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.

Global survey shows planners use widely varying sea-level rise projections for coastal adaptation

“…However, a single number is eventually needed in many contexts, especially by engineers designing coastal infrastructure. This number should arise out of careful consideration of a range of projections during the asset life cycle including high-end estimates for risk-averse decisions 50 and timing windows to exceed design thresholds 51 . We recognize that the use case 52 of our respondents would shed further light on the structures and selected projections.…”

“…Conversely, adoption of this extreme value in planning can lead the public and policy makers to mistakenly anticipate more expensive and socially disruptive adaptation measures than may be necessary 37,41,48 . To navigate these advantages and disadvantages to high-end SLR use, practitioners would benefit from more guidance concerning the use of high-end scenarios (see van de Wal, et al 2022 50 ).…”

Antarctic ice stream thickening under Pliocene warmth

“…For each of these seven ice-shelf basal melting scenarios, we perform three experiments that reflect different upper-boundary estimates of global sea-level rise, as determining local sea-level changes is not trivial [75]. Sea level is linearly raised over the first 300 model years from present day by a total of 10.4 m, 12.7 m, and 25.0 m. The first value is based on the high-end estimate of sea-level rise for the most pessimistic IPCC scenario by 2300 [76], the second reflects a full contribution from Greenland and the WAIS to sea-level rise [20], and the third is the same value used in the Pliocene experiments [66]. After the first 300 years, we keep the forcings constant until the simulation reaches 1000 years of model time.…”