Ominous signals from the global ocean

The headline climate news continues to be extraordinary summer heat over our planet’s northern half and – adding to Canada’s fire season from hell – wildfires all around the Mediterranean. And the world and Tasmania alike have just had the warmest July ever recorded.

Unendurable heat and spectacular fires and floods grab attention, and the lived experience of them will be locked into millions of memories as indicators of a new, unrecognisable climate. But they may turn out to be the least of our climate woes.

The global ocean covers 70 per cent of Earth’s surface. Right now, out of the public spotlight, it is going through profound, ominous changes of its own, on a vast scale – changes that spell trouble for the whole planet. 

Global average sea surface temperatures are now at an all-time high. In the Atlantic Ocean, heat is so far off the charts that even seasoned oceanographers can hardly believe the readings. Where anomalies above or below the long-term average for a given month are typically around 0.2C, the reading for June was 1.36C above. July is expected to go even higher.

On two successive days last week, surface waters off the southern tip of Florida hit what a Yale scientist called “hot tub” level: 37.9C (over 100F), the highest open-water sea surface temperature ever recorded, anywhere.

Then there’s the not-insignificant matter of ocean circulation, both lateral and vertical.

Nature, one of a select handful of global science publications which every scientist wants in their resumé, is not given to hyperbole. So when a paper titled “Warning of a forthcoming collapse of the Atlantic meridional overturning circulation” turns up, as happened last week, the world’s scientific community sits up and takes notice. 

And so should the rest of us. As the main Arctic-Antarctic connection, AMOC is a bedrock of the global circulatory system and a key regulator of the world’s climate. The Gulf Stream, which warms western Europe, is part of it. 

The mainstream view about AMOC is that collapse is unlikely before 2100. The new University of Copenhagen modelling challenges that, finding that a mid-century collapse is likeliest. But its range of uncertainty leaves the possibility that AMOC could shut down as soon as – wait for it – 2025. Two years away.

Besides inundating the east coast of North America, a past AMOC shutdown 12,000 years ago saw average Northern Hemisphere temperatures fluctuating by as much as 15 degrees over a decade. But this process could already be happening, indicated by the extreme year-to-year variation in current Arctic temperatures described in another paper from last week, in Current Biology.

A similar decline in vertical ocean circulation is happening off Antarctica, where “Antarctic bottom water” is formed from salt crystals rejected when seawater freezes. This dense, saline water sinks to the sea floor and carries oxygen and nutrients through the deepest parts of all the world’s oceans. It  has declined by over 30 per cent this century.

Yet another shock to Antarctic science this year involves the annual freezing of millions of square kilometres of the Southern Ocean during the southern winter – one of the world’s great seasonal events.

Science predicted early on that Antarctic sea ice would follow the Arctic example and slowly decline, but it defied those predictions until 2014, when it reached its greatest extent since satellite measurements started in 1979. Last year’s extent was lowest on record, but the 43-year trend has been flat.

This month, scientists keeping track of southern sea ice measurements – many based in Hobart – were shocked to see an unprecedented drop in sea ice cover, well over a million square kilometres below the range of all previous satellite records including last year. 

Antarctic sea ice was essentially a mystery to the world until analysis of satellite data from 1979 and logs from recent remote under-ice observations revealed its pivotal place in global systems.

Continuing studies underline its role as a buttress against the loss of land ice, as an insulating blanket between sea and air, as a means of getting nutrients and oxygen to the deep ocean, and as a home for ice-edge algae that nourish Antarctic krill – a vital marine food source – and draw carbon into the ocean.

We know the dangers of extreme weather, and while our part of the world is radically different from the global north, at least some of the heat, fire and fury of this northern summer may be our fate by year’s end. 

Thanks to science we can understand what’s happening and where we’re headed. It’s bad news, but dealing with bad news is what grown-ups do.

This entry was posted in Uncategorized. Bookmark the permalink.