North American “warming holes” and European heat during abrupt Holocene cooling events in the Atlantic

Domain: earth_science
Published: 2025-10-13
Editorial outcome: minor_revision

Abstract
In recent decades, Europe has warmed rapidly while a prominent ‘warming hole’ has muted changes in eastern North America. Paradoxically, the pattern coincides with cooling in the North Atlantic, even though ocean cooling is commonly expected to chill Europe, not North America. The regional departures from global trends and expectations may have informative precedents in millennial-scale climate anomalies during the Holocene. Here, circum-Atlantic records reveal that counter-intuitive European warming developed amid rapid cooling of the North Atlantic and North America from 5.8-4.8 ka and after 2.2 ka. The anomalies track rapid reductions in the flow of North Atlantic Deep Water southwest of Iceland and appear in independent but highly correlated records of mean sea-surface temperatures, continental temperatures, and hydroclimate changes from both sides of the Atlantic. The patterns confirm the importance of millennial climate variability during the Holocene and the risk of climate disruptions around the North Atlantic today.

Introduction
As current anthropogenic climate trends increase the risk of rapid climatic state shifts, unusual patterns such as ‘warming holes,’ intense heatwaves, and extreme droughts differentiate regional responses1–5 (Fig. 1). The patterns of interglacial climates may provide useful analogs for understanding such regional departures from global trends6,7. Looking beyond the slow, globalevolution of Holocene climates can reveal controls on abrupt shifts and regional differences possible during warm Earth system states8. Holocene climate changes were likely part of a power-law spectrum of variability, including significant millennial variability, particularly at regional scales7,9. Mismatches between paleoclimate reconstructions and orbitally-forced Holocene climate simulations underscore the need to re-examine any potential millennial-scale dynamics10,11 by identifying and evaluating benchmark patterns useful for testing numerical and conceptual models12,13.

The challenge is not simply recognizing that millennial changes took place during the Holocene, but rather to develop synoptic perspectives aimed at predicting their spatial patterns, regional impacts, and implications for the future. To do so, we examine a geographically gridded network of pollen-inferred summer warmth reconstructions, representing growing degree days above 5°C (GDD5)13, as well as an updated network of sea-surface temperature (SST) reconstructions spanning the North Atlantic Ocean. The SST reconstructions, updated from the Temp12ka dataset14, include all available reconstructions from 8.0–0.8 ka to maximize geographic coverage. We use the data to evaluate correlations among continental climates and broad oceanic changes within the western and central North Atlantic, although sparse data coverage prevents analyses of some relevant spatial patterns, such as those directly related to the Gulf Stream15.

Here, the network of records allows us to study millennial climate variations that were sufficiently large and extensive to coordinate impacts in disjunct ecosystems across two continents. We examine the hypotheses that interglacial instability affected North Atlantic climates and that North Atlantic cooling, such as that induced by slowed Atlantic Meridional Overturning Circulation (AMOC), can chill Europe amid a globally warm climate. Models are uncertain about such risks during interglacial climates12,16. We focus on summer patterns of the past 8 ka after the influence of the northern hemisphere ice sheets diminished and find patterns consistent with current North Atlantic cooling, the summer ‘warming hole’ in North America, coincident extreme heat in Europe, and affiliated hydroclimate changes (Fig. 1).

Results
Opposite sign millennial changes in Europe and North America Summer temperature (GDD5) changes over the past 8000 years include millennia with different direction anomalies across northwest Europe and mid-latitude eastern North America (ENA; Fig. 2). Over the long term, summer warmth declined across northwest Europe (orange line, Fig. 2A) and, to a lesser degree, mid-latitude ENA (blue line, Fig. 2A). Previous work showed that a stepwise increase in temperature interrupted the trends in the mid-Holocene13. However, the mid-Holocene increase developed primarily in Europe as ENA cooled from 5.8-4.8 ka (blue bar, Fig. 2A). Abrupt warming in Europe and cooling in ENA was then repeated at 2.2 ka (gray bars, Fig. 2A). Both events increased the thermal difference between the continents in summer.

The changes after 2.2 ka warmed Europe but cooled ENA and the North Atlantic during the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), demonstrating large-scale reorganization but not universal warmth17.

Mean SSTs across the central and western North Atlantic declined rapidly as Europe warmed at 5.8 and 2.2 ka (black line, Fig. 2B). The mean rate of central-western SST change (from southwest of Iceland to the Gulf of Mexico, as defined by the large inset box in Fig. 2D, E) correlates negatively with the European rate of GDD5 change per millennium (Fig. 2C; Pearson product moment correlation, r = −0.39, 95% CI: −0.58 - −0.15, df=61). Maps show expansive North Atlantic cooling, as well as warming like that today (Fig. 1C) west of Greenland and along the Labrador coast (Fig. 2D). Linear regression indicates that for every 1°C of cooling in the central-western North Atlantic mean, northwest European warmth increases by the equivalent of 1°C over ~100 days per year: ΔGDD5 = (−101±28°C days °C SST −1xΔSST) - 31±7°C days where Δ represent the rate of change per 1000 years (adjusted R2=0.16, df=61, p=0.0006). Other analyses of SSTs affirm the rapid mid-Holocene cooling8,18,19, although individual records commonly differ from the means because of spatial variability typical of the surface ocean.

