# Anthropocene tipping point reverses long-term Holocene cooling of the Mediterranean Sea

Nick Marriner, David Kaniewski, Majid Pourkerman, Benoît Devillers

► To cite this version:

Nick Marriner, David Kaniewski, Majid Pourkerman, Benoît Devillers. Anthropocene tipping point reverses long-term Holocene cooling of the Mediterranean Sea: A meta-analysis of the basin's Sea Surface Temperature records. Earth-Science Reviews, 2022, 227, pp.103986. 10.1016/j.earscirev.2022.103986. hal-03610062

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Anthropocene tipping point reverses long-term Holocene
cooling of the Mediterranean Sea: a meta-analysis of the basin's
Sea Surface Temperature records

Nick Marriner¹*†, David Kaniewski²,³†, Majid Pourkerman⁴ & Benoît Devillers⁵,⁶

¹CNRS, ThéMA, Université de Franche-Comté, UMR 6049, MSHE Ledoux, 32 rue Mégevand, 25030 10 Besançon Cedex,
France

*Corresponding author. Email: nick.marriner@univ-fcomte.fr

²TRACES, UMR 5608 CNRS, Université Toulouse Jean Jaurès, Maison de la Recherche, 5 allées A. Machado 12 31058
Toulouse Cedex 9, France

³Université Paul Sabatier-Toulouse 3, Département Biologie et Géosciences, 118 Route de Narbonne, 31062 Toulouse cedex
9, France

⁴Iranian National Institute for Oceanography and Atmospheric Sciences (INIOAS), 3 Etemad Zadeh St., Fatemi Ave., 14155-
4781 Tehran, Iran

⁵Université Paul-Valéry - Montpellier 3, CNRS, MC, UMR 5140 Archéologie des sociétés méditerranéennes, 34000, Montpellier,
France

⁶Labex ARCHIMEDE, ANR-11-LABX-0032-01, Université Paul-Valery, Montpellier 3, Route de Mende, 34199 Montpellier
Cedex 05, France

† These authors contributed equally to this work

Abstract

The Mediterranean is facing numerous socio-environmental challenges linked to global change, frequently compounded by rapid population growth. Within this framework, regional-scale Holocene temperature reconstructions are key to placing industrial-era warming into the perspective of natural climatic variability. Here, we present a new Mediterranean Sea Surface Temperature (SST) stack based on 54 records for the last 11,750 years, to evaluate millennial-to-centennial-scale climate variability and to contextualize present and future changes. The Holocene thermal maximum is reconstructed 9,400–3,000 years ago and is estimated to have been an average of 0.47 ± 0.2°C warmer than the mean for 1900-1960, followed by the onset of long-term cooling beginning around 3000 years BP. The coolest temperatures during the last ten millennia are recorded during the Little Ice Age. We find that Mediterranean Sea temperatures have risen from near the coldest to warmest
levels of the Holocene within the past century, with decadal warming rates that are ~16 times greater than the closest interglacial analogue. The basin's SSTs have consistently exceeded the full distribution of Holocene warmth since 2011.

**Keywords:** Sea Surface Temperatures, Holocene, Mediterranean, Anthropocene.

**Introduction**

The Mediterranean comprises one of the world's most prominent and vulnerable climate change “hotspots” due to its rapid response to atmospheric forcings (Tuel and Eltahir, 2020); current change and future scenarios consistently highlight the substantial and growing risks that the basin will face in coming decades (Guiot and Cramer, 2016; Tramblay et al., 2020). Within this context, Sea Surface Temperature (SST) is an important component of the Mediterranean climate system (Macias et al., 2013) and knowledge of past, present and future trends in its SSTs is crucial for future climate scenarios and to improve understanding of thermal and dynamical interactions between the Mediterranean Sea, the atmosphere (Alexander et al., 2018; Pastor et al., 2018) and steric-driven sea-level changes (Vacchi et al., 2021).

