Spatial patterns of Holocene temperature changes over mid-latitude Eurasia

Domain: earth_science
Published: 2024-02-19
Editorial outcome: major_revision

Abstract
The Holocene temperature conundrum, the discrepancy between proxy-based Holocene global cooling and simulated global annual warming trends, remains controversial. Meanwhile, reconstructions and simulations show inconsistent spatial patterns of terrestrial temperature changes. Here we report Holocene alkenone records to address spatial patterns over mid-latitude Eurasia. In contrast with long-term cooling trends in warm season temperatures in northeastern China, records from southwestern Siberia are characterized by colder conditions before ~6,000 years ago, thus long-term warming trends. Together with existing records from surrounding regions, we infer that colder air mass might have prevailed in the interior of mid-latitude Eurasian continent during the early to mid-Holocene, perhaps associated with atmospheric response to remnant ice sheets. Our results challenge the proposed seasonality bias in proxies and modeled spatial patterns in study region, highlighting that spatial patterns of Holocene temperature changes should be reconsidered in record integrations and model simulations, with important implications for terrestrial hydroclimate changes.

Introduction
Temperature changes during the Holocene epoch provide key insights into climate projections under warming scenarios. Globalsyntheses of proxy-based Holocene temperature records indicate a thermal maximum during the early to mid-Holocene, followed by a cooling trend toward the preindustrial period, coincident with declining boreal summer insolation1,2. However, transient climate model simulations indicate a long-term Holocene warming trend in global annual mean temperature, suggested to be forced by retreating Northern Hemisphere ice sheets and rising greenhouse gas concentrations3,4. This model-data discrepancy, known as the Holocene temperature conundrum3, is proposed to be associated with potential seasonal or spatial biases in proxy-based reconstructions4–6 or sensitivity of climate models7–9. Recent seasonal temperature reconstructions from East Asia and the Northern Hemisphere reveal an early to mid-Holocene thermal maximum in both summer and winter seasons10,11, not supporting the generally annual warming trends presented in model simulations or the proposed seasonality effects.

Spatial patterns of Holocene temperature changes provide important insights into dynamical responses to climate forcings and offer a regional perspective on current global warming. Yet, recent Holocene temperature syntheses and simulations have yielded inconsistent spatial patterns over the mid-latitude Eurasian continent9,11–13. A spatiotemporal analysis of annual temperature variability using data from model simulations suggests Holocene cooling trends over the Arctic Ocean and mid-to-high latitude Eurasia12. A data assimilation reconstruction combining proxy records and simulation results also yields high annual mean temperatures across continental Eurasia at 6000 a BP relative to the preindustrial period and, thus, a cooling trend in annual mean temperatures since the mid-Holocene9. Yet, a synthesis of pollen records indicates warm early to mid-Holocene conditions followed by generally annual cooling trends in Europe and 0–50°N of Asia, but long-term Holocene annual warming trends in 50–75°N of Asia11. Another recent pollen-based synthesis study shows Holocene annual warming trends in 40–50°N of Asia but annual cooling trends in 60–70°N of Asia13. The latitudinal patterns of Holocene temperature change over Eurasia inferred from pollen records11,13 appear to differ from the spatially unanimous annual cooling trends over the extratropical Eurasian continent in transient model simulations9,12. Given the uncertainties in regional temperature changes and poorly constrained climate responses to various forcings, our understanding of Holocene hydroclimatic changes and associated mechanisms also remains largely speculative14,15.

To advance our understanding of Holocene terrestrial temperature changes and their spatial patterns, here we present alkenone records from Lake Yihesariwusu (48.13°N, 118.68°E) in northeastern China, Lake Ebeyty (54.65°N, 71.73°E), Kuchuk (52.69°N, 79.84°E), and Maloye Yarovoye (53.03°N, 79.11°E) in southwestern Siberia (Methods, Fig. 1, Supplementary Figs. 1 and 2). Our results, together with temperature records from surrounding regions, allow us to identify spatial patterns of Holocene temperature changes over the mid-latitude Eurasian continent, which sheds new light on the Holocene temperature conundrum and associated terrestrial hydrological changes.

