OCEANOGRAPHY
Ocean-surface warming four times faster now than late-1980s
University of Reading
The rate of ocean warming has more than quadrupled over the past four decades, a new study has shown.
Ocean temperatures were rising at about 0.06 degrees Celsius per decade in the late 1980s, but are now increasing at 0.27 degrees Celsius per decade.
Published today (Tuesday, 28 January 2025) in Environmental Research Letters, the study helps explain why 2023 and early 2024 saw unprecedented ocean temperatures.
Professor Chris Merchant, lead author at the University of Reading, said: “If the oceans were a bathtub of water, then in the 1980s, the hot tap was running slowly, warming up the water by just a fraction of a degree each decade. But now the hot tap is running much faster, and the warming has picked up speed. The way to slow down that warming is to start closing off the hot tap, by cutting global carbon emissions and moving towards net-zero.”
Energy imbalance
This accelerating ocean warming is driven by the Earth's growing energy imbalance – whereby more energy from the Sun is being absorbed in the Earth system than is escaping back to space. This imbalance has roughly doubled since 2010, in part due to increasing greenhouse gas concentrations, and because the Earth is now reflecting less sunlight to space than before.
Global ocean temperatures hit record highs for 450 days straight in 2023 and early 2024. Some of this warmth came from El Niño, a natural warming event in the Pacific. When scientists compared it to a similar El Niño in 2015-16, they found that the rest of the record warmth is explained by the sea surface warming up faster in the past 10 years than in earlier decades. 44% of the record warmth was attributable to the oceans absorbing heat at an accelerating rate.
Expect more warming
The findings show that the overall rate of global ocean warming observed over recent decades is not an accurate guide to what happens next: it is plausible that the ocean temperature increase seen over the past 40 years will be exceeded in just the next 20 years. Because the surface oceans set the pace for global warming, this matters for the climate as a whole. This accelerating warming underscores the urgency of reducing fossil fuel burning to prevent even more rapid temperature increases in the future and to begin to stabilise the climate.
Journal
Environmental Research Letters
Method of Research
Computational simulation/modeling
Article Title
Quantifying the acceleration of multidecadal global sea surface warming driven by Earth’s energy imbalance
Article Publication Date
28-Jan-2025
Q&A: How rate of CO2 rise can affect a global ocean current
University of Washington
image:
The global ocean “conveyor belt” circulation, shown in part here as red and blue lines, circulates cooler seawater below the surface and warmer seawater at the surface throughout the world’s oceans. The Atlantic Meridional Overturning Circulation is part of this system of global ocean currents.
view moreCredit: NOAA
As we burn fossil fuels, the amount of carbon dioxide in Earth’s atmosphere is gradually rising, and with it, the planet’s average temperature. How fast the level of atmospheric carbon dioxide — and with it, the temperature — goes up matters for the ability of humans and ecosystems to adjust. A slower increase gives humans time to move away from low-lying coasts and animals time to move to new habitats.
It turns out the rate of that increase matters for non-living systems, too. A recent University of Washington study looked at how a major current in the Atlantic Ocean that includes the Gulf Stream will respond to a doubling of carbon dioxide from preindustrial levels. The study, published in the Proceedings of the National Academy of Sciences, found that when carbon dioxide levels rise more gradually, they have less impact on the ocean circulation.
Camille Hankel, a UW postdoctoral researcher in the Cooperative Institute for Climate, Ocean and Ecosystem Studies, answered questions about her research.
Why did you choose to study how the rate of rising CO2 affects the climate system?
Camille Hankel: In my PhD, some of my work was on “climate tipping points,” which emerge from the hypothesis that there might be some sort of critical thresholds of warming or CO2 change that can lead to very abrupt and irreversible change in some parts of the climate system. Through that work, I got exposed to some literature on “rate-induced tipping points,” which is the idea that instead of crossing a critical level, that there could be some critical rates of CO2 change that are important for the climate system.