The temperature changes after 5.8 and 2.2 ka also correspond to effective moisture changes (Figs. 3, 4), which are spatially similar to those that exist today (Fig. 1). The oceanic changes (Fig. 3A, B) and reduced ENA temperatures (Fig. 3C) correlate significantly with an increase in mean effective moisture across ENA (increased lake levels, reduced dust, Fig. 3D)20. The increase likely facilitated cooling there by reducing sensible heating2 (Fig. 3C). Reduced effective moisture may have produced the opposite outcome in other regions, such as California (Fig. 3D) and Europe (Fig. 4). Trees submerged in Sierra Nevada lakes have been interpreted as evidence of severe droughts in California from 5.8–4.5 ka and after 2.2 ka21,22 (histogram, Fig. 3D). From 4.8–3.5 ka, the reverse hydroclimate pattern developed with drying common in ENA20 (Fig. 3D) and sedimentary evidence of pluvial conditions in the Sierra Nevada23. Likewise, a previous synthesis of lake-level records from Europe24 indicates a strong correlation between summer warmth and effective moisture (Fig. 4A, B; Pearson product-moment correlation, r=-0.77, 95% CI: −0.88 - −0.60, df=35). Low effective moisture extended furthest across Europe during peak warmth from 5.8–4.6 ka. Afterwards, lakes rose widely except during renewed warming and drying from 2.2–1.4 ka (Fig. 4B). The correlations among European and North American temperature changes (Fig. 2A), with North Atlantic SSTs (Fig. 3B, C), and with the sedimentary evidence of hydroclimate changes (Figs. 3, 4) show no significant lags in the pollen response underlying the temperature reconstructions and indicating rapid terrestrial ecosystem consequences.

Context of AMOC reorganization

The millennial temperature anomalies during the Holocene appear related to AMOC reorganization, even if a large-scale shutdown did not take place (blue bar, Fig. 3). North Atlantic Deep Water (NADW) flow rates declined, at least in some areas, when much of the North Atlantic Ocean cooled (Fig. 3A, B). The flow rate inferred from the mean size of sortable silt from southwest of Iceland25 (core MD2251, Fig. 3A) correlates significantly with our mean SST reconstruction (Figs. 2B, 3B) (Pearson product moment correlation, r=0.78, 95% CI: 0.67–0.86, df=61). The correlation exceeds the 95% range of 1000 random surrogate time series with the same spectral properties (see Supplementary Information) and suggests that the MD2251 record could represent a broad North Atlantic dynamic. The detailed sortable silt record from core MD225125 linearly predicts not only the mean SST history (Fig. 3B) but also changes in North American temperature and hydroclimate (Fig. 3C, D) and the timing of the GDD5 changes in northwest Europe (Fig. 4).

Other mineralogical indicators of NADW flow26 and an additional set of sortable silt records from near Iceland27 confirm the correlation with SSTs (Fig. 5A, B) expressed in core MD2251 (Fig. 3A). They indicate that Iceland-Scotland (I-S) overflow, a component of the AMOC, slowed at 5.8 and 2.2 ka, especially at water depths of <1.6 km27, even though overflow flow remained persistently high elsewhere28,29 (see Supplementary Information). The continuous flow rates in these other areas agree with 231Pa/230Th records that indicate negligible Holocene changes in deep-water residence times30. However, carbon isotopes show limited NADW penetration near Bermuda31 and Ireland32 from 5.8–4.8 ka (see Supplementary Information). Changes afterwards involved a rapid acceleration of Labrador Sea Water flow, another component of AMOC, off eastern Canada33. The AMOC flow signals appear reproducible (Fig. 5B), but dependent on location, which reveals spatial variability and Holocene proxy differences tied to diverging flow, penetration, and residence time variations, which have analogs in variability among sites and proxies during Pleistocene abrupt changes34 and today35.

Methods
Modern climate trends
Maps of modern climate trends around the North Atlantic Ocean (Fig. 1) were generated using the NCAR/NCEP Reanalysis I56. The differences between CE 1951–2000 and 2001–2020 were calculated using online tools provided by the NOAA Physical Sciences Laboratory, Boulder, Colorado, USA, and obtained as NetCDF files from their website at https://psl.noaa.gov/. The focus on 21st century differences from the second half of the 20th century avoids effects of the warm, dry CE 1930s on the ‘warming hole’ and related North American temperature statistics2,4,5.

Holocene temperatures
The 2°x2° latitude/longitude grid of growing degree day base 5°C (GDD5) reconstructions from Marsicek et al.13 were extracted from the NOAA Paleoclimate database (https://www.ncei.noaa.gov/access/paleo-search/study/22992) and mapped using R version 4.3.157. The reconstructions include 100-year time steps spanning the Holocene based on GDD5 reconstructions from 584 pollen records that passed a significance screen based on comparisons of each reconstruction with a distribution of random null models for each site13. We map the changes for the 5.8 and 4.8 ka intervals (Fig. 2) as well as 2.2 ka (Fig. SI2) based on the change over 600 years by subtracting an initial interval (6.0, 5.3, or 2.5 ka) from a post-change interval (5.4, 4.9, or 1.9 ka, respectively). The changes were mapped after classifying the anomalies into zero change (−50 to 50°C days), moderate change (50–100°C days magnitude) and large changes (>100°C days magnitude).

To produce European and North American GDD5 timeseries, grid cells from the target regions were extracted and averaged per century. For each century, beginning at 9 ka, 75% of the available grid cells in each region were randomly selected and averaged using base functions in R. The process was repeated 100 times to generate a distribution of mean GDD5 reconstructions for each region, which accounts for spatial variability and differences in time and reconstruction uncertainty among the grid cells13; the timeseries in Fig. 2A show the median and 95% range of the regional means. The CE 1950 (0 ka) value was subtracted from each of the regional means prior to plotting as anomalies. GDD5 units represent the mean annual sum of degree days, which are total daily mean increases in degrees (°C or K) above 5°C per year.

To produce timeseries of North Atlantic Ocean sea-surface temperatures (SSTs), all SST reconstructions from the Temp12ka database14 were combined with additional SST reconstructions from the region that spanned from 8.0–0.9 ka43,58–60 (Table SI1). The additional records were either too new or did not span a sufficient duration of the Holocene for inclusion in the Temp12ka dataset. All data were obtained from either the PANGAEA or NOAA Paleoclimate databases, and most derived from two sources: Temperature 12ka Database (https://www.nature.com/articles/s41597-020-0445-3)14 and the database of Holocene temperature reconstructions for northeastern North America and the northwestern Atlantic (https://doi.org/10.1594/PANGAEA.905293)58. Additional individual records were then identified and added43,59,60.

