  Strategies to manage the risks posed by future sea-level rise hinge on a
sound characterization of the inherent uncertainties. One of the major
uncertainties is the possible rapid disintegration of large fractions of the
Antarctic ice sheet in response to rising global temperatures. This could
potentially lead to several meters of sea-level rise during the next few
centuries. Previous studies have typically been silent on two coupled
questions: (i) What are probabilistic estimates of this "fast dynamics"
contribution to sea-level rise? (ii) What are the implications for strategies
to manage coastal flooding risks? Here, we present probabilistic hindcasts and
projections of sea-level rise to 2100. The fast dynamics mechanism is
approximated by a simple parameterization, designed to allow for a careful
quantification of the uncertainty in its contribution to sea-level rise. We
estimate that global temperature increases ranging from 1.9 to 3.1{\deg}C
coincide with fast Antarctic disintegration, and these contributions account
for sea-level rise of 21-74 centimeters this century (5-95% range,
Representative Concentration Pathway 8.5). We use a simple cost-benefit
analysis of coastal defense to demonstrate in a didactic exercise how
neglecting this mechanism and associated uncertainty can (i) lead to strategies
which fall sizably short of protection targets and (ii) increase the expected
net costs.
