The fate of a critical ocean current may depend more on how fast our planet heats up than the temperature level we ultimately end up with.
Earth could warm by more than 5掳C above pre-industrial levels but still enjoy a functional Atlantic Meridional Overturning Circulation (AMOC), so long as this warming happens very slowly. But very fast warming, similar to the current rate, could push the AMOC towards collapse at just 2掳C above pre-industrial levels.
at the University of Utrecht in the Netherlands and his colleagues used a state-of-the-art climate model to simulate the stability of the AMOC under three different rates of warming. The first scenario modelled a very slow increase of carbon dioxide concentrations of 0.5 parts per million each year; the second a rate five times faster; and the final scenario a rate 10 times faster, which is comparable to today鈥檚 rate.
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The team found that, under very slow rates, the AMOC remained stable beyond 5掳C of warming. 鈥淲e were very surprised by this result,鈥� says van Westen.
But under current rates, the AMOC collapses at about 2掳C of warming. Unless the world dramatically reduces its emissions, this could happen around 2060, says van Westen. 鈥淲e need to consider that this is a scenario that can develop over the upcoming decades if we remain at these warming rates.鈥�
The modelling indicates the AMOC鈥檚 stability 鈥渋s not controlled by some magical temperature level鈥�, he says, but 鈥渂y how fast the climate is warming鈥�.
The AMOC carries warm, salty water from the Gulf of Mexico towards the north Atlantic Ocean, where it cools and sinks, flowing back south along the ocean floor.
Without the AMOC, much of Europe would suffer a 10 to 30掳C drop in wintertime temperatures, while rainfall patterns around the world would shift dramatically, with catastrophic consequences.
To drive the flow of the current, surface water around Greenland must sink into the deep ocean. This depends on the surface water cooling and becoming dense enough to descend into the deep ocean. 鈥淭here needs to be some kind of density overlap between the interior of your ocean and the surface,鈥� says van Westen. 鈥淭hat鈥檚 the only way to have a very strong AMOC.鈥�
But under fast rates of warming, surface waters become warmer and lose density far faster than the deep ocean, leaving the ocean layers stratified and unable to effectively mix.
Keeping the warming at a slow pace allows both the surface water and the deep water of the ocean to change at roughly the same pace, says van Westen. 鈥淭he whole ocean can keep up.鈥�
The findings suggest that a dramatic slowdown in the rate of CO2 pollution could rescue the AMOC from collapse, even if the world doesn鈥檛 manage to achieve net-zero emissions in the medium term.
But translating results from a climate model to the real world is tricky. For example, the model doesn鈥檛 account for factors such as the melting of the Greenland ice sheet and the effect of other greenhouse gas emissions like methane.
鈥淗igher-complexity models vary a lot in AMOC behaviour and have biases that affect AMOC stability, so we can鈥檛 yet take this as a clear-cut, real-world projection,鈥� says at the University of Sussex, UK.
But the main message is clear, he stresses: 鈥淚t still shows, though, just how much pressure the ocean is being put under by continued fossil fuel burning, and underlines that the faster these emissions fall, the better the chance of avoiding any potential nasty surprise from the AMOC.鈥�
鈥淲hat this study shows is that slowing the rate of CO2 increase immediately is perhaps more crucial for averting future AMOC weakening than limiting the total amount of CO2 we emit,鈥� says at the University of Washington. For policy-makers, she says it could mean that 鈥渟preading out our total emissions in time would be more favourable for maintaining a strong AMOC鈥�.
Nature Climate Change