For consistency with the GDD5 data, each SST record was interpolated to 100-yr time steps using base functions in R in accordance with recommendations61. Maps show their differences based on the same 600-yr time intervals as used for the GDD5 data. Likewise, all SST reconstructions from southwest of Iceland to the Gulf of Mexico (south of 65°N and west of 15°W) were averaged using the same iterative approach, based on 100 iterations of 75% of the records to create a mean SST timeseries and 95% confidence interval. The CE 1950 (0 ka) value was again subtracted from the mean prior to plotting as anomalies. To confirm the spatial extent and coherence of the anomalies, the same approach was used with the prcomp function in R to conduct a principal components analysis (PCA), except that all 77 SST records spanning from 8.0–0.9 ka north of 20°N were included (Fig. SI3). All SST reconstructions were included regardless of seasonality to ensure maximum geographic coverage and because the magnitudes of millennial anomalies are consistent across seasons, but the Supplementary Information includes versions of the analysis using only annual (Fig. SI1, n=26 in our core study region) and only summer SST reconstructions (Fig. SI2, n=7 in our core study region).

Rates of change and correlations
Rates of change (Fig. 2C) were calculated by subtracting the change in the regional mean GDD5 and SST reconstructions over 1000-year intervals, centered on the 500-year mid-point for each interval. Time series in Fig. 3 compare our mean SST and the previously published a) NADW flow rate reconstruction based on sortable silt size data from core MD2251 from the central Gardar Drift, south of Iceland25, b) eastern North America mean temperature of the warmest month (MTWM) reconstruction based on well-resolved fossil pollen stratigraphies between Wyoming and Maine20, and c) eastern North America mean effective moisture based on the average z-scores of dust and lake-level records from Wyoming to Massachusetts20. The correlations of the NADW flow rate with the SST, MTWM, and moisture reconstructions were compared with those calculated based on 1000 random surrogate time series with a power-law spectrum with the same spectral slope as the actual reconstructions; the surrogates were generated and correlations compared using the corit package in R61. The correlations were compared based on both the raw (Fig. SI4) and detrended timeseries (see Fig. SI5). To compare the different sign GDD5 changes in Europe and North America, the absolute values of the 1000-yr rates of change were also compared (Fig. SI6).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Shuman and Stefanescu makes some interesting observations about broad-scale patterns of temperature and hydroclimate change between North America, Europe, and the North Atlantic during the Holocene. They make a fairly compelling case that there are two times when these regions shift in relative directions that are broadly consistent with patterns of change during recent decades: at ~5.8 and 2.2 ka, Europe warmed, eastern North America cooled and got wetter, the western US dried, and SSTs cooled. I am supportive of eventual publication, but I have some substantial comments that should be addressed in revision.\nOne weakness is that the ocean fingerprint over recent decades, that of a ‘warming hole’ southwest of Iceland, cannot really be resolved with present paleo-SST data. Instead the authors combine the entire North Atlantic south and west of Iceland, for which “southwest of Iceland” is a misleading name (the box is referred to as black in the Fig 2 caption, but I think it’s the giant gray box). In attributing the modern SST pattern to an AMOC slowdown, the region of extreme warming off the northeast US is equally important, as it plausibly reflects the accumulation of Gulf Stream heat there. The paleo data do not show such a dipole (Fig 2D), though warming off west Greenland and east Labrador fits. I suggest that the authors explicitly acknowledge the limitations of the paleo SST data, particularly the lack of coverage in the modern Gulf Stream warming region, and that the data is overall too sparse to clearly reconstruct spatial patterns. Here a spatial infilling technique could be valuable, but that could be left to future work.\nThe subject of paleo AMOC as an explanation is really key to this paper, but the lede is kind of buried, because AMOC does not appear in the title or the abstract. The comparison in Fig 3 of the sortable silt record to regional records (SST, ENA temperature, ENA moisture) is quite striking. I would guess that the authors’ reticence is related to the fact that other sortable silt records from the region show different patterns (e.g. the gray line in Fig 4). Thornalley et al. (2013, Clim Past) do a nice job of showing that Holocene SS is reproducible, but highly dependent on which part of NADW the core sits in. I encourage the authors to provide a fuller context here, and talk about what, exactly, the Hoogaaker record likely records, relative to other records that show different histories. I believe Thornalley commented on that issue. He also provided a weighted ISOW stack that, although low resolution, shows a slowdown around 6 ka. It’s not clear why the authors show the gray record in Fig 4 rather than other options. The idea of ISOW compensating for LSW (p. 9) left me confused in terms of both supporting evidence and whether it is required.\nI’m not sure how the Florida Straits SST record in Figure 4 fits in: is it recording AMOC strength directly (northward flow of tropical waters)? The new Florida Straits O2 record is also mentioned as fitting the story, but I think the Holocene resolution and noise of that record is too poor to say anything.\nI think the Discussion falls a bit flat on the question of forcing. Arguably we have an AMOC slowdown with continental climate effects that resemble the pattern over recent decades. That would both strengthen the idea that the recent warming hole is due to AMOC, and that the continental patterns may be too. Again, a bit buried. But why did this all happen? Is it stochastic? The coincidental timing of the end of the African Humid Period is intriguing. That event has been interpreted as a positive feedback threshold response to slow orbital forcing. Might the same idea play out on AMOC? Or was the AMOC a response to AHP-related changes in hydroclimate?\nIn Fig 1B, why has the air over the warming Gulf Stream (off northeastern US) actually cooled?\nI may have missed it, but are there European paleo-hydroclimate indicators that might fit the strong pattern shown in Fig 1C? If there are indicators but they are ambiguous or don’t ffit, that should be briefly mentioned.