In the past 15 months global sea surface temperatures have surged to unprecedented levels, fuelling heatwaves and accelerated the melting of sea ice, alarming scientists and environmentalists alike.

These record-high temperatures in the north Atlantic waters, particularly around the UK and Ireland, were described as “beyond extreme” last year by the EU’s Earth observation service.

And if anything, this year’s temperatures are even higher. June marked the fifteenth consecutive month of record-breaking global sea temperatures. Forecasters warn that the unusually warm waters could lead to an exceptionally intense hurricane season which will suck up the energy from the bathwater hot seas. Last month, Hurricane Beryl, devastated the Caribbean, Mexico, and Texas, after it became the earliest maximum-severity storm on record.

Scientists are investigating the causes behind this rapid rise in sea temperatures, its persistence, and the potential for future cooling. A 2019 report from the Intergovernmental Panel on Climate Change (IPCC) indicated that marine heatwaves have doubled in frequency since 1982, becoming longer, more intense and more widespread.

The oceans play a crucial role in mitigating climate change, absorbing 90% of excess heat and about a quarter of human-caused carbon dioxide emissions since the industrial era. “The ocean has been doing this huge climate favour for us for decades,” one expert noted, but cautioned, “there is no guarantee it will continue doing so at the same rate.” Two primary factors – global warming and the El Niño-Southern Oscillation phenomenon – are broadly agreed to be key contributors.

“Like we have heatwaves in the atmosphere, we have heatwaves in the ocean. But we are seeing more of them; they are becoming more frequent and more intense,” explains the IPCC, which reported that between 84% and 90% of marine heatwaves from 2006 to 2015 were likely due to human-caused warming.

El Niño events, occurring every two to seven years, release significant heat from the tropical Pacific into the atmosphere, elevating global temperatures. The most recent El Niño was declared in June 2023. Prior to this, a rare “triple-dip” La Niña, which removes heat from the atmosphere, masked the anticipated rise in global temperatures from 2020 to 2022, despite increasing greenhouse gas (GHG) concentrations.

In 2023, the onset of El Niño resulted in a dramatic release of stored ocean heat, leading to consecutive record-breaking temperatures. Research published in May suggested that reduced sulphur dioxide emissions could potentially double the warming rate of seas in the 2020s compared to the period since 1980 as SO2 reflects more sunlight and prevents it reaching and warming the surface.

The World Meteorological Organization estimates a 70% chance of a shift to the La Niña cooling phase between August and November this year, which could eventually lower both ocean and land surface temperatures. However, a lag effect may sustain high temperatures this year, say scientists.

The rising sea temperatures are already wreaking havoc on marine ecosystems. In April, NOAA reported the world’s fourth global coral bleaching event, the second in just a decade. Bleaching, caused by elevated sea temperatures, forces corals to expel algae, leading to widespread coral death.

Extensive bleaching-level heat stress has been recorded across the Atlantic, Pacific, and Indian Oceans, affecting corals in Florida, the Caribbean, Brazil, Australia’s Great Barrier Reef, the Red Sea, the Persian Gulf and large areas of the tropical and south Pacific. Warmer, more acidic waters are dissolving the calcium structures of corals and impacting marine life. Since 1850, ocean acidity has increased by approximately 25%.

The fundamental concern remains whether the rapid spike in ocean temperatures signifies that the oceans are struggling to absorb heat and carbon dioxide as efficiently as before. If this absorption rate decreases, atmospheric temperatures could rise even more sharply, exacerbating the effects of climate change.