Climate change constitutes one of multiple drivers affecting Mediterranean anthroposcapes and human health (Cramer et al., 2018), either directly (e.g. through temperature extremes, drought and water security, storms) or indirectly (modifications in food provision and quality, air pollution, creation of urban heat islands). Future warming in the Mediterranean region is forecasted to exceed global rates by 25 %, with summer warming outpacing the global mean by 40 % (Lionello and Scarascia, 2018). In 2019, Mediterranean countries hosted >520 million people (United Nations, 2019) and population exposure to heat is increasing due to climate change and rapid rates of urbanization. For example, from 2000-2019, >65,000 people died due to Mediterranean heatwaves (EM-DAT, 2020). This figure is particularly alarming because the frequency of Mediterranean heatwave days is projected to evolve from an average of about two days per summer for the period 1961–1990 to around 13 days for 2021–2050 and 40 days for 2071–2100 (Fischer and Schär, 2010). The Mediterranean coastline, in particular, is extremely sensitive
to heatwaves and strong changes in ambient temperature and sea temperatures, notably the basin's densely populated urban centers.

Mediterranean SST data covering the Holocene can furnish crucial insight into natural decadal to centennial time-scale changes and put recent climate change into a longer-term perspective (McGregor et al., 2015; Abram et al., 2016). This is particularly important because semi-enclosed basins like the Mediterranean Sea are poorly resolved by the ~1° resolution of many global climate models. The Mediterranean's rich archaeological record also emphasizes the importance of recurring socio-cultural changes/transitions and demonstrates that the region's past societies faced climate-related challenges at various points during the Holocene (Kaniewski et al., 2017). Here, we place today's regional SST changes into the broader temporal context of the Holocene using a consistent framework based on spatially averaged estimates of Mediterranean SSTs. Mediterranean Sea temperatures are now ~1.4°C higher than at the turn of the 20th century, compared with an increase of ~1.2°C worldwide (NOAA, 2020). Understanding future trends in SSTs is, therefore, of paramount importance for Mediterranean climate adaptation policies, including public health, land-use and the conservation of biodiversity.

## Methods

The SST database gathers 54 records from 44 locations in the Mediterranean Sea (see **Table 1**). **Figure 1** displays the spatial distribution of the archives used in the database. The records derive from marine cores and are based on the following proxy methods (**Figures S1 and S2**): alkenones (n=34), long-chain diols (n=2), Mg/Ca (n=11), TEX86 (n=5), δ18Ow foraminifera (n=1) and Ri OH-GDGTs (n=1). They range in temporal length from 0 to 244,350 years BP, but only the Holocene section of each record was retained for the database. Details regarding the collection, analysis and interpretation of individual records are provided in the original publications. A list of sites in the database, including basic metadata, is presented in **Table 1**. To facilitate comparison of information on a common chronological grid and for shared spectral resolutions, all records were annualized using nearest-neighbor interpolation. Each initial record was converted into SST anomalies (in °C) by subtracting the mean temperature from each sequence. This method also helps to reduce analytical biases associated with individual geochemical reconstruction
techniques. The time series were subsequently summed and averaged to generate a
spatially averaged Holocene record of Mediterranean SSTs. To quantify analytical
uncertainties in the time series, we calculated Confidence Intervals, integrating the
number of records at a given point in time, the reconstructed annual mean and the
associated standard deviation (Figure S3). The mean CI for the entire time series is
0.23°C. We also incorporated the varying chronological uncertainties (2σ) associated
with each of the 54 time series used to construct the SST stack, by means of the
initial age models. Figure S4 plots the 2σ error average of all records used to
construct the SST record for each time point. The data are plotted at a resolution of
50 years for clarity. Mediterranean SSTs for 1854-2019 were extracted from the
International Comprehensive Ocean-Atmosphere Data Set (ICOADS)
(https://icoads.noaa.gov; 55). All SST data are reported as average temperature
anomalies. For data plotted on the B2K timescale, the year 2000 CE represents 0.
Negative time values are post-2000 (e.g. 2010 = -10).

