Fuelled by climate change, the world’s oceans have broken temperature records every single day over the past year, a BBC analysis finds.
Nearly 50 days have smashed existing highs for the time of year by the largest margin in the satellite era.
Planet-warming gases are mostly to blame, but the natural weather event El Niño has also helped warm the seas.
The super-heated oceans have hit marine life hard and driven a new wave of coral bleaching.
The analysis is based on data from the EU’s Copernicus Climate Service.
Copernicus also confirmed that last month was the warmest April on record in terms of global air temperatures, extending that sequence of month-specific records to 11 in a row.
For many decades, the world’s oceans have been the Earth’s ‘get-out-of-jail card’ when it comes to climate change.
Not only do they absorb around a quarter of the carbon dioxide that humans produce, they also soak up around 90% of the excess heat.
But over the past year, the oceans have displayed the most concerning evidence yet that they are struggling to cope, with the sea surface particularly feeling the heat.
From March 2023, the average surface temperature of the global oceans started to shoot further and further above the long-term norm, hitting a new record high in August.
Recent months have brought no respite, with the sea surface reaching a new global average daily high of 21.09C in February and March this year, according to Copernicus data.
Not only has every single day since 4 May 2023 broken the daily record for the time of year, but on some days the margin has been huge.
This human-driven ocean warming is having considerable impacts on global sea life and may even be shifting the seasonal cycle of sea temperatures, according to a recent study.
Perhaps the most significant consequence of the recent warmth has been the mass bleaching of coral globally.
These key ocean nurseries turn white and die because the waters they live in grow too hot. They are a critical element in the ocean ecosystem, home to around a quarter of all marine species.
One important factor that’s made the last year more impactful in seas all over the world has been the El Niño weather phenomenon, adding to human-driven emissions of warming gases.
El Niño sees warmer waters come to the surface of the Pacific. As a result, it tends to push up the global average.
El Niño kicked into gear in June 2023 – after a prolonged period of cooler La Niña conditions – and reached a peak in December, although it has since been fading away.
But other ocean basins that aren’t usually affected by El Niño have also experienced record marine heatwaves – leaving scientists trying to work out exactly what is going on.