\nThe nature of the SST proxies should be briefly mentioned in the introduction, and not left solely to the Methods. The reference numbers in Table S1 don’t seem to match the Methods or the reference list.\nI found the labels on Fig 2D-E confusing. I suggest changing them to “5.4 minus 6.0 ka” and “4.7 minus 5.3 ka.”\nSecond line of abstract: remove “also.”", "reviewer_2": "The manuscript by Shuman and Stefanescu presents a novel spatial analysis of Holocene climate anomalies of the last 8000 years. As the authors indicate, this analysis is urgently required for drawing lessons of relevance from the Holocene past for ongoing and future climate changes linked to the dynamics of the Atlantic system. The study builds on an existing pollen database that underpinned previous reconstructions through Holocene time, but presents the original findings of spatial analysis for N America and Europe, coupled to new analysis of palaeoceanographic records for the Atlantic. Overall, it is a powerful approach that permits a robust comparison across the Atlantic sector, with noteworthy detection of contrasting temperature anomalies between the regions. The methodology is sound, evidencing careful attention to statistical analysis of correlations between different records. The results are noteworthy and can provide a stimulus for the research community to consider the spatial component of Holocene climate changes in more detail. Overall, I would recommend publication of this manuscript with attention to some line-by-line detailed comments and some more general observations (>>) given below, generally with some attention to strengthening the final discussion.\nPage 1 - line 20 - can be more specific about which future risks this speaks to\n>>good emphasis on spatial patterns, spectrum of variability and departures from global trends\n>>emphasis on synoptic perspectives v important\n>>uses a previous pollen 'product' that the authors are well versed in\n>>sensible time interval choice to avoid deglaciation effects\np3 l21 - being critical the \"blue line, Fig 2A\" does not speak to \"parts of ENA\" - either need to clarify the evidence for \"parts of\" or simplify the statement\np4 l5 - might be worth linking this to the problem of no universal definition of these historical climate periods: Neukom, R., Steiger, N., Gómez-Navarro, J.J., Wang, J., Werner, J.P., 2019. No evidence for globally coherent warm and cold periods over the preindustrial Common Era. Nature 571, 550-554.\n>note that the start of the RWP is poorly defined, generally all the historical climate intervals are\np4 l21 - perhaps I am missing something but wouldn't submerged trees point to intervals of higher lake levels and tree death by waterlogging of roots? Please clarify the mechanism to be sure of the phasing with moisture/drought. I am picturing this phenomenon in lakes in MOrocco but perhaps the context is different...\nFigure 2D - is there a NW (cooler)-SE(warmer) gradient over European continent? Could be worth commenting and developing. It reminds me of AMO pattern, see O’Reilly, C.H., Woollings, T., Zanna, L., 2017. The dynamical influence of the Atlantic multidecadal oscillation on continental climate. Journal of Climate 30, 7213-7230. We wrote about this here (https://doi.org/10.1016/B978-0-323-99712-6.00026-X), in section 9.3. I am curious whether the authors have considered links to prevailing atmospheric modes of variability.\np6 l31 - please check - \"Fig 3-4\" should that be 3A? or, if this is Figure 3 and 4, then it would be clearer to point the reader specifically to each in turn\n>>I wonder why the authors don't include or show the Principal component time series associated with the second mode from SST proxies of HAMOC data (Ayache et al., 2018) since this supports the AMOC overview nicely. Or, seeing that their new analysis here (Supp Info) actually reproduces that second PC, perhaps show that or comment further on this here.\np7 l2 - \"could represent\" - typo\np7 check GDD5 - some typos\np7 l11 - \"areas including\" rather than \"like\"\np7 l 12 \"others such as\" rather than \"like\"\n>>Regarding the discussion of the ISOW, I think the authors must take into account the work of Paola Moffa-Sanchez, e.g. Moffa-Sanchez, P., Hall, I.R., Thornalley, D.J., Barker, S. and Stewart, C., 2015. Changes in the strength of the Nordic Seas Overflows over the past 3000 years. Quaternary Science Reviews, 123, pp.134-143. This works shows that there are antiphased components of the Nordic overflows east and west of Iceland, which can add some precision to the statement e.g. \"some components of Iceland-Scotland (I-S) overflow, part of the AMOC, declined near Iceland at ca. 6 and 2 ka\"\nP7 l22 beginning \"However, a spatially complex...\" I think the message of this sentence can be clarified, what is being compared and what is the key message?\nP8 l28 \"...related to a global array of anomalies\" - I find this statement a bit vague and the two references a bit underpowered. Refine or simply remove.\n>>The authors may be interested in a review of the 5.2 ka event we undertook here, although the emphasis there was intra-continental patterns in Europe: https://doi.org/10.1016/B978-0-323-99712-6.00027-1\nP9 l2 - yes, and also feedbacks and amplifiers\nP9 l 6 \"Potentially, as during recent extreme years today\"\n>> Paragraph beginning \"Additional feedbacks...\" I don't find the link to African dust supply very convincing here - although I don't doubt some possible teleconnections/interactions. There is a much larger debate about the timing and nature (abrupt/progressive/step-wise) aridification of N Africa in response to precessional forcing. I'd be cautious about using the ODP 658C dust curve uncritically, without noting, e.g. T.M. Shanahan, N.P. McKay, K.A. Hughen, J.T. Overpeck, B. Otto-Bliesner, C.W. Heil, J. King, C.A. Scholz, J. Peck. The time-transgressive termination of the African Humid Period. Nat. Geosci., 8 (2015). Overall, the selection of far-flung records in Figure 5 all illustrate a Middle Holocene Transition, or crossing of thresholds in diverse enviromental systems. However, it's very difficult to evidence that they are mechanistically connected. It's an interesting challenge, we recently discussed briefly here (section 7.3): https://doi.org/10.1016/B978-0-323-99712-6.00027-1.\n>>Overall, I'd be tempted to cut out this 'widest field' of connections to allow more space to develop the N America-Atlantic-Europe analysis and implications. However, I think it could be useful to look northward to Greenland - did the authors consider the reconstruction of Kobashi, T., Menviel, L., Jeltsch-Thömmes, A., Vinther, B.M., Box, J.E., Muscheler, R., Nakaegawa, T., Pfister, P.L., Döring, M., Leuenberger, M. and Wanner, H., 2017. Volcanic influence on centennial to millennial Holocene Greenland temperature change. Scientific reports, 7(1), p.1441? This shows a clear Middle Holocene cooling that would be worth discussing.