To quantify the potential role of geographic location in influencing the overall signal we applied the following three-step analysis. (1) The initial 54 records were divided into three separate entities East (21°E to 36°E), Central (10°E to 20.9999°E) and West (-4.7496°W to 9.9999°E) Mediterranean. The SST data were transformed into percentages of the total signal. For each calculation, the mean was subtracted by geographic entity using these percentages. (2) Next, we took the number of sequences by geographic entity (East, Central and West) and transformed these numbers into percentages of the total signal. For each calculation, we also subtracted the mean (per geographic entity) using these percentages. (3) To weight the overall signal using the three different zones, we multiplied the percentages produced in step 1 with those from step 2. The level of signal variation is low, with the exception of the first ~200 years of the Holocene, giving us confidence in the overall quality of the SST stack as a robust palaeoclimate record (Figure S5).

A homogeneity test was applied to the Holocene record of SSTs to detect shifts in
the long-term dynamics. Each discordant period was categorized and its average
indicated by “mu” (Figure 2). A polynomial regression was then applied (Figure 2)
and a sinusoidal regression was calculated to model potential periodicities in the time
series. The link between insolation and Mediterranean SSTs was tested, ranking the
insolation scores in ascending order and retaining the associated SST values.
Averages were calculated and are plotted with their standard deviations (Figure 3).
The same process was applied in testing the relationship between the Agassiz Ice
Cap melt record and the SSTs (Figure 4A-B). Warming and cooling phases during
the Holocene (expressed in °C per decade; Figures 5-7) were calculated by
selecting the highest and lowest values for each climate episode. Rates were defined
by subtracting the high and low values and dividing by time. A linear model was
applied to present-day values to project warming values up to 2100, based on each
of the past Holocene scenarios (with the associated standard deviation; Figure 7E).

A short-time Fourier transform was applied to the last two millennia (window = rectangle; **Figure S7**) to determine the sinusoidal frequency and the phase content of sections of the SST signal over time. Boxplots of Mediterranean SST variability during the Holocene (**Figure S8**) were plotted to highlight the extent of Global Warming since the 19th century AD. Each chronological window was defined by a series of homogeneity tests.

All of the raw data tables used to draft the figures are reported in the Excel file
(Supplementary Material) accompanying the manuscript.

**Results and discussion**

*Holocene evolution of Mediterranean SSTs*

**Figure 2** plots average SST anomalies on the B2K timescale. The record has successfully captured both first- and second-order pacemakers. Change-point analyses highlight three statistically significant periods. (1) The reconstruction shows a strong SST warming trend between 11,700 to 9400 B2K characterized by warming of ~2.8°C at an average rate of 0.12°C per century. The temperature anomalies range from -2.75 ± 0.59°C to 0.03 ± 0.26°C with a mean of -1.02 ± 0.39°C. This early Holocene period was characterized by a much greater range of variability than seen over later millennia. (2) Around 9400 years ago, the rates of SST change slowed rapidly. Between 9400 to 3000 B2K, temperatures plateaued in a phase of sustained warming with SST anomalies ranging from -0.01 ± 0.2°C to 0.58 ± 0.18°C, with an average anomaly of 0.32 ± 0.2°C. Maxima in the SST data occur at 4700-4950 B2K (average = 0.53 ± 0.23°C) and 7100-7200 B2K (average = 0.54 ± 0.19°C). (3) After
3000 BP, the reconstruction manifests an overall cooling trend of >1°C, from a maximum of 0.3 ± 0.19°C at 3000 B2K to a minimum of -0.77 ± 0.06°C at 110 B2K.

Following the disappearance of the major Northern Hemisphere ice sheets between 8000 B2K and 6000 B2K, the chief SST trends in the Mediterranean parallel the dominant climate forcings of the Holocene. Mediterranean SSTs during the last 8000 years are characterized by an overall decrease in temperatures, consistent with insolation forcing associated with decreasing orbital obliquity. The strongly decreasing boreal summer insolation induced a progressive SST cooling of ~1.4°C (Figure 3). A southerly zonal shift in the mean position of the intertropical convergence zone (ITCZ; Schneider et al., 2014) allowed an expansion of the Siberian High over Eurasia, ultimately resulting in cooler, drier winters and springs in the Mediterranean (Peyron et al., 2017). Our reconstruction demonstrates that cooling only became well expressed in the past three millennia, accompanied by a marked transition in seasonality that is clearly attested in terrestrial records from around the Mediterranean, concomitant with a gradual precession-driven decrease in summer insolation versus increases in winter values (Peyron et al., 2011; Kaniewski et al., 2020).

