what we know about why ocean temperatures keep breaking records

This article by Professor Thomas Wernberg from the School of Biological Sciences at the University of Western Australia, Senior Lecturer in the School of Biological, Earth and Environmental Sciences at UNSW Sydney Alex Sen Gupta, Postdoctoral Research Scientist at the Marine Biological Society Kathryn Smith, Scientia Originally published features Professor and Deputy Director of the Australian ARC Center for Excellence in Antarctic Science (ACEAS) at UNSW Sydney Matthew England, University of Tasmania Professor Neil Holbrook and Postdoctoral Researcher at the Center for Marine Science and Innovation at UNSW Sydney Zhi Li in Conversation on April 11.

Over the past year, our oceans have been hotter than any other time on record. Our instrumental record covers the last 150 years. But based on rough observations, we can say that our oceans are now hotter than they were long before the rise of human civilization, most likely for at least 100,000 years.

This is not entirely unexpected. Ocean temperatures have been steadily rising due to human-caused global warming, which in turn means the hottest years on record are becoming more common. The last time ocean temperature records were broken was 2016 and before that was 2015. The last year we experienced a record cold year was way back in the early 20th century.

But what is remarkable about the past year is the continued large increase in global ocean temperature that began in April of last year. Last year was warmer than the previous record year by 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 this does not explain why the increase in heat has been so great. Climate drivers such as El Niño likely play a role, as does the randomness of some weather events and perhaps reduced sulfur emissions from transportation. Researchers around the world are trying to figure out what's going on.

How big is the jump in heat?

You can see the increase in heat very clearly in the nearly global ocean surface temperature data.

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

These record temperatures have been widespread, with the oceans of the Southern Hemisphere, the Northern Hemisphere and the tropics all reaching record temperatures.

What is behind the growth?

We still don't have a full explanation for this record-breaking burst of warming. But several factors are likely involved.

The first and most obvious is global warming. Year after year, the ocean is gaining heat through the enhanced greenhouse effect – indeed over 90% of the heat associated with human-caused global warming has gone into the oceans.

The additional heat flowing into the oceans results in a gradual increase in temperature, with the trend possibly accelerating. But that alone doesn't explain why we've experienced such a big jump in the last year.

Then there are the natural drivers. The El Niño event that took place in June of last year certainly played an essential role.

El Niño and its partner, La Niña, are opposite ends of a natural oscillation, the Southern El Niño, which plays out in the tropical Pacific Ocean. This cycle moves heat vertically between the deeper ocean waters and the surface. When El Niño comes, warmer water comes to the surface. During La Niña, the opposite happens.

You can clearly see the impact of an El Niño on short-term temperature increases, even against a background of strong long-term warming.

But even climate change and El Niño together are not enough to explain it.

Other natural heat-transferring oscillations, such as the Indian Ocean Dipole or the North Atlantic Oscillation, may play a role.

It may also be that our successful efforts to reduce aerosol pollution from the transportation of the dirty fuel that fuel transportation relies on has had an unintended side effect: more warming. With fewer reflective aerosols in the atmosphere, more of the Sun's energy can reach the surface.

But there is probably a level of random chance as well. Chaotic weather systems over the ocean can reduce cloud cover, which can allow more solar radiation. Or these weather systems can weaken winds, reducing cooling evaporation.

Why is this important?

To us, a warmer ocean might feel nice. But the extra heat is manifesting underwater as an unprecedented series of massive marine heat waves. Ocean organisms are particular about their preferred temperature range. If the heat gets too high and too long, they have to move or die.

Marine heat waves can lead to mass die-offs or mass migration of marine mammals, seabirds, fish and invertebrates. They can cause the death of vital kelp forests and seagrass meadows, leaving the animals that depend on them without shelter or food. And they can disrupt species important to fisheries and tourism.

This year's heat stress has caused widespread coral bleaching around the world. Bleaching has been seen on reefs in the Caribbean, Florida, Egypt and the Great Barrier Reef.

In the colder waters of Tasmania, tremendous conservation efforts have been made to try and protect endangered fish species such as redfish from the heat, while in the Canary Islands, small-scale commercial fisheries have emerged for species that do not are normally found there. .

Last year, Peru's anchovy fishery – the country's largest – was closed for long periods, leading to export losses estimated at A$2.1 billion.

What will happen next?

Given that record temperatures stem from a combination of human-induced climate change and natural sources, it is highly likely that ocean temperatures will drop back to more “normal” temperatures. The normal now is, of course, much warmer than in previous decades.

In the coming months, forecasts suggest that we have a fair chance of heading into another La Niña.

If that happens, we could see slightly cooler temperatures than the new normal, but it's still too early to know for sure.

However, one thing is certain. As we struggle to curb greenhouse gas emissions, the steady march of global warming will continue to add more heat to the oceans. And another increase in global warming of the oceans will not be far away.