June 2026

Oceanography

THE OFFICIAL MAGAZINE OF THE OCEANOGRAPHY SOCIETY

VOL. 39, NO. 2, JUNE 2026

QUANTIFYING THE ROLE OF

CLIMATE CHANGE IN DRIVING

MASS CORAL BLEACHING

ECOLOGICAL BENTHIC UNITS:

A NEW CHARACTERIZATION OF

THE GLOBAL SEAFLOOR

TOWARD OCEAN MODEL-DRIVEN

ROBOTIC EXPLORATION

REVELLE LECTURE: THE QUEST TO

MAP THE GLOBAL OCEAN

AND MORE…

IN THIS ISSUE

Oceanography | Vol. 39, No. 2

POWERING SCIENCE-BASED DECISIONS

FOR A BETTER OCEAN.

June 2026 | Oceanography

1

contents VOL. 39, NO. 2, JUNE 2026

5 QUARTERDECK. Rule Changes Related to Publications Proposed by the US Office of Management

and Budget

By E.S. Kappel

6 COMMENTARY. Argo at a Crossroads: Achievements, Impacts, and the Imperative for Sustained

Global Ocean Observations

By L.A. Dove, G. Forget, M. Hell, V. Hormann, I. Marinov, M. Messié, A. Romanou, J. Steinberg, and L.D. Talley

12 FEATURE ARTICLE. Quantifying the Role of Climate Change in Driving Mass Coral Bleaching

By A.J. Pershing, J.F. Bruno, J. Giguere, and A. Khrizman

17 FEATURE ARTICLE. Ecological Benthic Units: A New Characterization of the Global Seafloor for

Ocean Spatial Planning and Management

By P.T. Harris, D.J. Wright, K. Butler, K. VanGraafeiland, M.J. Costello, K. Howell, G. Kågesten, V. Lucieer,

M. Macmillan-Lawler, and R. Sayre

34 FEATURE ARTICLE. Toward Ocean Model-Driven Robotic Exploration

By R. Mendes, A.F. Duarte, L. Azevedo, L. Bernacchi, J.B. de Sousa, J. Pereira, B. Gabriel, J. Bogas, M. Cunha,

C.F. Rodrigues, A. Subramaniam, F. Esteves, and K. Rajan

48 ROGER REVELLE COMMEMORATIVE LECTURE. The Quest to Map the Global Ocean

By L. Mayer

62 WORKSHOP REPORT. The PACE Data Hackweek Launches a New Wave of Open Science:

How We Did It (Twice), and Why

By A.E. Windle, I. Carroll, C. Poulin, S. Caplan, and P.J. Werdell

68 DIY OCEANOGRAPHY. Unlocking a Fish Finder for Benthic Habitat Characterization

By M.P. Scherer and V. Schmidt

73 THE OCEANOGRAPHY CLASSROOM. Students Co-creating Their Community of Inquiry

By M.S. Glessmer, A.D. Årvik, and K. Daae

76 CAREER PROFILES. Gabby Ahmadia, Vice President, Seascapes and Science, World Wildlife Fund

Oceans • Shane Guan, Oceanographer, Bureau of Ocean Energy Management, and Adjunct Assistant

Professor, Department of Mechanical Engineering, The Catholic University of America

48

Oceanography | Vol. 39, No. 2

2

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WHAT GETS OUR ATTENTION

FEATURE ARTICLES (<7,000 words) provide an outlet for making significant advances in oceanography acces­

sible to a broad readership. They can include review or synthesis papers, papers that discuss new findings and

how they significantly revise our thinking about a topic, or more traditional scientific research papers.

BREAKING WAVES (<3,500 words) articles describe novel approaches to multidisciplinary problems in ocean­

ography. These provocative papers present findings that have the potential to move the field of oceanography

forward or in new directions.

OCEAN EDUCATION (<3,500 words) articles should inspire teachers in higher education to try new active,

student-centered instruction (ranging from short activities to curricula) and provide ideas/materials to do so.

DIY OCEANOGRAPHY (<3,500 words) articles share all of the relevant information on a homemade sensor,

instrument, or software tool(s) so that others can build, or build upon, it. These articles also showcase how this

technology was used successfully in the field.

MEETING/WORKSHOP/CONFERENCE REPORTS (<3,500 words) describe the goals, activities, and accom­

plishments of meetings/workshops/conferences in all aspects of ocean science.

COMMENTARIES (<3,500 words) present analyses of issue of interest to Oceanography readers, written by

experts in the field.

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of oceanography.

LEARN MORE

See the online Oceanography Author Guidelines for a full listing of manuscript categories and

descriptions, publication fees, and details of the manuscript submission process.

ON THE COVER

Acropora coral, Howland Island, central

Pacific Ocean, affected by a global

bleaching event in 2010. Image credit:

Kevin Lino, NOAA Fisheries

CONTACT US

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OCEANOPTICSCONFERENCE.ORG

OCEAN OPTICS XXVII

SEPTEMBER 13–18, 2026 | GHENT, BELGIUM

Registration is still open for Ocean Optics XXVII, the twenty-seventh edition of

the biennial international Ocean Optics Conference. This event brings together

a diverse ocean optics community, including oceanographers, limnologists,

optical engineers, Earth observation scientists, resource managers, and policy

professionals from across the globe, all united by a shared passion for optics in

aquatic environments.

June 2026 | Oceanography

3

EDITOR

Ellen S. Kappel, Geosciences Professional Services Inc.

ASSISTANT EDITOR

Vicky Cullen

DESIGN/PRODUCTION

Johanna Adams

ASSOCIATE EDITORS

Claudia Benitez-Nelson, University of South Carolina

Emmanuel Boss, The University of Maine

Philip Bresnahan, University of North Carolina Wilmington

Luca Centurioni, Scripps Institution of Oceanography

Grace Chang, Integral Consulting Inc.

Tim Conway, University of South Florida

Kjersti Daae, University of Bergen

Mirjam S. Glessmer, Lund University

Charles H. Greene, University of Washington

Alistair Hobday, CSIRO Environment

Camille Pagniello, University of Hawai‘i at Mānoa

Helen R. Pillar, University of Texas at Austin

Carol Robinson, University of East Anglia

Amelia Shevenell, University of South Florida

Robert E. Todd, Woods Hole Oceanographic Institution

Peter Wadhams, University of Cambridge

Oceanography is an open-access, peer-reviewed journal pub­

lished quarterly by The Oceanography Society. Its articles

present significant research, noteworthy achievements, and

emerging tools and technologies, and connect ocean science

with public policy and education. By publishing articles that

are accessible to a broad audience, Oceanography promotes

cross-disciplinary communication in the ocean sciences.

Oceanography (Print ISSN 1042-8275; Online ISSN 2377-617X)

is published by The Oceanography Society, 1 Research Court,

Suite 450-117, Rockville, MD 20850 USA. Oceanography arti­

cles are licensed under a Creative Commons Attribution 4.0

International License, which permits use, sharing, adaptation,

distribution, and reproduction in any medium or format as long

as users cite the materials appropriately, provide a link to the

Creative Commons license, and indicate the changes that were

made to the original content. Third-party material used in arti­

cles are included in the Creative Commons license unless indi­

cated otherwise in a credit line to the material. If the material is

not included in the article’s Creative Commons license, users

will need to obtain permission directly from the license holder

to reproduce the material. Please contact Jennifer Ramarui at

info@tos.org for further information.

Oceanography

tos.org/oceanography

The Oceanography Society was founded in 1988 to advance

oceanographic research, technology, and education, and to dis­

seminate knowledge of oceanography and its application through

research and education. TOS promotes the broad understanding

of oceanography, facilitates consensus building across all the dis­

ciplines of the field, and informs the public about ocean research,

innovative technology, and educational opportunities throughout

the spectrum of oceanographic inquiry.

OFFICERS

PRESIDENT: Paula Bontempi

PRESIDENT-ELECT: Heidi Dierssen

PAST-PRESIDENT: Deborah Bronk

SECRETARY: Allison Miller

TREASURER: Susan Banahan

COUNCILORS

APPLIED TECHNOLOGY: Alana Sherman

AT-LARGE: Leonard Pace

BIOLOGICAL OCEANOGRAPHY: Grace Saba

CHEMICAL OCEANOGRAPHY: Phoebe Lam

EARLY CAREER: Hilary Palevsky

EDUCATION: Leilani Arthurs

GEOLOGICAL OCEANOGRAPHY: Jon Lewis

JUSTICE, EQUITY, DIVERSITY, AND INCLUSION:

Onjalé Scott Price

OCEAN DATA SCIENCE: Jeremy Werdell

OCEAN SOCIAL SCIENCE AND POLICY: Eric Wade

STUDENT REPRESENTATIVE: Moronke Harris

EXECUTIVE DIRECTOR

Jennifer Ramarui

CORPORATE AND INSTITUTIONAL MEMBERS

Ganey Science

Greenwater Marine Sciences Offshore

Integral Consulting Inc.