Results
Alkenone proxy interpretation

We use the alkenone unsaturation index (UK037) (Methods) as a terrestrial warm season temperature indicator. Long-chain alkenones produced by haptophytes are increasingly utilized as temperature and salinity indicators in saline lakes and marine environments. The UK037-temperature calibration from mid-latitude Asian lakes16 is largely consistent with a culture experiment17, and the UK037 index has been successfully applied to reconstruct Holocene temperature changes in this region18–20. Phylogenetic studies21,22 suggest that Group I haptophytes, with the typical presence of C37:3 isomer, tend to live in freshwater lakes, while Group II haptophytes live in brackish/saline lakes. Phylogenetic groups may have different temperature sensitivities23. However, alkenone C37:3 isomer has not been identified from all four lakes in our study, indicating that alkenone-based records in our study are not affected by potential phylogenetic effects during the Holocene. The alkenone UK37 index, which includes C37:4 in its definition, has been used to reconstruct temperature changes in high latitude lakes24,25, where C37:4 abundance appears to primarily respond to temperature variations. Nevertheless, alkenone C37:4 responds more sensitively to lake salinity changes in mid-latitude arid/semi-arid regions where lake salinity could vary substantially26–28, and the proportion of alkenone C37:4 index (%C37:4, Methods) has been successfully applied to reconstruct lake salinity changes in northwestern China18,20. A recent phylogeny study suggests that changes in species or subclade of alkenone-producing Isochrysidales appear to affect the relative abundance of alkenone compounds, which may confound the interpretation of alkenone-derived paleoclimatic indicators in saline lakes29. However, the evident spatial patterns of UK037 and %C37:4 records observed from investigated mid-latitude Asian lakes (Fig. 2, Supplementary Fig. 2) cannot be attributed to species effects due to contrasting associations of UK037 with %C37:4 changes in various regions. Thus, we consider that variations in alkenone-based records from these lakes primarily indicate regional climatic changes, rather than species factors, during the Holocene.

Lake Yihesariwusu is located in a marginal monsoon region, while Lake Ebeyty, Kuchuk, and Maloye Yarovoye are located in the westerlies-dominated region. Contrasting Holocene moisture changes between the two regions have been widely recognized30. Our published %C37:4 records from both regions18,31,32 are consistent with regional moisture changes inferred from various hydrological indicators18,32–41. %C37:4 records from Lake Yihesariwusu and Qinghai18,31,32 show wettest conditions during the mid-Holocene followed by generally drying trends towards the late Holocene (Supplementary Fig. 3a, b), consistent with hydrological changes inferred from pollen34,35 (Supplementary Fig. 3c, d), paleosol36 (Supplementary Fig. 3e), and stabilized dune sites records37 (Supplementary Fig. 3f) from marginal monsoon regions. %C37:4 records from Lake Ebeyty, Kuchuk, and Maloye Yarovoye in southwestern Siberia and Lake Sayram18 in northwestern China show overall Holocene wetting trends (Supplementary Fig. 4a, b). Such wetting trends are consistent with hydrological changes inferred from pollen records in the same sediment cores33,38 (Supplementary Fig. 4c, d) and synthesized moisture records39,40 (Supplementary Fig. 4e, f) from westerlies-dominated regions. Hence, we suggest that alkenone %C37:4 records from mid-latitude Asian lakes could successfully indicate lake salinity variations and, thus, hydroclimatic changes during the Holocene. The UK37 records from mid-latitude Asian lakes, following the structure of %C37:4 variations, could be largely confounded by salinity changes, while lake salinity has little effect on the UK037 index26–28. Therefore, we use UK037 index, instead of UK37 index, as a warm season temperature indicator in mid-latitude Asian lakes.

Spatial patterns of temperature changes

UK037 record from Lake Yihesariwusu (Fig. 2a) is characterized by a long-term Holocene cooling trend, with an average value of 0.21 before ~6000 a BP (years before 1950 AD) and 0.14 after that. However, UK037 records from Lake Ebeyty, Kuchuk, and Maloye Yarovoye (Fig. 2e) show an average value of 0.07 before ~6000 a BP and 0.14 after that, indicating extremely low temperature during the early to mid-Holocene and a warmer late Holocene. Such long-term warming trends in southwestern Siberia (Fig. 2e) are in striking contrast to the overall cooling trends from Lake Yihesariwusu (Fig. 2a) and Sayram18 (Fig. 2b) in northern China, while the warm that the interval of ~4000‒3000 a BP appears to be exceptional at Lake Sayram. Previously reported UK037 records from mid-latitude Asian lakes also exhibit variable long-term trends over the Holocene. Holocene UK037 record from Lake Hala42 (Fig. 2d), a high-elevation lake on the northeastern Tibetan Plateau, shows a slight warming trend. UK037 record from Lake Hurleg in the Qaidam Basin18,19 (Supplementary Fig. 5c) does not appear to show discernible long-term trends during the Holocene. UK037 records from Lake Balikun43 (Fig. 2c) and Lake Qinghai18,31,32 (Supplementary Fig. 5b) in northern China display a slight cooling trend. Hence, based on the same alkenone proxy, which indicates warm season temperature, a clear Holocene warming trend occurs in southwestern Siberia; such a warming trend disappears approaching northern China and is likely shifted to a cooling trend (Fig. 2).

