Gulf Stream weakest in 1,000 years, US East Coast may see more flooding if climate change worsens: Study
The Gulf Stream system — a system of ocean currents that redistributes heat on our planet and has a major impact on climate — is at its weakest in over 1,000 years and it is likely due to human-induced climate change. Also known as the Atlantic Meridional Overturning Circulation (AMOC), scientists warn that if global warming continues unabated, it will weaken further, and could reduce by about 34% to 45% by the end of this century, bringing it closer to a “tipping point”.
The AMOC is a key control on both regional and global climate, and its abrupt slowdown could trigger disruptions around the globe. The consequences could be manifold for people living on both sides of the Atlantic, with the east coast of the US vulnerable to flooding. A weakened Gulf Stream would increase sea levels on the Atlantic coast of the US, with potentially disastrous consequences, they emphasize. “If we continue to drive global warming, the Gulf Stream System will weaken further, by 34-45% by 2100, according to the latest generation of climate models. This could bring us dangerously close to the tipping point at which the flow becomes unstable,” warns Dr Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), who initiated the study.
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According to Dr Rahmstorf, the Gulf Stream System works like a giant conveyor belt, carrying warm surface water from the equator up north and sending cold, low-salinity deep water back down south. It moves nearly 20M cubic meters of water per second, almost a hundred times the Amazon flow, he adds.
The team from Ireland, Britain and Germany was able to reconstruct the flow history of the AMOC over the last 1600 years by using a range of several different and largely independent proxy indicators, including data from tree rings, ice cores, ocean sediments and corals, as well as from historical data, such as ship logs.
The analysis, published in Nature Geoscience, shows consistent evidence that the Gulf Stream’s slowdown in the 20th century is unprecedented in the past millennium, likely linked to human-caused climate change. It reveals that it has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, followed by a second, more rapid decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades.
“This study shows the increasing evidence in support of the modern Atlantic Ocean undergoing unprecedented changes in comparison to the last millennium, and in some cases longer. Alongside other human pressures on the marine environment, the changes we are observing in ocean circulation are impacting marine ecosystems in both the surface and deep ocean. We will need to take account of these ongoing changes in our efforts to conserve and manage marine resources,” writes co-author Dr David Thornalley, associate professor in the Department of Geography, University College London (UCL). He cautions: “Overall, this new study highlights that major changes are occurring, but it also draws attention to how in future work we must aim to resolve which components and pathways of the AMOC have changed, how, and for what reason – there is a lot we still need to learn.”
Dr Levke Caesar, the lead author from the ICARUS Climate Research Centre, Maynooth University, and guest scientist at PIK, explains that 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. “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,” says Caesar.
In Europe, a further slowdown of the AMOC could imply more extreme weather events like a change of the winter storm track coming off the Atlantic, possibly intensifying them.
What are experts saying?
According to Dr Laura Jackson, Met Office scientist specializing in AMOC, the paper adds to evidence that the AMOC has been weakening over the historical period, but there are still uncertainties associated with using these indirect observations. “It is very likely the AMOC will weaken with increasing greenhouse gases, but unlikely we would see a collapse. However, a collapse or a slowdown would be extremely impactful, so AMOC research remains a high priority,” notes Dr Jackson who was not involved in the study.
Dr Andrew Meijers, deputy science leader of polar oceans at British Antarctic Survey, who was not involved in the current research, believes that the work has provided new long term context and reveals that before the era of human-induced climate change, the Atlantic overturning circulation was relatively stable and stronger than it is now. According to him, this indicates that the slowdown is likely not a natural change, but the result of human influence.
“The AMOC has a profound influence on global climate, and 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. Additionally, the AMOC is frequently modeled as having a tipping point below some circulation strength, a point at which the relatively stable overturning circulation becomes unstable or even collapses,” says Dr Meijers. He adds, “The ongoing weakening of the overturning means we risk finding that point, which would have profound and likely irreversible impacts on climate.”