Superimposed on the insolation-based Holocene cooling of Mediterranean SSTs, the reconstruction manifests considerable variability linked to second-order forcing agents. Six periods of marked SST cooling are delineated at 8360-8100 B2K, 5400-5100 B2K, 4220-3900 B2K, 3130-2800 B2K, 1470-1250 B2K and 670-140 B2K (Figures 4 and 5). Our analysis shows that these rapid Mediterranean cooling phases mesh with internal oceanic shifts in the North Atlantic during which ice rafting and meltwater outburst events from Greenland and North America cooled and freshened ocean surface waters, slowing and modifying the geography of deep-water formation and disrupting the northerly transport of warm low-latitude waters by the thermohaline circulation (Fisher et al., 2012; Figure 4). These major changes in the Atlantic Meridional Overturning Circulation (AMOC) significantly affected SSTs in the Mediterranean with broader implications for the basin's palaeoclimates (Fletcher et al., 2013). Terrestrial-based records demonstrate that the cooling phases were frequently associated with reductions in precipitation and temperatures, and had wider social, cultural, and political consequences for some Mediterranean and West
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Asian societies at key points during the mid to late Holocene (Weiss, 2016; Sinha et al., 2019). For instance, the Late Bronze Age to Early Iron Age transition in the Eastern Mediterranean was characterized by a 30-40% reduction in precipitation which appears to have engendered food shortages in parts of the Levant, Egypt and Greece, and marked the onset of the Iron Age Dark Ages (Kaniewski et al., 2013; Kaniewski et al., 2017). The archaeological record documents significant cultural and socio-economic shifts in the Levant and Egypt during the Late Bronze Age, and several sites from Syria, Israel and the Nile Delta show a transition to cooler and more arid conditions at ~1200 BC (Kaniewski et al., 2019). Egypt withdrew from its province in Canaan and experienced food shortages (Shaw, 2004). In Greece, the population was reduced, and the world of organized state armies, kings, officials, and redistributive systems disappeared. During the Early Iron Age, Greece was divided into independent regions organized by kinship groups and the oikoi or households, the origins of the later poleis. This model appears to have provided citizens with greater opportunities for political participation and rendered these societies more resilient to climate change. Later, around 1500-1250 B2K, rapid climate cooling of 0.27°C coincided with the transformation of the eastern Roman empire into the Byzantine empire (Büntgen et al., 2016) and marked the end of a period of relatively stable climatic conditions during which Rome extended its hegemony over great swathes of Europe, the Near East and North Africa.

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The lowest SSTs during the past 10,000 years occurred during the 1600 to 1800s. At this time, many Mediterranean glaciers attained their maximum Holocene extent and even existed as far south as the High Atlas of Morocco and the Sierra Nevada of southern Spain (Hughes, 2018). All of the solar minima of the Little Ice Age are clearly captured by the Mediterranean SST stack. These cold episodes mesh with abrupt cooling events in the Bosphorus and colder winters in Istanbul (Turkey), both identified in historical data (Yavuz et al., 2007; **Figure S7**). Rapid temperature deviations are therefore important in fully comprehending the complex interplay between Mediterranean climate, landscapes and its societies.

