Citation: Rubinetti, S.; Zanchettin, D.; Gazzola, K.; Papa, A.; Rubino, A. Mid-XIX Century Estuary SST Time Series Recorded in the Venice Lagoon. Climate 2022, 10, 155. https:// doi.org/10.3390/cli10100155 Academic Editors: Delle Rose Marco, Maria Teresa Caccamo and Timothy G. F. Kittel Received: 22 September 2022 Accepted: 19 October 2022 Published: 20 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). climate Communication Mid-XIX Century Estuary SST Time Series Recorded in the Venice Lagoon Sara Rubinetti 1,2,3 , Davide Zanchettin 3, *, Kevin Gazzola 3 , Alvise Papa 4 and Angelo Rubino 3 1 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, 27570 Bremerhaven, Germany 2 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, 25992 List/Sylt, Germany 3 Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, 30172 Mestre Venezia, Italy 4 Centro Previsioni e Segnalazioni Maree, 30124 Venice, Italy * Correspondence: davidoff@unive.it Abstract: Sea surface temperature (SST) is of paramount importance for comprehending ocean dy- namics and hence the Earth’s climate system. Accordingly, it is also the most measured oceanographic parameter. However, until the end of the XIX century, no continuous time series of SST seems to exist, with most of the available data deriving from measurements on ships. Here, we present a continuous digitalized record of surface water measurements originally written in a book published in 1853. The measurements were retrieved thrice daily in the Venice lagoon, in the northeastern part of the Italian peninsula, from June to August 1851 and 1852. To the best of our knowledge, these data constitute the oldest time series of the entire world ocean. The measurements were performed by immersing a Réaumur thermometer a few meters deep in the lagoon water at 8 a.m., 12 p.m., and 8 p.m. Despite several limitations affecting these data (e.g., lacking information regarding the exact water depth where measurements were performed and instrumental metadata), they are of utmost significance, as they put many decades backward the date of the development of a fundamental aspect of oceanographic observations. Moreover, the data were collected close to the Punta della Salute site, where actual sea water temperature measurements have been performed since 2002. Therefore, a unique comparison between surface water temperatures within the Lagoon of Venice across three centuries is possible. Keywords: sea surface temperature; Venice lagoon; Réaumur thermometer 1. Introduction Sea surface temperature (SST) is an “essential climate variable” for understanding our climate system and its evolution [1]. On short timescales, it affects the meteorological weather, while, on interannual and longer periods, it fundamentally contributes to shaping prominent climate patterns like the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) (e.g., [2]). Its knowledge allows for quantifying the exchanges between the ocean and the overlying atmosphere [3]. Therefore, the estimate of its variabil- ity is crucial in the investigation of climate changes as well as in the comprehension of the functioning of the physics and the biogeochemistry of the ocean, including that of marine ecosystems [4]. Since the middle of the XX century, the availability of continuous SST time series has significantly increased, together with a rediscovery of the fundamental economic, political, and environmental relevance of the world ocean [5]. Thanks to the advent of remote sensing and, more recently, unmanned vehicles for in situ observations, we have witnessed in recent decades a frequency and coverage of SST measurements never seen before [6]. On the contrary, SST data are scarce and mostly non-continuous for the previous periods, and no time series of SST seem to exist until the end of the XIX century. Very noticeably, this is in opposition to what happened in meteorology starting from the middle of the XVII Climate 2022, 10, 155. https://doi.org/10.3390/cli10100155 https://www.mdpi.com/journal/climate