Lag in response of coastal barrier-island retreat to sea-level rise

The response of coastal barrier islands to relative sea-level rise (SLR) is a long-debated issue. Over centennial and longer periods, regional barrier retreat is generally proportional to the rate of relative SLR. However, over multi-decadal timescales, this simplification does not hold. Field observations along the USA East Coast indicate that barrier retreat rate has at most increased by similar to 45% in the last similar to 100 years, despite a concurrent >= 200% increase in SLR rate. Using a coastal evolution model, we explain this observation by considering disequilibrium dynamics-the lag in barrier behaviour with respect to SLR. Here we show that modern barrier retreat rate is not controlled by recent SLR (last decades), but rather by the baseline SLR of the past centuries. The cumulative effect of the baseline SLR is to establish a potential retreat, which is then realized by storms and tidal processes in the following centuries. When SLR accelerates, the potential for retreat is first realized through removal of geomorphic capital. After several centuries, barrier retreat accelerates proportionally to the increase in SLR. As such, we predict a committed coastal response: even if SLR remains at present rates, barrier retreat in response to SLR will accelerate by similar to 50% within a century. The lag dynamics identified here are probably general, and should be included in predictions of barrier-system response to climate change.