Financial advisers recommend a diverse portfolio to respond to market fluctuations across sectors. Similarly, nature has evolved a diverse portfolio of species to maintain ecosystem function amid environmental fluctuations. In urban planning, public health, transport and communications, food production, and other domains, however, this feature often seems ignored. As we enter an era of unprecedented turbulence at the planetary level, we argue that ample responses to this new reality — that is, response diversity — can no longer be taken for granted and must be actively designed and managed. We describe here what response diversity is, how it is expressed and how it can be enhanced and lost.