The ocean is hot. Too hot.
Humanity’s addiction to fossil fuels has driven up global temperatures and much of this heat has ended up in the ocean. Since early 2023, this warming has accelerated, outpacing what researchers predicted based on climate models.
What is happening with ocean warming?
For the past 18 months, global ocean temperatures have been rising at a rate that scientists say is deeply alarming. Researchers have predicted significant ocean warming as part of human-induced climate change, but they did not foresee the spike in ocean heat that began in early 2023. And they cannot fully explain it.
In March 2023, sea surface temperatures in the north of the Atlantic Ocean suddenly shot up, bringing a marine heatwave to much of the region.
That April, global sea-surface temperatures set a record. The same thing happened again in May. By June, temperatures across the North Atlantic were 1-3°C above the seasonal average. In Antarctica, sea ice had reached its lowest extent on record, at 17 per cent below average and significantly lower than the previous June.
Before long, every ocean basin on Earth was experiencing accelerated warming, with global average sea-surface temperatures reaching a new daily high of over 21°C in February and March 2024.
The spike in global ocean warming continued until August 2024. As of October, record sea surface temperatures were still widespread, affecting the Caribbean and parts of the Indian, Pacific and Southern oceans.
How does this compare historically?
Since the industrial era began in around 1850, the global average sea-surface temperature has increased by an average of 0.68-1.01°C, according to a 2023 report by the Intergovernmental Panel on Climate Change.
Unesco’s State of the Ocean report, published in June 2024, suggests this average has now jumped to 1.45°C. It also identifies clear hotspots of more than 2°C in the Mediterranean Sea, and the Tropical Atlantic and Southern oceans.
As well as gauging sea-surface temperatures from satellite data, scientists glean a more complete picture of ocean warming at different depths using in-situ instruments.
Chief among these is an array of 3,000 robotic “Argo” floats distributed throughout the global ocean. By measuring changes in ocean heat content, these instruments show the global ocean has warmed significantly since the 1950s.
Since the 1990s, the rate at which the ocean absorbs heat has also increased dramatically, and has doubled in the past 20 years. The Mediterranean Sea and the Atlantic and Southern oceans have warmed the fastest, but the mercury began to rise in every ocean basin on Earth in early 2023.
Ocean warming in 2023 was the highest since records began six decades ago. The warming, which continued into 2024, has outpaced what has been expected from modelling projections.
Is El Niño to blame?
Ocean warming has primarily come about from human activities that emit greenhouse gases, such as deforestation and the burning of fossil fuels. But that alone cannot explain the recent trend.
One likely contributor to warming since June 2023 is El Niño. This is a natural, irregular climatic phenomenon that starts with an increase in sea-surface temperature in the eastern Pacific Ocean. Over a period of months, the warming spreads to other regions of the world, where it can play havoc with sea-surface temperature, weather patterns and food production.
The current El Niño began in June 2023 and peaked late last year and early in 2024. This weather pattern is now weakening – and the tropical Pacific is starting to cool – but its impact on global ocean temperature is still being felt. But although 2023 was an El Niño year, this warming started before El Niño took hold that summer.
If not El Niño, what is behind the current temperature spike?
The current spike in temperature is so extreme that many scientists think there is more than climate change at play.
One possible contributor is a recent change in global shipping regulations. The International Maritime Organization – the agency that governs global shipping – forced ship owners to cut the sulphur content of their fuel from 3.5 per cent to 0.5 per cent from 1 January, 2020.
A year later, the agency declared the ruling a resounding success for dangerous air pollution, saying it had cut global shipping’s sulphur dioxide emissions by around 70 per cent. But this also means there is less particulate pollution in the atmosphere. These particles would ordinarily bounce some of the sun’s rays back into space, so this change has possibly contributed to the uptick in ocean warming.
Adding to this complicated picture is the recent eruption of the subsea Hunga Tonga-Hunga Ha‘apai volcano. Near the island of Tonga in the Pacific Ocean, it erupted in January 2022 and threw up to 150 billion kilograms of water vapour into the sky. Water vapour is a greenhouse gas, so this event may also have accelerated ocean warming.
