The full extent of the global coral reef crisis

Coral reefs around the world are largely in decline (Bellwood et al. 2004; Bruno & Selig 2007; De’ath et al. 2012), putting the roughly 7.5% of humanity that depends on them in jeopardy. Diagnoses of the extent of reef decline and debate about the causes, which collectively define the global coral reef crisis, are based largely on local-scale estimates of shallow-water reef condition measured as the percentage of living coral veneer per unit area of reef (i.e., coral cover) (Hughes et al. 2011; Sweatman & Syms 2011; Sweatman et al. 2011). Coral cover is a popular metric for reef monitoring because it can be determined rapidly in the field and because it has been recorded, with varying degrees of quality, for over 50 years (Bellwood et al. 2004; Bruno & Selig 2007). Other ecological metrics, such as species and functional trait composition and reef structural complexity, are likely better for documenting reef condition (Graham et al. 2015) but take substantial time to measure consistently and over the broad scales required for global assessment. Regardless of the ecological metric used, they are measured at small and standardized scales and therefore paint only a relative picture (e.g., per unit reef area) of whether the world’s reefs are declining or expanding over the long term (e.g., the decadal to millennial periods over which climate change will unfold). Capturing the full picture of how coral reefs globally are faring requires knowledge of the absolute amount of consolidated reef structure upon which these ecological estimates apply at broader spatial scales. Consolidated reef is hard calcium carbonate substrate largely made up of skeletons of dead corals that either currently supports living coral or is suitable for coral to grow upon (e.g., it includes consolidated reef that is currently covered by coral competitors, such as macroalgae, but could support coral recruitment and growth in their absence). However, the global extent of this valuable resource remains elusive. To accurately quantify global-scale trends in coral reef condition over the next century, local-scale ecological