Atmospheric and oceanic circulation altered by global mean sea-level rise

Zhongshi Zhang, Eystein Jansen, Stefan Pieter Sobolowski, Odd Helge Otterå, Gilles Ramstein, Chuncheng Guo, Aleksi Nummelin, Mats Bentsen, Caoyi Dong, Xijin Wang, et al.

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Zhongshi Zhang, Eystein Jansen, Stefan Pieter Sobolowski, Odd Helge Otterå, Gilles Ramstein, et al. Atmospheric and oceanic circulation altered by global mean sea-level rise. Nature Geoscience, 2023, 16, pp.321-327. 10.1038/s41561-023-01153-y. hal-04041322

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# Atmospheric and oceanic circulation altered by global mean sea-level rise

Received: 3 February 2022

Accepted: 21 February 2023

Published online: 16 March 2023

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Zhongshi Zhang*<sup>1,2,3</sup> †, Eystein Jansen<sup>4,5,6</sup>, Stefan Pieter Sobolowski<sup>5,6</sup>, Odd Helge Otterå<sup>5,6</sup>, Gilles Ramstein<sup>7</sup>, Chuncheng Guo<sup>5</sup>, Aleksi Nummelin<sup>5</sup>, Mats Bentsen<sup>5</sup>, Caoyi Dong<sup>1,3</sup>, Xijin Wang<sup>1,3</sup>, Huijun Wang<sup>8</sup> & Zhengtang Guo<sup>9</sup>

Over recent decades, the rate of global mean sea-level rise has increased, although the magnitude—tens of centimetres—remains small from a geological perspective. Such a modest rise in sea level presents a challenge when attempting to assess its global climate impacts, as the signal is weak. However, in previous warmer geological periods, sea levels reached up to tens of metres higher than the present levels. These palaeoclimate periods offer a unique opportunity to investigate the climate effects of higher sea levels. Here, using climate simulations of the Last Interglacial period and a set of present-day sea-level sensitivity experiments, we highlight the importance of global mean sea-level rise in modulating global climate. The lowering of terrestrial elevation and deepening of oceanic bathymetry due to a spatially uniform rise in sea level reorganizes atmospheric and oceanic circulations. Our simulations of the Last Interglacial show that considering this aspect of global mean sea-level rise in isolation from changes associated with land-sea masks or freshwater input reduces the long-lasting model-data mismatch in the Southern Hemisphere. Furthermore, the present-day sensitivity experiments demonstrate that a slight increase in global mean sea level causes substantial adjustments in the global climate, particularly at mid-high latitudes.

Global mean sea level (GMSL) has risen at an accelerated rate in recent decades<fnref n="1" />⁻⁵ due to oceanic thermal expansion, ice-mass loss and changes in terrestrial water storage³. The rate of GMSL rise has varied on a multi-decadal timescale throughout the twentieth century. Before the 1990s³, the rise velocity (with a mean value of -1.5 mm yr⁻<fnref n="1" />) remained below ~3 mm yr⁻<fnref n="1" />. Afterwards, it increased beyond ~3 mm yr⁻<fnref n="1" /> (refs. <fnref n="1" />⁻³). Today, GMSL is ~20 cm higher than at

the beginning of the twentieth century³. It is virtually certain that it will continue to increase throughout the twenty-first century<fnref n="1" />. It is projected to increase by between 0.38 m (0.28–0.55 m, likely range; Shared Socio-economic Pathways SSP1–1.9) and 0.77 m (0.63–1.01 m, likely range; SSP5–8.5) by 2100¹. Post-2100 projections, although they have low confidence, suggest that GMSL growth will probably reach several metres by 2300 (0.3–3.1 m under SSP1–2.6, 1.7–6.8 m

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e-mail: zhongshi.zhang@cug.edu.cn

<footnote n="1">¹Department of Atmospheric Science, School of Environmental Studies, China University of Geosciences, Wuhan, China. ²School of Geographic Science, Nantong University, Nantong, China. ³Centre for Severe Weather and Climate and Hydro-geological Hazards, Wuhan, China. ⁴Department of Earth Science, Bjerknes Centre for Climate Research, University of Bergen, Bergen, Norway. ⁵NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway. ⁶Centre for Early Sapiens Behaviour, University of Bergen, Bergen, Norway. ⁷Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France. ⁸Key Laboratory of Meteorological Disaster/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China.</footnote>

