Abstract
Abstract
Environmental baselines for deep-seabed mining must be sufficient to separate the effects of mining from a climate background that is itself changing. We used a five-model Coupled Model Intercomparison Project Phase 6 ensemble to ask when climate-driven change in the abyssal Clarion–Clipperton Zone becomes distinguishable from natural variability—its time of emergence—for temperature, dissolved oxygen and pH at 5000 m, and for particulate organic carbon export at 100 m as a proxy for food supply reaching the seafloor. We expressed the displacement of a ten-year running mean from a modelled 2015–2030 reference state as a signal-to-noise ratio (SNR) against preindustrial control variability; emergence required an SNR above 2 sustained for five consecutive years. Robust emergence was rare by 2035, occurring in one model and for temperature only. By 2050 it had begun in 17 of 34 model–variable–scenario combinations and been confirmed in 16. The changes at emergence were physically small: oxygen shifted by less than 1% of its background concentration and temperature by a few thousandths of a degree, well below plausible physiological thresholds. Food supply was the exception, with a 15% decline in the single case that emerged—one model, under the high-end SSP5-8.5 pathway only, and not before the late 2050s. Ten-year records proved too short to characterise deep-ocean variability, and Areas of Particular Environmental Interest differed from contract areas once depth and location were matched. The variables that emerge earliest are therefore those least likely to matter biologically, while the variable most likely to matter is one whose upper-ocean supply mining does not set, though mining can alter that supply in transit and at the seabed. That is where attribution will be hardest, and why a fixed historical baseline needs to be supplemented rather than relied upon.