National Oceanography Centre

Sea-Bird Scientific

Sequoia

tos.org

Oceanography | Vol. 39, No. 2

4

CELEBRATING 27 YEARS OF

ROGER REVELLE COMMEMORATIVE LECTURES

The Roger Revelle Commemorative Lecture Series was created by the Ocean Studies Board of the National Academies of

Sciences, Engineering, and Medicine to highlight the important links between ocean sciences and public policy. The series was named

in honor of the late Roger Revelle, a leader in the field of oceanography for over 50 years who spearheaded efforts to investigate the

mechanisms and consequences of climate change.

Oceanography has published these lectures annually, beginning with the first lecture in 1999. Links to the articles are provided below.

2026. The Quest to Map the Global Ocean — by Larry Mayer

2025. Oceanography in the Age of Intelligent Robots and a Changing Climate — by Chris Scholin

2023. Making Waves and Charting New Paths: A Fireside Chat with Kathy Sullivan and Margaret Leinen — by Taylor Goelz

2022. Bio-Inspired Ocean Exploration — by Nicole W. Xu and John O. Dabiri

2021. A Generational Shift in Ocean Stewardship — by Alfredo Giron-Nava and Harriet Harden-Davies

2020. The Story of Plastic Pollution: From the Distant Ocean Gyres to the Global Policy Stage — by Chelsea M. Rochman

2019. Sustainability in Deep Water: Climate Change, Human Pressures, and Biodiversity Conservation — by Lisa A. Levin

2018. Distress Signals: Historical Waypoints in Northwest Atlantic Fisheries Since 1850 — by W. Jeffrey Bolster

2017. Swells, Soundings, and Sustainability, but…“Here Be Monsters” — by Dawn J. Wright

2016. Managing Leviathan: Conservation Challenges for the Great Whales in a Post-Whaling World — by Phillip J. Clapham

2015. Overturning Assumptions: Past, Present, and Future Concerns about the Ocean’s Circulation — by M. Susan Lozier

2014. The Contemporary Challenge of the Sea: Science, Society, and Sustainability — by David M. Karl

2013. Melting Ice: What is Happening to Arctic Sea Ice, and What Does It Mean for Us? — by John E. Walsh

2012. Tsunamis: Are We Underestimating the Risk? — by Eddie Bernard

2011. Troubled Waters of the Gulf of Mexico — by Nancy N. Rabalais

2010. The Interconnected Biosphere: Science at the Ocean’s Tipping Points — by Jane Lubchenco and Laura E. Petes

2009. The Once and Future Ocean — by Paul G. Falkowski

2008. Looking Down on the Seas: How Satellites Are Revolutionizing Our Understanding of the Ocean — by Michael H. Frielich

2007. What Corals are Dying to Tell Us About CO2 and Ocean Acidification — by Ken Caldeira

2006. Disasters, Death, and Destruction—Making Sense of Recent Calamities — by Roger A. Pielke, Jr.

2004. Abrupt Climate Changes: Oceans, Ice, and Us — by Richard B. Alley

2003. Sustaining Our Oceans: A Public Resource, A Public Trust — by Admiral James D. Watkins

2002. Beyond the Freedom of the Seas: Ocean Policy for the Third Millennium — by Michael Orbach

2001. Ocean Exploration — by Marcia McNutt

2000. The Oceans and Human Health: The Discovery and Development of Marine-Derived Drugs — by Shirley A. Pomponi

1999. Contemplating Action: Storing Carbon Dioxide in the Ocean — by Peter G. Brewer

June 2026 | Oceanography

5

RULE CHANGES RELATED TO PUBLICATIONS

PROPOSED BY THE US OFFICE OF MANAGEMENT AND BUDGET

Ellen S . Kappel, Editor

ARTICLE DOI. https://doi.org/10.5670/oceanog.2026.e206

QUARTERDECK

In the United States, it has been difficult for scientists to ignore

the proposed new Office of Management and Budget (OMB) rules

related to scientific awards, international collaboration, confer­

ence attendance, and publication fees, among other activities. If

implemented as scheduled on October 1, 2026, the proposed rules

would significantly affect scientists’ ability to carry out work effec­

tively and efficiently. The Oceanography Society (TOS) has shared

these proposed rules with our membership, and TOS’s president,

Paula Bontempi, has urged everyone to submit public comments

by July 13 (see https://tos.org/omb-2026-0034). I certainly have.

Here, I focus on the proposed rules’ potential consequences for

publications.

From the Federal Register (v. 91, n. 103, May 29, 2026, p. 32232):

“OMB is revising the section to make publication costs unallowable

unless such costs are expressly required by statute or approved in

advance by the Federal agency on a case-by-case basis.”

This is a fundamental change to the current rules, which state:

“Page charges, article processing charges (APCs), or similar fees

such as open access fees for professional journal publications and

other peer-reviewed publications resulting from a Federal award

are allowable where the publications report work supported by

the Federal Government,” and the charges are levied impartially

by the journal, whether or not the research was supported under

a Federal award.

The ambiguous language makes the consequences of the pro­

posed change a bit unclear. On the one hand, it could mean that

as long as APCs are an approved item within the grant’s budget

(“approved in advance”), then authors can, as in the past, charge

the APC to their grant without needing to seek further approval.

On the other hand, the “case-by-case” wording suggests that for

every paper scientists wish to publish, they would need to ask

their agency program manager for permission to use grant funds.

If that is the government’s intent, then this requirement imposes

a huge new burden on the authors—and their program managers.

Without knowing how favorably such requests will be received, and

how quickly the program manager will respond, is it worth the time

and effort to submit such requests? Will there be a simple form and

portal to submit publication requests? Will agencies require letters

containing detailed justification for publishing? Can that letter con­

tain a request for a budget supplement to support APCs, or will

the grantee need to use funds budgeted for something else? Will

program managers need approval from a political appointee before

responding positively? If APCs cannot be paid from grants, where

will the funds to publish come from? If the burden of paying APCs

is shifted to universities, how much support will universities (and

other institutions) be able to provide to their researchers?

And, of course, if APCs are no longer permitted in federal

grants, will journals like Oceanography, which is published by a

small nonprofit society, be able to survive? Our APCs cover the

cost of humans doing work (editing, design, and web posting,

among other tasks), plus they allow us to make your work publicly

and openly accessible on the web. We do not have an advertising

base nor large donors to cover the costs of publishing and main­

taining an archive of past issues.

The text of the proposed new rule also provides OMB’s ratio­

nale for this rule change: “Publication costs are not inherently

necessary to carry out the core programmatic objectives of most

Federal awards. In many cases, such activities are discretionary,

vary widely in scope and costs, and may serve institutional, profes­

sional, or reputational interests rather than the specific objectives

of the Federal program.”

That statement represents a fundamental misunderstanding of

how science advances or an intentional misrepresentation of the

need for grants to support scientific publishing. As any scientist

knows, good experimental design relies on what you and your col­

leagues around the globe have learned from previous research, as

laid out in the peer-reviewed literature and as discussed at con­

ferences and elsewhere. Advancing the science also relies on your

colleagues’ evaluation of your experimental design and find­

ings, as shared in the peer-reviewed literature and at conferences.

Publishing results offers transparency in scientific background,

methodology, and results, and permits others to duplicate your

experiment to ensure its validity (reproducibility). Peer review and

sharing our results simply makes our science better. Importantly,

supporting APCs (and conference attendance) also allows pro­

gram managers and other agency representatives to see and evalu­

ate the results of the grants they supported.

If you haven’t already, I urge you to use this public comment

period to submit your thoughts on these proposed OMB rules,

whether you pick one or a few rules that are particularly meaning­

ful to you or you take on the whole batch of changes. With more

voices, perhaps we can make a difference.