Holocene temperature records inferred from various proxies also show spatially variable long-term trends over the mid-latitude Eurasian continent (Fig. 3). Branched fatty alcohol44 (Fig. 3b) and pollen records45 from central China, chironomid records from Russia Far East46 (Fig. 3c) and northern Finland47 (Fig. 3d), and alkenone records from Svalbard24 (Fig. 3e), representing annual, annual, July, July, and warm season temperatures, respectively, show relatively high annual mean/summer temperatures before ~6000 a BP followed by a general cooling trend. However, an archaeal record from Altai Mountains48 (Fig. 3g), oxygen isotope records from Siberian Arctic49 (Fig. 3h) and Ural Mountains50 (Fig. 3i), and chironomid record from central Sweden51 (Fig. 3j), representing annual, winter, winter, and summer temperatures, respectively, indicate low seasonal/annual temperatures before ~6000 a BP relative to the late Holocene. Additionally, records used to reconstruct global Holocene temperature changes2,11,45, inferred from various proxies potentially biased toward different seasons, show that Holocene seasonal/annual cooling trends prevailed in northern China, northern and western Europe (Methods, Fig. 4a), whereas central to eastern Europe and western to central Siberia are broadly characterized by long-term seasonal/annual warming trends (Fig. 4a). Moreover, a synthesis of pollen records reveals Holocene long-term cooling trends in 0–50°N of Asia but warming trends in 50–75°N of Asia in both summer and winter seasons11 (Fig. 5b, c), and such spatial patterns are broadly consistent with our results (Supplementary Fig. 5b, c). Yet, the climatic transitions from warm to cold (or cold to warm) conditions at the time interval of ~6000–5000 a BP, as indicated by our UK037 records, are not well documented by the synthesized pollen records11,13 (Fig. 5b, c).

Our alkenone UK037 records, together with published datasets, collectively show that Holocene temperature records inferred from the same proxy, alkenone (Fig. 2, Supplementary Fig. 5), chironomid (Fig. 3c, d, j), or pollen11 (Fig. 5b, c), display contrasting long-term trends in different regions of the mid-latitude Eurasian continent. Further, temperature records inferred from various proxies that are potentially biased toward different seasons show similar long-term trends in the same regions. For example, UK037 (Fig. 3f) and oxygen isotope records49,50 (Fig. 3h, i), which are suggested to represent warm and cold season temperatures, respectively, display spatially consistent long-term Holocene warming trends in Siberia. Contrasting Holocene temperature records over the mid-latitude Eurasian continent (Figs. 2, 3, and 4a) are thus unlikely to be attributed to seasonality bias in proxies but rather largely reflect spatial patterns of temperature changes. Such spatial patterns appear to persist throughout the year rather than being limited to a particular season (Figs. 3 and 4a). Therefore, we consider that colder air mass appears to have prevailed in the interior of the mid-latitude Eurasian continent during the early to mid-Holocene (Fig. 4a), a spatial feature that has been largely overlooked in previous synthesis studies1,2.

Implications for data synthesis and model simulations

A hotly debated issue regarding the Holocene temperature conundrum is the possible seasonality in proxy reconstructions3–6. Spatially variable temperature trends inferred from the same proxies in different regions (Figs. 2, 3, and 4a, Supplementary Fig. 5), combined with the replicability of regional temperature trends among various proxy types (Figs. 3 and 4a), provide compelling evidence of the sensibility of proxies to regional temperature variations and reliability of spatial patterns of Holocene temperature changes over the mid-latitude Eurasian continent. Our findings, in conjunction with recent seasonal temperature reconstructions10,11 (Fig. 5b, c), challenge the hypothesis that the model-data discrepancy might be associated with seasonality bias in proxies3,4,6,7, at least for the study region. The proposed seasonal origin for the Holocene thermal maximum in global temperature reconstructions, based on the assumption that sea-surface temperature records are primarily driven by local seasonal insolation4, could not account for the spatial patterns of Holocene terrestrial temperature changes and associated controlling factors. Further, regional temperature synthesis1,2,11,13, based on latitudinal binning, could also potentially mask the spatial patterns as it tends to smooth out the opposite trends in different regions for the same latitudes. Therefore, a greater focus on geographically distributed temperature reconstructions, along with regional syntheses of Holocene temperature records, could enable a more comprehensive assessment of dynamical responses to climate forcings, which may provide new insights into the Holocene temperature conundrum.

