Sea-level rise allowances for the UK

• Published in: Environmental Research Communications
• Main Authors: Howard, Tom, Palmer, Matthew D
• Other Authors: Department for Business, Energy and Industrial Strategy, Department for Environment, Food and Rural Affairs
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: IOP Publishing 2020
• Subjects: Antarctic; The Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1088/2515-7620/ab7cb4; https://iopscience.iop.org/article/10.1088/2515-7620/ab7cb4; https://iopscience.iop.org/article/10.1088/2515-7620/ab7cb4/pdf

Abstract We evaluate sea-level-rise allowances for UK tide-gauge locations based on the best available current description of present-day extreme still water return level curves and the United Kingdom Climate Projections 2018 (UKCP18) regionalised projections of future time-mean sea-level (MSL) change. We focus primarily on projections for 2100 under Representative Concentration Pathway 8.5. The practical benefit of an allowance is that it condenses a distribution of MSL projections into a single recommendation. Our basic allowances are founded on the probability distributions of all components of time-mean sea-level rise considered by UKCP18 except the Antarctic dynamic uncertainty. For the Antarctic dynamic contribution we include the mean projection. These basic allowances are suitable for use in situations where there is some tolerance of uncertainty. The allowance is always greater than the central estimate (50th percentile) of mean sea-level rise. Our basic allowance is found to lie below the 80th percentile in most cases, and below the 90th percentile in all cases, throughout the 21st century. We find significant sensitivity of the allowance to the uncertainty (but not the curvature) of the present-day return level plot. We find that the normal approximation is legitimate for the sum of all components of time-mean sea-level rise considered by UKCP18 except the Antarctic dynamic uncertainty. UKCP18 used a lognormal distribution to parameterise the Antarctic dynamic contribution. For a lognormal distribution of MSL rise, Hunter’s allowance is not finite. Modifying the lognormal distribution, we give an additional allowance for the Antarctic dynamic contribution uncertainty, which could be added to the basic allowance in situations where there is less tolerance of uncertainty. This additional allowance is small (less than 1 cm at any UK location before 2060) compared to the basic allowance, and, at least over the 21st century, it is virtually independent of greenhouse gas concentration pathway.