Oceanography | Vol. 39, No. 2

6

Argo AT A CROSSROADS

ACHIEVEMENTS, IMPACTS, AND THE IMPERATIVE FOR

SUSTAINED GLOBAL OCEAN OBSERVATIONS

By Lilian A. Dove, Gaël Forget, Momme Hell, Verena Hormann, Irina Marinov, Monique Messié,

Anastasia Romanou, Jacob Steinberg, and Lynne D. Talley

COMMENTARY

The ocean, covering more than two-thirds of our planet, exerts a

profound influence on Earth’s climate system by regulating regional

climates, driving weather patterns, and absorbing and redistrib­

uting vast amounts of heat and carbon dioxide from the atmo­

sphere. Despite its central role, the global ocean remains largely

unexplored, owing both to its immense scale and to the opacity of

seawater to the electromagnetic signals commonly used in atmo­

spheric and space-based observations. As a result, satellite cover­

age is largely restricted to the ocean’s surface and upper tens of

meters. In this context, the Argo program, through an extraordi­

nary international collaboration, has emerged as a transformative

force over the past 25 years, deploying a global fleet of autonomous

underwater profilers that have delivered sustained, frequent, and

widespread measurements of the ocean’s interior dynamics and

properties (Roemmich et al., 1999, 2019; Thierry et al., 2025).

As members of the Phenomena, Observations, and Synthesis

(POS) Panel of the US Climate Variability and Predictability

Program (US CLIVAR), we are inspired to highlight Argo as a

remarkable international success story. The mission of the POS

Panel is to improve understanding of climate phenomena, inform

the needs for a sustained global climate observing system, and

guide the development of global climate-relevant datasets and

model synthesis. Our range of expertise in ocean and climate sci­

ence enables us to assert that Argo affects virtually everyone who

depends on or cares about weather prediction, ecosystem health,

and national security, both in the United States and globally. Argo’s

impact resonates across various sectors, from academic research

to practical applications in industries and government, making it a

fundamental piece of modern Earth system science.

Here, we spotlight a critical juncture regarding the future of

monitoring Earth’s biosphere with Argo. We champion continued

investment in this critical infrastructure that advances and directly

supports US federal research priorities while serving as a model for

sustained multinational scientific cooperation. Through partner­

ships between governments, academia, nonprofit research insti­

tutions, and industry, Argo supports ocean data assimilation and

modeling, workforce development in data sciences, and ongoing

exploration of the ocean and polar regions. Together, these efforts

accelerate technological and scientific innovation and result in the

delivery of applied science products to stakeholders.

KEY OUTCOMES FROM 25 YEARS OF ARGO

For over two decades, Argo has been a core component of the

Global Ocean Observing System (GOOS), coordinated interna­

tionally through the United Nations Educational, Scientific, and

Cultural Organization (UNESCO). The Argo program has trans­

formed oceanography from a data-sparse discipline into one

grounded in sustained, global observations of the ocean’s interior

ABSTRACT. Over the past 25 years, the international Argo program has become the backbone of subsurface ocean observation and

a prime example of a highly successful, sustained global scientific collaboration. Through evolving technology, the program has become

more efficient while also expanding its suite of observations, with a subset of floats equipped with biogeochemical sensors or the abil­

ity to sample below 2,000 m water depth. By delivering continuous, global measurements essential for climate science and national secu­

rity operations, Argo observations underpin advances in understanding ocean heat uptake, circulation, carbon cycling, deoxygenation,

and marine ecosystem variability, while also supporting operational forecasting, ocean reanalysis, and coupled Earth system prediction.

Here, we synthesize Argo’s scientific, operational, and societal contributions while emphasizing the importance of sustained continuity

and expansion of the observing system. We, members of the US CLIVAR Phenomena, Observations, and Synthesis Panel, argue that Argo

has become an indispensable global infrastructure whose long-term value depends on maintaining consistent global coverage and data

quality. Emerging advances in biogeochemical sensing, deep-ocean observations, artificial intelligence, and data systems provide oppor­

tunities to further strengthen the network, but continued investment and international coordination are essential to preserve and expand

humanity’s capacity to monitor and predict a changing ocean and climate system.

June 2026 | Oceanography

(Figure 1). Numerous comprehensive reviews (e.g.,  Riser et  al.,

2016; Roemmich et  al., 2019; Wong et  al., 2020; G.C. Johnson

et al., 2022; Thierry et al., 2025) have documented these advances

in detail. Rather than reiterating individual findings, we synthesize

Argo’s principal outcomes across three domains: scientific discov-

ery, operational capability, and education (Figure 2).

Argo has fundamentally reshaped understanding of the ocean’s

role in the climate system by providing continuous, global mea-

surements of temperature and salinity in the upper 2,000 m of

the ocean. These observations underpin quantification of key cli-

mate indicators, including ocean heat content, sea level rise, and

changes in the global hydrological cycle. More recently, through

the Biogeochemical-Argo (BGC-Argo; Claustre et al., 2020) and

Deep Argo (Zilberman et  al., 2023) expansions, data coverage

has been extended to include biogeochemical parameters and the

full ocean depth. Together, the core observations of temperature

and salinity, BGC-Argo, Deep Argo, and proposed polar expan-

sions of Argo are referred to as “OneArgo” (Thierry et al., 2025).

With its expanded capabilities, Argo has enabled advances in

understanding full-depth ocean circulation and mixing, while

also transforming observation of the ocean carbon system,

biological productivity, and oxygen distribution (Claustre et  al.,

2020; G.C. Johnson et al., 2022). As highlighted in prior review arti-

cles, Argo now provides the dominant share of global subsurface

observations for many essential climate variables (Figure 1), estab-

lishing an internally consistent benchmark for evaluating Earth

system variability and change.

Beyond its scientific contributions, Argo provides a cornerstone

for climatology, operational oceanography, and environmental pre-

diction. Argo profiles are routinely assimilated into ocean and cou-

pled Earth system models to constrain initial conditions, reduce

uncertainty, and improve forecasts spanning short-range weather

to seasonal and decadal climate variability (Figure 3). These obser-

vations enhance prediction of phenomena such as the El Niño-

Southern Oscillation (ENSO), marine heatwaves, and tropical

cyclone intensity, and are operationally integrated into forecast-

ing systems at US agencies that include the National Hurricane

Center of the National Oceanic and Atmospheric Administration

(NOAA; Halliwell et al., 2017) and the US Navy (Chen et al., 2017),

as well as those at international modeling centers (Balmaseda

et al., 2024). Argo data are also central to ocean reanalyses, pro-

viding coherent reconstructions of the evolving ocean state and

FIGURE 1. Status and contribution of the

Argo network. (a) Dots represent oper-

ating floats contributed to the Argo net-

work by various countries as of May

2026. (b) Oxygen profiles are plotted per

year in the NOAA World Ocean Database

collected by Argo as opposed to tradi-

tional observation methods (bottle, CTD).

Panel (a) reproduced from ocean-ops.org.

Panel (b) reproduced from Thierry et  al.

(2025), under CC BY license

Oceanography | Vol. 39, No. 2

8

critical benchmarks for climate models; in many regions, Argo is

the dominant source of subsurface ocean observations for these

systems. Its global, internally consistent data record further sup­

ports emerging essential satellite measurement calibration and

bias correction efforts, as well as artificial intelligence and machine

learning (AI/ML) approaches that rely on large training datasets to

improve prediction of ocean and climate variability (Forget, et al.,

2015; Jiménez-Esteve et al., 2025). At the same time, the growing

biogeochemical Argo record enables detection of declining ocean

oxygen and hypoxic regions, with implications for marine eco­

systems, fisheries management, and nitrogen cycling (Claustre

et al., 2020; Thierry et al., 2025).

Finally, Argo’s openly accessible, near-real-time data have also

had a profound impact on education and workforce development.

The program has supported hundreds of graduate theses across

oceanography, climate science, and related fields (Argo, 2024) while

also serving as a widely used resource in undergraduate curricula.

At the K–12 level, initiatives such as Adopt-a-Float and curated

educational materials have enabled primary and secondary school

students worldwide to engage directly with real ocean data, foster­

ing early interest in Earth science, engineering, and data literacy.

More broadly, Argo exemplifies the value of open, international

scientific infrastructure, providing equitable access to high-quality

environmental data and enabling participation from a global com­

munity of researchers, educators, and students.

ARGO’S RETURN ON INVESTMENT

Given its immense and wide-ranging impacts (Figure 2), Argo

delivers unparalleled value for its cost. In the last 25 years, Argo

floats have collected over three million temperature and salinity

profiles, greatly exceeding the number of subsurface (>1,000 m)

observations available from the historical record prior to Argo

(Wong et al., 2020; Thierry et al., 2025). BGC-Argo has helped fill

the gap in oxygen data created by declining shipboard measure­

ments (Figure 1b). This prolific output is achieved at a fraction

of the traditional cost: producing one core Argo profile costs less

than $250 (in present-day US dollars) compared to over $10,000

for one ship-based temperature and salinity profile (Jayne et al.,

2017). Likewise, a biogeochemical profile by BGC-Argo costs

between $800 and $900, while collecting an equivalent suite of

biogeochemical observations from a research vessel typically costs

orders of magnitude more due to ship time and analytical expenses

(K.S. Johnson and Claustre, 2026).

