What was behind extreme Atlantic heatwave of 2023?
A marine heatwave in the North Atlantic Ocean in 2023 was caused by record-breaking weak winds combined with increased solar radiation due to a lack of clouds – all on the back of continued climate change, scientists found.
From Greenland to the Sahara and across to the Americas, the waters of the North Atlantic Ocean warmed at an unprecedented speed in the summer of 2023 according to a new study.
'The intensity of the warming in that single summer was equivalent to about two decades worth of warming for the North Atlantic,' said co-author Prof 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 researching the so-called 'cold blob', a region of cooling in the North Atlantic south-east of Greenland which is one of the more unusual consequences of global warming. It is a sign of the Atlantic Meridional Overturning Circulation (AMOC) slowing down, which is a scenario made famous by the Hollywood film, The Day After Tomorrow.
The waters had been cooling for the past 50 to 100 years.
'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 said.
The heatwave is essentially measured by the depth of the upper layer of the ocean – directly affected by solar radiation – which sits above the cooler deep ocean. With more exposure to the sun during spring and summer, the upper ocean gradually warms.
Co-author of the report published in Nature, Prof Alex Sen Gupta, also from UNSW, said the rate of warming depends on the thickness of the ocean's upper layer. The thickness is set by the winds that churn up the surface waters and mix heat throughout it.
'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,' Prof Sen Gupta said.
In June and July of 2023, the North Atlantic winds were weaker than ever recorded, 'so the upper layer of the ocean was thinner than ever recorded', he said. In some areas it was only 10 metres deep, compared to the usual 20 to 40 metres deep.
An extra factor was lack of cloud cover. In 2020, new international rules were introduced to reduce the sulphur pollution emitted by ships, aiming to improve air quality around the world's major shipping lanes.
But clearer skies means more sunlight can reach the sea surface – especially in the North Atlantic, which is a high-traffic shipping area.
However, Prof England says this effect was secondary, only contributing to localised regions of enhanced warming. Most of the blame he says, still lay with the lack of wind.
'Reducing sulphate emissions is good for reducing air pollution,' he said. 'Though it has the unfortunate effect of allowing additional warming of the ocean's surface, because less sunlight is reflected back to space.'
Co-author Dr Zhi Li, also from UNSW, who led the analyses of ocean observations said the temporary thinning caused by weaker-than-average winds was also bolstered by global warming.
Long-term warming causes the surface ocean to become less dense, suppressing the ability of winds to mix the upper ocean.
'So we were also dealing with a long-term thinning of the upper layer,' he said.
The combination of weak winds with reduced mixed layer depths and clearer than average skies meant the rapid warming became a full-basin marine heatwave which the scientists described as 'off the scale'.
As the warming waters radiated heat back into the atmosphere, this triggered a series of consequences on land. Air masses travelling over the top of the ocean were picking up heat to scorch cities across Europe.
Deadly heatwaves of more than 40°C across Germany, France and Italy broke temperature records, wildfires broke out, while torrential rains devastated parts of Spain and Eastern Europe.
Back underwater, the coral reefs of the Caribbean were bleaching under severe heat stress. Hurricanes, which only occur during summer, fuelled by ocean heat, intensified into disasters. That season, Hurricane Idalia hit Florida – causing eight deaths and damages worth $3.6bn.
'This wasn't just a small area of warm water off one coast,' said Prof 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.'
Prof England said marine heatwaves in the North Atlantic are only set to 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 said.
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