The Guardian looks at one of the less discussed elements of climate change – with a once-dependable ocean current slowing, slowing, slowing down in a way that could change more than just the surf breaks on the East Coast:
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.
“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 than thought.
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.