"AMOC isn't an on-or-off phenomenon: it speeds up and slows down, and almost every 21st-century projection shows it slowing down, as a consequence of global warming."
Planetary warming is affecting how the world’s systems work, potentially pushing a crucial mechanism to a ‘point of no return’, writes Ilona Amos, Press and Communications Lead for Stop Climate Chaos Scotland
Here in Scotland our climate is classed as temperate and oceanic, with changeable but rarely extreme weather. Although it may not always feel like it, the country enjoys milder conditions than many other places with a similar northerly latitude, and that’s thanks to the influence of the Gulf Stream – a fast, warm current from the Atlantic.
But experts are warning that all this could change, and we in northern Europe could possibly face a frozen future – despite, and as a result of, the planet warming overall.
Why? It’s all because of disruption to the Atlantic Meridional Overturning Circulation (AMOC), a major conveyor belt of ocean currents that includes the Gulf Stream and plays a crucial role in regulating the global climate.
The AMOC is in danger from human-caused climate change, as warming seas, melting ice and increased rainfall upset the balance of temperature and salt in the north Atlantic.
So what is the AMOC and why is it important?
The AMOC is driven by a massive pump – created by the cold at the poles, which freezes fresh water on the surface brought there on warm currents and sends the residual heavier, saltier water to the ocean floor, where it flows back towards the equator. AMOC’s strength is determined by variations in seawater density, controlled by temperature and salinity, which relies on this action of freezing water.
By transporting warm surface water from the tropics northwards and then pushing cold water through the deeps back southwards, the system keeps Europe warm and plays a role in controlling rainfall worldwide. It connects into an even larger network of ocean currents that continuously moves water, nutrients and carbon around the globe.
The ocean and the atmosphere are interlinked. As climate change raises air temperature, the ocean surface warms too. This heating causes more freshwater to enter subpolar regions due to increased rain and melting ice. And because fresher, warmer water is less dense than cold, salty water it does not sink as deeply as it would under cooler conditions. If this process slows, the ocean cannot pull carbon dioxide from the atmosphere into the deep ocean as quickly. This leaves behind greater quantities of greenhouse gases in the atmosphere, in turn speeding up climate change.
Should we worry?
Scientists have previously set out how rising heat is affecting the seas and could cause collapse of the AMOC in the future, with severe repercussions for the environment. Now, though, several recent international studies suggest a dangerous “tipping point” could be reached much sooner than predicted.
Evidence shows the currents are slowing down. Researchers believe that rising heat could force the circulatory system to a crux that would leave it in a weak state for many centuries.
In its sixth report (AR6), the Intergovernmental Panel on Climate Change (IPCC) summarised the impact of shutdown as follows: “If an AMOC collapse were to occur, it would very likely cause abrupt shifts in the regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and could result in weakening of the African and Asian monsoons, strengthening of southern hemisphere monsoons and drying in Europe.”
Scotland-based oceanographer Dr Neil Banas specialises in climate impacts on ecosystems and is part of the team assessing the present and future of AMOC for the forthcoming JPI AMOC in Focus report. He explains that the system “isn’t an on-or-off phenomenon”.
He says: “It speeds up and slows down, and almost every 21st-century projection shows it slowing down, as a consequence of global warming.”
Breakdown could have knock-on impacts worldwide, affecting food supplies, pushing up sea level and upsetting rainfall patterns. It could also potentially act as a catalyst that triggers a series of other devastating climate tipping points.
Collapse could have wide-reaching consequences, with the effects being felt across the planet – including here.
The scenario could affect temperatures in Europe, eventually causing them to plummet – even with global warming. Projections suggest it could set us on course for a future where it’s normal to see winter extremes in London hitting –20C, while Norway’s capital city could be shivering in –48C of cold.
Dr Banas continues: “For every degree of additional global warming, Scotland goes further into a maze of diverging futures where AMOC-weakening and climate-warming interact in ways that are sometimes opposing and sometimes reinforcing.
“So the question of whether our local climate will be warmer or cooler is just the starting point: we should also be thinking about drought as well as flooding, changes in the storm track and possibly extreme declines in the productivity of the ocean.”
When could the AMOC collapse?
Estimates for when this danger level could be reached vary. The IPCC had concluded that a breakdown before 2100 was unlikely. However, recent studies have warned that the point of no return could be reached much sooner, over the next few decades, pushing the system towards a potentially irreversible shutdown next century. Once in this new, frail state, the currents could not simply return to their previous pattern, even if conditions reverted to their original state.
Knock-on effects
Dr Banas warns that climate warming is already having an impact on the ocean and how it functions, but this could get worse.
“I’m increasingly concerned that we haven’t identified Scotland’s exposure to AMOC change in many sectors, like fisheries, food systems, nature conservation, transportation – everything affected by climate, really,” he says.
“It wouldn’t take a sudden, catastrophic collapse – whatever probability one assigns to that – for us to feel these impacts.”
So what can be done?
It’s not just the potential collapse of the AMOC. In myriad other ways the planet is showing us the perilous state human activities have pushed it to.
Atmospheric concentrations of greenhouse gases are 50% higher today than in pre-industrial times, and the planet has heated by around 1.4C. Emissions must be slashed if climate warming is to be halted, or even slowed. This requires a global effort, including ending the burning of fossil fuels – the biggest culprit of carbon dioxide build-up, which pushes temperatures up.
And Scotland needs to play its part. This means politicians of all parties and people across the country pulling together to deliver on national and international emissions-reduction goals, with the Scottish Government’s new Climate Change Plan providing a routemap.
But action must be stronger, faster and more effective to realise the full benefits of a greener world. Done right, and in a fair way, the move to a low-carbon society will bring multiple improvements for Scots – including warm, energy-efficient homes; public transport that is cheaper, cleaner and more efficient; agriculture that provides good-quality, healthy food and supports nature; new skilled jobs in a greener industry; and carbon-removal methods that are proven to work.
There is no time to waste. Let’s do it, now – for people and for the environment. ⏹