<footnote n="9">⁹Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China.</footnote>
<table><caption>**Table 1 | Experimental design**</caption><thead><tr><th>Experiment name</th><th>Orbit</th><th>CO<sub>2</sub><sup>a</sup></th><th>CH<sub>4</sub><sup>a</sup></th><th>Bathymetry</th><th>Topography</th></tr></thead><tbody><tr><td colspan="6"><strong>Group 1</strong></td></tr><tr><td>picontrol</td><td>1990</td><td>284.70</td><td>791.60</td><td>pi</td><td>pi</td></tr><tr><td colspan="6"><strong>Group 2</strong></td></tr><tr><td>lig126</td><td>126k</td><td>274.99</td><td>652.52</td><td>pi</td><td>pi</td></tr><tr><td>lig126sl5m</td><td>126k</td><td>274.99</td><td>652.52</td><td>pi+5m</td><td>pi-5m</td></tr><tr><td>lig126sl10m</td><td>126k</td><td>274.99</td><td>652.52</td><td>pi+10m</td><td>pi-10m</td></tr><tr><td colspan="6"><strong>Group 3</strong></td></tr><tr><td>co<sub>2</sub>400</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi</td><td>pi</td></tr><tr><td>co<sub>2</sub>400sl0.625m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+0.625m</td><td>pi-0.625m</td></tr><tr><td>co<sub>2</sub>400sl1.25m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+1.25m</td><td>pi-1.25m</td></tr><tr><td>co<sub>2</sub>400sl2.5m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+2.5m</td><td>pi-2.5m</td></tr><tr><td>co<sub>2</sub>400sl5m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+5m</td><td>pi-5m</td></tr><tr><td>co<sub>2</sub>400sl10m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+10m</td><td>pi-10m</td></tr><tr><td>co<sub>2</sub>400sl20m</td><td>1990</td><td>400.00</td><td>791.60</td><td>pi+20m</td><td>pi-20m</td></tr></tbody></table>

under SSP5-8.5 in the absence of marine ice-cliff instability, and by up to 16 m under SSP5-8.5 considering marine ice-cliff instability)¹.

## Modelling climate impacts of sea-level rise

Increasing GMSL—a major factor contributing to coastal hazards and risks—has captured the public imagination and is a frequent topic of discussion in the scientific literature and popular media. Previous studies have estimated the rate of increase³,⁴,⁶, improved the understanding of its drivers³ and closed the budget of GMSL change³,⁶. Others have demonstrated the increased frequency of future extreme sea-level events⁷,⁸ and investigated regional hazards⁹, ecosystem responses¹⁰ and potential adaptation measures¹¹ in response to sea-level growth in coastal regions.

The influence of GMSL rise is potentially far-reaching, with global-scale effects. GMSL growth lifts the sea-level datum (the coordinate reference surface between land topography and ocean bathymetry), deepens ocean gateways, expands ocean surfaces¹², reshapes regional relative sea levels¹³,¹⁴ and reorganizes ocean density structures and dynamics. However, the recent GMSL rise has occurred over a relatively short time span, with a magnitude limited to centimetres. Due to the low signal-to-noise ratio, it is difficult to observe or investigate its climate influences on a global scale.

Palaeoclimate modelling provides a unique opportunity¹⁵⁻¹⁸ to investigate the impact of sea-level change on the climate system. GMSL fluctuated in past glacial and interglacial periods, closing and opening ocean gateways such as the Bering Strait and imprinting on global climate¹⁸⁻²⁰. GMSL fluctuations also modified global ocean bathymetry by changing sea surface heights. In past warm climates with smaller ice sheets on land, GMSL was higher, and the ocean was deeper than today. However, to reduce complexity, palaeoclimate simulations often neglect or simplify changes in bathymetry. For example, in the Paleoclimate Modelling Intercomparison Project-Phase 4 (PMIP4), the experimental protocol²¹ of the Last Glacial Maximum (LGM ~21 ka, when the GMSL was ~120 m lower than today; ka, thousand years ago) suggested decreasing ocean bathymetry in the boundary conditions. However, many LGM experiments²²,²³ did not make this modification. Other examples include experiments targeting the Pliocene (~3 Ma, million years ago) or the Last Interglacial (LIG, ~130–115 ka). During the Pliocene, when the atmospheric CO₂ concentrations were comparable to those of today²⁴,²⁵, the GMSL was probably 5–25 m higher¹, although this estimated range is highly uncertain²⁶⁻²⁸. During the LIG, the peak GMSL was ~5–10 m higher¹, and the Northern (Southern

a

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b

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c

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**Fig. 1 | Comparison between simulated last interglacial annual surface temperature and reconstructions.** a. Responses of annual SAT (in °C) to the orbital configuration of 126 ka, relative to the pre-industrial control. b. Changes in annual SAT due to the orbital parameters of 126 ka and a global mean sea-level rise of 5 m, compared to the pre-industrial control. c. As in b, but the sea-level rise is 10 m. The filled contours show significant changes with a confidence level higher than 95% (t-test on 200 annual means). The white contours highlight changes with a spacing of 1°. The dots show the SST (in °C) differences in the Southern Ocean between the LIG and the present day (2005–2017). These Southern Ocean SST anomalies come from the recent reconstruction by Chandler and Langebroek³⁶. Extended Data Fig. 1 shows a point-to-point comparison at the reconstruction sites.

