- New research shows that ocean temperatures are hotter than ever in the modern era due to human-driven global warming.
- High ocean temperatures are placing a strain on marine life and biological processes while also increasing extreme weather events on land.
- The world is also seeing an escalation in the frequency and intensity of marine heat waves, events in which sea temperatures exceed a certain threshold for five days or more.
Human actions are rapidly changing the world’s oceans, whether through overfishing, pollution or coastal development. But among the most intense pressures placed on the seas right now is humanity’s ongoing burning of fossil fuels, pumping dangerous amounts of greenhouse gases into the atmosphere, which in turn has pushed sea temperatures to record levels.
The global ocean, which covers 70% of the planet’s surface, currently absorbs 90% of the solar heat trapped by humanity’s carbon emissions. This greatly moderates rising atmospheric temperatures and helps temper the intensity of the climate crisis. Put another way, the world would be a lot hotter by now if the ocean wasn’t taking in all this heat.
But the ocean’s absorption of this anthropogenic heat still has serious consequences.
One clear result has been an unprecedented rise in global sea temperatures, which has placed strains on marine life and biological processes, and increased extreme weather on land. Rising ocean temperatures are also resulting in an escalation in marine heat waves, placing even more pressure on marine organisms and ecosystems already struggling to cope with other changes brought on by fossil fuel burning, such as ocean acidification, along with other human stressors.
Without urgent action to curb carbon emissions, experts say ocean heat will continue to increase, which will, in turn, impact the very planetary systems necessary for humanity’s survival and for maintaining life on Earth as we know it.
The oceans are hotter than ever in modern history
New research published Jan. 11 in Advances in Atmospheric Sciences finds that the oceans are hotter than they’ve ever been in modern times. The sea’s heightened temperatures have now smashed previous heat records for at least seven years in a row (or eight years depending on data interpretation), according to data collected by the Institute of Atmospheric Physics at the Chinese Academy of Sciences. Similar data was collected by the U.S. National Oceanic and Atmospheric Administration (NOAA), reinforcing these findings.
“It’s year after year that we’re setting heat records in the ocean,” study co-author John Abraham, professor of thermal sciences at the University of St. Thomas in Minnesota, told Mongabay. “The fact that this process is continuing apace every single year is illuminating for us because it drives home how the oceans are connected to global warming, and how we can use the oceans to measure how fast the earth is warming.”
In 2023, the oceans absorbed about 287 zettajoules of heat, which Abraham says is the equivalent of eight Hiroshima atomic bombs detonating every second of every day into the ocean. Last year’s heat was 15 zettajoules greater than what the ocean absorbed in 2022.
The researchers detected increased heat in many parts of the ocean — from the surface to a depth of 2,000 meters (6,600 feet) — though Abraham says higher temperatures were most evident in the surface waters, or the top 20 m (66 ft).
During the first half of 2023, temperatures were 0.1° Celsius (0.2° Fahrenheit) above the 2022 temperatures; over the second half of the year, they were 0.3°C (0.5°F) higher than 2022 temperatures, according to the study.
“We really blew the record off ocean surface temperatures last year,” Abraham said. “It was just mind-boggling hot.”
According to Abraham, the surface temperatures were particularly high in 2023 due to the combined effect of long-term global warming and a strong El Niño climate pattern currently playing out. El Niño is a regularly recurring natural phenomenon that weakens winds along Earth’s equator, leading to a rise in sea temperatures and atmospheric temperatures.
What happens when the oceans heat up?
Higher-than-normal sea temperatures have widespread effects across the ocean. They cause sea level rise as the water experiences thermal expansion. They can also stress or kill coral reef systems, accelerate the melting of polar sea ice, redistribute fish populations, and deplete oxygen levels.
They can also influence what happens on land. Once the surface of the sea is hot, it creates the perfect conditions for terrestrial extreme weather events to develop, such as intense rainfall, droughts, and severely destructive storms.
“The oceans drive weather,” Abraham said. “As the oceans warm, the air flows over the oceans and heat and moisture gets transferred. So it’s the oceans that help heat the atmosphere and the oceans that provide the humidity to the atmosphere … and weather is dictated by temperature and humidity.”
With oceans and atmosphere experiencing record heat, extreme weather events are happening with more frequency and intensity. In 2023, for instance, there was a spate of extreme events, including deadly heat waves in China, Europe and North America, an extreme fire season in Canada, and record-breaking rain and flooding in Libya, the Democratic Republic of Congo, and Australia.
“Our weather is going back and forth like a swing between extremes,” Abraham said. “And those extremes are getting bigger. And those extremes are costly.”
Just how costly are extreme weather events? A study published in Nature Communications last year suggested that the damage from extreme weather events costs about $16 million every hour. Other data showed that in 2023, the U.S. alone saw 28 billion-dollar weather and climate disasters, totaling $92.9 billion in damages, surpassing the previous record of 22 billion-dollar events in 2020.
Marine heat waves are surging
High ocean temperatures don’t only drive extremes on land — they also influence extreme events in the water. As the global ocean becomes hotter, the frequency and intensity of marine heat waves increase.
