NEW YORK – A surge in the temperature of the world’s oceans is continuing into 2024. What is behind the unprecedented readings, and what do they mean for people, wildlife and the wider climate?

1. Just how high are these temperatures?

Alarmingly high. Since early May 2023, the average global sea surface temperature – excluding waters around the poles – has been at record seasonal highs every day, in data that goes back to 1979. It hit a new record in February, surpassing August’s peak. It generally tops out in March, the end of summer in the Southern Hemisphere, which contains the most ocean.

Historical records indicate that waters have been much warmer millions of years ago – although being accurate about what was happening in the distant past can be tricky.

2. Why are the oceans getting warmer? 

The main long-term cause is the continuing increase in concentrations of greenhouse gases. More than 90 per cent of the extra heat in the planet’s climate system that is caused by human-made global warming is in the ocean. Since the 1980s, ocean waters have absorbed roughly a quarter of our carbon dioxide pollution. The start of the El Nino phenomenon in 2023 also contributed to warmer waters, but does not explain the whole thing. 

Slowing wind speeds have also been discussed by experts: This impacts both the mixing of ocean waters and the amount of Saharan dust drifting over the North Atlantic, which typically protects the waters below from solar radiation. 

A four-year-old environmental rule known as IMO 2020 aimed at slashing ships’ sulphur emissions has also been discussed as a contributing factor to warming seas. Sulphur emissions are generally known to have a cooling effect by making clouds brighter and reflecting sunlight – but clarifying the relationship with ocean temperatures can be tricky. 

3. What do warmer waters mean for wildlife and the fishing industry? 

Because fish regulate their body temperatures by adjusting their latitude and depth in the ocean, warmer waters are pushing many populations – like mackerel – towards the earth’s poles or deeper waters. 

Some locations are likely to benefit, with high-latitude areas potentially seeing a 30 per cent to 70 per cent increase in commercially valuable species. In the Mediterranean, the arrival of rabbit fish and lionfish in significant numbers is considered invasive, with knock-on impacts for fishermen and the local ecosystem. Warmer temperatures have also been linked to the rise of marine bacteria called Vibrio, which can cause vibriosis in humans, bringing symptoms such as diarrhoea, vomiting, fever and, in some cases, death. 

Continued absorption of CO2 by the ocean also makes it more acidic, making it harder for many creatures to maintain their shells and skeletons. And warmer water contains less oxygen – restricting supply for marine life and contributing to rising sea levels through thermal expansion.

One of the most memorable high-water-temperature occurrences was The Blob, a massive marine heatwave that developed off the west coast of North America in the mid-2010s. The event was linked to changing salmon migration routes, seabird die-offs, starved California sea lion pups washing up on beaches and the shutdown of crab fisheries worth millions of dollars.

Warmer ocean – and air – temperatures are generally expected to reduce sea ice worldwide. That is bad news not just for Emperor penguins: It is feared melting ice will help weaken what is known as the Atlantic Meridional Overturning Circulation, or Amoc – a system of currents, including the Gulf Stream, that plays a crucial role in regulating the planet’s climate. 

While a shutdown of Amoc this century is considered unlikely, at least one study estimates a collapse around mid-century. Another said Europe’s climate would be greatly affected by a collapse, with several cities seeing temperature drops of 5 deg C to 15 deg C. 

Warmer waters have also been found to reduce monsoon rainfall over the central Indian subcontinent and in 2023 helped push US scientists to raise their forecast for Atlantic hurricanes.

Additionally, coral can be bleached and even killed by warmer waters. Hundreds of thousands of marine species are thought to be found in coral reefs – they offer food, shelter, nurseries and breeding grounds, and are a magnet for tourists. Global warming of 1.5 deg C is forecast to result in a 70 per cent to 90 per cent decline in coral reefs, according to a 2018 IPCC report. Marine heat can also damage and ultimately kill kelp, which absorb CO2 and are an important food source. 

4. What’s going on in different regions? 

As different parts of the planet receive varying amounts of sunlight, ocean surface temperatures can vary widely, from 30 deg C in the tropics to minus 2 deg C near the poles. As at Feb 19, the surface temperature in the North Atlantic was at a seasonal high in data going back to 1979, and that has been the case since May 2023. Marine heatwaves persist in the Mediterranean, another area that saw record-breaking heat last summer, and are also seen in the tropical Pacific and elsewhere. At a buoy off Florida in 2023, the temperature reportedly surpassed 37.8 deg C, about the same temperature as a hot tub. Temperatures also change significantly depending on depth. Measuring warmth in the deep ocean is difficult and expensive, although one study published in 2020 found a fractional increase in temperatures in the Atlantic’s Argentine Basin at depths of more than 4,500m. 

5. What’s next for ocean temperatures? 

If ocean surface temperatures remain as high as they are now, accelerated atmospheric warming is possible, according to marine systems modelling associate head Joel Hirschi at Britain’s National Oceanography Centre. That said, the most likely scenario for the rest of 2024 is for El Nino to continue to wane, followed by a switch to La Nina, he continued. Subsurface ocean temperatures suggest a La Nina event will start developing later in 2024. “This would lead to a lowering of global ocean surface temperatures and surface air temperatures below 2023 and current levels. A sustained warming anomaly exceeding 1.5 deg C is unlikely as we move further into 2024.”

Longer term, the IPCC has said it is virtually certain the ocean will continue to soak up heat throughout this century, with the rate of uptake determined by emissions. It also expects an increase in the frequency, duration, spatial extent and intensity of marine heatwaves. The European Environment Agency has issued a series of long-term forecasts for 2071 to 2100 relative to temperatures for recent years, which show a global rise of 0.5 deg C to 3.8 deg C for the ocean, depending on the emissions scenario.

“I very much hope that humanity will see sense and that a sea surface temperature increase by 3.8 deg C is a scenario we successfully avert,” Dr Hirschi said. If not, he said, the Greenland and West Antarctic ice sheets would melt, raising sea levels significantly, and hurricane-type storms could develop in regions that are not normally prone to them. Even before 3.8 deg C sea surface temperature warming is reached, many coral reefs would have died. BLOOMBERG