Hemisphere received more (less) insolation in the boreal summer (winter) half-year²⁹⁻³¹ than today. Although the atmospheric CO₂ concentration remained below 280 ppm (ref. ³²), the LIG global mean

<footnote>aCO₂ in units of ppm and CH₄ in units of ppb. pi, pre-industrial.</footnote>
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**Fig. 2 |** Surface temperature changes due to global mean sea-level rise in present-day sensitivity experiments. **a,** Comparison between the co₂400sl0.625m and co₂400 experiments in the response of annual SAT (in °C) to a sea-level rise of 0.625 m. **b–f,** As in **a,** but the sea-level uplifts are 1.25 m (**b**), 2.5 m

(c), 5 m (**d**), 10 m (**e**) and 20 m (**f**). The filled contours show significant changes with a confidence level higher than 95% (*t*-test on 200 annual means). The white contours highlight changes with a spacing of 1°.

temperature was ~0.5–1.5 °C (refs. ³³⁻³⁶) warmer than the period from 1850 to 1990. Again, modelling these warm climates²⁵,²⁹ neglected the effects of high GMSL.

In this Article, using the fast version of the Norwegian Earth System Model (NorESM1-F)³⁷, we conduct LIG snapshot and present-day sensitivity experiments (Table 1) to investigate the climate impact of GMSL rise on its own. In addition to Earth's orbital parameters³¹ and greenhouse gas levels³², which are set to values at 126 ka, we lift GMSL by 5 and 10 m (the lower and upper bounds of the LIG GMSL estimate¹) in the LIG simulations. In the present-day sensitivity experiments, we combine an atmospheric CO₂ concentration of 400 ppm (the level used in the Pliocene Model Intercomparison Project, PlioMIP²⁵), the present-day orbital configuration and land-sea distribution, and sea-level increases of 0.625, 1.25, 2.5, 5, 10 and 20 m (a geometric sequence). The value of 0.625 m is close to the upper (lower) bound of the future GSML estimate at the end of the twenty-first century under the SSP1-1.9 (SSP5-8.5) scenario. The values from 1.25 to 20 m are comparable to the GMSL rise by 2300. In these experiments we aim to isolate the climate responses to sea-level datum uplifts—one

aspect of GMSL rise. Potential climate responses to land-sea masks and freshwater input are not considered. (See Methods for details of the model and experimental design.)

## Reduction in model-data mismatch

A longstanding problem in simulating the LIG climate is underestimation of surface temperature in the Southern Hemisphere compared to available reconstructions. These LIG reconstructions³³⁻³⁶ showed a substantial warming south of 40°S in the Southern Hemisphere. In contrast, the PMIP4 LIG experiments that only considered the Earth's orbital parameters and greenhouse gas levels generated much smaller regional responses²⁹. During the LIG, the orbital configuration made the Southern Hemisphere receive less insolation than today²⁹,³⁰. This decreased insolation, particularly in austral summer, cannot fully explain³⁰ the reconstructed warming in the Southern Hemisphere. Other mechanisms, such as freshwater input³⁸, are needed to solve this inconsistency.

Our LIG simulations demonstrate that the above model-data mismatch reduces when considering the GMSL uplifts (Fig. 1)³⁶. Following
the classical LIG experimental protocol, our experiment for 126 ka (Fig. 1a) again displays a cooling in the Southern Hemisphere relative to the pre-industrial control run. However, once the experiments include the high GMSL as part of the boundary conditions, a significant surface warming in annual mean sea surface temperature (SST) and surface air temperature (SAT) appears over the whole Southern Ocean (Fig. 1b,c). As a result, the root-mean-square error between the reconstructed and simulated temperature anomalies at sites decreases by ~9–12% (Extended Data Fig. 1).

In our LIG sensitivity experiments, the GMSL uplifts weaken the near-surface Southern Hemisphere westerlies, causing a reduced northward Ekman transport and a surface warming over the Southern Ocean (Extended Data Fig. 2; see Supplementary Information for the mechanism of the weakened Southern Hemisphere westerlies). As a result, poleward (southward) ocean heat transport (Extended Data Fig. 3) increases by ~5% and ~11% in the Southern Hemisphere mid-latitudes when the GMSL increases by 5 and 10 m. Meanwhile, the GMSL uplifts lower the simulated Atlantic meridional ocean circulation (AMOC) strength (Extended Data Fig. 4) by ~2% and ~3%, favouring more southward heat transport.

