publications

2025

1. 

   Record sea surface temperature jump in 2023–2024 unlikely but not unexpected

   Jens Terhaar, Friedrich A. Burger, Linus Vogt, Thomas L. Frölicher, and 1 more author

   Nature, Mar 2025

   Abs DOI

   Global ocean surface temperatures were at record levels for more than a year from April 2023 onwards, exceeding the previous record in 2015–2016 by 0.25 °C on average between April 2023 and March 20241. The nearly global extent and unprecedented intensity of this event prompted questions about how exceptional it was and whether climate models can represent such record-shattering jumps in surface ocean temperatures2. Here we construct observation-based synthetic time series to show that a jump in global sea surface temperatures that breaks the previous record by at least 0.25 °C is a 1-in-512-year event under the current long-term warming trend (1-in-205-year to 1-in-1,185-year event; 95% confidence interval). Without a global warming trend, such an event would have been practically impossible. Using 270 simulations from a wide range of fully coupled climate models, we show that these models successfully simulate such record-shattering jumps in global ocean surface temperatures, underpinning the models’ usefulness in understanding the characteristics, drivers and consequences of such events. These model simulations suggest that the record-shattering jump in surface ocean temperatures in 2023–2024 was an extreme event after which surface ocean temperatures are expected to revert to the expected long-term warming trend.

2. 

   Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s

   Jens Terhaar, Linus Vogt, and Nicholas P. Foukal

   Nature Communications, Jan 2025

   Abs DOI

   The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global ocean carbon and heat uptake, and controls the climate around the North Atlantic. Despite its importance, quantifying the AMOC’s past changes and assessing its vulnerability to climate change remains highly uncertain. Understanding past AMOC changes has relied on proxies, most notably sea surface temperature anomalies over the subpolar North Atlantic. Here, we use 24 Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to demonstrate that these temperature anomalies cannot robustly reconstruct the AMOC. Instead, we find that air-sea heat flux anomalies north of any given latitude in the North Atlantic between 26.5°N and 50°N are tightly linked to the AMOC anomaly at that latitude on decadal and centennial timescales. On these timescales, air-sea heat flux anomalies are strongly linked to AMOC-driven northward heat flux anomalies through the conservation of energy. On annual timescales, however, air-sea heat flux anomalies are mostly altered by atmospheric variability and less by AMOC anomalies. Based on the here identified relationship and observation-based estimates of the past air-sea heat flux in the North Atlantic from reanalysis products, the decadal averaged AMOC at 26.5°N has not weakened from 1963 to 2017 although substantial variability exists at all latitudes.

2024

1. 

   Stratification and overturning circulation are intertwined controls on ocean heat uptake efficiency in climate models

   Linus Vogt, Jean-Baptiste Sallée, and Casimir Lavergne

   EGUsphere, Nov 2024

   Preprint; manuscript currently undergoing peer review

   Abs DOI

   \textlessp\textgreater\textlessstrong class="journal-contentHeaderColor"\textgreaterAbstract.\textless/strong\textgreater The global ocean takes up over 90 % of the excess heat added to the climate system due to anthropogenic emissions, thereby buffering climate change at the Earth’s surface. A key metric to quantify the role of the oceanic processes removing this heat from the atmosphere and storing it in the ocean is the ocean heat uptake efficiency (OHUE), defined as the amount of ocean heat uptake per degree of global surface warming. Despite the importance of OHUE, there remain substantial uncertainties concerning the physical mechanisms controlling its magnitude in global climate model simulations: ocean mixed layer depth, Atlantic meridional overturning circulation (AMOC) strength, and upper ocean stratification strength have all been previously proposed as controlling factors.\textless/p\textgreater \textlessp\textgreaterIn this study, we analyze model output from an ensemble of 28 climate models from the Coupled Model Intercomparison Project, phase 6 (CMIP6), in order to resolve these apparently divergent explanations. We find that stratification in the mid-latitude Southern Ocean is a key model property setting the value of OHUE due to its influence on Southern Ocean overturning. The previously proposed role of the AMOC for OHUE is explained by a linkage of stratification model biases between the subpolar North Atlantic and the Southern Ocean. Our analysis thus reconciles previous attempts at explaining controls on OHUE, and highlights the importance of interlinked model biases across variables and geographical regions.\textless/p\textgreater

2. 

   Increased future ocean heat uptake constrained by Antarctic sea ice extent

   Linus Vogt, Casimir de Lavergne, Jean-Baptiste Sallée, Lester Kwiatkowski, and 2 more authors

   Apr 2024

   Preprint; manuscript currently undergoing peer review

   Abs DOI

   The ocean moderates global warming by absorbing most of the excess heat resulting from anthropogenic climate change. However, this ocean heat uptake also causes sea level rise, more frequent extreme events, ocean deoxygenation, and multiple impacts on marine ecosystems. Despite their importance for informing climate mitigation and adaptation measures, ocean heat uptake projections still strongly differ between climate models. Here, we provide improved global ocean heat uptake projections by identifying a relationship between present-day Antarctic sea ice extent and future ocean heat uptake across an ensemble of 28 state-of-the-art climate models. Models with a higher present-day sea ice extent also simulate greater sea ice loss over the 21st century, a stronger global cloud feedback and atmospheric warming, and hence increased ocean heat uptake. By combining this relationship with satellite observations of Antarctic sea ice, we reduce the uncertainty of ocean heat uptake projections under realistic future emissions scenarios by 12-33%, and show that models underestimate present-day sea ice extent and consequently future ocean heat uptake by 5-7%. Our findings indicate that accounting for biases in the Southern Ocean mean state in the latest generation of climate models implies larger future climate warming.