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Climate change is not only manifest in mean SST trends but also in shifts in the anatomy of past abrupt warmings and coolings recorded in the Mediterranean SST stack. We probed the duration and amplitude of cooling/warming transitions in the
SST record, focusing on four warming phases and nine cooling episodes (Figure 6). We note large variability in the transition duration from one event to the other. The longest warming transition was for the Holocene Warm Period with a warming of 0.43 ± 0.17°C over ~510 years. The longest cooling-phase transition was the 3.2 Event characterized by cooling of -0.38 ± 0.19°C over ~320 years which contrasts markedly with the short cooling-phase transition during the Dalton Minimum of the Little Ice Age with cooling of -0.45 ± 0.23°C in just ~45 years. The absence of a systematic pattern in the duration of SST transitions could result from internal climate variability superimposed on broader forcing mechanisms. Nonetheless, our data do suggest that warming and cooling phases have been particularly rapid and of high amplitude during the past 1000 years.

Present and future SSTs trends and implications

Our results demonstrate that the mean of Mediterranean SSTs for the decade 2010–2019 exceeds the warmest temperatures of the early to mid-Holocene (9000 to 3000 BP) by 0.31 ± 0.09°C. By contrast, the decadal SST mean of the early 20th century (1900–1909) was cooler than 86% of the Holocene distribution. Mediterranean SSTs have therefore risen from near the coldest to the warmest levels of the Holocene within the past century (Figures 6 and 7), reversing the long-term cooling trend that began ~3000 years ago.

During the past 46 years, Mediterranean SSTs have increased by ~1.54°C ± 0.09°C at an average warming rate of 0.33°C per decade. Prior to the ongoing Industrial-era warming, phases of rapid warming played out at 1070-705 B2K, 5350-4840 B2K and 8300-7150 B2K (Figures 6 and 7), although, as we highlight above, the tempo of warming rates during these periods contrast markedly with the recent Industrial period. For instance, average warming rates were (i) 0.021°C per decade during the Medieval Climate Anomaly (0.36 ± 0.15°C in 172 years), (ii) as low as 0.008°C per decade during the Holocene Warm Period (0.43 ± 0.17°C in 510 years) and (iii) 0.015°C per decade during the Holocene Climate Optimum (0.5 ± 0.2°C in 330 years). To test to what extent current warming rate is unparalleled in relation to the last 11,750 years, we calculated temperature changes using moving 46-years windows for the entire sequence (Figure S9). The 46-year window corresponds to the present ongoing phase of rapid Industrial-era warming (1973 to 2019); our
analysis confirms that the current rate of Mediterranean Sea warming is without precedent at the Holocene timescale.

These past warm periods are particularly relevant for modelling future changes in Mediterranean SSTs. Our reconstruction shows that Mediterranean SSTs have consistently exceeded the full distribution of Holocene warmth since 2011 (Figure 7E). If the rates of temperature increases observed over the past five decades were to remain constant for the rest of the twenty-first century, Mediterranean SSTs could attain anomalies of around 3.59 ± 0.08°C by 2100. This analysis suggests that Industrial-era Mediterranean SST warming rates are unprecedented at the scale of the Holocene, with Mediterranean SSTs projected to warm ~16 times faster than the closest analogue during the past ~11,000 years.

## Conclusion

Our analysis provides further evidence that low carbon emissions are critical to mitigate contributions of Mediterranean SST increases to local and regional sea-level rise. Our multi-millennial assessment also clearly proves the need to assimilate pre-twentieth-century data into comprehensive assessments of industrial-era warming. At the scale of the Mediterranean, societal and economic impacts are likely to propagate in a nonlinear manner as climate-related hazards reach tipping points beyond which the affected physiological, anthropogenic, or ecological systems function less effectively or break down altogether. Many of these effects are likely to become even stronger in the near future because of the Mediterranean's growing human population levels and economic activity, particularly in coastal areas. Our analysis demonstrates that there are no direct Holocene analogues to assess the resilience of human societies in the face of such rapid current warming.

## Acknowledgments

This work was supported by MITI CNRS Evénements rares (2022-2023) project AQUASANMARCO. We wish to thank an anonymous reviewer for constructive suggestions that helped to significantly improve earlier versions of the manuscript.

## Figure and table captions
**Figure 1:** Location of the 54 records included in the Mediterranean database of SSTs. The numbers correspond to the references in Table 1.