## Global circulation response

Compared to the LIG experiments, a GMSL rise of 5 or 10 m in the present-day sensitivity experiments causes more complicated behaviours in the Southern Hemisphere, with surface cooling appearing in most parts of the Southern Ocean (Fig. 2 and Extended Data Fig. 2). Additionally, the GMSL uplifts of 5 and 10 m lower the AMOC strength by ~1% and ~3%, thereby reducing the northward ocean heat transport in the Atlantic, which has the same sign but is weaker than the responses in the LIG experiments (Extended Data Figs. 3 and 4).

The simulated Southern Ocean surface cooling, a prevailing response in our present-day sensitivity experiments, results from intensified near-surface westerlies in the Southern Hemisphere. The GMSL uplifts cause adjustments in atmosphere pressure (Extended Data Fig. 5) and ocean-atmosphere interactions (Extended Data Figs. 6 and 7). As a result, the further deepened sea-level pressure in the Antarctic relative to other regions (Extended Data Fig. 2d) favours intensification of the Southern Hemisphere westerlies (see Supplementary Information for more discussions on the mechanism of wind responses). The strengthened westerlies enhance the northward Ekman transport of cold surface water, thus cooling large parts of the Southern Ocean (Extended Data Fig. 8), where the mixed layer becomes shallow (Extended Data Fig. 7). In contrast, regional warming appears in the Amundsen-Bellingshausen Sea and the South Atlantic, where the mixed layer deepening enlarges the upward mixing of subsurface heat³⁹.

Meanwhile, a significant surface warming (Fig. 2 and Extended Data Fig. 8) takes place in the North Pacific due to the increased northward ocean heat transport (Extended Data Fig. 3c) when increasing GMSL in the experiments. The North Pacific warming and the Southern Ocean cooling cause a north-south temperature dipole, indicating an enhanced inter-hemispheric meridional gradient. The temperature dipole is evident even when the GMSL rise is tens of centimetres and strengthens with further increase in GMSL (Fig. 3c).

Moreover, the GMSL uplifts drive more Pacific water flow into the Arctic via the deepened Bering Strait (Extended Data Fig. 9a), tending to weaken the AMOC. However, the response of the AMOC is nonlinear (Fig. 3d). A sea-level uplift of ~5 m appears to be a threshold. When the GMSL increase is less than 5 m, there are no significant changes in the water flow from the Arctic via the Canadian Archipelago (Extended Data Fig. 9b) relative to the present-day experiment forced with an atmospheric CO₂ level of 400 ppm. Although water flow adjusts through the gateways connecting the Arctic, the Nordic Seas and the Atlantic (Extended Data Fig. 9c-f), the change in the outflow through the Denmark Strait remains (Extended

<figure><img src="image_8.png" /></figure>

**Fig. 3** | Impacts of global mean sea-level rise in present-day sensitivity experiments. a, Globally averaged annual SAT (in °C). b, Plots of the globally averaged annual SAT (in °C) on land. c, The SST (in °C) contrast between the North Pacific (NP, 50–65° N and 120° E–120° W) and the South Pacific/Southern Ocean (SO, 50–70° S and 120° E–120° W). d, Plots of the maximum values of AMOC (in Sv). Dots show the mean values, and the lengths of the error bars represent s.d. Filled (open) dots indicate that sea-level rise leads to a significant (insignificant) change (higher than the 95% confidence level with the *t*-test) in the mean value compared to the co₂400 experiment. Solid (dotted) error bars indicate that sea-level rise leads to a significant (insignificant) response (higher than the 95% confidence level with the *F*-test) in the s.d. relative to the co₂400 experiment. Statistical analyses are based on 200 annual means for each experiment. The grey dotted lines show the trend, which is insignificant in **a** and significant in **b–d** at the 95% confidence level.

Data Fig. 9f) insignificant. The subsequent responses in surface water salinity (Fig. 4c) and convection (Extended Data Fig. 7a-c) in the Labrador Sea are not large enough to weaken AMOC (Fig. 3d). Once the GMSL change exceeds 5 m, more Pacific water enters into the Labrador Sea (Extended Data Fig. 9b,d), reducing the regional surface salinity (Fig. 4d) and convection (Extended Data Fig. 7d-f). The AMOC strength hence lowers (Fig. 3d), and the surface temperature drops in the subpolar North Atlantic (Fig. 2d-f and Extended Data Fig. 8d-f).