The value of these observations extends across sectors, support­

ing applications in weather forecasting, fisheries, shipping, and

national security. While difficult to quantify precisely, Argo’s cen­

tral role in GOOS implies economic benefits on the order of hun­

dreds of millions of dollars annually (Kite-Powell, 2009). Crucially,

no other observing system provides sustained, global subsurface

coverage at comparable scale and cost. Satellite (Barale et al., 2010)

and ship-based (Sloyan et al., 2019) observations are complemen­

tary but cannot replace this capability; a resilient ocean observ­

ing enterprise depends on the integration of all these observa­

tional data types.

Argo’s commitment to free and open data further amplifies its

impact, ensuring that these observations are immediately accessi­

ble worldwide and fully utilized across scientific, operational, and

societal applications (Roemmich et al., 1999). The freely available

data guide actionable science such as detection of extreme weather

and early warning signals for disaster response, maritime safety,

and global shipping route optimization, as well as aquaculture and

fisheries management. Open access to high-quality ocean observa­

tions also helps expand scientific capacity in developing countries,

where freely available data are essential for advancing research

programs, supporting education and workforce development, and

enabling participation in the global ocean observing community.

FIGURE 2. Operational, societal,

and scientific goals and approaches

enabled by Argo. Research themes

include measurements of essential

Earth system variables, both those

directly observed and those calcu­

lated. Operational themes include

capacity building and technological

breakthrough potentials. Societal

themes include international and

national priorities for safety, secu­

rity, and cooperation.

Ocean Carbon

Cycling &

Acidification

Hydrological

Cycle Shifts &

Extremes

Ocean

Circulation

Changes

Ocean

Deoxygenation

Net Community

Production

Characterization

Ocean Heat

Uptake

Major Themes of Argo

Improved

Weather

Forecasts

Sea Level Rise

Assessment

Sensor

Technology

Advancement

Marine

Resources

Management

Cost-Effective

Monitoring

Global

Cooperation &

Education

Early Warning

for Disaster

Preparedness

National

Security &

Operations

Research

Operational

Societal

June 2026 | Oceanography

9

THE NECESSITY OF CONTINUITY

TO THE ARGO MISSION

Sustained continuity is essential to Argo’s value. The program’s

impact depends on not only global coverage but also the consistency

of its long-term record. Interruptions in sampling or shifts in mea­

surement systems risk degrading the climate data record in ways

that cannot be recovered retrospectively (Lidström and Wickberg,

2025). Reduced coverage below scientifically determined targets

for the Argo program—Core Argo (2,500 floats), Biogeochemical

Argo (1,000 floats), and Deep Argo (1,200 floats)—will dimin­

ish the system’s ability to resolve seasonal-to-interannual variabil­

ity, including early warning signals for extreme events. It will also

weaken constraints on ocean heat and carbon uptake and increase

uncertainty in estimates of large-scale circulation and water mass

changes (G.C. Johnson et  al., 2015; Chamberlain et  al., 2023).

Because the configuration of the Argo array is already minimal for

meeting climate-scale objectives (Roemmich et al. 2009), further

reduction would not only degrade resolution but also risk crossing

thresholds beyond which key signals become aliased or undetect­

able (Sivareddy et al., 2017; Gasparin et al., 2023). Maintaining full

coverage is not an aspirational goal but a requirement for preserv­

ing the integrity, usability, and long-term societal value of GOOS.

Reduction of the Argo program would undermine basic as well

as actionable research and their critical roles in protecting ecosys­

tems and societies.

The continuity of Argo is critical because systems, such as fore­

casting and reanalysis, and emerging AI/ML applications, depend

on Argo as their primary source of subsurface observations (Forget

et al., 2015; Lellouche et al., 2021). Disruptions would propagate

directly into reduced forecast skill and increased uncertainty. The

implications of a reduced Argo program extend beyond science

to economic stability and national security. Given the reliance on

Argo data across a wide range of economic sectors, from shipping

and fisheries to disaster preparedness and national defense, gaps in

coverage represent a form of strategic vulnerability. Argo provides

not only observations of the ocean but also the basis for decisions

that affect billions of dollars in economic activity and public safety.

Ensuring continuity is therefore essential to maintaining reliable

environmental intelligence in a changing climate.

The continued development and deployment of new technol­

ogies, including improved sensors that can expand observational

capabilities, increase accuracy, and reduce costs, are key aspects of

the Argo system. However, given the importance of data continu­

ity, advances in sensor technology must be integrated carefully. The

strength of the Argo program lies in the stability and comparabil­

ity of its long-term record, which depends on consistent measure­

ment practices across decades (Roemmich et al., 1999; Riser et al.,

2016; Thierry et al., 2025). Introducing new sensors inevitably risks

introducing biases, calibration offsets, or shifts in data character­

istics that can mask real climate signals. To avoid this, sensor evo­

lution must be accompanied by careful cross calibration, overlap

between old and new technologies, and rigorous validation against

reference standards (World Meteorological Organization, 2015;

Biogeochemical-Argo Planning Group, 2016). At the same time,

failing to adopt improved sensors would limit the system’s abil­

ity to address emerging risk factors and scientific questions, from

ocean carbon uptake to deoxygenation (Ito et al., 2026). The chal­

lenge, therefore, is not to choose between consistency and innova­

tion, but to manage their coexistence and intensify effort, ensur­

ing that technological progress strengthens the observing system

without compromising sustainability or the integrity of the envi­

ronmental record it is designed to sustain.

FIGURE 3. Temperature correction

estimated from Argo float observa­

tions for depths of 300–700 m. This

correction represents how model

temperatures are adjusted over time

to better match real-world ocean

measurements, as applied within

the ORAS4 ocean reanalysis (a com­

bined

model-observation

ocean

reconstruction). Figure reproduced

from Balmaseda et al. (2013)

100°E

50°S

0°

50°N

160°W

60°W

Temperature Bias Correction from Argo (°C hr–1)

Oceanography | Vol. 39, No. 2

10

NEW FRAMEWORKS FOR AN

INDISPENSABLE PROGRAM

Meeting the research, operational, and educational demands of

the coming decades may require rethinking Argo not simply as a

research program, but as an enduring infrastructure. Currently, in

the United States, which funds half of the international Argo pro­

gram, it is funded through National Science Foundation (NSF),

National Aeronautics and Space Administration (NASA), and

NOAA research programs rather than through a dedicated long­

term operational infrastructure line. As reliance on Argo data

grows across forecasting, climate monitoring, and decision-making

systems, its sustained operation must be treated with the same pri­

ority as other core environmental observing networks. This shift

in framing from project-based funding toward long-term infra­

structure investment would provide the stability needed to main­

tain continuity while enabling strategic and sustainable innovation

(NASEM, 2017; European Marine Board, 2021).

Recognizing the need for a more scalable global observing net­

work, the Argo community is advancing efforts to streamline tech­

nical infrastructure and operations. Argo-affiliated international

partners, data system developers, and observing system engi­

neers have already begun to develop new technical frameworks

to improve efficiency and scalability. These efforts include mod­

ernizing data management pipelines, integrating cloud-based and

AI-enabled workflows, improving interoperability across observ­

ing platforms, and developing more autonomous quality con­

trol systems to accommodate the rapidly growing volume and

complexity of ocean observations. Efforts to standardize com­

munication interfaces and data systems across float platforms

are reducing operational complexity and lowering costs while

also improving interoperability and data accessibility within the

broader GOOS program. These advances position Argo to more

seamlessly incorporate new technologies without fragmenting

the observing network.

Emerging AI/ML approaches in data assimilation and ocean

modeling are creating new pathways for scientific and opera­

tional advances, with Argo positioned as a primary source of the

observations needed to train, constrain, and evaluate these next-

generation systems. Throughout the program’s life, Argo observa­

tions have been routinely integrated into operational and research

forecasting frameworks, underpinning ocean reanalyses and cou­

pled climate predictions. Advances in AI/ML therefore build on

an already mature assimilation infrastructure, offering pathways

to improve how Argo data are used, potentially through more effi­

cient data assimilation, model optimization, improved error char­

acterization, and the capacity for adaptive sampling strategies that

respond to evolving ocean conditions. Looking ahead, develop­

ments in high-performance computing systems and increased use

of graphics processing units (GPUs) may further enhance weather

prediction and ocean state estimation systems that already rely

heavily on Argo. Rather than requiring a fundamental shift, these

innovations position Argo to remain central to a new generation

of forecasting systems that will support a more flexible and respon­

sive observing network while preserving the continuity and reli­

ability that make it indispensable.

