The Need for Multi‐Century Projections of Sea Level Rise

The latest assessment report of the Intergovernmental Panel on Climate Change (IPCC) provided scenario‐based local sea level projections to 2150 and characterized the long‐term committed global mean sea level rise on 2,000‐ and 10,000‐year time horizons associated with peak surface warming levels. Turner et al. build on the scientific assessment of the IPCC to provide time‐continuous projections of future sea level rise to 2500. These projections fill an important knowledge gap to help inform coastal decision‐making processes and more fully quantify the benefits of mitigation actions in terms of limiting future sea level rise. However, limited understanding of ice instability processes remains a key scientific challenge and improved observational and modeling capability are critical to reducing uncertainties and monitoring the trajectory of observed change.

Plain Language Summary Sea level rise presents a major societal challenge for coastal communities and decision-makers around the world.Due to the slow response of the oceans and ice sheets to global warming, sea level rise will continue for many centuries even under scenarios with strong reductions in future greenhouse gas emissions.In this issue Turner et al. provide a pioneering study to estimate the sea level rise to 2500 to support coastal adaptation planning and demonstrate the long-term benefits of reductions in greenhouse gas emissions.
The Intergovernmental Panel on Climate Change (IPCC) recently reported a 0.2 m rise in global mean sea level (GMSL) over the period 1901 to 2018, with a persistent acceleration since the 1960s and human influence very likely (in the calibrated uncertainty language of IPCC, very likely indicates a 90% or greater probability) the main driver of GMSL rise since at least 1971 (IPCC, 2021).Many coastal cities and settlements are already experiencing adverse impacts associated with sea level rise and climate variability, such as high-tide flooding, watertable salinization and increased erosion and coastal flood damage (Pörtner et al., 2022).With 10% of the world's population and assets lying less than 10 m above sea level and worldwide examples of coastal settlements with high inequality and/or limited adaptive capacity, sea level rise remains one of the primary threats associated with anthropogenic climate change.A key question then is: how high and how quickly will sea levels rise in the future?
Scenario-based climate model projections have tended to focus on the period up to 2100 (e.g., O'Neill et al., 2016), partly motivated by the computational expense associated with running simulations beyond this time-frame.However, unlike surface temperature rise, which responds readily to reductions in greenhouse gas forcing, sea level rise is characterized by substantial commitment associated with past greenhouse gas emissions (Fox-Kemper et al., 2021).Committed sea level rise is associated with the slow response of the oceans and ice sheets to a warming world and means that sea level rise will continue for many centuries even under the most ambitious emissions reduction scenarios (e.g., Fox-Kemper et al., 2021;Nauels et al., 2017;Palmer et al., 2020).The IPCC Sixth Assessment report (AR6) provided local sea level projections that extended to 2150 and also quantified the long-term committed change associated with peak global warming level on 2,000-and 10,000-year time-horizons (Figure 1).
The IPCC AR6 likely range GMSL projections show that the dependency of sea level rise to emissions scenario only becomes apparent in the latter half of the 21st century.This is one illustration of the sea level rise commitment associated with past emissions (Figure 1a).By 2150 the central estimates of GMSL rise under the lowest and highest emissions scenarios are about 0.6 and 1.3 m, respectively, relative to the 1995-2014 average.Furthermore, GMSL rise of about 5 m at 2150 cannot be ruled out under high emissions due to poorly understood ice sheet instability processes (Fox-Kemper et al., 2021).The slow response of the ice sheets to a warmer climate is evident in the committed GMSL change estimates by warming level (Figure 1b).Even limiting surface temperature rise to 1.5C above pre-industrial levels will result in a 2,000 year committed GMSL rise of about 2-3 m, increasing to about 6-7 m on a 10,000 year timescale.Under higher peak warming levels, these numbers increase dramatically, with a peak warming of 5C potentially yielding more than 30 m of committed GMSL rise on a 10,000-year timescale.
