THE SHUTDOWN OF OCEAN CURRENTS COULD FREEZE EUROPE / THE ECONOMIST

Climate tipping-points

The shutdown of ocean currents could freeze Europe

When climate change poses a strategic threat, it needs a strategic response

The Ilulissat Icefjord in Greenland / Photograph: Getty Images

Those who think about national security love to bandy acronyms such as ATACMS (Army Tactical Missile System) and WOMBAT (Weapon of Magnesium, Battalion, Anti-Tank). 

They need to add AMOC to the list. 

The Atlantic Meridional Overturning Circulation is not a weapons system. 

But it could lay waste a continent—specifically, Europe—to an extent that only a nuclear war could outmatch.

AMOC is part of a system of currents which move heat around the oceans of the world. 

It delivers a stupendous flow of that heat—more than 1,000 terawatts—to the North Atlantic. 

That sounds like the sort of planetary juggernaut it would be incredibly hard for humans (whose global civilisation runs at a mere 20 terawatts) to do anything about. 

Alas, no. 

AMOC is a curiously delicate thing. 

Changes in sea-surface temperature and salinity caused by global warming could conceivably make it stall; such abrupt shutdowns are clearly visible in the geological record. 

For Europe that could mean a sudden, severe cooling—even as the rest of the world keeps warming.

Europeans sweltering through yet another summer heatwave might think such cooling would be just the ticket. 

Again, alas, no. 

A complete AMOC shutdown could see Brussels hitting -20°C (-4°F) in a bad winter. 

In Oslo the figure would be almost -50°C (-58°F); not quite Yakutsk, but not far off. 

February sea ice in the North Sea could come as far south as the Humber estuary and the Frisian Islands north of Holland. 

Average rainfall in parts of northern Europe would drop precipitously; according to one estimate as much as 80% of England’s arable land would no longer be farmable without irrigation. 

Storms would get worse; so, in some models, might summer heatwaves. 

This would be the worst of all worlds.

And it’s not just Europe. 

By cooling the northern hemisphere as a whole, an AMOC collapse would push the band of rain which girdles the tropics towards the south. 

That would be very bad for the African countries on the south edge of the Sahara; it could also be devastating to the Amazon.

Cold, dry and sudden

These ghastly prospects are one of the reasons that AMOC takes a starring role in worries about climate “tipping points”—effects of warming that might be dramatic, damaging and irreversible. 

Another reason is the strong suggestion, in both theory and models, that after a (currently unknown) temperature threshold is passed, the collapse could take just a few decades. 

A third is that AMOC, or at least parts of it, may already be in slow decline.

This is well known to people who think about climate change—as is the level of uncertainty about how far away the threshold actually is and the spirited debate over how complete a collapse might ensue. 

But there is no evidence that such possibilities are feeding into government planning processes.

You might argue that they shouldn’t: that the response to the risk should be to redouble all efforts which might keep the temperature low enough to avoid a tipping point. 

But preparedness makes sense. 

The Advanced Research and Invention Agency in Britain is funding prototype monitoring schemes that might make possible early warnings of accelerating collapse. 

If it could be made robust enough, such a system could make possible years of preparation.

If this were a military threat, such risk-reduction would be second nature, as would table-top analysis of vulnerabilities and contingency plans for softening impacts. 

Larger outlays are not, as yet, necessary. 

But larger imaginations are.