A marine heat wave is defined as an event in which sea temperatures exceed a certain threshold — specifically the 90th percentile of 30-year historic values — for a period of five or more days. These extreme events are caused by the influx of greenhouse gas emissions, which causes the ocean to absorb and store more heat, and also by the movement of warmer water via sea currents.
While the transport of ocean water through global current systems is a natural occurrence, these processes, too, are being impacted by climate change.
“Climate change is making ocean currents do different things,” Alistair Hobday a research scientist at Australia’s CSIRO Climate Science Centre and the University of Tasmania’s Centre for Marine Socioecology, told Mongabay. “Currents that flow from the equator towards the poles are getting stronger, and they’re bringing more warm water further south or further north, depending on which hemisphere you live in, and that can give you marine heat waves.”
The 2023-24 El Niño is also reinforcing heat in some parts of the ocean, and heightening the possibility for marine heat wave development, Hobday said.
Last year, the world saw several marine heat waves, including a record-high event in the North Atlantic that led to an alarming phytoplankton decline, and a blob of heat that killed off swaths of coral reefs in Florida, including all of the corals in a restoration project.
Research published in Frontiers in Marine Science in 2019 suggested that the number of marine heat wave days increased globally by more than 50% from 1925 to 2016, and that the intensity of these events also shot up since the start of satellite data collection in 1982.
“Marine heat waves are natural, but the increase we’re seeing is not natural,” Thomas Wernberg, a marine ecologist at the University of Western Australia (UWA), who co-authored the Frontiers in Marine Science study along with Hobday, told Mongabay.
“We see the events increasingly occur, but we also see increasing impacts from them, both in terms of the species themselves and the ecosystem services that the oceans provide, such as changes in fisheries, fisheries yields, shifting fisheries,” Wernberg said.
The 2019 study also suggested that the world could move into a near-permanent marine heat wave by the end of the century if global warming isn’t reined in. This would result in most parts of the global ocean experiencing some level of extreme heat for the majority of the year.
Karen Filbee-Dexter, a UWA researcher who wasn’t involved in the 2019 study, called the idea of moving into a near-permanent marine heat wave “quite frightening,” but said the trend of increasing marine heat waves shows we’re on our way to this reality.
“You can imagine that if all the oceans become warmer and become a little more extreme, you will essentially have a reorganization of marine life,” Filbee-Dexter told Mongabay. “This will happen most obviously in the shallow waters, but we still find marine heat waves going all the way into the deeper regions as well.”
While some organisms will be able to adapt or migrate to more suitable conditions for their survival, others will not. This is especially the case with sessile, or immobile, organisms that are attached to the ocean bottom, such as corals and oysters, Filbee-Dexter said.
She noted that increased ocean heat and marine heat waves will lead to the die-off of ecosystems like coral reef systems and kelp forests, and redistribute fish populations, impacting global fisheries.
Will the oceans continue to warm?
The oceans are currently seeing a strong warming trend, increasing the frequency and intensity of extreme events on both land and sea. But will these trends continue? Experts say this depends, in part, on humanity’s actions in response to the global climate crisis.
But, according to Hobday, even if humanity stopped emitting fossil fuels today, we’re still locked into a period of “committed warming.”
“We’re probably locked into the year 2050 with warming … because the methane and carbon dioxide will have a lifetime in the atmosphere,” Hobday said. “So even if you turned off the tap today, they’re still going to have an effect before the ocean can draw down that carbon dioxide or before the methane breaks down.”
Abraham said he’s especially keeping an eye on whether global ocean heating accelerates, a trend he says would be “concerning” if it develops.
“Right now, we don’t have enough data to determine whether it’s speeding up,” Abraham said. “But my gut feel [is that] there is a slight acceleration that we are now starting to detect … but we can’t make the claim that there’s an acceleration until we get more data.”
While the future looks dismal for the ocean — and, by extension, the entire planet — Abraham said we can use the information provided by ocean heating to make positive changes that can help slow down some global warming effects.
“There’s really two things that are in our power to do right now,” Abraham said. “It’s use energy more wisely and maximize our clean energy draw. Let’s get as much of that energy as we can with clean green energy that doesn’t emit greenhouse gases.”
Banner image: A lemon shark in the mangroves in the Bahamas. Image by Anita Kainrath / Ocean Image Bank.
Cheng, L., Abraham, J., Trenberth, K. E., Boyer, T., Mann, M. E., Zhu, J., … Lu, Y. (2024). New record ocean temperatures and related climate indicators in 2023. Advances in Atmospheric Sciences. doi:10.1007/s00376-024-3378-5
Newman, R., & Noy, I. (2023). The global costs of extreme weather that are attributable to climate change. Nature Communications, 14(1). doi:10.1038/s41467-023-41888-1
Oliver, E. C., Burrows, M. T., Donat, M. G., Sen Gupta, A., Alexander, L. V., Perkins-Kirkpatrick, S. E., … Smale, D. A. (2019). Projected marine heatwaves in the 21st century and the potential for ecological impact. Frontiers in Marine Science, 6. doi:10.3389/fmars.2019.00734