Our present-day sensitivity experiments demonstrate that GMSL growth—even by tens of centimetres—generates significant climate responses, particularly at mid-high latitudes, where the ocean is more active in interacting with the atmosphere. When moving a sea-level datum up, lowering topography and deepening bathymetry both appear crucial in influencing atmospheric and oceanic circulations (see the experiments described in Supplementary Table 3). Because the atmosphere adjusts quickly to elevation reductions, GMSL growth probably imprints on climate on the decadal timescale. The impact of GMSL rise on AMOC (0.2–2.4 Sv) remains relatively weak (Fig. 3d) compared to the decrease of AMOC only due to the increase in atmospheric CO₂ levels⁴⁰ or the large variability of AMOC⁴¹. However, other regional imprints, such as the temperature dipole in the Pacific or the Pacific-Arctic water exchange through the Bering Strait⁴², are more substantial and more easily detectable.
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<figure><img src="image_11.png" /></figure>

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**Fig. 4 | Simulated surface ocean currents and salinity at the North Pole.**

**a,** Annual mean sea surface salinity (in PSU (practical salinity units); shaded colours) and the ocean currents (in cm s⁻¹; arrows) averaged between 0 and 50 m in the pre-industrial control experiment. These results agree well with observations⁴⁵,⁴⁶. **b,** Differences between the co₂400 and the picontrol experiment, and the changes due to an increased atmospheric CO₂ level to

400 ppm. **c,** Plots of the differences between the co₂400sl0.625m and the co₂400 experiment, and the changes caused by the global mean sea-level rise of 0.625 m. **d,** As in **c**, but the sea-level rise is 10 m. The filled contours show significant salinity changes with a confidence level higher than 95% (t-test on 200 annual means). The thin black contours in **b–d** show salinity changes with a spacing of 0.4 PSU.

## Sea-level feedback on past and future climate

In this Article we highlight the importance of considering GMSL change in modelling palaeoclimate, particularly past warm climates. GMSL fluctuations reshape land-sea distribution and adjust ocean bathymetry and land topography. Both can influence past climate evolutions. Although the impact of GMSL change depends on the background climate state and is complex and nonlinear, GMSL (rise) tends to result in positive (warming) feedback in the climate system. Considering GMSL variations can potentially resolve model-data mismatches and debates in understanding palaeoclimate evolutions. For example, our simulations demonstrate that the high sea-level stand allows more relatively fresh Pacific water to enter the North Atlantic, helping explain the weakened convection in the Labrador Sea⁴³ and the high variability in AMOC strength⁴⁴ evident in sediments during the LIG.

Our study further suggests that the effect of GMSL rise on the global climate will become non-negligible in the future. Although our experiments are idealized and only consider one aspect of GMSL rise, they demonstrate that a slight spatially uniform uplift in sea level is enough to alter atmospheric and oceanic circulations and heat

transport. Whether the present GMSL variations have imprinted on the global climate remains unknown. However, in light of our results and the ongoing nature of GMSL increase, determination of its impact on global climate is critical and may be possible towards the end of the twenty-first century or into the next century. The Southern Ocean is one crucial region in which to determine the impact, although observations and in situ investigations remain challenging there. Other key areas for observing the effect are the Bering Strait and the Bering Sea. Indeed, the processes through which GMSL growth influences the global climate are more complicated than simply lifting a sea-level datum. To further understand and evaluate the climate impact of GMSL rise, it is vital to develop a new generation of climate models that consider dynamic changes in bathymetry and topography together with atmospheric responses to relative sea-level variations. Given that GMSL growth brings adjustments to large-scale atmospheric and oceanic circulations and associated synoptic systems, both coastal and inland countries are exposed to these changes. If anthropogenically driven global warming continues unabated, the role of GMSL rise could become an essential forcing factor for our future climate system at decadal and centennial timescales.
## Online content

Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41561-023-01153-y.

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# Methods

## Introduction to NorESM1-F

NorESM1-F, a computationally efficient model in the Norwegian Earth System Model family, simulates the global climate reasonably well³⁷. It was built on the Community Climate System Model, version 4⁴⁷, and developed based on the version for the fifth phase of the Coupled Model Intercomparison Project (CMIP5), NorESM1-M. Like NorESM1-M⁴⁸, NorESM1-F uses the same grid with a horizontal resolution of 2° and 26 vertical levels in the atmosphere component, and a new tripolar grid with a nominal 1° horizontal resolution and 53 vertical layers in the ocean component. The tripolar grid provides a higher horizontal resolution (-40 km) and is more isotropic at high northern latitudes. The simulated surface properties and circulation in the Arctic region (Fig. 4a) agree well with observations⁴⁵,⁴⁶. Model documentation³⁷ presents a more detailed description and evaluation of NorESM1-F.