MAINTAINING UNITED STATES LEADERSHIP

IN A GLOBAL SUCCESS STORY

Argo’s path has been one of remarkable success and collaboration,

but the journey ahead demands critical decisions, renewed com­

mitment, and potentially new organizational frameworks. In our

view, augmenting Argo, not just maintaining it, is of the utmost

importance for science and society. By extending observations to

the full depth of the ocean, sustaining the newly built global BGC

float array, and enhancing coverage in polar regions, the proposed

expansion to a complete OneArgo by 2030 (Thierry et al., 2025)

would transform our mechanistic and quantitative understand­

ing of the world ocean as well as our ability to predict its future

evolution. This evolution is not merely an enhancement of exist­

ing capabilities but a necessary leap to address emerging scientific

questions and environmental risks while continuing to support

existing data-assimilating and machine-learning infrastructure.

Recently in the United States, leaders of the ocean science and

technology agencies on the National Science and Technology

Council have reaffirmed the value that data from Argo floats bring

to all agency missions (National Science and Technology Council

Subcommittee on Ocean Science and Technology, 2024). This

reaffirmation presents an opportunity for the United States to

strengthen its commitment, for fellow leaders in the Argo com­

munity (the European Union, United Kingdom, Japan, Canada,

India, Australia) to expand their investment in the program, and

for emerging leaders to play a larger role in this global initiative.

As we look ahead, the future of Argo presents several possi­

ble paths. In an optimistic scenario, sustained and reliable fund­

ing would support a technologically advanced, publicly accessi­

ble network that delivers unparalleled insights into our ocean and

its societal impacts. By contrast, maintaining the status quo with

flat or even decreased funding risks stagnation and an erosion

of scientific and operational capacities. Because Argo is already

deeply embedded in operational forecasting and climate monitor­

ing frameworks, such degradation would directly reduce predic­

tive skill for weather, climate, and ocean hazards, thus increasing

the risk to humanity, societies, and individuals. If the United States

does not rise to the challenge, international partners will need to

shoulder the responsibility of sustaining momentum and shaping

the next era of global ocean observations.

At this critical juncture, Argo represents a story of great past

achievements through strong US involvement and international

collaboration as well as expected future accomplishments with

renewed dedication and financial support. The path we choose will

determine not only the future of global ocean measurements but

also our ability to understand and respond to the challenges posed

by a changing climate and environment.

June 2026 | Oceanography

11

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ACKNOWLEDGMENTS

Any opinions or views expressed or implied herein are those of the authors and do

not necessarily reflect the views of NASA, NOAA, the Department of Commerce, or

their individual employers. The authors thank Alyssa Johnson, Alyssa Cannistraci, and

Mike Patterson of the US CLIVAR program office for their assistance in the prepara­

tion of this manuscript.

AUTHORS

Lilian A. Dove (dove@gatech.edu), School of Earth and Atmospheric Sciences,

Georgia Institute of Technology, Atlanta, GA, USA. Gaël Forget, Department of

Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,

Cambridge, MA, USA. Momme Hell, Woods Hole Oceanographic Institution,

Woods Hole, MA, USA. Verena Hormann, Scripps Institution of Oceanography,

University of California San Diego, La Jolla, CA, USA. Irina Marinov, Department of

Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA.

Monique Messié, Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA.

Anastasia Romanou, NASA-Goddard Institute for Space Studies, New York, NY, USA.

Jacob Steinberg, NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ,

USA. Lynne D. Talley, Scripps Institution of Oceanography, University of California

San Diego, La Jolla, CA, USA.

ARTICLE CITATION

Dove, L.A., G. Forget, M. Hell, V. Hormann, I. Marinov, M. Messié, A. Romanou,

J. Steinberg, and L.D. Talley. 2026. Argo at a crossroads: Achievements, impacts, and

the imperative for sustained global ocean observations. Oceanography 39(2):6–11,

https://doi.org/10.5670/oceanog.2026.e207.

COPYRIGHT & USAGE

This is an open access article made available under the terms of the Creative

Commons Attribution 4.0 International License, which permits use, sharing, adapta­

tion, distribution, and reproduction in any medium or format as long as users cite the

materials appropriately, provide a link to the Creative Commons license, and indicate

the changes that were made to the original content.

Oceanography | Vol. 39, No. 2

12

QUANTIFYING THE

ROLE OF CLIMATE CHANGE IN DRIVING

MASS CORAL BLEACHING

FEATURE ARTICLE

ABSTRACT. Four global mass coral bleaching events have now been recorded. The first, in 1998, was associated with that year’s strong

El Niño. Subsequent global-scale bleaching occurred in 2010 and then over the periods 2014–2017 and 2018–2025 (and likely ongoing).

All these events also coincided with El Niños for at least part of the period. While El Niño is an important factor, each event has also

occurred during a period of rising global temperatures. Here, we use attribution of daily sea surface temperatures to show that the most

recent global coral bleaching event is entirely due to human-caused warming. Furthermore, all previous events, including the original one

in 1998, had strong fingerprints of anthropogenic climate change. Our analysis shows that global warming is the main driver of bleach­

ing and that global coral bleaching simply would not occur without human-caused climate change.

By Andrew J. Pershing, John F. Bruno, Joseph Giguere, and Alexandra Khrizman

Coral reefs are the most diverse marine ecosystems, housing

approximately 32% of all named marine species (Fisher et  al.,

2015). Reef-building corals have adapted to their local annual

cycle of environmental temperature, but when temperatures

exceed these conditions for several weeks, bleaching or even death

can occur (Hoegh-Guldberg, 1999). Extensive mortality leads to a

transformation in the ecosystem from one dominated by coral to

one dominated by algae, sponges, or other taxa (Norström et al.,

2009), and this is typically accompanied by a loss of species rich­

ness and diversity (Bellwood et al., 2004).

Global mass coral bleaching events—coincident bleaching across

many reefs around the world—have occurred periodically since

1998 (Goreau et al., 2000). These events, which also occurred in

2010, 2014–2017, and 2018–2025 (Spady et al., 2026), all coincided

wholly or in part with El Niño conditions, making it difficult to iso­

late the influence of climate change. Here, we use attribution of daily

ocean temperatures to quantify the role of anthropogenic ocean

warming in driving global mass bleaching of the world’s coral reefs.

ISOLATING THE SIGNAL OF CLIMATE CHANGE

IN OCEAN TEMPERATURE RECORDS

Our ability to detect climate change at finer spatial and tempo­

ral scales has increased in the last decade. This is most apparent in

the advent of extreme event attribution (Otto, 2017; Swain et al.,

2020), which has been used to diagnose the contribution of cli­

mate change to individual heatwaves on land (Stott et al., 2004)

and in the ocean (Hope et  al., 2024), extreme rainfall events

(Van Oldenborgh et al., 2017; Reed et al., 2022), and economic

damages from coastal flooding (Strauss et al., 2021).

Gilford et al. (2022), extended extreme event attribution meth­

ods (e.g., Otto, 2017; Swain et al., 2020) to daily air temperatures.

Their approach integrates information from climate models run

with and without anthropogenic greenhouse gas forcing and from

the observed local response to changing global temperatures.

Giguere et  al. (2024) applied this same multi-method approach

to sea surface temperatures from NOAA’s Optimum Interpolation

Sea Surface Temperature (OISST) long-term Climate Data Record

(Reynolds et al., 2007). The Gilford-Giguere approach reaches sim­

ilar conclusions about how climate change increased the likelihood

of terrestrial heatwaves (Gilford et al., 2024b) and marine heat­

waves (Giguere et al., 2024) as did other studies (e.g., Laufkötter

et al.’s [2020] attribution of marine heatwaves). This approach has

an additional advantage in that it can be easily automated and

applied to any day’s temperature, not just extremes.

This daily, local-scale perspective provides new capabilities for

considering that climate change has and is impacting marine eco­

systems such as coral reefs. For this study, we use Giguere et al.’s

(2024) approach to assess the influence of climate change on sea

surface temperatures, specifically using NOAA’s OISST (Reynolds

et al., 2007). This approach is built around a statistical reconstruc­

tion of the distribution of daily temperatures expected at a location

and time of year under a specific global climate condition (detailed

methods are provided in the online supplementary material). We

use a smoothed version of HadCRUT5 (Morice et al., 2021) global

mean temperature (GMT) as our estimate of the global conditions.

We refer to this as GMT15 because it approximates a centered

15-year running average. Global temperatures are reported relative

to the 1850–1900 baseline used by the Intergovernmental Panel on

Climate Change (IPCC). For example, assessing a temperature in

2025 would contrast the distribution under GMT15 = 1.35°C with

the counterfactual (no global warming) distribution represented

by GMT15 = 0°C.