Several model and data assimilation studies have reported Holocene temperature changes at a regional scale9,12,52–55. However, the spatial patterns of Holocene annual temperature changes observed in reanalyzed model data12, which show marked long-term annual cooling trends over the extratropical Eurasian continent (Fig. 4d), suggested to respond to increased Arctic sea ice coverage, are opposite to what the proxy-based results suggest here (Figs. 2, 3, and 4a). Similarly, recent data assimilations9,52 fail to capture the substantial temperature discrepancies over the extratropical Eurasia during the mid-Holocene, which show spatially unanimous higher annual mean temperatures over this region at 6000 a BP than 500 a BP (Fig. 4b). Simulated annual mean temperatures from PMIP4-CMIP6 models53 show lower temperatures over the mid-latitude Asia at 6000 a BP than the preindustrial period, but higher temperatures over the Arctic Ocean (Fig. 4c). Therefore, the spatial patterns over mid-latitude Eurasia identified here are in contrast with those in both model simulations and data assimilation results (Fig. 4). TraCE-21ka simulation results54 show spatially consistent Holocene summer cooling and annual/winter warming trends in northeastern China and southwestern Siberia (Fig. 5a, d), thus not capturing the spatial patterns observed from proxy-based reconstructions (Fig. 5b, c). The simulated seasonal temperature anomalies generated by multiple climate models53,55 also indicate spatially unanimous higher summer temperatures and lower winter temperatures over the extratropical Eurasian at 6000 a BP than the preindustrial period, and thus consistent summer cooling and winter warming trends since the mid-Holocene. Current climate models appear to exhibit limitations in simulating the spatial patterns of Holocene temperature changes over the extratropical Eurasian continent.

Spatial patterns of hydroclimate changes over the mid-latitude Eurasian continent have been well documented throughout the Holocene30. Our alkenone %C37:4 record from Lake Yihesariwusu displays a relatively dry early Holocene and the wettest mid-Holocene (Methods, Supplementary Fig. 2b), while those from southwestern Siberia show overall Holocene wetting trends (Supplementary Fig. 2e), following the hydrological changes in marginal monsoon regions and westerlies-dominated regions, respectively30 (Supplementary Figs. 3 and 4). Based on compilations of Holocene climatic records from the Northern Hemisphere, changes in latitudinal temperature gradient are proposed to influence the strength of westerlies flow and, in turn, mid-latitude moisture variability during the Holocene14. Reduced mid-latitude annual net precipitation during the early to mid-Holocene is thus attributed to decreased temperature gradient14. However, the spatial complexity of temperature and hydrological changes over the mid-latitude Eurasian continent could confound this hypothesis, and hydrological variations in this region, particularly in westerlies-dominated regions, did not correspond to latitudinal temperature gradient changes14. Rather, prevailing colder air mass could have controlled regional hydroclimate changes during the early to mid-Holocene.

Possible controls on spatial patterns

The spatial feature over mid-latitude Eurasia, well identified here (Fig. 4a), remains to be explained physically as it has not been captured by current transient model simulations. Extratropical Northern Hemisphere received enhanced annual insolation during the early to mid-Holocene56 (Fig. 6a), while the total solar irradiance remained at a relatively low level57 (Fig. 6b). The spatial patterns of Holocene temperature changes do not appear to be directly associated with decreasing boreal insolation or increasing greenhouse gas concentration (Fig. 6a) because such global-scale forcings could induce spatially consistent temperature variations if other forcings/feedbacks are not involved. Current model simulations12,53–55 have included changes in both forcings yet are unable to capture the substantial temperature discrepancies over extratropical Eurasia during the early to mid-Holocene (Figs. 4c, d and 6g). Therefore, additional climate forcings or feedbacks are required to explain the spatial patterns of Holocene temperature changes over this region.

Examination of climate forcings or feedbacks inferred from model simulations and climatic records might provide a plausible interpretation of the spatial patterns observed from proxy reconstructions. Supported by model simulations55, a stable anticyclonic system over northern high latitudes associated with Northern Hemisphere ice sheets could have altered regional atmospheric circulation dynamics and regional climatic conditions during the Last Glacial Maximum58. Shown in a coupled atmosphere-ocean model simulation55, remnant ice sheets during the early to mid-Holocene (Fig. 6c), suggested to contribute to the simulated Holocene annual warming trend3, could have still exerted an influence on atmospheric circulation dynamics over the mid-latitude Eurasian continent as they could have induced pressure and anticyclonic winds anomalies over northern mid-to high latitudes. A large-scale atmospheric and oceanic reorganization could have occurred in response to the demise of the Laurentide ice sheet and the reduction of meltwater flux59. Moreover, a blocking index reconstruction50 (Fig. 6d) suggests persisted atmospheric blocking over Scandinavia associated with ice sheet forcing during the early to mid-Holocene, causing relaxation of (sub)polar cold air mass and depressed surface temperature in the interior of the mid- to high-latitude Eurasian continent.

