The heat is on: what we know about why ocean temperatures keep smashing records

By Alex Sen Gupta, UNSW Sydney, Kathryn Smith, Marine Biological Association, Matthew England, UNSW Sydney, Neil Holbrook, University of Tasmania, Thomas Wernberg, The University of Western Australia, Zhi Li, UNSW Sydney

Over the last year, our oceans have been hotter than any time ever recorded. Our instrumental record covers the last 150 years. But based on proxy observations, we can say our oceans are now hotter than well before the rise of human civilisation, very likely for at least 100,000 years.

This isn’t wholly unexpected. Ocean temperatures have been steadily rising due to human-caused global warming, which in turn means record hottest years have become increasingly common. The last time ocean temperature records were broken was 2016 and before that it was 2015. The last year we experienced a record cold year was way back at the start of the 20th century.

But what is remarkable about the past year is the huge ongoing spike in global ocean temperature which began in April last year. Last year was hotter than the previous record year by a whopping 0.25°C. In contrast the margins of other previous record years were all less than 0.1°C.

Why? Global warming is the main reason. But it doesn’t explain why the heat spike has been so large. Climate drivers such as El Niño likely play a role, as do the random alignment of certain weather events and possibly the reduction in sulfur emissions from shipping. Researchers around the world are trying to understand what’s going on.

How big is the jump in heat?

You can see the surge in heat very clearly in the near-global ocean surface temperature data.

The trend is clear to see. Earlier years (in blue) are typically cooler than later years (in red), reflecting the relentless march of global warming. But even with this trend, there are outliers. In 2023 and 2024, you can see a huge jump above previous years.

These record temperatures have been widespread, with the oceans of the southern hemisphere, northern hemisphere and the tropics all reaching record temperatures.