June 2026 | Oceanography

13

We divide the year into 24 periods. We then use output from

13 climate simulations and two empirical approaches to charac­

terize the local temperature distributions for each period and how

they change with GMT15. We downscale and debias each cli­

mate model dataset based on the 1991–2020 period. We then fit

a skew-normal distribution to the 30 years of daily output when

each model’s global temperature anomaly first crosses the desired

GMT15 value. We also fit a distribution to the model’s control run

without anthropogenic greenhouse gas forcing. This produces

13 paired distributions.

The Gilford-Giguere approach also uses two empirical meth­

ods to estimate how local temperatures respond to changes in

GMT. The first empirical method uses linear regression to estimate

how the median daily temperature responds to a change in GMT.

The slope of this regression, called the scale factor, is used to shift

the skew-normal distribution built with data from 1991 to 2020

(GMT15 = 0.88°C) to the climate of the observations and to the

counterfactual climate. The second empirical method calculates

the scale factor for 21 evenly spaced quantiles between 0.01 and

0.99. This allows for the variance and skewness of the distribution

to change in response to global forcing.

For impact studies, we need to estimate the counterfactual tem­

perature, not the probability. Following Giguere et al. (2024), we

define the counterfactual temperature (Tcf ) as the temperature with

the same exceedance probability in the GMT15 = 0° distribution as

the observed temperature (T) has in its climate. We calculate Tcf for

each of the 13 climate models and average them. We then calcu­

late Tcf for the two empirical models and average. Finally, we aver­

age the two averages. This gives us two sets of temperatures, one

corresponding to the observations and another corresponding to

the observations but with the influence of climate change removed.

These can then be used to drive a process model, allowing us to esti­

mate how climate change has influenced that process. For example,

Gilford et al. (2024a) applied a model of hurricane maximum inten­

sity to daily sea surface temperature counterfactuals to identify the

role of climate change in increasing Atlantic hurricane intensity.

ATTRIBUTION OF CORAL BLEACHING

For this study, our process of interest is the risk of coral bleach­

ing. We applied the bleaching risk model developed by NOAA’s

Coral Reef Watch (CRW) program (Liu et al., 2014; Skirving et al.,

2019) to both the observed and the counterfactual temperatures.

This model is based on degree heating week (DHW) accumulation

above a local maximum monthly mean climatological tempera­

ture. We calculated the maximum monthly mean climatology for

OISST using the same years (centered on the year 1998.5) as CRW.

We mapped the 12-week DHW onto risk levels (Heron et al., 2016;

Manzello et al., 2025): moderate (DHW ≥4°-week) implies a risk of

reef-wide bleaching, severe (DHW ≥ 8°-week) translates to a risk

of mortality of sensitive species, and extreme (DHW ≥ 12°-week)

indicates risk of multi-species mortality. To highlight patterns at

lower DWH, we split NOAA’s “low” category into two bins: very

low (DHW < 1°-week) and low (1°-week ≤ DHW < 4°-week) cor­

respond to a risk of possible bleaching. For 100 coral reef contain­

ing regions, we calculated the annual maximum value (DHWmax)

for the years 1982–2025.

Spady et al. (2026) aggregated reports of bleaching during the

most recent global bleaching event. For each of the 71 regions

where they determined that bleaching had occurred, we found the

maximum DHWmax over the period 2018–2025. Consistent with

their analysis, we find that bleaching risk was elevated in all the

regions they identified (Figure 1a). Risk was most intense in the

southern Caribbean and along the Central and South American

coasts. Throughout this region, DHWmax reached extreme levels

(DHWmax ≥ 12°C-weeks) at which large-scale mortality is expected

and has been documented in some regions (Manzello et al., 2025).

FIGURE 1. Bleaching risk attributable to anthropogenic climate change for the fourth global mass coral bleaching event. (a) Risk levels based on maximum

degree heating weeks (DHWmax) for 71 regions where bleaching was reported during 2018–2025 (Spady et al., 2026). The extreme risk level is split into three

sublevels to highlight the gradation. (b) Risk levels calculated using counterfactual temperatures. (c) Plot of observed (circles) and counterfactual (squares)

DHWmax. The regions are ranked based on the observed values and colored according to risk levels as in panel a. The DHWmax values for each region are in

online supplementary Table S1.

30°S

0°

30°N

180°

150°W 120°W

90°W

60°W

30°W

0°

30°E

60°E

90°E

120°E

150°E

180°

30°S

0°

30°N

20

40

60

Rank

12

16

20

24

Degree Heating Weeks (°-week)

RISK LEVEL

Very Low

Low

Moderate

Severe

X1

X2

X3

Oceanography | Vol. 39, No. 2

14

Other regions that reached the extreme level include the Red Sea,

central and eastern Polynesia, and the coast of Japan. Risk reached

at least moderate levels (DHWmax > 4°C-weeks) in all other regions,

a risk level associated with reef-wide bleaching.

Our analysis indicates there would be essentially no coral

bleaching and certainly not a global-scale event without human-

caused climate change. Based on DHWmax calculated with counter-

factual temperatures, we find a substantial reduction in the risk

of bleaching (Figure 1b). Only one region, the central Kuroshio,

had moderate bleaching risk in the counterfactual climate. This

region had an observed DHWmax value exceeding 17.4°C-weeks,

well above the extreme risk threshold (Figure 1c). All the remain­

ing regions had little temperature-based risk, including 51 regions

with DHWmax < 1°C-week.

The influence of climate change is also apparent in previous

global coral bleaching events. During each of these events, at

least 40 regions were at moderate risk (Figure 2). In 1998, only

one of these regions had bleaching risk that was not attributable,

meaning that the region reached the moderate risk level under

the counterfactual conditions (in the absence of global warming).

For all other global events, the other regions would not have had

this level of risk in the counterfactual climate. The rising trend in

the number of regions experiencing each risk level suggests that

many regions are now experiencing stressful temperatures multi­

ple times in a decade.

GLOBAL WARMING VS. EL NIÑO

The attributability of the 1998 event is especially noteworthy

given the perceived importance of El Niño in that event (Hoegh-

Guldberg, 1999; Bruno et  al., 2001). Our approach to calculat­

ing the counterfactual temperature is designed to account for

natural variability from the El Niño-Southern Oscillation (ENSO)

and other sources. It does this by characterizing the distribution

of temperatures during a 30-year reference period (in this case,

1991–2020) and then shifting that distribution using the observed

trends and climate models (Giguere et al., 2024). This means that

the counterfactual temperatures for 1998 or any El Niño year

would reflect El Niño conditions in the absence of warming.

To further explore the hypothesis that global warming and not

El Niño is the main driver of large-scale bleaching, we use linear

regression to fit a statistical model of DHWmax for each region:

DHWmax(year) = a*GMT15(year) + b*ONI(year-L) + c,

where ONI(year) is the monthly Oceanic Niño Index (Glantz and

Ramirez, 2020; NOAA, 2025) from the month when the region’s

maximum monthly climatological temperature occurs. The param­

eter L is the lag in months (from 0 to 12) that gives the best fit.

We found 23 regions where only the temperature term was signif­

icant and 74 out of 100 regions where both the global temperature

and ONI effects were significant (Figure S1 in the online supple­

mentary material). For the regions where both factors were signif­

icant, we calculated GMTeqiv = 2*b/a (i.e., the global mean tem­

perature that leads to a change in DHWmax comparable to a strong

El Niño; ONI = 2).

According to the statistical modeling, the effect of global tem­

perature became stronger than El Niño in half the regions where

both factors are significant when global mean temperature was only

0.5°C above the preindustrial (Figure 3a, blue curve). These con­

ditions occurred in the year 1989. In 1998, during the first global

bleaching event, the global mean temperature was 0.72° above

preindustrial conditions, and warming was stronger in 61 out of

74 regions. Under current conditions in 2025 (GMT15 = 1.35°),

only three regions, Cocos Islands, Northern Galápagos Islands,

and Line Islands, have a stronger El Niño effect (Figure 3b). Under

current warming rates (SSP3-7.0), warming will outweigh ENSO

in all regions where both factors are significant by 2028.

The statistical models also demonstrate that climate change has

been the main driver of coral bleaching risk over the last 40 years.

By 2000, when GMT15 = 0.77°C, the effect of warming emerged

from natural variability in half of the 95 regions with significant

GMT15 terms (Figure 3a). In the current climate of 1.35°C, forc­

ing from global temperature exceeds natural variability in 93% of

regions (90/97). All regions are expected to exceed their natural

variability by 2040 (Figure 3c).

