When warming cools | La Presse

Stronger storms caused by climate change could abruptly cool parts of the oceans. That’s causing carnage in some southern seas. And it’s not just marine cold snaps that pose a threat.

Sharks, rays and squid

In March 2021, more than 260 large fish and marine animals washed up on South African beaches in just a few days. The event intrigued Nicolas Lubitz, a shark expert at James Cook University in Australia.

In April, in the magazine Nature Climate ChangeMr. Lubitz showed that, unlike mass mortalities usually linked to ocean heatwaves, this event – ​​and others in Australia – is linked to sudden and extreme cold. This cold snap is also a result – paradoxically – of global warming.

More frequent storms are bringing up cold water from the depths of two Indian Ocean currents, which are decimating large fish and mollusks accustomed to warm surface waters. In 24 hours in March 2021 in South Africa, surface water dropped by 11 degrees Celsius.

Analyzing 41 years of ocean temperatures and 33 years of wind measurements, Lubitz and his colleagues showed that the problem will get worse with climate change. “We show an increase in the intensity and frequency of deep water upwelling,” he explained on the social network X.

Salmons

A “super-chill” has occurred in recent years in Newfoundland in winter, causing excessive mortality on salmon farms. Wild species, however, are not vulnerable in the Atlantic provinces because the water there is already close to freezing in winter, according to Frédéric Cyr of Fisheries and Oceans Canada.

“There are occasional upwellings from deep where the temperature near the surface drops rapidly, say from 10°C to near 0°C, but I guess the ecosystem is used to it because it happens regularly.”

On the other hand, species that move further north as surface waters warm, such as silver hake and white sharks, could suffer from these sudden coolings, according to Mr. Cyr.

Gulf Stream

An even more extreme cold snap would be the shutdown of the Atlantic Meridional Overturning Circulation (AMOC), an ocean current that includes the Gulf Stream. This current brings heat to northern Europe, which explains why Lyon is much warmer than Montreal despite being at the same latitude. In the spring, researchers at Utrecht University stated in Nature Climate Change that once a “point of no return” is passed, the AMOC would collapse in less than 100 years. And in June, they published a study on the scientific preprint site arXiv concluding that this point of no return would be reached before 2050. “The climatic and societal repercussions of this phenomenon are serious,” says the lead author of both studies, René van Westen of Utrecht University.

Natural variations

There have been many worrying studies about the AMOC over the past 20 years, particularly because of an area south of Greenland where the ocean is cooling instead of warming like everywhere else on the planet. But last fall, in a special issue of the journal Philosophical Transactions A of the British Royal Society, skeptical oceanographers and climatologists have fought back. Their standard-bearer is Susan Lozier, an oceanographer at Georgia Tech University and a former president of the American Geophysical Union (AGU). “We only have 20 years of direct measurements of the strength of the AMOC,” says M.^me Lozier in an interview. I think we underestimate the natural variability of the AMOC.

The complexity of the debate lies in the multiplicity of ways of analyzing the situation. Yuxin Zhou, an oceanographer at the University of California at Santa Barbara, for example, published in May in the journal Science a study concluding that the rate of melting of glaciers and Arctic sea ice was not fast enough to affect the AMOC, compared to what has happened over the past 50,000 years.

But Mr Zhou still calls Mr van Westen’s results “alarming if confirmed”.

The Canadian contribution

Frédéric Cyr of Fisheries and Oceans plays a role in the AMOC debate. “I am involved in a research group that is interested in long hydrographic observations. Within the group and from what we observe, there is no clear evidence of an immediate collapse, although this possibility cannot be ruled out.” He knows the Utrecht researchers and considers them “serious.”

The “Working Group on Ocean Hydrography” (WGOH) in which he participates monitors oceanographic conditions in the North Atlantic from several networks of measurement buoys. WGOH is a project of the International Council for the Exploration of the Seas (ICES), a scientific NGO based in Copenhagen.

There are several networks of oceanographic buoys crossing the North Atlantic. One of them goes from Scotland to Labrador via Greenland, another from Brittany to Newfoundland and a third from West Africa to Florida.

The ABCs of AMOC

The water in the Gulf Stream is very salty, due to evaporation in the Caribbean, and therefore heavier than the rest of the water in the North Atlantic. When it cools, this density of salty water is no longer compensated by heat (warm water is less dense than cold water). It therefore sinks into the depths of the ocean and begins a journey south. However, the melting of the ice in the Arctic brings a lot of fresh water into the North Atlantic. The water in the Gulf Stream is diluted by the fresh water from the Arctic, it will not sink, which will weaken the “pump” feeding the AMOC. In 2004, the disaster film The Day After Tomorrow described an ice age paradoxically caused by global warming, due to a slowdown of the AMOC.

Watch the trailer for The Day After Tomorrow (in English)