Both colder conditions in central to eastern Europe and western to central Siberia inferred from our alkenone results and other temperature records (Figs. 2, 3, and 4a), and the suggested atmospheric circulation dynamics, characterized by the presence of high-pressure system and frequent atmospheric blocking events during the early to mid-Holocene over the mid-latitude Eurasian continent50,55,59, broadly resemble current winter conditions (Fig. 1). Interestingly, more records showing lower temperatures before ~6000 a BP to the south of Scandinavia than north also resemble current winter conditions (Fig. 4a). Additionally, most Arctic Ocean sea-ice records show less extensive sea–ice cover during the early to mid-Holocene9,60 (Fig. 6h). Hence, the overall high temperature conditions over the Arctic Ocean, accompanied with low temperature conditions over the central Eurasian continent, during the early to mid-Holocene, might be broadly comparable to the recent Arctic warming and Eurasian cold winters61,62.

Given the resemblances in atmospheric circulation patterns and temperature conditions over the mid-latitude Eurasian continent during current winters and the early to mid-Holocene, we consider that the enhanced wavy pattern of westerlies, associated with the persistent anticyclone system, perhaps induced by the remnant ice sheets before ~6000 a BP, is a plausible interpretation of spatial patterns of Holocene temperature changes in this region. Cold conditions in the interior of the mid- to high-latitude Eurasian continent, in contrast to the high temperatures in surrounding regions, during the early to mid-Holocene, shaped the spatial patterns of long-term Holocene temperature changes over the mid-latitude Eurasian continent.

As ice sheet-induced anticyclone might have persisted over northern mid-high latitudes50 (Fig. 6d), we suggest that large-scale changes in land-sea temperature and pressure gradients could induce more meridional, or wavy patterns of westerlies, and thus cold (sub)polar air mass reaching the interior of continental Eurasia. Consequently, regions affected by cold air masses show depressed surface temperatures during the early to mid-Holocene (Figs. 4a and 6e), and the climatic transition towards a warmer late Holocene occurred in response to the demise of Northern Hemisphere ice sheets (Fig. 6c). Regions outside the influence of (sub)polar air masses should have followed the decreasing boreal insolation, displaying Holocene long-term cooling trends (Fig. 6f). The boundary between the two distinct temperature patterns appears to be roughly situated along northern Xinjiang and may extend to the northeastern Tibetan Plateau (Fig. 4a). Consequently, temperature records from these regions show subtle or indiscernible long-term trends over the Holocene (Fig. 2c, d, Supplementary Fig. 5b, c). The enhanced anticyclone, rather than reduced latitudinal temperature gradient14, could also well explain drier conditions in the interior of the mid-high latitude Eurasia continent during the early to mid-Holocene, which may further affect hydroclimate changes in marginal monsoon regions (Supplementary Fig. 3).

In summary, here we report spatial patterns of Holocene temperature changes over the mid-latitude Eurasian continent. Colder air mass appears to have prevailed in the interior of the mid-latitude Eurasian continent during the early to mid-Holocene, associated with the wavy pattern of westerlies perhaps modulated by remnant ice sheets, although the exact physical mechanism remains to be explored. Our results challenge the hypothesis of seasonality bias in proxies, at least for this region and the simulated regional spatial patterns of Holocene temperature changes, with important implications for the Holocene temperature conundrum and terrestrial hydroclimate changes.