\np10 l15 : \"For example, major tree taxa like Ulmus (elm) declined widely at ca. 5.8 ka\" - implies several taxa, but I don't know of others than Ulmus. Clarify or reword. The text here only implies a linkage to climate so could be more explicit about what is inferred here.\np10 l15 \"In North America...\" I think this complex sentence might better be split into two - one about the past, one about implications for the \"current agricultural center\"\nP11 l4. I think that the authors might try to build the evidence base from literature more strongly about spatially variable imprint of e.g. the MCA - unlike the Middle Holocene, this topic has been widely researched, tree-rings, etc. I'm thinking of the work , e.g. Cook, B.I., Cook, E.R., Smerdon, J.E., Seager, R., Williams, A.P., Coats, S., Stahle, D.W. and Díaz, J.V., 2016. North American megadroughts in the Common Era: Reconstructions and simulations. Wiley Interdisciplinary Reviews: Climate Change, 7(3), pp.411-432.\n>>Having noted my confusion about the submerged trees above, I don't follow the \"stark warning\" as clearly as the authors probably hoped! I would unpack this sentence to clarify to the reader what the current ocean heat transport trends are, and which wider consequences might be inferred to follow thanks to this new analysis. p11 l5.\np11 l6 \"They also confirm\" - I'd shift the agency away from the trees to the authors \"We\" \"Our analysis\" etc\nMethods\nLine 15 - please clarify the nature of the \"significance screen\"\n>>Did the authors try to drill down to sub-continental regions? I'm guessing that if they did then maybe the significance or patterns weren't clear or convincing? Whatever the case, might be worth a comment on that.", "reviewer_author_response": "Response to REVIEWER COMMENTS\nWe greatly appreciated the constructive reviews and have tried to respond to all\nof the points that we raised. Several of these points helped to refine the\nmanuscript significantly and have resulted a major changes to figures and\nDiscussion. The bullet points below explain our responses to each point raised in\nthe review.\nReviewer #1 (Remarks to the Author):\nThe manuscript by Shuman and Stefanescu makes some interesting observations\nabout broad-scale patterns of temperature and hydroclimate change between North\nAmerica, Europe, and the North Atlantic during the Holocene. They make a fairly\ncompelling case that there are two times when these regions shift in relative directions\nthat are broadly consistent with patterns of change during recent decades: at ~5.8 and\n2.2 ka, Europe warmed, eastern North America cooled and got wetter, the western US\ndried, and SSTs cooled. I am supportive of eventual publication, but I have some\nsubstantial comments that should be addressed in revision.\nOne weakness is that the ocean fingerprint over recent decades, that of a ‘warming\nhole’ southwest of Iceland, cannot really be resolved with present paleo-SST data.\nInstead the authors combine the entire North Atlantic south and west of Iceland, for\nwhich “southwest of Iceland” is a misleading name (the box is referred to as black in the\nFig 2 caption, but I think it’s the giant gray box).\n We have revised to provide a more accurate description of the geographic extent\nof our SST mean by referring to the region as ‘the central and western North\nAtlantic’.\n The box is black in the figure, but if it appears gray, we could be revise to another\ncolor if appropriate. For clarification, we now state that it is the “large inset box.”\nIn attributing the modern SST pattern to an AMOC slowdown, the region of extreme\nwarming off the northeast US is equally important, as it plausibly reflects the\naccumulation of Gulf Stream heat there. The paleo data do not show such a dipole (Fig\n2D), though warming off west Greenland and east Labrador fits. I suggest that the\nauthors explicitly acknowledge the limitations of the paleo SST data, particularly the lack\nof coverage in the modern Gulf Stream warming region, and that the data is overall too\nsparse to clearly reconstruct spatial patterns. Here a spatial infilling technique could be\nvaluable, but that could be left to future work.\n This is a very good point. We have added text describing this limitation to the\nsecond paragraph of the Introduction\nThe subject of paleo AMOC as an explanation is really key to this paper, but the lede is\nkind of buried, because AMOC does not appear in the title or the abstract.\nThe comparison in Fig 3 of the sortable silt record to regional records (SST, ENA\ntemperature, ENA moisture) is quite striking. I would guess that the authors’ reticence is\nrelated to the fact that other sortable silt records from the region show different patterns\n(e.g. the gray line in Fig 4).\n We felt cautious about making a strong claim about AMOC for precisely this\nreason, but appreciate the encouragement to emphasize the strength of the\nrelationships in the data.\n We have revised the title to better draw attention to the main patterns in the data,\nincluding the role of the North Atlantic changes.\n We have similarly updated the Abstract to be specific about the correlation with\nNADW changes and to avoid burying the lede.\nThornalley et al. (2013, Clim Past) do a nice job of showing that Holocene SS is\nreproducible, but highly dependent on which part of NADW the core sits in. I encourage\nthe authors to provide a fuller context here, and talk about what, exactly, the Hoogaaker\nrecord likely records, relative to other records that show different histories. I believe\nThornalley commented on that issue. He also provided a weighted ISOW stack that,\nalthough low resolution, shows a slowdown around 6 ka. It’s not clear why the authors\nshow the gray record in Fig 4 rather than other options. The idea of ISOW\ncompensating for LSW (p. 9) left me confused in terms of both supporting evidence and\nwhether it is required.\n We appreciate these suggestions and have revised our figures accordingly. We\nremoved, but still cite, the gray record that had been shown in Fig. 4, and replace\nit instead with the Thornalley stack and other evidence supporting the ISOW\nslowdown in Figure 5A-B. (Figure 4 is changed, and Figure 5 expanded, to\naddress other suggestions below).\n We have streamlined the related text (“Context of AMOC reorganization” section\nof the Results and first 2 paragraphs of Discussion) to acknowledge but not\noverly dwell on the differences across sectors of the AMOC.\n We modified the reference to the compensation of flow across different areas, but\nretained an acknowledgement of the idea to be consistent with Mjell et al. (2015),\nreference #31.