Ocean warming is accelerating, and hotspots reveal which areas are absorbing the most heat
A new study reveals increasing warming rates in the world’s oceans in recent decades and the locations with the greatest heat uptake
Ocean warming has accelerated dramatically since the 1990s, nearly doubling during 2010–2020 relative to 1990–2000, according to new UNSW Sydney-led research.
The study, published this week in Nature Communications, also shows some areas of the ocean are doing more of the work in heat uptake or absorption, which has implications for our understanding of sea-level rise and climate impacts.
Increasing concentrations of greenhouse gases in the atmosphere from human activity traps heat within the climate system, warming air, the land surface, the oceans, and melting polar ice. Oceans do by far the most work, absorbing more than 90 per cent of the excess human-generated heat accumulated in the Earth’s climate system, moderating atmospheric temperature rises.
While ocean warming helps slow the pace of climate change, it is not without cost, says Scientia Professor Matthew England, co-author of the study from the UNSW Centre for Marine Science and Innovation.
“The world ocean, in 2023, is now the hottest ever recorded, and sea levels are rising because heat causes water to expand and ice to melt,” says Prof. England. “Ecosystems are also experiencing unprecedented heat stress, and the frequency and intensity of extreme weather events are changing rapidly, and the costs are enormous.”
“Right now, the ocean is warming at a dramatically accelerating rate, nearly doubling during the 2010s relative to the 1990s,” says Dr Zhi Li, lead author of the study from the UNSW Centre for Marine Science and Innovation. “What we wanted to do in this study was to figure out exactly where this ocean heat uptake has been occurring.”
Hotspots of ocean heat uptake
For the study, the researchers evaluated all available observations of ocean warming activity spanning modern Argo float data – an international ocean research program that collects information using robotic instruments – to those taken in the 1950s when only sparse measurements were made from ship-borne devices. They then analysed the heat uptake across water masses and quantified each water mass’s role in ocean heat content change.
They found oceanic warming has been pervasive worldwide, spreading from the surface to the deep-sea regions known as the abyssal layers and spanning each basin from the tropics to the polar regions. However, the distribution of ocean warming by region was far from uniform.
The Southern Ocean saw the largest increase in heat storage over the past two decades, holding almost the same excess anthropogenic heat as the Atlantic, Pacific, and Indian Ocean combined. This includes two large masses of water in the Southern Ocean that combine to fill a depth range of 300 – 1500 metres.
“Melting ice caps, extreme weather, and marine ecosystems, including coral reefs, are all highly sensitive to ocean temperature changes,” says Dr Sjoerd Groeskamp, co-author of the study from the Royal Netherlands Institute for Sea Research. “It is critical we understand exactly how and where the ocean warms – both now and into the future.”
Exactly how heat uptake plays out over the coming decades and beyond remains highly uncertain. For example, if the oceans develop a reduced capacity to absorb heat, it will have profound implications for the rate of future climate change.
The scientists say their findings highlight an urgent need to increase monitoring of the global oceans, especially in remote locations like the polar oceans, as well as key regions of the subtropical and coastal seas to better understand and predict sea-level rise and impacts on marine ecosystems.
“Without Argo floats, for example, this study would not have been possible,” says Prof. England.
The team also call for more international action from big-emitting nations to meet their net zero carbon targets as soon as possible and limit the damage from uncontrolled ocean warming.
“Without any action, these net zero pledges are just meaningless,” says Dr Groeskamp.
JOURNAL
Nature Communications
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Recent acceleration in global ocean heat accumulation by mode and intermediate waters
Research explores whether coral islands could survive the impact of rising sea levels
A major international research project is to explore the potential for low-lying coral atoll islands to survive the predicted rise in sea level.
The islands, commonly found in the Indian and Pacific Oceans, are widely acknowledged to be among the world’s most vulnerable environments to climate change.
Most of them are presently predicted to be uninhabitable by the mid-21st century, but those forecasts are based on relatively simple hydrodynamic models.
A £2.8million ARISE project, funded through UK Research and Innovation’s Horizon Europe Guarantee programme, aims to improve our understanding of the processes that will threaten and preserve these island nations. It also aims to aid in the formulation, development and implementation of climate change adaptation strategies.
The project is being led by the Coastal Processes Research Group at the University of Plymouth, which has previously led studies suggesting that island ‘drowning’ may not be inevitable in the face of sea-level rise.
Gerd Masselink, Professor of Coastal Geomorphology at the University of Plymouth, is the project’s Principal Investigator. He said: “The rise in sea levels as a result of climate change is going to place many coastal communities under threat. Within that, it has largely been assumed that these coral atoll islands could just disappear. Our previous research has suggested that is not a foregone conclusion, and this project will establish the processes at play and as well as supporting the communities that call these islands home by identifying and evaluating adaptation strategies.”