Methods
Material and chronology

Lake sediment cores were retrieved from Lake Yihesariwusu in northeastern China and Lake Ebeyty, Kuchuk, and Maloye Yarovoye in southwestern Siberia (Fig. 1). Lake Yihesariwusu (48.13°N, 118.63°E) is a hydrologically closed saline lake located in the center of Hulun Buir dune field, China. The lake is mainly recharged by meteoric precipitation, with an area of ~5 km2 and a maximum water depth of ~4.0 m63. The lake catchment is surrounded by semi-fixed and mobile dunes to the north and sandy land to the south. The mean annual temperature of this region is −1.3 °C and the mean annual precipitation is 349 mm63. Two overlapping sediment cores were retrieved from Lake Yiheshariwusu using a piston corer63. The original core chronology was established using 210Pb and thirteen 14C dates without considering the reservoir ages63. Here, we updated the core chronology using an additional 14C date (Supplementary Data 1) and subducted a reservoir age of 570 years determined by the age difference between plant remains at 14 cm and bulk organic matter at 14.5 cm (Supplementary Data 1). Three anomalously old 14C ages at 437 cm, 506 cm, and 582 cm may indicate old terrestrial carbon carried by meltwater to the lake during the deglaciation and early Holocene64. The updated age-depth model suggests a basal age of ~9600 a BP for the core from Lake Yihesariwusu65. The core from Lake Ebeyty (54.65°N, 71.73°E, 50 m, asl) is a hydrologically closed hypersaline lake located in southwestern Siberia, Russia, with an area of 90–113 km2 and a periodically changeable depth of 0.6–3 m66. The mean annual temperature of this region is 2 °C, and the mean annual precipitation is 420 mm. A ~3.0 m-long core was retrieved from the lake at a water depth of 0.6 m67. The core chronology was established using 14C dates, including 13 reported previously67 and an additional date (Supplementary Data 1). Two anomalously old 14C dates below 270 cm depth (Supplementary Data 1) were excluded from the depth-age model constructed using Bacon software65. A reservoir age of 700 years was determined by the intercept of linear regression of four 14C dates from the upper 50 cm of the core. Here, we analyzed samples above 270 cm, spanning the last ~9600 years based on the depth-age model for the lake (Supplementary Fig. 1b).

Lake Kuchuk (52.69°N, 79.84°E, 98 m, asl) and Maloye Yarovoye (53.03°N, 79.11°E, 96 m, asl) are also located in southwestern Siberia, Russia. Lake Kuchuk is a hydrologically closed hypersaline lake with an area of 166 km2 and a maximum depth of 3 m33. Lake Maloye Yarovoye, located 40 km north of Lake Kuchuk, is a hydrologically closed hypersaline lake with an area of 35 km2 and a maximum depth of 5 m33. The mean annual temperature of this region is 0 °C, and the mean annual precipitation is 300 mm33. A ~3.1 m-long core was taken from Lake Kuchuk at a water depth of 2.9 m, and a ~5.0 m-long core was taken from the lake center of Maloye Yarovoye at a water depth of 3.6 m33. The original core chronologies were established using accelerator mass spectrometry 14C dates of bulk organic matter33. Here, updated reservoir ages of 1000 and 1200 years were determined by the intercepts of linear regressions of 14C dates from Lake Kuchuk and Maloye Yarovoye, respectively, slightly different from the originally proposed ones33. The updated core chronologies suggest a basal age of ~14,000 a BP for the core from Lake Kuchuk and ~12,000 a BP for the core from Lake Maloye Yarovoye. Here, we report records over the Holocene from the two lakes.

Alkenone analysis

We have analyzed long-chain alkenones in 172 samples from Lake Yihesariwusu, 235 samples from Lake Ebeyty, 19 samples from Lake Kuchuk, and 15 samples from Lake Maloye Yarovoye. Freeze-dried samples were extracted ultrasonically (3×30 min) with a solvent of dichloromethane:methanol (9:1, v/v, 15 ml each time). The total lipids were dried under N2 and saponified with 6% KOH in MeOH for at least 12 h at room temperature. Subsequently, 1.5 ml of NaCl in water (5%, w/w) was added, and the neutral solution was extracted with 4 ml n-hexane. The neutral solution was then separated into three fractions with silica gel column chromatography using 4 ml n-hexane, 4 ml dichloromethane, and 4 ml methanol, respectively, while the dichloromethane fraction contains alkenones. The alkenone fraction was analyzed on an Agilent 7890 Gas Chromatography (Agilent DB-1 column: 60 m × 250 μm × 0.10 μm film thickness) with Flame Ionization Detection (FID) at The University of Hong Kong, using n-C36 n-alkane as an internal standard for quantification. Each sample was injected in a splitless mode, with H2 as carrier gas. The oven temperature program for alkenone analysis was 60 °C (1 min) to 270 °C at 13 °C/min, and then to 310 °C at 3 °C/min (held 30 min). The laboratory standards were repeatedly analyzed to assess the analytical precision.

The relevant alkenone-based proxies UK03768 and %C37:469 were calculated as follows:

UK037 = [C37:2] / ([C37:2] + [C37:3]) (1)

%C37:4 = [C37:4] / ([C37:2] + [C37:3] + [C37:4]) * 100% (2)

where [C37:2], [C37:3] and [C37:4] are the concentrations of di-, tri- and tetra-unsaturated C37 alkenones, respectively. The estimated analytical error is typically within 0.01 for UK037, and 5% for %C37:4.