\nI’m not sure how the Florida Straits SST record in Figure 4 fits in: is it recording AMOC\nstrength directly (northward flow of tropical waters)? The new Florida Straits O2 record\nis also mentioned as fitting the story, but I think the Holocene resolution and noise of\nthat record is too poor to say anything.\n Given the complexity of this point, we have removed the references to the Florida\nStrait record.\nI think the Discussion falls a bit flat on the question of forcing. Arguably we have an\nAMOC slowdown with continental climate effects that resemble the pattern over recent\ndecades. That would both strengthen the idea that the recent warming hole is due to\nAMOC, and that the continental patterns may be too. Again, a bit buried. But why did\nthis all happen? Is it stochastic? The coincidental timing of the end of the African Humid\nPeriod is intriguing. That event has been interpreted as a positive feedback threshold\nresponse to slow orbital forcing. Might the same idea play out on AMOC? Or was the\nAMOC a response to AHP-related changes in hydroclimate?\n We really appreciate this critique and have substantially revised the Discussion to\ndiscuss the forcing question. Hopefully, the revision suitable considers the three\nmain categories of drivers (external forcing, internal climate dynamics, and\nfeedback/threshold-driven changes potentially including the end of the AHP).\n We also revised Fig. 5 to help provide the broader context here.\n We moved the previous Fig. 5 to the Supplementary Information (now Fig. SI8)\nand revised it to show, however, that the millennial anomalies here raise a\nquestion about whether a positive feedback threshold drove the end of the AHP –\nor whether the rapid change was simply a function of interference between long-\nterm orbital trends and millennial-scale variability involving AMOC and SST\nchanges. The idea may be provocative, but is a potential implication of our\nanalysis: millennial-scale cooling of the North Atlantic would likely shift sub-\ntropical moisture southward during the same period, accelerating the orbitally\nforced trends in west Africa (while temporarily reversing them in Ghana, see\nrevised Fig. 5).\nIn Fig 1B, why has the air over the warming Gulf Stream (off northeastern US) actually\ncooled?\n This is an intriguing question but requires additional analyses of the modern\nclimate system beyond the scope of our study. (It is fascinating, though,\nespecially because New England experienced significant cooling when AMOC\nslowed during the Younger Dryas – even though, as noted above, the spatial\nfingerprint of AMOC changes should have included a warm anomaly associated\nwith Gulf Stream heat off the NE coast. Was there a similar disconnect between\nocean and atmospheric temperature changes at that time? We hope to explore\nthis topic in another manuscript).\nI may have missed it, but are there European paleo-hydroclimate indicators that might fit\nthe strong pattern shown in Fig 1C? If there are indicators but they are ambiguous or\ndon’t ffit, that should be briefly mentioned.\n Thank you for asking this question, which we should have thought to explore\ninitially. We turned to an existing synthesis of European lake-level records and\nfound that the relationship to temperature was quite strong.\n We have revised Figure 4 to emphasize this relationship and modified text\nthroughout to point out that temperature-moisture relationships appear consistent\nwith the modern pattern on both continents, not just North America.\nThe nature of the SST proxies should be briefly mentioned in the introduction, and not\nleft solely to the Methods. The reference numbers in Table S1 don’t seem to match the\nMethods or the reference list.\n We have added text briefly describing the SST dataset to the second paragraph\nof the Introduction.\n The reference numbers have been corrected.\nI found the labels on Fig 2D-E confusing. I suggest changing them to “5.4 minus 6.0 ka”\nand “4.7 minus 5.3 ka.”\n Changed the labels\nSecond line of abstract: remove “also.”\n The abstract has been revised.\nReviewer #2 (Remarks to the Author):\nThe manuscript by Shuman and Stefanescu presents a novel spatial analysis of\nHolocene climate anomalies of the last 8000 years. As the authors indicate, this\nanalysis is urgently required for drawing lessons of relevance from the Holocene past\nfor ongoing and future climate changes linked to the dynamics of the Atlantic system.\nThe study builds on an existing pollen database that underpinned previous\nreconstructions through Holocene time, but presents the original findings of\nspatial analysis for N America and Europe, coupled to new analysis of palaeoceanographic\nrecords for the Atlantic. Overall, it is a powerful approach that permits a robust\ncomparison across the Atlantic sector, with noteworthy detection of contrasting\ntemperature anomalies between the regions. The methodology is sound, evidencing\ncareful attention to statistical analysis of correlations between different records. The\nresults are noteworthy and can provide a stimulus for the research community to\nconsider the spatial component of Holocene climate changes in more detail. Overall, I\nwould recommend publication of this manuscript with attention to some line-by-line\ndetailed comments and some more general observations (>>) given below, generally\nwith some attention to strengthening the final discussion.\nPage 1 - line 20 - can be more specific about which future risks this speaks to\n The Abstract has been substantially revised\n>>good emphasis on spatial patterns, spectrum of variability and departures from global\ntrends\n>>emphasis on synoptic perspectives v important\n>>uses a previous pollen 'product' that the authors are well versed in\n>>sensible time interval choice to avoid deglaciation effects\np3 l21 - being critical the \"blue line, Fig 2A\" does not speak to \"parts of ENA\" - either\nneed to clarify the evidence for \"parts of\" or simplify the statement\n This section has been revised.\np4 l5 - might be worth linking this to the problem of no universal definition of these\nhistorical climate periods: Neukom, R., Steiger, N., Gómez-Navarro, J.J., Wang, J.,\nWerner, J.P., 2019. No evidence for globally coherent warm and cold periods over the\npreindustrial Common Era. Nature 571, 550-554.\n We appreciate the suggestion and have added the citation in the revised text.\n>note that the start of the RWP is poorly defined, generally all the historical climate\nintervals are\np4 l21 - perhaps I am missing something but wouldn't submerged trees point to intervals\nof higher lake levels and tree death by waterlogging of roots? Please clarify the\nmechanism to be sure of the phasing with moisture/drought. I am picturing this\nphenomenon in lakes in MOrocco but perhaps the context is different...