Barrier islands represent about 10% of the world’s coastline, sustain rich ecosystems, host valuable infrastructure and protect mainland coasts from storms. Future climate-change-induced increases in the intensity and frequency of major hurricanes and accelerations in sea-level rise will have a significant impact on barrier islands--leading to increased coastal hazards and flooding--yet our understanding of island response to external drivers remains limited. Here, we find that island response is intrinsically bistable and controlled by previously unrecognized dynamics: the competing, and quantifiable, effects of storm erosion, sea-level rise, and the aeolian and biological processes that enable and drive dune recovery. When the biophysical processes driving dune recovery dominate, islands tend to be high in elevation and vulnerability to storms is minimized. Alternatively, when the effects of storm erosion dominate, islands may become trapped in a perpetual state of low elevation and maximum vulnerability to storms, even under mild storm conditions. When sea-level rise dominates, islands become unstable and face possible disintegration. This quantification of barrier island dynamics is supported by data from the Virginia Barrier Islands, USA and provides a broader context for considering island response to climate change and the likelihood of potentially abrupt transitions in island state.