Holocene temperature dataset

Holocene terrestrial temperature datasets over the extratropical Eurasian continent shown in Fig. 4a were obtained from ref. 2, 11, 45 and listed in Supplementary Data 2. Pollen, GDGTs, and fatty alcohol-based records were (re)calibrated to represent mean annual temperature changes2,11,45, whereas chironomid and alkenone/oxygen isotope records indicate warm/cold season temperature changes2,11,45. Records with locations shown in Fig. 1 are also presented in Fig. 4a. Dots, triangles, and inverted triangles in Fig. 4 are present records indicating annual mean, summer, and winter temperature changes, respectively. The spatial patterns of Holocene temperature changes over the extratropical Eurasian continent appear to be persistent throughout the year rather than being limited to a particular season. Therefore, we present temperature records inferred from different proxies and compare such patterns with annual mean temperature changes inferred from model and data assimilation in Fig. 4. As a general warming trend has been identified in the early Holocene (~11,000–7000 a BP) over the Eurasian continent11, we choose to define temperature trends since ~8000 a BP, to focus on spatial patterns of Holocene temperature changes. We classify those records into two types, i.e., warming and cooling types (pink and blue colors in Fig. 4a, respectively) based on their overall trends since ~8000 a BP. Records with insignificant long-term trends are not included in our synthesis. Despite uncertainties in reconstructions and calibrations, it can be observed that records with a warming trend are predominantly concentrated within the interior of the mid-latitude Eurasian continent.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents the Holocene temperature reconstruction based on alkenone UK37 in four\nlakes in northern Eurasia. The temperatures show opposite trends between southwestern Siberia\n(three sites in warming trend) and northeastern China (one site in cooling trend). This opposite\ntrend seems to be also consistent with some other temperature proxies of reconstructions.\nTherefore, the new reconstructions highlight the spatial pattern of the temperature trend in the\nHolocene. The paper also propose that the contrasting temperature trend is caused by the impact\nof residual ice on atmospheric circulation. Personally, I think the data analysis itself (including the\nsynthesis with other proxies) is interesting. The argument that the same proxy should represent\nthe same seasonal temperature change is also legitimated (although the comparison with other\nproxies makes this seasonality messy, because the unknown seasonality of other proxies).\nThe mechanism part is, however, too speculative and confusing, at best. Overall, the paper is poorly\npresented as it stands. Therefore, I recommend a major revision before it can be published. The\nmajor point of the paper, seems to me, is that they proposed a regional pattern of temperature\ntrend in northern Eurasia region and this pattern, if true, remains to be explained physically.\nHere are my major concerns.\n1. There are many parts of the paper ambiguous about the seasonality. Since the warming/cooling\ninvolves potentially seasonality, it is important to be very clear when warming/cooling is stated for\nboth data and model, is it annual mean, or a seasonal change (which season)? In addition, since\nthe Holocene temperature conundrum is for global mean and annual mean temperature, it should\nalso be clear if the temperature is regional or global mean. Here is one example of the ambiguity.\n(There are too many to list in the paper!)\nIn the introduction section,\nL41: “However, the long-term Holocene warming trend observed in transient climate model\nsimulations is suggested to be forced by retreating Northern Hemisphere ice sheets and rising\ngreenhouse gas concentrations”. Is this annual mean, or seasonal, it is global mean or regional?\n2. The last section on the mechanism of the regional pattern is, at best, confusing. Indeed, I don’t\nsee any physical sensible physical argument, except the speculation of the role of residual ice\nsheet. For example,\nL178: “Remnant ice sheets appear to have exerted a continuous influence on atmospheric\ncirculation dynamics over the mid-latitude Eurasian continent during the early to mid-Holocene\nas a coupled atmosphere-ocean model simulation shows that they could have induced pressure and\nanticyclonic winds anomalies over northern mid-high latitudes, and a large-scale atmospheric and\noceanic reorganization could have occurred in response to the demise of Laurentide ice sheet and\nreduction of meltwater flux. Moreover, a blocking index reconstruction (Fig. 4d) suggests persisted\natmospheric blocking over Scandinavia associated with ice sheet forcing during the early to mid-\nHolocene, causing relaxation of polar jet stream and depressed surface temperature in the interior\nof the mid- to high-latitude Eurasian continent. Both colder conditions in central to eastern Europe\nand western to central Siberia “\nI don’t quite understand these arguments. First, do you mean local ice sheet over Eurasia or\nremote ice sheet from North America? Either way, how much residual ice sheet is left in the\nHolocene in Eurasia and North America? Second, what is the evidence that the residual ice sheet\nfrom North America can cause the contrasting temperature response pattern in northern Eurasia?