The five-year project will include a series of extensive field tests in both the Maldives and the Pacific, beginning in January 2024 and continuing through 2027, using their state-of-the art coastal process research instrumentation and autonomous survey equipment.
There will also be laboratory experiments in the spring and summer of 2024 in the largest wave flume in the world - the Delta Flume at Deltares in the Netherlands.
Combined, these tests will enable researchers to explore the impact of overwashing on the islands’ beaches and any natural processes that are adding to their resilience.
The research teams will also be on standby to travel to the atoll island systems and analyse their response to cyclones and other extreme wave events.
The datasets generated through these tests will be used to develop, calibrate and validate a series of numerical models, which will then be used to evaluate how they might respond – both in the short and long-term – to sea level rise.
Much of the Coastal Processes Research Group’s previous work has been conducted with communities in the Maldives, and this project will extend this collaboration to the Pacific.
They will be working closely together with the Maldives Government, Secretariat of the Pacific Community and the Maldives National University.
The researchers also aim to work with communities and government bodies in the island nations, enabling them to implement adaptation strategies that maximise opportunities for continued habitation.
Professor Masselink added: “Atoll islands have been created over hundreds to thousands of years by ocean waves, and their future is intrinsically connected to it. The ecology of the reefs they sit on is also under threat, but their survival is critically important to the island’s survival. The big question is whether all of that can keep up with sea level rise, and answering that is crucial for both the islands and the people who live on them.”
In addition to academics and technicians from the Coastal Processes Research Group, and a number of international partners, the project has recruited six PhD candidates. They will be working with the researchers to explore the many and varied processes impacting the islands, and also the ways in which they might adapt to them.
For more about the project, and the research team leading it, visit https://www.plymouth.ac.uk/research-and-expertise/coastal-processes-research-group/arise
Investigators examine shifts in coral microbiome under hypoxia
Washington, D.C.—A new study published in Applied and Environmental Microbiology, a journal of the American Society for Microbiology, provides the first characterization of the coral microbiome under hypoxia, insufficient oxygen in the water. The research is an initial step toward identifying potential beneficial bacteria for corals facing this environmental stressor.
The researchers conducted the study because of the increasing awareness of the impact of the microbiome on host health. For example, a healthy human gut microbiome plays key roles in digestion, immune system response, and even mental health. As in humans, the coral microbiome has beneficial impacts on its host, the coral animal. These include disease prevention, nutrient uptake and resistance to environmental stressors like rising temperatures and acidification.
Despite this, scientists know far less about the role of the microbiome when corals experience hypoxia. The researchers wanted to understand how the microorganisms living on the coral's surface react to hypoxia. They thought the work may provide insights on how symbiotic microbes respond to host and environmental stress.
The researchers conducted their experiments in Bahiá Almirante, Bocas del Toro, Panama. “We picked this site because we have seen hypoxic events here associated with human activity, including agriculture and coastal development,” said lead study author Rachel Howard, Ph.D. candidate, Department of Soil, Water and Ecosystem Sciences, University of Florida. “We established experimental chambers which lowered dissolved oxygen on patches of coral reef. We then sampled the microorganisms living on corals in those chambers and corals outside the chambers after 2 days to see how the community of microbes differed with and without the stress of low oxygen.”
The researchers found that when oxygen levels dropped, the overall coral microbiome changed after only 48 hours, and the number of some specific types of bacteria increased. The bacteria that increased are those that can survive without oxygen and are ready to take advantage of a change in resources. When there is not enough oxygen in the water, it throws the community of microorganisms on the coral out of balance, and some of the suspected harmful bacteria, such as Desulfovibrionaceae or Clostridia, become more active.
“Because corals vary in their sensitivity to deoxygenation and given the crucial role of microorganisms in coral health, we suggest that changes in the microbiome may influence coral resilience to low oxygen conditions. Episodes of low oxygen, along with other impacts of climate change, pose a threat to coral and other marine organisms,” Howard said.
The researchers say that the microbiome is key to understanding the response of corals to stressors including warming seas. This study is a first step toward understanding the response of coral microbiomes to deoxygenation which, along with warming and ocean acidification, represent the “triple threat” of climate change to the ocean’s ecosystem.
The researchers plan to look more closely at the health of corals and relate that to the response of coral microbiomes when challenged by the stress of low oxygen.
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The American Society for Microbiology is one of the largest professional societies dedicated to the life sciences and is composed of 36,000 scientists and health practitioners. ASM's mission is to promote and advance the microbial sciences.
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JOURNAL
Applied and Environmental Microbiology