Specifically, I read this study that was looking at this idea in the context of the AMOC, the Atlantic Meridional Overturning Circulation, which is this large-scale ocean circulation. That study was using what we call a box model — a simplified, mathematical representation of the ocean circulation. And I thought, hey, I can run these global models, which are much more realistic representations of the Earth’s climate, including ocean, atmosphere, land, and sea ice, and test whether the rate of CO2 change really is that important.
What is the Atlantic Meridional Overturning Circulation, which includes the Gulf Stream ocean current, and why is it so important for Earth’s climate?
CH: It’s one of the large-scale, key features of the climate system. In particular, it transports a lot of heat from the low latitudes in the South Atlantic to the higher latitudes closer to the North Pole. So it delivers a lot of heat, primarily to Northern Europe. It also distributes nutrients around through this sort of sinking motion that characterizes the circulation — it draws the surface waters down into the deep ocean, and recirculates deep water up to the surface. It’s a big feature of the climate system, particularly in the North Atlantic, but also globally.
We’ve heard about a potential slowdown of the Gulf Stream current that could affect European weather. This was dramatized (perhaps not accurately) in the 2004 disaster movie ‘The Day After Tomorrow.’ Are we actually seeing a slowdown in Atlantic Ocean circulation?
CH: We have a pretty short observational record of the AMOC current, and it's sparse. You have to imagine, this is a 3D circulation in the entire Atlantic basin, and we have a couple little slices of data in particular parts of the Atlantic. We are seeing a modest slowdown so far, but it's a pretty noisy and uncertain observational record, so it's hard to tell.
I would say, however, that slowdown seen in current observations would match the model predictions of future slowdowns. And we also see a pattern in temperature changes where, while the rest of the globe is warming right now as we increase CO2, there's what people call a “warming hole” over the North Atlantic: We’re not seeing as much warming in that North Atlantic region compared to the rest of the globe. And it's hard to conclusively attribute what's causing it in the Earth's climate, but the idea is that the modest slowdown of the AMOC that we've seen so far could be one contributing factor to that lack of warming we're seeing in the North Atlantic.
So the observations suggest some slowdown, though much less dramatic than what was depicted in that movie.
Why is the AMOC expected to slow down under climate change?
CH: One way of thinking about what drives this major ocean current is differences in ocean density. You have this really important zone in the North Atlantic where the waters sink because the surface waters are heavier than the waters below. When you change CO2 levels, you do two things. You start to warm the ocean’s surface, and by melting glaciers as well as changing sea ice, you add freshwater to the surface of the otherwise salty ocean. Both warming and freshening reduce the density of that upper ocean water and potentially inhibit or disrupt that critical sinking motion.
There are other ways of looking at it, but the one I look at in the study is understanding how those density perturbations happen in a higher-CO2 climate and how they modulate the sensitivity to the rate of CO2 change that I find in the AMOC’s response to CO2.
Your study finds that if atmospheric carbon dioxide doubles from pre-industrial levels more slowly, there’s less slowdown in the Atlantic Ocean compared to if CO2 doubles more quickly. Is that because everything is happening more slowly?
CH: Yes, that’s part of it. The different parts of the climate system — the ocean, atmosphere, and ice — all have different response timescales to CO2 changes, meaning they respond to perturbations with different lag times. Then how these components of the climate interact with each other under slower or faster CO2 changes can look very different, and in this case influence the ocean circulation.
Specifically, I found what’s known as a positive feedback — a sort of self-amplifying cycle — that helps explain why the level of AMOC weakening depends on the rate of CO2 change. In this feedback cycle, the initial modest amount of AMOC slowdown leads to a reduction of heat transport into the Arctic, which in turn cools the region and leads to a temporary period of Arctic sea ice expansion. This sea ice expansion causes more ice to be exported to the North Atlantic, where it melts and adds freshwater to the ocean, causing the AMOC to slow down even more: hence the self-amplifying cycle. It turns out that this feedback cycle is more effective at amplifying AMOC weakening under more rapid CO2 changes than under gradual CO2 changes.
