Source-composition sensitivity in historical marine wind observations: a CLIWOC case study, 1750–1855

Abstract. Historical ship-logbook observations are widely used to reconstruct pre-instrumental marine climate, but aggregate trends can be sensitive to changes in the source composition of the underlying archive. We reproduce a recent article's principal annual aggregate from CLIWOC release 2.1 and find that this unstandardized, record-weighted series does not on its own identify a global atmospheric change. The observations contributing to it change materially in nationality, archival coverage, and – within Dutch material – format and documented processing history of wind-force values. BIC-based segmentation of the annual aggregate selects working boundaries at 1783, 1816, and 1833. For adjacent selected regimes, descriptive Oaxaca--Blinder decompositions show that the 1816 onset contrast is highly sensitive to nationality weights: under the stated reference convention, the nationality-weight component comprises a large share of the observed contrast, although a within-nationality component remains. The 1833 recovery contrast is likewise highly sensitive to whether Dutch records are pooled or classified by the operational numeric-versus-verbal 'AllWindForces' rule. These results demonstrate non-comparability risks in a pooled CLIWOC aggregate; they do not identify the causal effects of nationality, war, archival practice, or conversion pathway. The earlier 1783 decline is not accounted for by the nationality or geographic specifications examined here, and its cause remains unresolved. The appropriate conclusion is therefore methodological: a raw historical logbook aggregate requires an explicit target population, support and comparability diagnostics, and independent validation before it can support a global-atmospheric interpretation. The analysis provides a source-comparability diagnosis for this CLIWOC aggregate; it does not provide a replacement reconstruction of atmospheric circulation.