A High‐End Estimate of Sea Level Rise for Practitioners

Abstract

Sea-level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate for the deep ocean and ice sheets. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientists and practitioners builds on a framework of discussing physical evidence to quantify high-end global SLR for practitioners. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2 ˚C in 2100 (RCP2.6/SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for a (RCP8.5/SSP5-8.5) we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2 ˚C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while here we emphasize the importance of the timing of ice shelf collapse around Antarctica. This is highly uncertain due to low understanding of the driving processes. Hence both process understanding and emission scenario control high-end SLR.

Illustrative Multi‐Centennial Projections of Global Mean Sea‐Level Rise and Their Application

“…A more sophisticated approach would be preferred in future, particularly when the studies disagree substantially (see Discussion). Other studies have considered the effect of assuming full correlation or independence of contributions on total sea level projections: for example, van de Wal et al (2022) found that assuming independence reduced their global mean sea level projections at 2300 by 1.8 m under the very high emissions scenario SSP5-8.5, compared with full correlation, and by 0.3 m under the SSP1-2.6 scenario that we present here.…”

“…Projections under SSP1-2.6 and SSP2-4.5 could be sufficient for a first assessment in these case studies, because SSP1-2.6 corresponds to the upper target of the Paris Agreement and SSP2-4.5 is often used for (2021). A scenario is also needed for stress testing of critical infrastructure, but this may be already covered to a large extent by the low-likelihood, high-impact (i.e., high-end) projections to 2150 provided by the AR6 (IPCC, 2021a, 2021b) and others (van de Wal et al, 2022).…”

Through an Ice Sheet, Darkly

“…Uncertainties associated with the East Antarctic ice sheet, these authors find, play an important role only for longer timescales and high warming. van de Wal et al (2022) generate high-end estimates of sea-level rise for risk-averse practitioners. Coproduced by scientists and practitioners using a framework introduced by Stammer et al (2019), the estimates generated by this study are distinct from, but complementary to, the corresponding values reported in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (Fox-Kemper et al, 2021) because different approaches are taken to quantify the contribution of land-ice wastage to future sea-level rise.…”

“…Bamber et al (2022) argue that uncertainties on future Greenland surface mass balance largely stem from uncertain albedo processes, such as changing cloud cover, surface darkening and debris cover, and the seasonality of air temperature and precipitation, whereas uncertainties on future ice discharge from West Antarctica are mainly controlled by ice-shelf buttressing and initiation of marine ice sheet and ice cliff instabilities. Also important, underscore van de Wal et al (2022), are uncertainties associated with melt-albedo and height-surface mass balance feedbacks, atmospheric rivers and blocking events, tipping points and timing of ice-shelf collapse around Antarctica, ice-ocean interactions and circulation of circumpolar deep water, and basal friction near grounding lines, among other processes.…”

“…It will also be imperative to make best use of recent advances in geophysical modeling and remote sensing to anticipate relative sea-level rise from coastal land subsidence (Buzzanga et al, 2020;Shirzaei et al, 2021;Wöppelmann & Marcos, 2016). The most useful science to support risk management will therefore result from interdisciplinary sea-level researchers working to address open questions related to the evolution of the cryosphere in a warming world, as highlighted by Bamber et al (2022), Grinsted et al (2022), andvan de Wal et al (2022), as well as the ocean, atmosphere, and solid Earth (Hamlington et al, 2021;Kopp et al, 2019).…”