3. 

   The role of the upper ocean for global ocean heat uptake and climate

   Linus Vogt

   Sorbonne Université, Dec 2024

   Ph.D. thesis

   Abs

   The Earth’s climate is currently undergoing rapid and widespread changes. Human activities in the industrial era, in particular the emission of CO2 into the atmosphere through the burning of fossil fuels, have led to an enhanced greenhouse effect which has caused an increase in the global average surface air temperature of 1.1°C in 2011-2020 relative to 1850-1900. A further consequence is the warming of the global ocean: it has absorbed over 90% of the excess energy stored in the Earth system due to the increased radiative forcing. This global ocean heat uptake (OHU) is a critical climate process and plays a dual role for anthropogenic climate change. On the one hand, OHU is a measure of the cumulative effects of transient climate change, and scales with negative impacts such as sea level rise and the frequency of oceanic extreme events. On the other hand, OHU provides a crucial service by shielding the atmosphere from large amounts of heat that would otherwise cause much greater global warming than currently observed. Despite their importance, many of the physical processes controlling OHU are still poorly understood, including in state-of-the-art numerical climate models used for international climate change assessments. In this thesis, we address this problem using climate simulations of models participating in the Coupled Model Intercomparison Project (CMIP). In a first study, we provide improved future projections of global OHU by the end of the 21st century by identifying an emergent relationship across an ensemble of CMIP models linking the simulated baseline climate state of the Southern Hemisphere to future global OHU. By combining this relationship with observational data, we obtain constrained projections showing that future OHU is likely larger than previously thought. In a second study, we clarify the processes involved in setting the ocean heat uptake efficiency (OHUE) which quantifies the amount of OHU per degree of global surface warming. We reconcile a number of previous attempts at explaining controls on OHUE, and show that the upper ocean stratification in the Southern Ocean is a key property setting its value in CMIP climate models. Last, we present an exploratory analysis combining the approaches of these two studies, and perform a statistical analysis of simulations from a large multi-model ensemble with the goal of constraining OHUE. Beyond these concrete results concerning global OHU, we also discuss some of the methodological issues related to the interpretation of uncertainties arising from multi-model ensembles more generally.

2023

1. 

   Robust acceleration of Earth system heating observed over the past six decades

   Audrey Minière, Karina Schuckmann, Jean-Baptiste Sallée, and Linus Vogt

   Scientific Reports, Dec 2023

   Abs DOI

   Global heating of the Earth system is unequivocal. However, detecting an acceleration of Earth heating has remained elusive to date, despite suggestive evidence of a potential increase in heating rates. In this study, we demonstrate that since 1960, the warming of the world ocean has accelerated at a relatively consistent pace of 0.15 ± 0.05 (W/m2)/decade, while the land, cryosphere, and atmosphere have exhibited an accelerated pace of 0.013 ± 0.003 (W/m2)/decade. This has led to a substantial increase in ocean warming, with a magnitude of 0.91 ± 0.80 W/m2 between the decades 1960–1970 and 2010–2020, which overlies substantial decadal-scale variability in ocean warming of up to 0.6 W/m2. Our findings withstand a wide range of sensitivity analyses and are consistent across different observation-based datasets. The long-term acceleration of Earth warming aligns qualitatively with the rise in CO2 concentrations and the decline in aerosol concentration during the same period, but further investigations are necessary to properly attribute these changes.

2. 

   Southern ocean carbon and heat impact on climate

   SO-CHIC Consortium, J. B. Sallée, E. P. Abrahamsen, C. Allaigre, and 50 more authors

   Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, May 2023

   Abs DOI

   The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

2022

1. 

   Local Drivers of Marine Heatwaves: A Global Analysis With an Earth System Model

   Linus Vogt, Friedrich A. Burger, Stephen M. Griffies, and Thomas L. Frölicher

   Frontiers in Climate, May 2022

   Abs DOI

   Marine heatwaves (MHWs) are periods of extreme warm ocean temperatures that can have devastating impacts on marine organisms and socio-economic systems. Despite recent advances in understanding the underlying processes of individual events, a global view of the local oceanic and atmospheric drivers of MHWs is currently missing. Here, we use daily-mean output of temperature tendency terms from a comprehensive fully coupled coarse-resolution Earth system model to quantify the main local processes leading to the onset and decline of surface MHWs in different seasons. The onset of MHWs in the subtropics and mid-to-high latitudes is primarily driven by net ocean heat uptake associated with a reduction of latent heat loss in all seasons, increased shortwave heat absorption in summer and reduced sensible heat loss in winter, dampened by reduced vertical mixing from the non-local portion of the K-Profile Parameterization boundary layer scheme (KPP) especially in summer. In the tropics, ocean heat uptake is reduced and lowered vertical local mixing and diffusion cause the warming. In the subsequent decline phase, increased ocean heat loss to the atmosphere due to enhanced latent heat loss in all seasons together with enhanced vertical local mixing and diffusion in the high latitudes during summer dominate the temperature decrease globally. The processes leading to the onset and decline of MHWs are similar for short and long MHWs, but there are differences in the drivers between summer and winter. Different types of MHWs with distinct driver combinations are identified within the large variability among events. Our analysis contributes to a better understanding of MHW drivers and processes and may therefore help to improve the prediction of high-impact marine heatwaves.