What is the importance of this work?
CH: We know about AMOC slowdowns — there's a ton of work on that, and the mechanisms that drive such an AMOC slowdown. But what’s new is this sensitivity of the circulation changes to the rate of CO2 increase, independent of the total change in concentration of CO2.
When we think about policy and basic science, we tend to focus a lot on how the level of global warming can impact the climate system. I'm trying to bring a new perspective by thinking about the rate of increase as a driver itself, that could have a lot of impacts.
You can imagine that if multiple different climates are possible for the same level of warming, then limiting us to 1.5 C or 2 C could have different meanings, right? I do think the most important thing for the climate system is always how much CO2 have you put into the atmosphere, but how quickly you got to that point clearly matters as well.
For more information, contact Hankel at crhankel@uw.edu.
Journal
Proceedings of the National Academy of Sciences
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
The effect of CO2 ramping rate on the transient weakening of the Atlantic Meridional Overturning Circulation
The deadly pathogen that decimated sea urchins in Eilat has now spread to the Indian Ocean
Concerns over the expansion of coral reef destruction worldwide
Tel-Aviv University
image:
Infected sea urchin on Reunion Island
view moreCredit: Jean-Pascal Quod
Research team: “This is an extremely violent global pandemic. The Caribbean, Red Sea, and the Indian Ocean are critical regions for the world’s coral reefs, and mortality rates for sea urchins in these areas are very high — over 90 percent. As of now, we have no evidence of this pathogen in Pacific Ocean sea urchins (where some of the world’s largest and most vital coral reefs are located), but this is something we are actively investigating.”
An international team of researchers, led by scientists from Tel Aviv University, has discovered that the pathogen responsible for the mass deaths of sea urchins along the Red Sea coast is the same one responsible for mass mortality events among sea urchins off the coast of Réunion Island in the Indian Ocean. This finding raises fears that the pathogen – a waterborne ciliate - could spread further, to the Pacific Ocean. The researchers warn that this is a highly aggressive global pandemic, and are now spearheading an international effort to track the disease and preserve sea urchins, which play a crucial role in the health of coral reefs.
The study, led by Dr. Omri Bronstein from the School of Zoology at Tel Aviv University’s Wise Faculty of Life Sciences and the Steinhardt Museum of Natural History, was published in the prestigious journal Ecology.
“This is a first-rate ecological disaster,” explains Dr. Bronstein. “Sea urchins are vital to the health of coral reefs. They act as the ‘gardeners’ of the reef by feeding on algae, preventing it from overgrowing and suffocating the coral, which competes with algae for sunlight. In 1983, a mysterious disease wiped out most of the Diadema sea urchins in the Caribbean. Unchecked, the algae there proliferated, blocking sunlight from the coral, and the region shifted from a coral reef ecosystem to an algae-dominated one. Even 40 years later, the sea urchin population — and consequently the reef — has not recovered."
In 2022, the disease reemerged in the Caribbean, targeting the surviving sea urchin populations and individuals. This time, armed with advanced scientific and technological tools to collect and interpret the forensic evidence, researchers at Cornell University successfully identified the pathogen as a ciliate Scuticociliate parasite. A year later, in early 2023, Dr. Bronstein was the first to identify mass mortality events among long-spined sea urchins, a close relative of the Caribbean Sea urchins, in the Red Sea.
“Until recently, this was one of the most common sea urchins in Eilat’s coral reefs — the familiar black urchins with long spines,” says Dr. Bronstein. “Today, this species no longer exists in significant numbers in the Red Sea. The event was extremely violent: within less than 48 hours, a healthy population of sea urchins turned into crumbling skeletons. In some locations in Eilat and the Sinai, mortality rates reached 100 percent. In follow-up research, we demonstrated that the Caribbean pathogen was the same one affecting populations in the Red Sea.”
