Climatic distance to cultivar origin predicts Deglet Noor date quality across two continents

Background. Premium-grade cultivars of terroir-controlled crops are tied to specific climatic origins, yet the quantitative relationships linking climate and quality remain poorly resolved across whole production landscapes. Deglet Noor date palm — the historically dominant premium cultivar of North African oases, named for the optical translucency of its top-grade fruits — exemplifies this knowledge gap. Methods. A single-optimum, climatic-distance-to-origin framework is tested on a controlled-cultivar Maghreb–Jerid fitting panel of thirteen sites (n = 13) spanning the Algerian Septentrional Sahara and the Tunisian Jerid–Nefzaoua, based on 35 years of ERA5 / Open-Meteo reanalysis (1990–2024). A four-dimensional climatic distance to the cultivar's documented origin (Tolga, Algeria) is regressed against site quality grade under leakage-free nested leave-one-out cross-validation with a permutation test. A floating-optimum analysis tests every panel site as the candidate optimum. A trans-Atlantic diaspora site (Coachella Valley, USA) is held out entirely from model fitting and predictor standardisation as an independent out-of-sample check. Results. The four-dimensional distance to Tolga explains 57 % of cross-site quality variance in-sample and 33 % under nested LOO (permutation p = 0.005, n = 13). The floating-optimum analysis identifies Tolga (tied with neighbouring Biskra — the same Zibans basin) as the best-fitting optimum. The held-out Coachella site is predicted at grade 2.5 from climatic distance alone; its independently market-anchored observed grade is 2. Three climatically opposite regimes — Saharan dryness (El Menia), Mediterranean humidity (Gabes) and Californian desert heat (Coachella) — converge on the same marginal grade by departing from Tolga along different dimensions, empirical evidence of a non-monotonic (Goldilocks) reaction norm. An AR6 regional delta-method projection indicates that Tolga itself drifts 1.3–2.1 grade points from its own historical optimum by 2050 (n = 13 calibration), with the geographical optimum migrating toward the Tunisian Jerid–Gafsa corridor; under SSP5-8.5, 12 of the 13 Maghreb panel sites fall below grade 3. Implications. A climatic-optimum framework anchored on a cultivar's documented origin parsimoniously captures terroir–quality relationships across continents, is testable in wine, coffee, tea and olive where origin is documented and consistent quality grading exists, and provides a quantitative basis for projecting climate-change vulnerability of premium cultivars. This preprint is manuscript version 20 (28 June 2026), deposited on the eve of submission to PLOS ONE. The full analytical pipeline (Python 3.14) and derived data are released as a companion Zenodo archive: https://doi.org/10.5281/zenodo.21399151.