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EXtreme upper TAil of SEA level rise: constraints from geological records

Periodic Reporting for period 1 - ExTaSea (EXtreme upper TAil of SEA level rise: constraints from geological records)

Reporting period: 2020-03-01 to 2022-02-28

Sea-level rise is a major societal concern, with potential impacts on population, infrastructure and coastal environments. Global sea levels rose ~20 cm in the last 120 years, but the rate of rise is faster than for anytime in the last 3,000 years, and continues to accelerate. In recent decades mass loss has accelerated from both the Greenland and Antarctic Ice Sheets at rates not seen in our instrumental records, and knowing how much and how fast sea level could rise due to melt of these ice sheets is key to assessing risk, and improving resilience.

Approx. 50 million people in Europe live within low-elevation coastal zone (<10 m elevation above of present sea level) and recent studies suggest that the cost of coastal flooding could increase 2 to 3 orders of magnitude by 2100. Without mitigation and adaptation, the annual damage from coastal flooding in the EU and UK could increase from 1.4 €billion today to almost 240 €billion by 2100 (Vousdoukas et al., 2018). Current projections suggest a low probability of loss from the polar ice sheets, but they cannot be ruled out. Knowing how much and how fast sea level could rise with melting of the polar ice sheets is crucial information for risk analysis and adaptation, particularly for stakeholders with a low tolerance of risk, or where asset life cycles are long (>100 years), and coastal infrastructure safety is paramount (e.g. nuclear energy). These stakeholders need credible “worst case” scenarios to assess the greatest potential risk and costs associated with the long-term, high-end component of future sea-level rise. However, the large uncertainties in long-term projections can make adaptation and planning activities challenging. ExTaSea should help these stakeholders by providing this credible “worst case” scenario.

The ExTaSea project will address two sources of uncertainty in current projections of future sea-level: (1) our understanding of the system and, (2) the degree to which we can simulate natural variability. ExTaSea will do this by producing well quantified natural bounds on both the rate and magnitude of sea level rise (objective 1) and probability distributions that include specific information on high-end extremes for global mean and regional sea levels (objective 2). These will contribute to our understanding of these sources of uncertainty and will form the basis for policy-relevant extreme sea-level scenarios (objective 3) that account for the dynamic response of the ice sheets to climate forcing. ExTaSea will produce statistical distributions by: (1) collating and quality checking already available geological data from past time intervals of the last 200,000 years that are useful analogues for future change; (2) novel statistical techniques (e.g. modified Bayesian partition modelling) and (3) modelling of solid Earth deformation processes (GIA) that will allow absolute magnitudes of sea levels to be determined. As the geological record integrates all processes, the statistical distributions on the natural bounds of sea-level will include the high-impact (extreme) tail associated with mass loss from the polar ice sheets.
The geological record is useful in that it provides context for present and future scenarios, includes real-world episodes of large-scale and rapid ice sheet mass loss, and a suite of data beyond the relatively short instrumental record (~20 years satellite data,~120 years from tide gauges). ExTaSea uses six intervals in Earth’s recent past (last 200,000 years). These natural test intervals provide an extensive range of scenarios in which climate and ice volumes were similar or warmer/smaller than present, as well as intervals of rapid ice-sheet mass loss (and sea-level rise). They also include all processes governing ice-sheet mass loss, not just those seen in the recent observational records, and so provide a complete picture of the long-term impacts, particularly as continued warming is moving the climate towards states outside of societal context. This allows natural sea-level variability to be investigated, and data-informed bounds on the rates and magnitudes of low-probability, high-impact sea-level rise. Both of these are crucial to planning and adapting to future sea-level rise. Geological records, combined with statistical analysis were used to produce regional and global reconstructions of past sea levels. The ExTaSea dataset contains >5,900 data points from around the globe which we statistically analysed to produce robust records of how much and how fast sea level has risen in the past. Some additional modelling was needed to account for the spatial heterogeneity in sea level due to gravitational, rotational and solid Earth deformation effects of melting ice.

Results from ExTaSea have been disseminated at major international conferences and workshops, and in published and forthcoming peer-reviewed scientific articles. However, dissemination and stakeholder activities were severely curtailed by the COVID-19 pandemic. Datasets will be made publicly available once associated manuscripts have been published. These provide key targets (and data for validation/testing of models and simulations) for climate and ice sheet modellers.
Coastal flooding is often the result of extremely high water level events due to the combined contributions of large waves, storm surge, high tides, and mean sea-level anomalies. ExTaSea only addresses the potential for low, probability but high-end (long-term) changes in mean sea level driven by mass loss from the polar ice sheets. This information is needed by some stakeholders (e.g. engineers and coastal/ecosystem managers; urban planners especially for urbanised estuaries). For example, in the nuclear power sector flood risk must be taken into account at all stages of planning, and for the full lifetime of the installation (at least 160 years). This means that infrastructure designed and built today must account for substantially different levels of risk that may be faced in 2100 and beyond. Additionally, given ongoing safety is paramount, they need to consider more unlikely potential future sea-level scenarios. The policy relevant sea-level scenarios (natural scenarios) of potential future extreme sea-level rise produced by ExTaSea will be realistic 'worst-case' global and regional sea-level scenarios that can be used to test coastal vulnerability to high-impact, low-probability sea-level rise. Outputs from ExTaSea include:

1. Global, sea-level database which provide valuable, quality checked datasets for reconstructing past sea levels, as well as for modelling, archaeology, palaeo-anthropology and biogeography communities.
2. New statistical methodology for robustly identifying structure within geological sea-level records that more fully captures the complex uncertainty structure.
3. Estimates of past sea levels, and contraints on natural rates and magnitude of sea level rise. These provide insights of future ice-sheet contributions, ice-dynamics and mechanisms of ice sheet loss.
4. Policy relevant extreme scenarios that are realistic ‘worst-case’ global and regional sea-level scenarios that can be used to test coastal vulnerability to future high-impact, low-probability sea-level change.
5. Publications and datasets: Dissemination within scientific and non-specialist audiences will increase awareness of the magnitude and rates of change associated low probability, high impact sea-level rise.
floating Antarctic iceberg