Now, using genetic tools, Dr. Bronstein and his international colleagues have shown that the same ciliate parasite is responsible for similar mortality events off the coast of Réunion Island in the Indian Ocean. “This is the first genetic confirmation that the same pathogen is present in all these locations,” he says. “Now it’s a global event, a pandemic. The Caribbean, Red Sea, and the Indian Ocean are critical regions for the world’s coral reefs, and mortality rates for sea urchins in these areas are very high — over 90 percent. As of now, we have no evidence of this pathogen in Pacific Ocean sea urchins, but this is something we are actively investigating. Although we’ve developed genetic tools for the specific identification of the pathogen, it’s difficult to monitor such rapid extinction events in the vast underwater environment. We are terrestrial creatures, and some reefs are located in deep or remote areas. If we miss the mortality event by even a couple of days, we might find no trace of the extinct population.”
To track the progression of the pandemic, Dr. Bronstein has established an international network of collaborators. He provides them with alerts about the likelihood of mortality events in their regions and sends them the necessary equipment to sample and preserve affected sea urchins for comparison with samples from other locations. These kits are then sent back to the laboratory at Tel Aviv University.
“For populations that are already infected, we really have no tools to help them,” says Dr. Bronstein with regret. “There is no Pfizer or Moderna for sea urchins — not because we don’t want one, but because we simply can’t treat them underwater. Our focus must be on two entirely different tracks. The first is prevention. To prevent further spread of the pandemic, we need to understand why it erupted here and now. We’ve developed two hypotheses for this. The first is the transportation hypothesis — that the pathogen from the Caribbean was transported by humans to new and distant regions after being carried in the ballast water of ships, infecting sea urchins in the Red Sea before spreading to the Western Indian Ocean. Incidentally, if this hypothesis is correct, we would expect to see mortality events in West Africa as well — as many cargo ships from the Caribbean stop there on their way to the Mediterranean and then through the Suez Canal to the Red Sea. Indeed, just in the past few weeks, we’ve discovered widespread mortality events in West Africa, as we predicted, and we’ve managed to obtain a limited number of samples collected during these events, which we are currently analyzing in the lab. If ships are indeed the source of the spread, then we could think of mitigation strategies. It’s not simple, and ships will never be completely sterile, but there are measures we can take. The second possibility is even more concerning: that the pathogen has always been present, and climatic changes have triggered its virulence and outbreak. That’s a challenge of an entirely different magnitude, one that we, as marine biologists, have very limited means to address.”
In parallel with global efforts, Dr. Bronstein has recently established a breeding nucleus for the affected sea urchins at the Israel Aquarium in Jerusalem, in collaboration with the Biblical Zoo and the Israel Nature and Parks Authority. This breeding population will serve as a reserve to restore affected populations and advance research into infection mechanisms and possible treatments.
“The pathogen is transmitted through water, so even sea urchins raised for research purposes in aquariums at the Interuniversity Institute for Marine Sciences and the Underwater Observatory in Eilat became infected and died. That is why we established a breeding nucleus with the Israel Aquarium, whose aquariums are completely disconnected from seawater. We genetically test the urchins transferred to the nucleus to ensure they are not carriers of the disease and that they genetically belong to the Red Sea population, enabling us to rehabilitate the population in the future. At the same time, we are using them to develop sensitive genetic tools for early disease detection from seawater samples — essentially creating ‘underwater COVID tests’ for sea urchins.”
Link to the article:
https://pmc.ncbi.nlm.nih.gov/articles/PMC11731499/
Journal
Ecology
Sea urchin mortalities on Reunion Island
Diadema group Zanzibar
The sea urchin Diadema setosum before (left) and after (right) mortality. The white skeleton is exposed following tissue disintegration and loss of spines.
Credit
Tel Aviv University.
