Deep-sea climate futures are already observable in the Red Sea

<title>Abstract</title> <p>The deep oceans are warming, but we lack reference systems for observing community restructuring without exploitation confounds. Here, we assess the deep Red Sea as a natural analogue for mesopelagic and upper-bathypelagic climate futures. Below 150 m, Red Sea temperatures stabilise at 21–22°C to the seafloor (3,040 m), conditions the global mesopelagic will not reach until at least 2100–2200. Deep-sea fisheries are effectively absent: over 70 years, catch composition has remained among the most coastally dominated globally (6th percentile). We identify 38 additional species, including conspicuous commercial taxa, potentially present but undocumented. Functional-space truncation relative to the Arabian Seas source pool is concentrated along axes of extreme depth-specialisation, while feeding and life-history diversity are retained. No other warm deep basin combines equivalent thermal extremity, oxygenation, and an unexploited baseline. The deep Red Sea is a globally unique, time-limited system for predicting mesopelagic restructuring under sustained ocean warming.</p>