The progress of science is tied to the standardization of measurements, instruments, and data. This is especially true in the Big Data age, where analyzing large data volumes critically hinges on the data being standardized. Accordingly, the lack of community‐sanctioned data standards in paleoclimatology has largely precluded the benefits of Big Data advances in the field. Building upon recent efforts to standardize the format and terminology of paleoclimate data, this article describes the Paleoclimate Community reporTing Standard (PaCTS), a crowdsourced reporting standard for such data. PaCTS captures which information should be included when reporting paleoclimate data, with the goal of maximizing the reuse value of paleoclimate data sets, particularly for synthesis work and comparison to climate model simulations. Initiated by the LinkedEarth project, the process to elicit a reporting standard involved an international workshop in 2016, various forms of digital community engagement over the next few years, and grassroots working groups. Participants in this process identified important properties across paleoclimate archives, in addition to the reporting of uncertainties and chronologies; they also identified archive‐specific properties and distinguished reporting standards for new versus legacy data sets. This work shows that at least 135 respondents overwhelmingly support a drastic increase in the amount of metadata accompanying paleoclimate data sets. Since such goals are at odds with present practices, we discuss a transparent path toward implementing or revising these recommendations in the near future, using both bottom‐up and top‐down approaches.

Decadal changes in Pacific sea surface temperatures (SSTs) and upper ocean heat content (OHC) remain poorly understood. We present an annual average composite coral Sr/Ca‐derived SST time series extending back to 1791 from Fiji, Tonga, and Rarotonga (FTR) in the Pacific Decadal Oscillation (PDO) sensitive region of the southwest Pacific. Decadal SST maxima between 1805 and 1830 Common Era (C.E.) indicate unexplained elevated SSTs near the end of the Little Ice Age. The mean period of decadal SST variability in this region has a period near 25 years. Decades of warmer (cooler) FTR SST co‐occur with PDO negative (positive) phases since at least ~1930 C.E. and positively correlate with South Pacific OHC (0–700 m). FTR SST is also inversely correlated with decadal changes in equatorial Pacific SST as measured by coral Sr/Ca. Collectively, these results support the fluctuating trade wind‐shallow meridional overturning cell mechanism for decadal modulation of Pacific SSTs and OHC.

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[1] In the South Pacific Convergence Zone (SPCZ), the variability in a sub-seasonally resolved microatoll Porites colony Sr/Ca record from Tonga and a previously published high-resolution record from Fiji are strongly influenced by sea surface temperature (SST) over the calibration period from 1981 to 2004 (R 2 = 0.67-0.68). However, the Sr/Ca-derived SST correlation to instrumental SST decreases back in time. The lower frequency secular trend (~1 C) and decadal-scale (~2-3 C) modes in Sr/Ca-derived SST are almost two times larger than that observed in instrumental SST. The coral Sr/Ca records suggest that local effects on SST generate larger amplitude variability than gridded SST products indicate. Reconstructed d 18O of seawater (d 18 O sw ) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64-0.67) but not local precipitation (r = À0.10 to À0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on d 18O sw in this region. The Sr/Ca records indicate SST warming over the last 100 years and appears to be related to the expansion of the western Pacific warm pool (WPWP) including an increasing rate of expansion in the last~20 years. The reconstructed d 18 O sw over the last 100 years also shows surface water freshening across the SPCZ. The warming and freshening of the surface ocean in our study area suggests that the SPCZ has been shifting (expanding) southeast, possibly related to the southward shift and intensification of the South Pacific gyre over the last 50 years in response to strengthened westerly winds.

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Abstract. Reconstructions of global hydroclimate during the Common Era (CE; the past ∼2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ2H) isotopic compositions of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 759 isotope records from the terrestrial and marine realms, including glacier and ground ice (210); speleothems (68); corals, sclerosponges, and mollusks (143); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and nonexperts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate-model-simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model–data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593 (last access: 30 July 2020).

Although reef coral skeletal carbon isotopes (δ 13 C) are routinely measured, interpretation remains controversial. Here we show results of a consistent inverse relationship between coral δ 13 C and skeletal extension rate over the last several centuries in Porites corals at Fiji, Tonga, Rarotonga and American Samoa in the southwest Pacific. Beginning in the 1950s, this relationship breaks down as the atmospheric 13 C Suess effect shifts skeletal δ 13 C > 1.0‰ lower. We also compiled coral δ 13 C from a global array of sites and find that mean coral δ 13 C decreases by −1.4‰ for every 5 m increase in water depth ( R = 0.68, p < 0.01). This highlights the fundamental sensitivity of coral δ 13 C to endosymbiotic photosynthesis. Collectively, these results suggest that photosynthetic rate largely determines mean coral δ 13 C while changes in extension rate and metabolic effects over time modulate skeletal δ 13 C around this mean value. The newly quantified coral δ 13 C-water depth relationship may be an effective tool for improving the precision of paleo-sea level reconstruction using corals.

Here we present a new composite record from two well-dated speleothem records from two caves in Northern Morocco. The high-resolution record covers the last millennium allowing to detect multi-decadal to centennial periodicities. Over the industrial period, δ18O values of our speleothems are shown to be dominated by the main mode of decadal variability in the North Atlantic region: the North Atlantic Oscillation (NAO). Statistical analyses confirm the previously reported multi-decadal variability related to the influence of the Atlantic Multidecadal Oscillation (AMO) in the region. High power and persistent centennial-scale periodicities, similar to the Vries-Suess 200-year solar cycle, are observed as well. Indeed, comparison between solar activity reconstructions and our record confirms the in-phase relationship on centennial time-scales. Low δ18O values, and hence negative phases of NAO that bring precipitation towards the Western Mediterranean, are observed during well-known solar minima periods. The results are consistent with previous models which describe low irradiance as a  trigger for southward shifts of precipitation-bearing westerlies during winter.