NorESM has contributed to the palaeoclimate modelling community for more than one decade. Compared to an early version (NorESM-L) used in palaeoclimate simulations⁴⁹, NorESM1-F exhibits an improved representation of climate at high latitudes. Additionally, NorESM1-F took part in the Modelling Intercomparison Projects (MIPs) for the Eocene⁵⁰, the Pliocene²⁵ and the LIG²⁹,⁵¹. Within these MIPs, NorESM1-F has consistently performed well in simulating various palaeoclimate periods. For example, of the models in the LIG model ensemble, NorESM1-F simulations of sea ice cover⁵¹ for the Nordic Seas agreed best with available reconstructions.

## Experimental design

We designed three groups of fully coupled experiments to investigate the climate impact of GMSL rise (Table 1). When GMSL growth is only tens of centimetres to a few metres, almost no land-sea masks change in an ocean model with a medium horizontal resolution (for example, 1°). Meanwhile, regional sea levels deviate significantly from GMSL. However, atmosphere models generally do not consider relative sea-level variations. It is thus essential to note that our experiments are idealized and only address sea-level datum uplifts—just one aspect of GMSL rise.

The first experiment is the pre-industrial control experiment (picontrol). It employs the Earth's orbital parameters of 1990 CE and atmospheric CO₂ level 284.70 ppm and CH₄ level 791.60 ppb. The picontrol experiment is initialized from the Levitus temperature and salinity³² and runs for 2,200 model years. Here, we analyse the model output for the last 200 years.

The second group (lig126 group) includes three simulations: lig126, lig126sl5m and lig126sl10m. The lig126 experiment only considers changes in the Earth's orbital configuration and greenhouse gas levels, using the orbital parameters of 126 ka (ref. ³¹), an atmospheric CO₂ level of 274.99 ppm (ref. ³²) and an atmospheric CH₄ level of 652.52 ppb (ref. ³²). It branches from the picontrol experiment in model year 1891 and runs for an additional 1,700 model years. In addition to the orbital parameters and greenhouse gas levels, lig126sl5m and lig126sl10m further address GMSL uplifts of 5 and 10 m. In the lig126sl5m (lig126sl10m) experiment, we add 5 m (10 m) in the ocean bathymetry and reduce by 5 m (10 m) in the land topography. To avoid the modelling uncertainties introduced by ice-sheet reconstructions, we do not further modify the topography of Greenland and Antarctica. This approach is consistent with the LIG experimental protocol suggested by PMIP4²⁹. (See Supplementary Table 1 and Supplementary Figs. 1 and 2 for further assessment of uncertainties in the LIG Antarctic ice-sheet reconstruction¹⁶,⁵³,⁵⁴.) The lig126sl5m and lig126sl10m experiments are initialized from the Levitus temperature and salinity³² (same initial conditions as used in picontrol) and run for 2,200 model years. Note that these experiments could not branch directly from the lig126 or the picontrol experiment due to the energy and salinity conservation requirement. Here, we analyse the model output for the last 200 years (see the time series in Supplementary Fig. 5).

The third group (co₂400 group) includes seven simulations: co₂400, co₂400sl0.625m, co₂400sl1.25m, co₂400sl2.5m, co₂400sl5m, co₂400sl10m and co₂400sl20m. In co₂400, the atmospheric CO₂ level is 400 ppm, and all other boundary conditions (including orbital parameters, CH₄ level, bathymetry and topography) are identical to picontrol. It branches from the picontrol experiment in model year 1,891 and runs for an additional 1,700 model years. Compared to co₂400, we lift GMSL by 0.625, 1.25, 2.5, 5, 10 and 20 m in the other experiments. When we increase the bathymetry in the ocean, we decrease topography by the same amount on land. We do not further modify the topography of Greenland and Antarctica. All these sea-level sensitivity experiments are initialized from the Levitus temperature and salinity³² (same as those used in picontrol) and run for 2,200 model years. Here, we analyse the model output for the last 200 years (see the time series in Supplementary Fig. 5).

## Data availability

The temperature results from the sea-level experiments are publicly available on Zenodo (https://doi.org/10.5281/zenodo.7365287). The annual mean atmosphere and ocean results are publicly available on Zenodo (https://doi.org/10.5281/zenodo.7650523). More model output can be provided upon request. In addition, the LIG SST reconstructions are available in ref. ³⁶.

## Code availability

The NorESM code is available on GitHub (https://github.com/NorESMhub/NorESM).

## References

47. Gent, P. R. et al. The Community Climate System Model Version 4. *J. Clim.* **24**, 4973–4991 (2011).

48. Bentsen, M. et al. The Norwegian Earth System Model, NorESM1-M—Part 1: description and basic evaluation of the physical climate. *Geosci. Model Dev.* **6**, 687–720 (2013).

