Currents of change seen in melting Antarctic ice

This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

Melting ice around Antarctica will cause a rapid slowdown of a major global deep ocean current by 2050 that could alter the world’s climate for centuries and accelerate sea level rise, according to scientists behind some new research.

The research suggests if greenhouse gas emissions continue at today’s levels, the current in the deepest parts of the ocean could slow down by 40 per cent in only three decades.

This, the scientists said, could generate a cascade of impacts that could push up sea levels, alter weather patterns and starve marine life of a vital source of nutrients.

A team of Australian scientists looked at the deep ocean current below 4,000 metres that originates in the cold, fresh and dense waters that plunge down off Antarctica’s continental shelf and spread to ocean basins around the globe.

Prof. Matt England of the Climate Change Research Centre at the University of New South Wales, and a co-author of the research published in Nature, said the whole deep ocean current was heading for collapse on its current trajectory.

“In the past, these circulations have taken more than 1,000 years or so to change, but this is happening over just a few decades. It’s way faster than we thought these circulations could slow down.

“We are talking about the possible long-term extinction of an iconic water mass.”

The research looked at what would happen in the deep ocean around Antarctica if the fresher water melting from ice sheets was added to climate modelling.

The modelling for the study assumed that global greenhouse gas emissions remained on their current path, but England said lower emissions could lessen the amount of ice melting, which, in turn, could slow the decline.

The slowdown in the deep ocean current relates to the amount of water that sinks to the bottom and then flows north.

Dr. Qian Li, formerly of the University of New South Wales and now at the Massachusetts Institute of Technology, was the lead author of the research, co-ordinated by England.

The study did not attempt to explain or quantify the knock-on effects, but the authors wrote the slowdown would “profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come.”

In a briefing, the authors said the deep ocean current influenced the climate around the world, with the potential to radically shift rainfall.

England said the slowdown of the deep ocean current caused those deep waters to heat up.

But as that deep water becomes isolated, it could then cause the upper ocean around the continent to get hotter, kicking off a feedback loop where more melting causes accelerated slowing of the current, which then causes more heating and more ice sheet melt.

The deep water that warms the fastest in the study, England said, was in the same areas — particularly in west Antarctica — where ice sheets were already vulnerable and melting.

“We don’t want to set off a self-reinforcing mechanism in those places,” he said, adding that the slowdown effectively stagnates the deep ocean, starving it of oxygen.

When ocean organisms die, they add nutrients to the ocean water that sinks to the bottom and circulates around the world’s oceans. Those nutrients get returned in upwells that feed phytoplankton — a foundation for the marine food chain.

Dr. Steve Rintoul, an oceanographer and expert on the Southern Ocean at the Australian government’s Commonwealth Scientific and Industrial Research Organisation, said as the deep ocean circulation slows, fewer nutrients would be returned to the upper layers of the ocean — affecting the production of phytoplankton. This process would play out over century-long timescales.

He said: “Once that overturning circulation slows down, we can only get it back again by no longer releasing meltwater around Antarctica, which means we need a cooler climate and then have to wait for it to restart.

“The longer we go on with higher rates of greenhouse gas emissions, the more changes we commit ourselves to.”

In a briefing, the scientists said the modelling was in line with recent observations of changes to the deep ocean circulation that was suggesting a slowdown may already be taking place.

Rintoul added: “Going back 20 years, we thought the deep ocean wasn’t changing that much. It was too far away to be responsive. But the observations and the models have shown that’s not the case.”

Scientists also think another major ocean circulation in shallower waters that spans the entire Atlantic Ocean — known as the Atlantic Meridional Overturning Circulation — is also slowing down.

Prof. Stefan Rahmstorf, an oceanographer and head of Earth system analysis at the Potsdam Institute for Climate Impact Research, said the new study — with which he was not involved — showed “a dramatic further weakening is likely around Antarctica in the coming decades.”

He said models presented in major UN climate reports had a “long-standing and major shortcoming” because they didn’t capture how meltwater influenced the deep ocean.

The ocean depths were refreshed in only a few places on the planet close to major ice sheets in Greenland and Antarctica.

“Unfortunately, these locations are all close to the ice sheets of Greenland and Antarctica, which are melting away as a result of fossil-fuel-caused global warming.

“The meltwater dilutes the salt content of these ocean regions, thereby making the ocean waters less dense and thus not heavy enough to sink down and push away the waters already there.”

A slowdown in the deep ocean current could also affect the amount of CO₂ that the deep oceans could lock away, Rahmstorf said.