A 2023 marine heatwave in the North Atlantic was the equivalent of 20 years of warming, the study says. Photo: CSIRO
A record-breaking marine heatwave in the North Atlantic Ocean in 2023 could very well prove to be a glimpse of the future, University of NSW scientists say.
In a UNSW-led Nature study (see link below) that was recently published, researchers say the off-the-scale heatwave in the North Atlantic Ocean was caused by the combination of historically weak winds, higher levels of solar radiation and climate change.
The paper explains that from Greenland to the Sahara and to the Americas, the North Atlantic warmed with unprecedented speed in summer 2023.
“The intensity of the warming in that single summer was equivalent to about two decades worth of warming for the North Atlantic,” says lead author Professor Matthew England from UNSW Sydney.
“While these extreme temperature events are typically only temporary, we can expect they’ll become more frequent in the future.”
At the time, Prof. England was studying a region of cooling in the North Atlantic, the university says.
UNSW says this ‘cold blob’ southeast of Greenland is a more unusual consequence of global warming – it is a sign of the Atlantic Meridional Overturning Circulation (AMOC) slowing down (made famous in The Day After Tomorrow).
These waters had been cooling for 50–100 years so when water temperatures suddenly spiked across the region, Prof. England’s team realised something unusual was unfolding.
“We even asked ourselves if this was the circulation making a temporary comeback but the rate of warming was far too rapid for that,” Prof. England says.
WEAK WINDS
The university says the ocean has two layers: an upper layer directly affected by solar radiation and the cooler, deep ocean.
With more exposure to sunlight during spring and summer, the upper ocean layer gradually warms.
Co-author and UNSW Associate Professor Alex Sen Gupta says the rate of warming depends on the thickness of the upper layer.
“A thin layer will warm faster, much in the same way that a pan of water on a stove with less water will warm faster than a pan with more (water),” he says.
He says the layer’s thickness in summer is set by winds that churn up the surface waters and mix heat throughout it.
In June and July 2023, North Atlantic winds were weaker than ever recorded “so the upper layer of the ocean was thinner than ever recorded”, A/Prof. Sen Gupta says.
In some areas, this layer was only 10 metres deep compared to the usual 20–40 metres, says Dr Zhi Li, who led the analyses of ocean observations for the study.
“This meant that the Sun heated the ocean’s surface more rapidly than normal, which led to those record-breaking temperatures,” he explains.
He says the temporary thinning caused by the very weak winds was boosted by global warming: long-term warming causes the surface ocean to become less dense, suppressing the ability of winds to mix the upper ocean.
“We were also dealing with a long-term thinning of the upper layer,” Dr Li says.
A SUNBURNT OCEAN
There was another unexpected, and local, factor in play that summer, the UNSW says.
In 2020, new shipping rules had been introduced to reduce sulphur pollution from ships and improve air quality around major traffic lanes.
But clearer skies mean fewer ‘seeds’ for clouds and less cloud cover means more sunlight warming the sea surface, especially in the North Atlantic.
Prof. England points out that this effect was secondary; most of the ‘blame’, he says, lies with the lack of wind.
“Reducing sulphate emissions is good for reducing air pollution though it allows additional warming of the ocean’s surface because less sunlight is reflected back to space,” he says.
Weak winds, thinner layer depths and very clear skies turned rapid warming into a full-basin marine heatwave, the UNSW says.
As the waters warmed, it radiated heat back into the atmosphere, the consequences of which were felt on land.
CONTINENTAL IMPACT
Air masses travelling over the top of the ocean were picking up heat which went on to scorch cities across Europe.
Deadly 40+ degree Celsius heatwaves across Germany, France and Italy broke temperature records while torrential rains devastated parts of Spain and Eastern Europe.
Underwater, the coral reefs of the Caribbean were bleaching. Hurricanes, which only occur during summer when fuelled by ocean heat, intensified into disasters. That season, Hurricane Idalia hit Florida – causing eight deaths and damages worth US$3.6 billion.
“This wasn’t just a small area of warm water off one coast,” says Professor Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), another co-author of the study.
“This was the entire North Atlantic, with impacts on weather systems, human lives, marine ecosystems and society.”
FUTURE-PROOFING
As climate change thins the ocean’s surface mixed layer, heat from the Sun will warm it more readily.
Prof. England says this means marine heatwaves in the North Atlantic will get worse in the future with extremely costly consequences for ecosystems and society.
“Severe marine heatwaves often only last for a few weeks or months but this one in the North Atlantic left a legacy that persisted for more than a year,” he says.
“Unfortunately, the frequency and intensity of marine heatwaves is only set to worsen in the coming decades and beyond.
“The only way to stop this trend is to phase out our use of fossil fuels. Net zero cannot come soon enough.”
