Warm current that has historically caused dramatic changes in climate is experiencing an unprecedented slowdown and may be less stable than supposed – with potentially severe consequences
The warm Atlantic current links between severe and abrupt changes in the climate in the past is now at its weakest in at least 1,600 years, new research presents. The findings, based on multiple lines of scientific proof, throw into question previous predictions that a catastrophic breakdown of the Gulf Stream would take centuries to occur.
Such a collapse would see western Europe suffer far more extreme wintertimes, sea levels rise fast on the eastern seaboard of the US and would disrupt vital tropical rainfalls. The new research shows the current is now 15% weaker than around 400 AD, an exceptionally large difference, and that human-caused global warming is a matter of at the 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 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 analyzes. He said current climate models do not replicate the observed slowdown, is recommended 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 hour 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 “its not” going to happen? It is one of these tip-off points that is relatively low likelihood, but high impact .”
The study by Thornalley and colleagues, published in Nature, utilized 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 employed the shells of tiny marine beings 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 utilized the characteristic pattern of temperatures, but assessed this using thermometer data collected over the last 120 years or so.
Both surveys found that Amoc today is about 15% weaker than 1,600 years ago, but there were also differences in their conclusions. The first analyse determined significant Amoc weakening after the end of the little ice age in about 1850, the result of natural climate variability, with farther weakening caused afterward by global warming.
The second study suggests most of the weakening arrived later, and can be squarely blamed on the burning of fossil fuel. 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, “were supposed to” expect a further long-term slowdown of the Atlantic overturning ,” told Alexander Robinson, at the University of Madrid, and one of the team that conducted the second study. He advised:” We are only beginning to understand the consequences of this unprecedented process- but they might be disruptive .”
A 2004 tragedy movie, The Day After Tomorrow, saw a rapid shutdown of Amoc and a devastate freeze. The basics of the social sciences were portrayed correctly, told Thornalley:” Plainly it was overstated- 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 .”
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