49. Zhang, Z. S. et al. Pre-industrial and mid-Pliocene simulations with NorESM-L. *Geosci. Model Dev.* **5**, 523–533 (2012).

50. Lunt, D. J. et al. DeepMIP: model intercomparison of early Eocene climatic optimum (EECO) large-scale climate features and comparison with proxy data. *Clim. Past* **17**, 203–227 (2021).

51. Kageyama, M. et al. A multi-model CMIP6-PMIP4 study of Arctic sea ice at 127ka: sea ice data compilation and model differences. *Clim. Past* **17**, 37–62 (2021).

52. Levitus, S. & Boyer, T. P. World ocean atlas 1994 volume 4: temperature. In *NOAA Atlas NESDIS* (US Government Printing Office, 1994).

53. Turney, C. S. M. et al. Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica. *Proc. Natl Acad. Sci. USA* **117**, 3996–4006 (2020).

54. Golledge, N. R. et al. Retreat of the Antarctic ice sheet during the Last Interglaciation and implications for future change. *Geophys. Res. Lett.* **48**, e2021GL094513 (2021).

## Acknowledgements

This study was jointly supported by the National Natural Science Foundation of China (grants nos. 41888101 and 42125502), the National Key Research and Development Program of China (grant no. 2018YFA0605602), the SapienCE (project no. 262618) and other projects (projects nos. 314371, 229819 and 221712) from the Norwegian Research Council, as well as the computing resources from Notur/Norstore projects NN9133/NS9133, NN9486/NS9486 and NN9874/NS9874.

## Author contributions

Z.Z. designed and performed the simulations and wrote the draft of the paper. S.P.S. and O.H.O. contributed to the
analyses of atmosphere dynamics, and C.G., A.N. and M.B. contributed to investigations into ocean dynamics. E.J., G.R., H.W. and Z.G. helped strengthen the palaeo and future climate link. C.D. and X.W. prepared some figures. All authors contributed to discussing the results and writing the paper.

**Competing interests**

The authors declare no competing interests.

**Additional information**

**Extended data** is available for this paper at
https://doi.org/10.1038/s41561-023-01153-y.

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41561-023-01153-y.

**Correspondence and requests for materials** should be addressed to Zhongshi Zhang.

**Peer review information** *Nature Geoscience* thanks Daniel Lunt and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Tom Richardson, in collaboration with the *Nature Geoscience* team.

**Reprints and permissions information** is available at www.nature.com/reprints.
<figure><img src="image_13.png" /></figure>

**Extended Data Fig. 1** | Comparison between last interglacial simulations with reconstructions³⁶ at sites. The blue dots and boxes show the simulated annual mean surface air temperature (SAT, in °C) or sea surface temperature (SST, in °C) anomalies (at data sites and the range) without the global mean sea-level (GMSL) rise. The red dots and boxes show the simulated annual mean SAT/SST anomalies (at data sites and the range) with the GMSL rise. The last interglacial (LIG) sensitivity experiments show strong warming between 60°S and 70°S when the GMSL rises. While most sites used in the reconstruction are located between 40°S and 50°S, complicating direct comparison. The LIG experiments demonstrate

that the model-data mismatch is reduced by considering the GMSL rise. Without the GMSL rise, the root mean square error (RMSE) between the simulated SAT (SST) and the reconstruction is 2.83 (2.85). When the GMSL rise is 5 m (10 m), the RMSE between the simulated SAT and the reconstruction becomes 2.48 (2.50), and the RMSE is 2.56 (2.58) between the simulated SST and the reconstructions. The box and whisker show the range of simulated surface temperature change at 17 sites, with the maximum, the 75% percentile, the mean, the 25% percentile, and the minimum value from top to bottom.
<figure><img src="image_14.png" /><figcaption>Extended Data Fig. 2 | See next page for caption.</figcaption></figure>
**Extended Data Fig. 2 | Comparison between last interglacial and present-day experiments.** (a) and (b) show the surface pressure changes (in Pa) due to the global mean sea-level rise of 10 m. (c) and (d) plot the sea-level surface pressure changes (in Pa). (e) and (f) illustrate the 850hPa winds anomalies (in unit m/s). (g) and (h) display the anomalies in surface air temperature (SAT, in °C). Only significant differences with a confidence level higher than 95% (*t*-test) are

shaded. The grey and blue contours highlight pressure changes with a spacing of 40 Pa in (a), (b), (c), and (d). In (e) and (f), the arrows plot the changes in winds, and the blue-filled contours show the changes in wind speed with a confidence level higher than 95% (*t*-test). The supplementary information presents detailed explanations for this figure.
<figure><img src="image_15.png" /><figcaption>Extended Data Fig. 3 | See next page for caption.</figcaption></figure>
**Extended Data Fig. 3 | Comparison in ocean heat transport between last interglacial and present-day experiments.** (a) shows the changes in global ocean heat transport (OHT, in PW) between the lig126sl5m and the lig126 experiment (green), between the lig126sl10m and the lig126 experiment (red), between the co₂400sl5m and the co₂400 experiment (blue), and between

the co₂400sl10m and the co₂400 experiment (black). The solid (dash) lines indicate the significant (insignificant) changes higher (lower) than a 95% confidence level (*t*-test). (b) and (c) as in (a), but for the Atlantic and the Indian and Pacific, respectively.
<figure><img src="image_16.png" /></figure>