\n We revised the text here to clarify that we are relying on the published\ninterpretations of the trees as representing drought periods. Space limitations\nprevent further clarification, although if it would be useful, we could add a\nsentence stating that the trees are rooted well below the modern water levels\nindicating that they were likely killed by rising (but much lower than modern)\nwater levels at the end of prolonged periods of drought that enabled them to\nestablish and grow in currently submerged settings.\nFigure 2D - is there a NW (cooler)-SE(warmer) gradient over European continent?\nCould be worth commenting and developing. It reminds me of AMO pattern, see\nO’Reilly, C.H., Woollings, T., Zanna, L., 2017. The dynamical influence of the\nAtlantic multidecadal oscillation on continental climate. Journal of Climate 30, 7213-7230. We\nwrote about this here (https://doi.org/10.1016/B978-0-323-99712-6.00026-X), in section\n9.3. I am curious whether the authors have considered links to prevailing atmospheric\nmodes of variability.\n Space prevented us from further examining some of the spatial patterns within\neach continent, but we agree that there may be spatial fingerprints of dynamics\nlike the AMO or NAO.\n We have added a related sentence in the new paragraph about the driving\ndynamics in the Discussion on p. 10, although for space we did not add this\nspecific citation.\np6 l31 - please check - \"Fig 3-4\" should that be 3A? or, if this is Figure 3 and 4, then it\nwould be clearer to point the reader specifically to each in turn\n Figures and their callouts have been revised.\n>>I wonder why the authors don't include or show the Principal component time series\nassociated with the second mode from SST proxies of HAMOC data (Ayache et al.,\n2018) since this supports the AMOC overview nicely. Or, seeing that their new analysis\nhere (Supp Info) actually reproduces that second PC, perhaps show that or comment\nfurther on this here.\n We agree that this analyses seems quite consistent with ours. We did not show it\nsimply for space and clarity, but we do cite it in the section on AMOC.\np7 l2 - \"could represent\" – typo\n Thank you. Typo corrected\np7 check GDD5 - some typos\n Thank you. We believe that the typos have been corrected\np7 l11 - \"areas including\" rather than \"like\"\n This section of text has been substantially revised.\np7 l 12 \"others such as\" rather than \"like\"\n This section of text has been substantially revised.\n>>Regarding the discussion of the ISOW, I think the authors must take into account the\nwork of Paola Moffa-Sanchez, e.g. Moffa-Sanchez, P., Hall, I.R., Thornalley, D.J.,\nBarker, S. and Stewart, C., 2015. Changes in the strength of the Nordic Seas\nOverflows over the past 3000 years. Quaternary Science Reviews, 123, pp.134-143. This works\nshows that there are antiphased components of the Nordic overflows east and west of\nIceland, which can add some precision to the statement e.g. \"some components of\nIceland-Scotland (I-S) overflow, part of the AMOC, declined near Iceland at ca. 6 and 2\nka\"\n We now cite this work in the Discussion, p. 10, line 11.\nP7 l22 beginning \"However, a spatially complex...\" I think the message of this sentence\ncan be clarified, what is being compared and what is the key message?\n We have revised the sentence.\nP8 l28 \"...related to a global array of anomalies\" - I find this statement a bit vague and\nthe two references a bit underpowered. Refine or simply remove.\n We have revised the beginning of the Discussion and removed these references.\n>>The authors may be interested in a review of the 5.2 ka event we undertook here,\nalthough the emphasis there was intra-continental patterns in\nEurope: https://doi.org/10.1016/B978-0-323-99712-6.00027-1\n Wonderful! We are glad to know about this review and have included citations to\nit, specifically at the end of the Discussion where we build further on the idea of a\nMid-Holocene Transition. See also Fig. 5.\nP9 l2 - yes, and also feedbacks and amplifiers\n We removed this point here, but expand on feedbacks now in another section of\nDiscussion.\nP9 l 6 \"Potentially, as during recent extreme years today\"\n Suggested change made\n>> Paragraph beginning \"Additional feedbacks...\" I don't find the link to African dust\nsupply very convincing here - although I don't doubt some possible\nteleconnections/interactions. There is a much larger debate about the timing and nature\n(abrupt/progressive/step-wise) aridification of N Africa in response to precessional\nforcing. I'd be cautious about using the ODP 658C dust curve uncritically, without noting,\ne.g. T.M. Shanahan, N.P. McKay, K.A. Hughen, J.T. Overpeck, B. Otto-Bliesner, C.W.\nHeil, J. King, C.A. Scholz, J. Peck. The time-transgressive termination of the African\nHumid Period. Nat. Geosci., 8 (2015). Overall, the selection of far-flung records in\nFigure 5 all illustrate a Middle Holocene Transition, or crossing of thresholds in diverse\nenviromental systems. However, it's very difficult to evidence that they are\nmechanistically connected. It's an interesting challenge, we recently discussed briefly\nhere (section 7.3): https://doi.org/10.1016/B978-0-323-99712-6.00027-1.\n We have tried to amplify our rationale for including this connection in the revised\nparagraphs at the end of the Discussion about feedbacks and the drivers of\nchange, including by citing climate model simulations that would suggest a North\nAtlantic linkage to the end of the African Humid Period.\n Additionally, we revised Fig. 5 to show that the 658c dust curve is not unique but\nrepresents a broadly synchronous (or at least not clearly time transgressive)\npattern across several degrees of latitude. The coincident abrupt change\nextends, in a climatologically logical way consistent with rapid Atlantic cooling, to\nthe data from Shanahan et al., which we also now include in Fig. 5.\n We have added some expanded explanation of this potential linkage by revising\nthe original Fig. 5 and moving it to become Fig. SI8.\n>>Overall, I'd be tempted to cut out this 'widest field' of connections to allow more space\nto develop the N America-Atlantic-Europe analysis and implications. However, I think it\ncould be useful to look northward to Greenland - did the authors consider the\nreconstruction of Kobashi, T., Menviel, L., Jeltsch-Thömmes, A., Vinther, B.M., Box, J.E.,\nMuscheler, R., Nakaegawa, T., Pfister, P.L., Döring, M., Leuenberger, M. and Wanner,\nH., 2017. Volcanic influence on centennial to millennial Holocene Greenland\ntemperature change. Scientific reports, 7(1), p.1441? This shows a clear Middle\nHolocene cooling that would be worth discussing.