Even with all of these factors taken into account, however, scientists cannot yet account for the record-breaking ocean temperatures of the past 18 months.
Why is so much heat ending up in the ocean?
As of 2023, 91 per cent of the heat generated by human activity is absorbed by the ocean, significantly lessening the impact of climate warming on humans. By comparison, only 1 per cent of the heat generated by human-induced climate change has ended up in the atmosphere; 5 per cent has been taken up by land and 3 per cent by melting ice.
The ocean has absorbed the lion’s share of this heat for a few reasons. Firstly, the ocean covers 70 per cent of Earth’s surface. Secondly, the ocean is less reflective than land and therefore absorbs more of the sun’s energy. Water also has four times the heat capacity of air; in practical terms, that means it takes four times as much energy to heat a kilogram of water compared to a kilogram of air.
Water is also much denser than air – it is roughly 1,000 times heavier per square metre. And the mass of the ocean is roughly 250-300 times that of the atmosphere.
With a larger heat capacity and a larger mass than the atmosphere, the ocean has an incredible ability to soak up and store heat.
How much energy are we talking about?
Nasa calculates the ocean absorbed around 360 zettajoules of energy in warming between 1955 and the end of 2023. All of the energy that humans produce in a year (from nuclear power plants, solar plants, coal-fired plants and so forth, combined) amounts to around 0.1 of a zettajoule.
Ocean warming is currently increasing year on year by around 10 zettajoules – meaning heat equivalent to 100 times global energy production is dumped into the ocean every year.
There are roughly 1.335 billion cubic kilometres of water in the ocean. It takes four kilojoules of energy to warm one single kilogram of water by 1°C.
These are extraordinary volumes of water. Now that this heat has entered the ocean, it is going to stay there for hundreds to thousands of years. There is no quick way of getting rid of the heat that is building up in the ocean.
What does all of this mean for the ocean and for people?
The ocean is not warming uniformly – most of the warming is happening in the uppermost 700 metres. Some of that heat is slowly penetrating down to two kilometres. In some regions, the heat is reaching much deeper down: in parts of Antarctica’s Southern Ocean, for instance, waters below two kilometres are warming at five times the average rate of the global ocean.
This is attributed to changes in winds and currents that drive deep ocean circulation in the region. As this water warms, it is losing density, which could have knock-on effects for global ocean circulation.
Once heat reaches the deep ocean, it can take hundreds or even thousands of years to resurface. This is good for life on Earth, in that it buffers us from the worst possible effects of climate change.
But ocean warming is not without consequence. Warm water is bleaching coral reefs worldwide, threatening entire marine ecosystems. And as water warms, it expands, contributing to sea-level rise. Warming is also changing conditions in the polar regions, melting glaciers which also causes sea-level rise.
How does this warming impact ocean currents?
One of the most concerning possibilities is that ocean warming will slow part of the “global ocean conveyor belt”, or even eventually it shut down. This is the system that moves heat and salt around Earth.
The portion of this loop centred on the Atlantic Ocean is known as the Atlantic Meridional Overturning Circulation (AMOC). It begins in the far north. Confronted with near-freezing Arctic temperatures, seawater here becomes chilled. Some of this water freezes into sea ice, and sheds salt.
The now colder, more salty water is denser and it sinks. As this cold water sinks it heads south; warm surface water that has been heated at the equator travels northward to replace it in this loop.
The AMOC has operated with varying intensities for at least three million years, slowing at times and plunging northern latitudes into a deep cold, while warming the southern hemisphere. The system depends on cold, dense water being present in the Arctic.
In late October, a group of 42 scientists wrote an open letter to the Nordic Council of Ministers warning that the AMOC could collapse entirely in the coming decades: “While the impacts on weather patterns, ecosystems and human activities warrant further study, [a collapse] would potentially threaten the viability of agriculture in northwestern Europe.”
Already, there are signs of this system slowing in the North Atlantic Ocean (most prominent perhaps is a persistent patch of cool water, referred to as the “cold blob”), in precisely the region where the AMOC ordinarily delivers much of its heat. Without the AMOC, Northern Europe would be much colder and the equator would be much hotter.
This article was originally published on Dialogue Earth under a Creative Commons licence.