**Extended Data Fig. 4** | Comparison in meridional ocean circulations between last interglacial and present-day experiments. (a) shows the stream function (in Sv) of Atlantic meridional ocean circulation (AMOC) for the lig126sl5m (filled colors and black contours) and the lig126 experiment (red and blue contours). The contours display stream function with a spacing of 4 Sv. (b) plots the

differences in AMOC between the lig126sl5m and the lig126 experiment. (c) and (d) show the AMOC comparison between the lig126sl10m and the lig126 experiment. (e) to (h) plot the comparison for the co₂400, co₂400sl5m, and co₂400sl10m experiments.
<figure><img src="image_17.png" /></figure>

**Extended Data Fig. 5** | Surface pressure changes due to global mean sea-level rise in present-day sensitivity experiments. (a) shows the comparison between the co₂400sl0.625 m and the co₂400 experiment, the response of annual surface pressure (in Pa) to the sea-level rise of 0.625 m. (b) to (f) as in (a), but the sea-level

rise is 1.25, 2.5, 5, 10, and 20 m, respectively. Only the significant differences with a confidence level higher than 95% (t-test) appear in the filled contours. The grey contours highlight pressure changes with a spacing of 40 Pa.
<figure><img src="image_18.png" /></figure>

**Extended Data Fig. 6** | Near-surface wind changes due to global mean sea-level rise in present-day sensitivity experiments. (a) shows the comparison between the co₂400s10.625 m and the co₂400 experiment, the response of 850hPa winds (in m/s) to the sea-level rise of 0.625 m. (b) to (f) as in (a), but the

sea-level rise is 1.25, 2.5, 5, 10, and 20 m, respectively. The arrows plot the changes in winds, and the blue-filled contours show the changes in wind speed with a confidence level higher than 95% (t-test).
<figure><img src="image_19.png" /></figure>

**Extended Data Fig. 7** | Mixed layer depth changes due to global mean sea-level rise in present-day sensitivity experiments. (a) shows the comparison between the co₂400sl0.625 m and the co₂400 experiment, the response of mixed layer depth (in m) to the sea-level rise of 0.625 m. (b) to (f) as in (a), but the sea-level

rise is 1.25, 2.5, 5, 10, and 20 m, respectively. Only the significant differences with a confidence level higher than 95% (t-test) appear in the filled contours. The black outlines highlight the changes with a contour spacing of 10 m.
<figure><img src="image_20.png" /><figcaption>**Extended Data Fig. 8** | Sea surface temperature changes due to global mean sea-level rise in present-day sensitivity experiments. (a) shows the comparison between the co₂400sl0.625 m and the co₂400 experiment, the response of sea surface temperature (in °C) to the sea-level rise of 0.625 m. (b)</figcaption></figure>

to (f) as in (a), but the sea-level rise is 1.25, 2.5, 5, 10, and 20 m, respectively. Only the significant differences with a confidence level higher than 95% (t-test) appear in the filled contours. The black outlines highlight temperature contours of negative and positive changes of 0.5, 1, and 2 degrees.
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

**Extended Data Fig. 9** | Statistics of annual water volume in seaways in high northern latitudes in present-day sensitivity experiments. (a) Bering Strait.

(b) Canadian Archipelago seaways. (c) Fram Strait. (d) Danmark Strait. (e) Barents Sea open. (f) Iceland-Faroe-Scotland channels. Positive means flow northward, while negative means flow southward (in Sv). The dots show the mean values, and the length of the error bars represent s.d. Filled (open) dots indicate the global mean sea-level rise leads to a significant (insignificant) change (higher

than the 95% confidence level with *t*-test) in the mean value compared to the co₂400 experiment. Solid (dotted) error bars indicate the sea-level rise leads to a significant (insignificant) response (higher than the 95% confidence level with *F*-test) in the s.d. relative to the co₂400 experiment. Statistical analyses are based on 200 annual means for each experiment. The grey horizontal lines show the mean values simulated in the pre-industrial control experiment, which agree with observations⁴⁵,⁴⁶.