**Figure 2:** Stack of Mediterranean Sea Surface Temperature (SST) anomalies for the Holocene. SST anomalies are the spatial average of the SST minus the mean of each individual time series in the database and are plotted on the B2K timescale. Change-point analysis was used to separate the time series into three periods: 11,750 to 9426 B2K (mu3 = -1.03), 9425 to 2951 B2K (mu2 = 0.32) and 2950 B2K to present (mu1 = 0.01). mu1-3 denote the averages for each statistically significant period. The light blue shading denotes the 68% Confidence Interval.

**Figure 3:** A. Mediterranean SSTs for the past ~10,000 years. B. Mediterranean SSTs plotted against June-July-August (JJA) insolation (orange line and shading) and December-January-February (DJF) insolation (green line and shading). C. Correlation between grouped SSTs and insolation for JJA and DJF. These analyses underline the role of changing Holocene seasonality in driving SST changes.

**Figure 4:** A. Mediterranean SST temperature anomalies (purple lines) plotted against Agassiz and Renland average temperature change record (in °C, green lines, record from Fisher et al., 2012) and Agassiz melt record (% melt, light-blue lines, record from Fisher et al., 2012). B. Grouped Mediterranean SST temperature anomalies versus grouped melting rates from the Agassiz record. The analysis demonstrates that increased meltwater inputs into the North Atlantic are correlated with cooling of Mediterranean SSTs.

**Figure 5:** Industrial-era warming compared and contrasted with other Holocene phases of rapid cooling (blue) or warming (orange).

**Figure 6:** A. Average annual rates of warming/cooling phases (in °C) for key Holocene periods. The analysis demonstrates that warming and cooling phases have been particularly rapid and of high amplitude during the past 1000 years. There are no direct Holocene analogues for the current rapid warming of Mediterranean SSTs. B. Onset and endpoints (dots) of key Holocene climate transitions (oblique lines) in Mediterranean SSTs. The vertical amplitude between the onset and the end of each
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warming/cooling phase is in °C (positive for warming and negative for cooling), and
represents the true T°C amplitude of change.

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**Figure 7:** **A-D.** Industrial-era SST warming rates compared to other periods of rapid Holocene warming, including the Medieval Climate Anomaly (B), the Holocene Warm Period (C) and the Holocene Climate Optimum (D). **E.** Modelled Mediterranean SSTs up to 2100 based on current and, for comparison, Holocene warming rates. The data suggest that decadal warming rates are ~16 times greater than the closest Holocene analogue and that Mediterranean SSTs have persistently exceeded the full distribution of Holocene warmth since 2011.

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**Table 1:** Basic metadata of the 54 records used in the Mediterranean SST compilation.

**Supplementary figures**

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**Figure S1:** Location of the 54 records in the Mediterranean database, grouped by proxy type.

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**Figure S2:** **A.** Temporal length and proxy type of the 54 records used to construct the Mediterranean SST stack. **B.** Number of records used in the Mediterranean SST stack through time.

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**Figure S3:** **A.** Stack of Mediterranean Sea Surface Temperature (SST) anomalies for the Holocene, plotted on the B2K timescale. The light blue shading denotes the 68% Confidence Interval. The red line is a 200-year resampling, based on the coarsest chronological resolution of all the 2σ error margins used to construct the SST stack. The yellow line represents the 68% Confidence Interval associated with the 200-year time step. **B.** Same as (A) but excluding the annual resolution and associated Confidence Intervals.

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**Figure S4:** Stack of Mediterranean Sea Surface Temperatures (SST) for the Holocene, plotted at 50-year intervals. The light blue error bars denote the 68% Confidence Intervals. The green error bars denote the average 2σ error of all records
used to reconstruct the SST record at time point x. The grey inset is a boxplot of the 2σ chronological errors in the initial 54 records (in years) used to construct the SST stack.