The above discussion clearly points out an information gap: what happens in the intervening time between 2150 and the multi-millennial committed sea level rise?Some of the sectors and applications that require multi-century level rise information include planning for: nuclear power installations and fuel disposal sites; new settlements and transport infrastructure; re-location of coastal communities; assessments of future coastal erosion.In a survey carried out as part of the latest UK national projections, more than 50% of stakeholder respondents had medium to high interest in time-horizons beyond 2100 (Weeks et al., 2023).Moreover, while the design lifetime of many societal assets may only be decades, in practical terms they often last for centuries or longer.Many countries around the world still make use of buildings and infrastructure that was built during the nineteenth century, or earlier.For many planning purposes it is important to have plausible trajectories of future sea-level rise that are time-continuous to inform adaptive planning approaches (e.g., Haasnoot et al., 2013;Ranger et al., 2013).Such approaches typically incorporate a lead time in advance of making a particular decision (such as constructing a new barrier or sea wall), which may add decades to the planning process.
Building on the work reported in IPCC AR6, Turner et al. (2023) address this need in their pioneering study on sea level rise projections that extend to 2500.Turner et al. (2023) provide new probabilistic, model-based, timecontinuous projections of GMSL rise across centuries for emissions scenarios, which align with the Nationally Determined Contributions under the Paris Agreement (i.e., RCP2.6/SSP1-2.6and RCP4.5/SSP2-4.5).The projections combine estimates of future changes in the amount of ice and water stored on land, and of thermal expansion, all of which contribute to changes in GMSL.Turner et al. extend physically based model projections of these contributions, complemented by extrapolation where necessary to 2500.For the Antarctic and Greenland ice sheets, estimates from multiple studies are combined, reflecting the deep uncertainty associated with modeling future ice sheet processes.Turner et al. show substantial differences in projected GMSL rise at 2500, dependent on the emissions scenario, demonstrating the importance of extended time horizons in informing mitigation action, as well as for long-term adaptation planning and decision-making.
The multi-century GMSL projections presented by Turner et al. are characterized by large uncertainties.For example, the 5th-95th percentile range presented spans from 1 m to over 7 m under SSP2-4.5 at 2500.The Antarctic ice sheet dominates both the total contribution to sea level change and the uncertainty on these timescales, consistent with previous studies that have explored the drivers of multi-century GMSL rise (Kopp et al., 2014(Kopp et al., , 2017;;Palmer et al., 2020).Even under strong mitigation scenarios, ice sheet instabilities can't be ruled out, and there is widespread evidence of tipping point behavior for both the Greenland and Antarctic ice sheet, which could lead to self-sustaining (and potentially irreversible) ice mass loss (e.g., Lenton et al., 2023).A recent review of the East Antarctic ice sheet-a region that has until recently been considered highly stable under climate change-could contribute several additional meters to GMSL rise by 2500 in the absence of strong greenhouse gas mitigation efforts (Stokes et al., 2022).Advancing modeling and observational capability to improve our understanding of the key ice sheet processes is imperative to reduce uncertainty in long-term sea level projections.Continued and improved monitoring of sea-level change and component processes will also be important over the coming decades to centuries.Long-term adaptation planning necessitates effective monitoring of the sea level trajectory and the development of early warning systems associated with ice sheet instability processes.For example, this would be highly beneficial in a dynamic adaptive planning approach, where changing rates of sea level rise are monitored to determine whether certain thresholds have been reached that may trigger key actions to be taken.
The range of sea level rise information required by coastal planners and decision-makers is broad and highly context-specific.However, there are clear sectoral needs for sea level rise information spanning the coming centuries to provide a more complete picture of committed sea level rise and the benefits of mitigation actions to reduce greenhouse gas emissions.The work by Turner et al. (2023) represents a significant step forward in meeting this information need by providing time-continuous projections of GMSL rise to 2500.Combined with methods to localize these GMSL projections (e.g., Kopp et al., 2023;Palmer et al., 2020), future advances in monitoring and modeling capability will refine our understanding of how high, and how quickly, sea levels will rise.