13 May 2024

One event could wreak global climate havoc. Neither side of Australian politics has got a clue about it.

This a case study from the report, Too hot to handle, recently published by the Australian Security Leaders Climate Group.

There is no greater disruptive physical climate risk than the collapse of the Atlantic meridional overturning circulation (AMOC), the main current system in the South and North Atlantic Oceans, which is linked to circulation in the Southern Ocean. 

There is a non-trivial and unacceptable risk that the AMOC flow will collapse this century, with devastating consequences for global food production, for sea levels and for flooding in Australia. Shifts in global weather patterns would likely deprive Asia of vital monsoon rains, with enormous security consequences for the region and for Australia.

Yet in the Australian Government’s analysis of climate risks, no attention has been paid to the AMOC collapse. In fact, it does not get a mention in the Defence Strategic Review, or the first report of the current National Climate Risk Assessment (NCRA). 

No minister or member of either major party has even mentioned it in parliament since the ALP came to power in 2022. Neither side of Australian politics has got a clue about it. One of the greatest climate-related threats to our future appears completely absent from the Australian Government’s thinking. 

"No minister or member of either major party has even mentioned it in parliament since the ALP came to power in 2022. Neither side of Australian politics has got a clue about it. One of the greatest climate-related threats to our future appears completely absent from the Australian Government’s thinking."

AMOC slowdown:  AMOC is the complex web of ocean current conveyor belts which, driven by differences in water density, transports water, heat and nutrients throughout the Atlantic Ocean. AMOC takes “a lot of heat from the tropics in the Atlantic, moves that heat up to Northern Europe, and releases that heat to the atmosphere, keeping that part of the world much more mild in its climate”.  It has been in a steady state for thousands of years, but climate change is melting Greenland at an accelerating rate, adding more fresh water to the Atlantic Ocean and gradually slowing the circulation strength.  At a critical point, with continuing high emissions, this slow-down will accelerate past a tipping point, resulting in the AMOC collapse, or cessation. This system has already slowed by 15 per cent since the mid-20th century. 

Collapse: In 2021 researchers concluded that there is “strong evidence that the AMOC is indeed approaching a critical, bifurcation-induced transition” (in other words, a tipping point) but the timing was unclear.  A July 2023 study estimated “a collapse of the AMOC to occur around mid-century under the current scenario of future emissions”, with a high confidence (95 per cent probability) of it occurring between 2025 and 2095.  Potsdam University’s Prof. Stefan Rahmstorf observes that while there is still “large uncertainty where the tipping point of the AMOC is… the scientific evidence now is that we can’t even rule out crossing a tipping point already in the next decade or two”. 

Antarctic connection: Consistent with this analysis, Australian researchers in March 2023 published projections showing Antarctic deep ocean warming and changes in deep ocean circulation contributing to a slowing of the AMOC over the next few decades, with physical measurements confirming these changes already well underway.  They warn that the currents “may even collapse” and, if this happens, this would “deprive the deep ocean of oxygen, limit the return of nutrients back to the sea surface, and potentially cause further melt back of ice as water near the ice shelves warms in response. There would be major global ramifications for ocean ecosystems, climate, and sea-level rise.”

Likelihood: The authors of this 2023 paper write that their “central estimate, situated at the mid-century, is where we believe that there is the highest risk of a collapse should we continue greenhouse gas emissions at the current rate”. Rahmstorf warns that “when several studies with different data and methods point to a tipping point that is already quite close, I think this risk should be taken very seriously” and “increasingly the evidence points to the risk being far greater than 10 per cent during this century – even rather worrying for the next few decades”.

Consequences:  Modelling by Prof. Peter Ditlevsen of the University of Copenhagen shows that AMOC slowdown would cool London by an average of 10°C and Bergen, Norway by 15°C.  A breakdown of this system could plunge the UK and large parts of the Northern Hemisphere into a new ice age, with temperatures in parts of Europe dropping by 3°C each decade and sea levels rising by a metre on both sides of the North Atlantic, while the wet and dry seasons in the Amazon would flip and severely disrupt the rainforest’s ecosystem. 

Global food and water security crisis: Ditlevsen and his co-authors conclude that a collapse of the AMOC heat-transporting circulation would be a going-out-of-business scenario for European agriculture: “You cannot adapt to this”. In addition, writes Prof. Tim Lenton, the director of the Global Systems Institute at the University of Exeter, the monsoons that typically deliver rain to West Africa and South Asia would become unreliable, and huge swaths of Europe and Russia would be devastated by drought. As much as half of the world’s viable area for growing corn and wheat could dry out. He says that “in simple terms [it] would be a combined food and water security crisis on a global scale.”

Consequences for Australia: The southern hemisphere, including Australia, would become warmer and more prone to flooding. A regional food crisis would have huge impacts on the global price of food, leading to large-scale regional people displacement and contributing to state breakdown and regional conflict. 

Download Too hot to handle
The NCRA is limited to looking only at adaptation responses, even though scientists working for government agencies privately acknowledge that 3°C of warming — which would be the result of the current high-emissions trajectory — is likely beyond adaptation. The only rational response to possible AMOC collapse is a global emergency effort to reduce emissions to zero far sooner than policymakers’ 2050 timeframe, along with whatever other measures can be applied to prevent the levels of warming triggering such an event. 

These necessary actions should already be the subject of discussion for action at the highest levels in the Australian government, and already flagged by Australia’s climate risk assessment processes, but there is no sign that has yet occurred.