**Figure S5:** Comparison between SST minus mean average (cold colours) and SST spatial average (warm colours).

**Figure S6:** Potential role of geographic location in influencing the overall SST signal for the Holocene. **A.** Each SST record (West, Central and East Mediterranean) is presented as deviations from the mean. **B.** The number of sequences used per geographic location is shown as deviations from the mean. **C.** The weight of each geographic location in the final SST curve is subtracted (using **A.** x **B.**) and the resulting three curves (West, Central and East Mediterranean) are compared and contrasted with each other.

**Figure S7:** **A.** Mediterranean SST anomalies during the past 2000 years and broad climatic periods. The stars denote freezing events in the Bosphorus and cold winters in Istanbul, based on historical records (Yavuz et al., 2007). **B.** Short-time Fourier transform of the data highlighting the periods of marked climatic variability.

**Figure S8:** Boxplots of Mediterranean SST variability during the Holocene, highlighting the magnitude and rate of ongoing industrial-era warming. Recent warming contrasts markedly with long-term Holocene cooling of the Mediterranean Sea.

**Figure S9:** Holocene changes in Mediterranean SST°C using moving 46-years windows for the complete sequence. The 46-year window corresponds to the current phase of rapid Industrial-era warming (1973 to 2019). The first value covers the temperature rise between 1973 and 2019, the second between 1972 and 2018, the third between 1971 and 2017 and so forth for the entire sequence. The associated confidence interval comprises the average of the confidence intervals for each 46-yr time-span.

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<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
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<figure><img src="image_19.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S1:** Location of the 54 records in the Mediterranean database, grouped by proxy type.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S2:** A. Temporal length and proxy type of the 54 records used to construct the Mediterranean SST stack. B. Number of records used in the Mediterranean SST stack through time.</figcaption></figure>
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**Figure S3:** A. Stack of Mediterranean Sea Surface Temperature (SST) anomalies for the Holocene, plotted on the B2K timescale. The light blue shading denotes the 68% Confidence Interval. The red line is a 200-year resampling, based on the coarsest chronological resolution of all the 2σ error margins used to construct the SST stack. The yellow line represents the 68% Confidence Interval associated with the 200-year time step. B. Same as (A) but excluding the annual resolution and associated Confidence Intervals.
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**Figure S4:** Stack of Mediterranean Sea Surface Temperatures (SST) for the Holocene, plotted at 50-year intervals. The light blue error bars denote the 68% Confidence Intervals. The green error bars denote the average 2σ error of all records used to reconstruct the SST record at time point x. The grey inset is a boxplot of the 2σ chronological errors in the initial 54 records (in years) used to construct the SST stack.
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**Figure S5:** Comparison between SST minus mean average (cold colours) and SST spatial average (warm colours).
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**Figure S6:** Potential role of geographic location in influencing the overall SST signal for the Holocene. A. Each SST record (West, Central and East Mediterranean) is presented as deviations from the mean. B. The number of sequences used per geographic location is shown as deviations from the mean. C. The weight of each geographic location in the final SST curve is subtracted (using A. x B.) and the resulting three curves (West, Central and East Mediterranean) are compared and contrasted with each other.
<figure><img src="image_27.png" /><figcaption>B.</figcaption></figure>

**Figure S7:** A. Mediterranean SST anomalies during the past 2000 years and broad climatic periods. The stars denote freezing events in the Bosphorus and cold winters in Istanbul, based on historical records (Yavuz et al., 2007). B. Short-time Fourier transform of the data highlighting the periods of marked climatic variability.
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**Figure S8:** Boxplots of Mediterranean SST variability during the Holocene, highlighting the magnitude and rate of ongoing industrial-era warming. Recent warming contrasts markedly with long-term Holocene cooling of the Mediterranean Sea.
<figure><img src="image_29.png" /><figcaption>**Figure S9:** Holocene changes in Mediterranean SST°C using moving 46-years windows for the complete sequence. The 46-year window corresponds to the current phase of rapid Industrial-era warming (1973 to 2019). The first value covers the temperature rise between 1973 and 2019, the second between 1972 and 2018, the third between 1971 and 2017 and so forth for the entire sequence. The associated confidence interval comprises the average of the confidence intervals for each 46-yr time-span.</figcaption></figure>