The Impact of Uncertainties in Ice Sheet Dynamics on Sea-Level Allowances at Tide Gauge Locations

Abstract

Sea level is projected to rise in the coming centuries as a result of a changing climate. One of the major uncertainties is the projected contribution of the ice sheets in Greenland and Antarctica to sea-level rise (SLR). Here, we study the impact of different shapes of uncertainty distributions of the ice sheets on so-called sea-level allowances. An allowance indicates the height a coastal structure needs to be elevated to keep the same frequency and likelihood of sea-level extremes under a projected amount of mean SLR. Allowances are always larger than the projected SLR. Their magnitude depends on several factors, such as projection uncertainty and the typical variability of the extreme events at a location. Our results show that allowances increase significantly for ice sheet dynamics' uncertainty distributions that are more skewed (more than twice, compared to Gaussian uncertainty distributions), due to the increased probability of a much larger ice sheet contribution to SLR. The allowances are largest in regions where a relatively small observed variability in the extremes is paired with relatively large magnitude and/or large uncertainty in the projected SLR, typically around the equator. Under the RCP8.5 (Representative Concentration Pathway) projections of SLR, the likelihood of extremes increases more than a factor 10 4 at more than 50-87% of the tide gauges.

Projected 21st Century Sea-Level Changes, Observed Sea Level Extremes, and Sea Level Allowances for Norway

“…We have made some tests of the method by assuming that our regional sea-level projections are normally distributed. If the distribution were skewed, however, this would presumably result in higher allowances than those presented here [71]. A disadvantage of using the ACER method (as opposed to the Gumbel method) is that our allowances are dependent on the return height of interest.…”

Applying Principles of Uncertainty within Coastal Hazard Assessments to Better Support Coastal Adaptation

“…The longer the planning timeframe, the increasing dominance of SLR on the outcome [45], and neither a best estimate, nor statistical uncertainty, can be robustly derived for SLR [18,29]. In any case the optimal risk will occur at a higher SLR than the best estimate of the hazard, due to the tail in the probability distribution for SLR for any of the RCPs [12]. In such situations, the likelihood component of risk must be handled some other way, such as using adaptive approaches like the DAPP process [16,19].…”

“…In the USA, SLR is causing deeper floods during extreme sea-level events, and more regular "nuisance" flooding during high tides, resulting in millions of dollars of insurance claims [10]. The rate of SLR is projected to accelerate over this century and beyond [3][4][5], which will greatly increase the frequency of flooding, e.g., [9][10][11][12], and exacerbate coastal erosion, e.g., [13] forcing communities to adapt in some way. Communities will need to decide when and how to adapt.…”

Coastal Sea Levels, Impacts, and Adaptation

“…Slangen et al [14] undertake a global assessment of extreme sea-level allowances (i.e., the height a coastal structure needs to be elevated to keep the same frequency and likelihood of sea-level extremes under a certain sea-level rise scenario). Simpson et al [15] and Malagon Santos et al [16] undertake regional assessments of mean and extreme sea levels around Norway and extreme waves around the UK, respectively.…”

“…Malagon Santos et al [16] carry out a spatial footprint and temporal clustering analysis of extreme storm-wave events around the coast of the UK using measurements from wave buoys. As the authors point out, economical, societal, and environmental impacts from extreme events are often correlated spatially.…”