DISCUSSION

Both the attribution analysis and the linear modeling indicate

that rising greenhouse gas emissions and the ensuing rise in

global temperatures are the main drivers of mass coral bleach­

ing. Without human-induced climate change, reef-wide bleach­

ing would be exceedingly rare, and global bleaching events simply

would not occur. According to our analysis, large El Niño events

add to the risk from long-term warming, but temperature changes

FIGURE 2. Number of coral regions exposed to elevated risk of bleaching.

(a) Number of regions reaching a particular bleaching risk level (indicated by

colors as in Figure 1a). The dashed lines show the number of regions where

the risk is attributable to human-induced warming, meaning that the region’s

DHWmax calculated with observed temperatures is above the threshold but

the value using the counterfactual temperatures is below the threshold.

(b) The number of regions at each risk level in 2024 re-calculated using the

1992–2023 baseline.

1980

1990

2000

2010

2020

Year

20

40

60

80

100

Number of Regions

Attributable Risk

Low

Moderate

Severe

Extreme

2024

June 2026 | Oceanography

15

from El Niño would not be strong enough on their own to result in

widespread bleaching.

The main advance in our study is the ability to quantify the

influence of global warming and to isolate it from natural climate

variability, including El Niño events. The multi-method, daily

approach developed by Giguere et  al. (2024) is currently being

used to provide operational assessments of the influence of climate

change on sea surface temperatures (see https://csi.climatecentral.

org/ocean). This approach provides a convenient pathway for gen­

erating time series of counterfactual temperatures that can be used

for impact attribution. The main requirement for impact attribu­

tion is a model linking sea surface temperature with the process of

interest. Marine species distributions (e.g., Morley et al., 2018) and

fishery population dynamics (e.g., Pershing et al., 2015) have all

been modeled using sea surface temperature and would be partic­

ularly suited to attribution studies.

The DHW risk model follows from the understanding that

reef-building corals are adapted to the prevailing conditions in

their regions. Exposure to stressful temperatures can cause bleach­

ing; however, it can also lead to physiological acclimatization and

genetic adaptation, and at the community level, to the loss of the

species most sensitive to warming. This may explain why some

reefs do not bleach when exposed to conditions that they have

previously experienced. For example, in the Chagos Archipelago,

high DHW values in 2005 and 2010 were comparable in magni­

tude to the 1998 event, yet coral cover did not decline and instead

increased until 2015 (Sheppard et al., 2017).

Ultimately, a region will only be able to retain a coral reef eco­

system if the rate of adaptation (defined broadly to include pro­

cesses ranging from acclimatization and genetic changes to species

replacement) is faster than the rate of warming (Pershing et al.,

2019). Understanding the process of adaptation to rising tempera-

tures is the defining goal of climate change ecology (Witman et al.,

2023). The rate of adaptation in coral reef communities should be

slow due to the long lifespan and slow growth of most reef-building

corals and will be further exacerbated in isolated reefs where spe­

cies replacement is slower.

Defining bleaching risk relative to the original baseline period

echoes the fixed-baseline definition of marine heatwaves (Hobday

et al., 2016). Jacox (2019) argued for defining marine heatwaves

after removing the long-term trend. From the perspective of marine

ecology, removing the trend assumes that adaptation processes

are fast enough to keep pace with the trend. Coral communities

lie somewhere between these perspectives. If we shift the baseline

period for the DHW calculations from the standard NOAA period

(mean year = 1982.3) to the most recent 30 years (mean year =

2008.5), there is a reduction in the number of reefs at each risk level

(Figure 2b); however, the number of reefs exposed in each category

remains high and is similar to 1998 and 2010. In other words, we

would still expect a significant global mass bleaching event.

Our analysis suggests that anthropogenic warming was already

driving coral bleaching in the 1990s, the period when the scien­

tific consensus on human-caused climate change was coming into

focus. The failure to reduce greenhouse gas emissions after that

time led to an additional 0.7° of warming, increasing the expo­

sure of reef ecosystems to dangerously warm temperatures. The

International Union for Conservation of Nature (IUCN) has doc­

umented that 1,008 species have gone extinct since 1600, most due

to human activities (IUCN, 2025). Continued emissions and addi­

tional warming raise the potential that human actions may soon

eliminate coral reef ecosystems as we know them from the planet.

SUPPLEMENTARY MATERIALS

The supplementary materials are available online at https://doi.org/10.5670/

oceanog.2026.e210.

FIGURE 3. Drivers of bleaching risk.

(a) The number of regions (expressed

as a percentile) for which the influence

of global mean temperature (GMT) on

DHWmax exceeds that of a strong El Niño

(blue dots) or natural variability (orange

dots). The year when the equivalent GMT

value was or is expected to be reached is

indicated on the right. The GMT values for

the start of the four global mass bleach­

ing events are indicated in bold. One set

of thin lines indicates the equivalent GMT

when the GMT effect is stronger in 50% of

regions. The other set highlights the pro­

portion of regions where GMT is stron­

ger in 2025 (current climate of 1.35°C).

The maps show the decade in which the

effect of GMT exceeds that of El Niño (b)

or natural variability (c).

0.0

0.2

0.4

0.6

0.8

1.0

Percentile

Equivalent GMT (°C)

GMT > El Niño

GMT > Variability

Corresponding Year

1980

2000

2020

2040

0.72

0.96

1.04

1.15

1998

2010

2014

2018

0.0

0.5

0.8

1.0

1.3

1.5

2.0

1880

1989

2002

2012

2024

2031

2047

Oceanography | Vol. 39, No. 2

16

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ACKNOWLEDGMENTS

This work synthesizes climate models and Earth system observations produced by

agencies all over the world, but especially by NOAA in the United States. We are

deeply grateful for these valuable contributions to our ability to track changes on the

planet. This manuscript and the analysis were improved by helpful suggestions from

Erick F. Geiger, Derek Manzello, Blake L. Spady, Alistair Hobday, and two anonymous

reviewers. Financial support for this work was provided by the Bezos Earth Fund and

the CO2 Foundation (AJP, JG).

AUTHORS

Andrew J. Pershing (apershing@climatecentral.org), Climate Central Inc., Princeton,

NJ, USA. John F. Bruno, Department of Biology, University of North Carolina at

Chapel Hill, Chapel Hill, NC, USA. Joseph Giguere, Climate Central Inc., Princeton,

NJ, USA. Alexandra Khrizman, Department of Earth System Science, Doerr School of

Sustainability, Stanford University, Stanford, CA, USA.

ARTICLE CITATION

Pershing, A.J., J.F. Bruno, J. Giguere, and A. Khrizman. 2026. Quantifying the role

of climate change in driving mass coral bleaching. Oceanography 39(2):12–16,

https://doi.org/10.5670/oceanog.2026.e210.

COPYRIGHT & USAGE

This is an open access article made available under the terms of the Creative

Commons Attribution 4.0 International License, which permits use, sharing, adapta­

tion, distribution, and reproduction in any medium or format as long as users cite the

materials appropriately, provide a link to the Creative Commons license, and indicate

the changes that were made to the original content.

June 2026 | Oceanography

17

FEATURE ARTICLE

ECOLOGICAL BENTHIC UNITS

A NEW CHARACTERIZATION OF THE GLOBAL SEAFLOOR

FOR OCEAN SPATIAL PLANNING AND MANAGEMENT

By Peter T. Harris, Dawn J. Wright, Kevin Butler, Keith VanGraafeiland, Mark John Costello, Kerry Howell,

Gustav Kågesten, Vanessa Lucieer, Miles Macmillan-Lawler, and Roger Sayre

INTRODUCTION

Society’s interest in exploiting deep-sea resources is growing rap­

idly, but the same cannot be said for our ability to effectively con­

serve and manage the deep ocean (>500 m depth) and high seas

(i.e., international waters beyond national jurisdiction). The recent

ratification of the United Nations (UN) High Seas Treaty (formally

the Agreement on Marine Biological Diversity of Areas Beyond

National Jurisdiction or BBNJ Agreement; High Seas Alliance,

2025) indicates that there is a growing need to conserve the ocean’s

resources and protect the biodiversity that it contains. There have

been discussions of what aspects should be considered when plan­

ning conservation strategies for the deep ocean realms (Ceccarelli

et al., 2021), but there remains a lack of sufficient tools and data

to guide decision-making processes (McQuaid et al., 2023; Misiuk

and Brown, 2024).