\n We appreciate the suggestion to consider data from Greenland. We have added\ncitations to Kobashi et al., but found that a number of other Greenland records\nspanning a wide range of locations and different proxies made a stronger case\nthan the ice core data for mid-Holocene cooling on the island.\n We have updated Fig. 5 to show these results and added related text in the\nDiscussion.\np10 l15 : \"For example, major tree taxa like Ulmus (elm) declined widely at ca. 5.8 ka\" -\nimplies several taxa, but I don't know of others than Ulmus. Clarify or reword. The text\nhere only implies a linkage to climate so could be more explicit about what is inferred\nhere.\n This makes sense. For simplicity, we re-worded just to focus on Ulmus. We have\nalso added Ulmus decline data to the revised Fig. 4 focusing on Europe.\np10 l15 \"In North America...\" I think this complex sentence might better be split into two\n- one about the past, one about implications for the \"current agricultural center\"\n Sentence split into two now\nP11 l4. I think that the authors might try to build the evidence base from literature more\nstrongly about spatially variable imprint of e.g. the MCA - unlike the Middle Holocene,\nthis topic has been widely researched, tree-rings, etc. I'm thinking of the work , e.g.\nCook, B.I., Cook, E.R., Smerdon, J.E., Seager, R., Williams, A.P., Coats, S., Stahle,\nD.W. and Díaz, J.V., 2016. North American megadroughts in the Common Era:\nReconstructions and simulations. Wiley Interdisciplinary Reviews: Climate Change,\n7(3), pp.411-432.\n This is a great point, but for space, we did not pursue this point further in the text.\n>>Having noted my confusion about the submerged trees above, I don't follow the\n\"stark warning\" as clearly as the authors probably hoped! I would unpack this sentence\nto clarify to the reader what the current ocean heat transport trends are, and which\nwider consequences might be inferred to follow thanks to this new analysis. p11 l5.\n We revised the final paragraph of the Main Text, including this sentence, to try to\nbe more specific.\np11 l6 \"They also confirm\" - I'd shift the agency away from the trees to the authors \"We\"\n\"Our analysis\" etc\n This sentence has been revised substantially.\nMethods\nLine 15 - please clarify the nature of the \"significance screen\"\n We have expanded the information here.\n>>Did the authors try to drill down to sub-continental regions? I'm guessing that if they\ndid then maybe the significance or patterns weren't clear or convincing? Whatever the\ncase, might be worth a comment on that.\n We have added a brief comment on this point on p. 11, line 5. There are some\ninteresting patterns, but sufficiently complex that they seemed beyond the scope\nof the current manuscript."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "I previously assessed the manuscript by Shuman and Stefanescu as Reviewer #1. I have read carefully through the response to reviewers, and the revised manuscript. I am glad to see that the link to sortable silt holds up when additional records are included. Overall, I think this is a really nice demonstration of coherent patterns of climate change in the circum North Atlantic over the Holocene, with plausible links to the AMOC. It is an important advance for understanding Holocene climate history, with potential relevance for the future. I have no further significant suggestions. I am glad to recommend that the revised manuscript be accepted for publication in its current form.", "reviewer_2": "The authors have done a thorough and serious revision with careful attention to the comments of both reviewers. I find the revised manuscript to be much more convincing and coherent in the overall argument. The additional datasets and revised figures are all worthwhile and helpful. I accept that some of my pointers could not be addressed within the length and scope of the paper but overall the authors have clearly reflected on all the reviewer comments and used them to substantially refine the paper. In particular, the manuscript reads more fluently and the argument flows much more clearly from beginning to end. I am happy to recommend that the manuscript proceed to publication.\nI have only one minor suggestion regarding the elm decline:\nLine 300-301. It would be good to emphasise here that a climatic cause of the elm decline was largely abandoned in the research literature but appears coherent with the new findings; so could extend the sentence to read e.g. \"... thus reinvigorating the hypothesis of climatic causality for this much debated biogeographical event\". or similar. citing the same ref 28"}, "author_response": "RESPONSE TO REVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI previously assessed the manuscript by Shuman and Stefanescu as Reviewer #1. I have read carefully\nthrough the response to reviewers, and the revised manuscript. I am glad to see that the link to sortable\nsilt holds up when additional records are included. Overall, I think this is a really nice demonstration of\ncoherent patterns of climate change in the circum North Atlantic over the Holocene, with plausible links\nto the AMOC. It is an important advance for understanding Holocene climate history, with potential\nrelevance for the future. I have no further significant suggestions. I am glad to recommend that the\nrevised manuscript be accepted for publication in its current form.\n Thank you we appreciate the interest in the paper.\nReviewer #2 (Remarks to the Author):\nThe authors have done a thorough and serious revision with careful attention to the comments of both\nreviewers. I find the revised manuscript to be much more convincing and coherent in the overall\nargument. The additional datasets and revised figures are all worthwhile and helpful. I accept that some\nof my pointers could not be addressed within the length and scope of the paper but overall the authors\nhave clearly reflected on all the reviewer comments and used them to substantially refine the paper. In\nparticular, the manuscript reads more fluently and the argument flows much more clearly from\nbeginning to end. I am happy to recommend that the manuscript proceed to publication.\n We appreciated the help reviews and are glad that revisions seem helpful.\nI have only one minor suggestion regarding the elm decline:\nLine 300-301. It would be good to emphasise here that a climatic cause of the elm decline was largely\nabandoned in the research literature but appears coherent with the new findings; so could extend the\nsentence to read e.g. \"... thus reinvigorating the hypothesis of climatic causality for this much debated\nbiogeographical event\". or similar. citing the same ref 28\n We have inserted the proposed text, which we agree makes sense."}]

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