it may be a response to increased melting ice and is likely to continue.
Researchers say that could have an impact on Atlantic ecosystems.
Scientists involved in the Atlas project - the largest study of deep Atlantic ecosystems ever undertaken - say the impact will not be of the order played out in the 2004 Hollywood blockbuster The Day After Tomorrow.
But they say changes to the conveyor-belt-like system - also known as the Atlantic Meridional Overturning Circulation (Amoc) - could cool the North Atlantic and north-west Europe and transform some deep-ocean ecosystems.
That could also affect temperature-sensitive species like coral, and even Atlantic cod.
Scientists believe the pattern is a response to fresh water from melting ice sheets being added to surface ocean water, meaning those surface waters "can't get very dense and sink".
"That puts a spanner in this whole system," lead researcher Dr David Thornalley, from University College London, explained.
The concept of this system "shutting down" was featured in The Day After Tomorrow.
"Obviously that was a sensationalised version," said Dr Thornally. "But much of the underlying science was correct, and there would be significant changes to climate it if did undergo a catastrophic collapse - although the film made those effects much more catastrophic, and happening much more quickly - than would actually be the case."
Nonetheless, a change to the system could cool the North Atlantic and north-west Europe and transform some deep-ocean ecosystems.
That is why its measurement has been a key part of the Atlas project.
Scientists say understanding what is happening to Amoc will help them make much more accurate forecasts of our future climate.
Prof Murray Roberts, who co-ordinates the Atlas project at the University of Edinburgh, told BBC News: "The changes we're seeing now in deep Atlantic currents could have massive effects on ocean ecosystems.
"The deep Atlantic contains some of the world's oldest and most spectacular cold-water coral reef and deep-sea sponge grounds.
"These delicate ecosystems rely on ocean currents to supply their food and disperse their offspring. Ocean currents are like highways spreading larvae throughout the ocean and we know these ecosystems have been really sensitive to past changes in the Earth's climate."
Media captionAtlas is the biggest ever deep Atlantic exploration mission launched by UK scientists.
To measure how the system has shifted over long timescales, researchers collected long cores of sediment from the sea floor.
The sediment was laid down by past ocean currents, so the size of the sediment grains in different layers provided a measure of the current's strength over time.
The results were also backed up by another study published in the same issue of Nature, led by researchers from the Potsdam Institute for Climate Impact Research in Germany.
This work looked at climate model data to confirm that sea-surface temperature patterns can be used as an indicator of Amoc's strength and revealing that it has been weakening even more rapidly since 1950 in response to recent global warming.
The scientists want to continue to study patterns in this crucial temperature-regulating system, to understand whether as ice sheets continue to melt, this could drive further slowdown - or even a shutdown of a system that regulates our climate.