Marine spatial planning in the ocean, including the high seas

areas, involves making decisions about managing human activities

in the ocean. For example:

1. Sustainable harvesting of living marine resources and conser­

vation of biodiversity: There is growing interest in harvesting

deep-sea biological resources for food and the genetic materials

used in the development of pharmaceuticals (Bisson et al., 2023).

2. Understanding impacts of pollution: For example, deep-sea

sediments are the final resting place for plastic pollution lost in

the ocean. However, our understanding of pathways and what

environments are most impacted is very poor (Harris et  al.,

2023). This includes creating more accurate environmental

impact assessments for potential deep-sea resource extraction.

3. Establishment and monitoring of high seas marine protected

areas: To create an ecologically representative global network of

marine protected areas (MPAs) as per the new UN Highs Seas

Treaty, we first need to understand the spatial distribution of

different seabed environments and the biodiversity they con­

tain. Our understanding of deep-sea habitats is still extremely

limited, illustrated by the recent extraordinary discovery of the

electrochemical production of “dark oxygen” on the abyssal sea­

floor by polymetallic nodules (Sweetman et al., 2024).

4. Understanding impacts of climate change: Research shows

that global climate change is driving changes in deep ocean pH

(Sulpis et al., 2018), bottom currents, and patterns of sedimen­

tation. Further, the shape of the seafloor combines with these

factors to influence the amount of heat flowing underneath ice

shelves and contributing to melting (Richter et al., 2025). Deep-

sea biota will respond to these ecosystem changes by moving

and/or recolonizing different locations that suit their needs

(Ramirez-Llodra et al., 2011).

In this paper, we present a new benthic classification that we

call “ecological benthic units” (EBUs) for the world ocean. Our

aim is to provide a new tool to improve global understanding and

ABSTRACT. Effective management of deep-sea ecosystems and the high seas is hindered by the absence of a globally consistent frame­

work for characterizing benthic habitats. Here we present the first global ecological classification of the seafloor, comprising 250 unique

ecological benthic units (EBUs), distributed on the seafloor as nearly 700,000 EBU polygon occurrences, generated by intersecting a

high-resolution geomorphic map with multivariate environmental seascapes. Using 17 million seafloor data points and 0.05° resolution

biophysical datasets—including bottom temperature, dissolved oxygen, pH, carbon flux, sediment thickness, crustal age, and bottom

currents—we identified 57 benthic regions across six major geomorphic groups (shelves, slopes, seamounts/guyots, spreading ridges,

abyssal/hadal areas, plateaus). The resulting EBUs reveal previously unrecognized ecological gradients, quantify global patterns of ben-

thic heterogeneity, and expose large-scale environmental vulnerabilities. Notably, we find that 95.6 million km2 (26% of the ocean area)

of abyssal seafloor lies below the carbonate compensation depth, that 4.16 million km2 (1% of the ocean) of continental slopes intersect

severe oxygen minima, and <1% of seamounts occur in seascapes most environmentally favorable to life. These insights provide a power­

ful basis for identifying rare habitat configurations, assessing exposure to climate-driven stressors, and prioritizing areas for high seas

marine protected area planning, as well as a policy-relevant foundation for environmental impact assessment and biodiversity baseline

proxies under the new United Nations High Seas Treaty.

Oceanography | Vol. 39, No. 2

18

decision-making capability for a range of global issues, as listed

above. EBUs build upon an earlier map of ecological marine units

(EMUs) that characterizes the ocean water column as a proxy

for pelagic ecosystems (Sayre et al., 2017). As abiotic surrogates

for benthic biodiversity (McArthur et al., 2010; Astudillo-Scalia

et al., 2021), the EBUs are environmentally distinct areas occur­

ring within physiographically stratified geomorphic types, and

they represent the ecological settings that control the distribution

of benthic biota.

APPROACH TAKEN

The global map of seafloor geomorphic features (Harris et  al.,

2014) provides the basis for the present study. It contains 29 sea­

floor feature categories, such as submarine canyons, seamounts,

mid-ocean ridges, troughs, rift valleys, escarpments, and basins.

The map divides the global ocean floor into more than 120,000

separate polygons and provides the basis for quantitative classifica­

tion of seafloor areas. Seafloor geomorphology, mapped and mea­

sured by marine scientists, has proven to be a very useful physical

attribute for ocean management. This is because different geo-

morphic features (e.g.,  submarine canyons, seamounts, spread­

ing ridges, escarpments, plateaus, trenches) are commonly associ­

ated with particular suites of habitats and biological communities

(Harris and Baker, 2020). The features themselves, be they can­

yons, sea valleys, rocky reefs, or sand banks, become the short­

hand descriptor (if not the direct focus) of conservation efforts and

marine management processes because they are easily understood

by the broader community and are commonly associated with a

particular kind of benthic community.

To advance the science underpinning ocean conservation,

better classification resolution and spatial resolution are needed to

sub-classify geomorphic features into more meaningful EBUs. We

used existing spatial datasets for environmental parameters rele­

vant to the deep-sea ecosystem. For example, six ecologically rel­

evant biophysical variables (depth, seabed slope, sediment thick­

ness, primary production, bottom water dissolved oxygen, and

bottom temperature) were used by Harris and Whiteway (2009) to

classify the ocean into 53,713 separate polygons comprising 11 dif­

ferent categories called ‘‘seascapes.’’ A similar approach was used

by Sayre et al. (2017) to create the EMU ocean water column clas­

sification. In this paper, we subclassify the existing map of seafloor

geomorphic features into different sub-units with existing spatial

data using multivariate statistics.

ECOLOGICAL SIGNIFICANCE OF VARIABLES

USED AND DERIVED

Reviews of the ecological significance of biophysical variables

used for global seascape type analyses such as those conducted

for the present study have been published previously (e.g., Harris

and Whiteway, 2009). The input variables used may be grouped

into four broad categories: (1) physical/structural (depth, sea­

floor gradient [slope)], feature height above seafloor, crustal age,

spreading rate, sediment thickness); (2) chemical (water tempera­

ture, pH, dissolved oxygen, salinity); (3) biological (primary pro­

ductivity, seafloor downward organic carbon flux); and (4) hydro­

dynamic (sea ice cover, significant wave height, tidal range,

benthic current velocity).

The use of static variables (e.g., annual mean value when sea­

sonal variations are present) to characterize the ocean has clear

limitations. The occurrence of biota at any given location on the

seafloor will not necessarily be in response to the mean value of

any variable. Dynamic aspects of the benthic environment include

natural changes in variables like current speed, temperature, dis­

solved oxygen, and food supply. It is now an established fact that

there are “benthic storms” (e.g.,  Woodgate and Fahrbach, 1999)

and seasons in the deep sea. Detrital-feeding fauna and suspension-

feeding organisms have evolved to exploit the seasonal, vertical flux

of surface matter sinking to the seabed. Seasons are recorded by

moored sediment traps (e.g., Juniper et al., 2013) and in thick ver­

sus thin growth rings in the shells of small abyssal molluscs (Gage

and Tyler, 1991). A key driver for longer-frequency (>1 year) vari­

ations is change in deep ocean (near bottom) current speed and/or

direction. Because water properties are essentially carried passively

over benthic habitats in the moving water column, changes in bot­

tom currents can trigger changes in (for example) the quantity and

quality of particulate organic carbon (POC) flux (food) to the abys­

sal seafloor (Smith et al., 2008). For this reason, we include modeled

bottom water current speed and direction in our analysis.

Examples of derived variables include local benthic habitat het­

erogeneity (e.g., number of unique EBU types per neighborhood)

and orientation of seafloor features to the prevailing current.

Several authors have reported evidence that seafloor heterogene­

ity is an indicator of areas with high species richness (e.g., Harris,

2012; Zeppilli et  al., 2016; Riehl et  al., 2020; Zhao et  al., 2020;

Romoth et  al., 2023). In this study, we quantify the number of

polygons per unit area to compare the heterogeneity of seascapes

and their associated geomorphic features.

Some seafloor features rise up prominently, providing a hard,

rocky substrate that is habitat for sessile benthic animals that rely

upon filter-feeding plankton and organic detritus from the water

column for their food supply. The optimum orientation of such a

substrate is one that faces into the prevailing current, thus bring­

ing the most amount of suspended detritus (food) to the wait­

ing tentacles (Baynes and Szmant, 1989). In this study we include

an analysis of seafloor feature orientation relative to the mod­

eled bottom current.

METHODS

CREATING SEAFLOOR GEOMORPHOLOGY

AS A SINGLE MAP LAYER

The project builds upon the foundation of an existing 1 km global

grid of seafloor geomorphic features (Harris et al., 2014) that was

based on bathymetric data available at the time, in particular,

the SRTM 30_PLUS model of Becker et al. (2009). Although the