The Atlantic Ocean circulation system that underpins the Gulf Stream, the weather system that pulls warm water from tropical regions up into the northern Atlantic and cold water back towards the south, is now at the weakest it’s been in more than 1 000 years, and climate change is the probable cause, according to a new study.
What is Happening?
- The study, published in Nature Geoscience, says that after studying sediments, Greenland ice cores and weather patterns, the current weakening had not been seen in at least the last 1 000 years.
- The study found an initial weakening of the AMOC in the mid-1800s, corresponding to the end of a relatively cool period known as the Little Ice Age, and a sharper decline beginning in the mid-1900s as human effects on the climate accelerated.
- 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 is accompanied by winds that also help bring mild and wet weather to Ireland, the UK and other parts of western Europe.
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What Does This Mean?
- Further weakening of the Atlantic Meridional Overturning Circulation (AMOC) could result in freezing winters in western Europe and an increase in damaging heatwaves and droughts across Europe.
- If global heating continues, scientists predict that the AMOC will weaken further and reduce by about 34-45% by 2100, which could spur a tipping point. A weakened Gulf Stream circulation system would also raise sea levels on the Atlantic Ocean coast of the US.
Referring to the potential of sea level rise in the eastern coast of the US, Dr Levke Caesar of Maynooth University in Ireland, and the lead author of the paper, says, “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.”
Stefan Ragmstorf, of the Potsdam Institute for Climate Impact Research, co-author of the study, told the Guardian that the circulation had already slowed by about 15% since the mid-1900s, and the impacts were being seen. He says, “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.”
- The study found that a tipping point is likely decades away, but that continued high greenhouse gas emissions would bring it closer. Rahmstorf says, “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.”
- The AMOC is a large part of the Gulf Stream, often described as the “conveyor belt” that brings warm water from the equator. The circulation has broken down before, in different circumstances, for example at the end of the last ice age.
- The Gulf Stream is separate from the jet stream that has helped bring extreme weather to the northern hemisphere in recent weeks, however it is also affected by rising temperatures in the Arctic.
- Normally, the frigid temperatures over the Arctic create a polar vortex that keeps a steady jet stream of air currents keeping that cold air in place. However, higher temperatures over the Arctic have resulted in a weak and wandering jet stream, helping 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 melting Arctic ice, which brings 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.
- The weakening AMC could also affect Atlantic marine ecosystems, disrupting fish populations and other marine life.
Featured image by: Flickr