\nWhat caused the change of the blocking index (assuming the reconstruction is correct)? Is it\nproduced by models or your speculation? In either case, it is not obvious why the jet shifts\nsouthward in response to the retreating of ice sheet. This southward migration is opposite to the\nrobust results of model experiments on the effect of Laurentide ice sheet at LGM: the retreat of\nLGM Laurentide ice sheet causes the jet moving northward (from more a southern more zonal\nposition at LGM northward to a northeast-southwest orientation as in the present day), instead of\nsouthward. Third, what is the evidence of “a large-scale atmospheric and oceanic reorganization”\nin response to the residual ice sheet and the associated (small) flux of meltwater? The\nreorganization in the coupled system is usually discussed for the early deglacial period, when both\nthe ice sheet and the meltwater flux are large.\n3. It will be helpful to clarify the point of the paper by presenting some model results. If model\nshows homogenous warming/cooling for seasonal and/or annual mean, it presents a clear model-\ndata inconsistency of the regional temperature response. The simplest is to check some regional\ntemperature trend in the available model experiments or data assimilation reanalysis products. Are\nall of the models generating a uniform temperature change? It will be more fruitful if the authors\ncollaborate with some modelers for the model-data comparison. The model result of Extended\nData Fig.5b seems to me not much relevant to the point of the paper. This model is used, not\nbecause it explains the physical mechanism, but because it looks like what the authors want.\nMinor points:\n“A spatiotemporal analysis of annual temperature variability using data from model simulations\nsuggests a Holocene cooling mode over the Arctic Ocean and mid-to-high latitude Eurasia . A data\nassimilation reconstruction combining proxy records and simulation results also yields warmer\nconditions across continental Eurasia during the mid-Holocene relative to the preindustrial era”\nBe consistent, or at least very careful, in specifying the trend of cooling/warming against the\ncolder/warmer condition in the mid-Holocene. They are of the opposite signs. For example, the\nsentence above is unambiguous. The first part “cooling” mode seems to refer to the cooling\ntowards late Holocene (so it is warmer condition in the mid-Holocene). The second part “warmer\ncondition during the mid-Holocene” also implies a cooling towards late Holocene. Are they the\nsame or not? It may be more clear to use either the trend or the mid-Holocene condition relative\nto the present throughout the paper.\n“Yet, a recent synthesis of pollen records indicates a warm early to mid-Holocene followed by\ngenerally cooling trends in southern Asia and Europe, but long-term Holocene warming trends in\nnorthern high-latitude Asia. Given the uncertainties in regional temperature changes and poorly\nconstrained climate responses to various forcings, the Holocene temperature conundrum remains\nhighly contentious, and our understanding of Holocene hydroclimatic changes and associated\nmechanisms remains largely speculative”\nIt appears the authors try to say models show the same sign of cooling towards late Holocene, but\na recent synthesis shows opposite trend in different regions in Eurasia continent. That is fine, but\ni) this point should be clarified explicitly, ii) this is not much relevant to the conundrum, which is\nabout global mean and annual mean.\nExtended Data Fig.4: What does it mean by “southern Asia” here? Lake Yihesariwusu is in\nnortheastern China, which is NOT the southern Asia we usually refer to (or I never heard this\nregion is called southern Asia). Be careful on your regional reference. In the region of your interest\n“northern Eurasia”, this is the “southern region”, while southern Siberia is the “northern region”.\nYou can predefine these regions first (give approximate latitude/longitude ranges…).\nL99- “warm conditions prevailed in northern China, northern and western Europe (Fig. 3a)”. This\ndescription is confusing. To my eye, the warming is dominant in central Europe, while cooling\noccurs in the surrounding area. Certainly, northern and western Europe are dominated by cooling\n(blue), instead of warming. It seems the authors claimed this pattern in Europe because they want\nto show a coherent pattern. But, the European pattern does not seem to me related to the pattern\nof their lake sites. Again, are these temperature change of seasonal (which season if yes) or\nannual mean?\nL142 “However, the spatial patterns of Holocene temperature changes observed in reanalyzed\nmodel data , which show marked long-term cooling trends over the extratropical Eurasian\ncontinent (Fig. 4g, Extended Data Fig. 5b), suggested to respond to increased Arctic sea ice\ncoverage, are opposite to what the proxy-based results suggest (Fig. 2, 3).” Again, is Fig.3g for\nannual mean or seasonal? If your data is seasonal, why don’t you compare the seasonal\ntemperature? Same when other models/data assimilation products are mentioned. Refer to my\nmajor comment 3 on this.\nL170 “The spatial patterns of Holocene temperature changes do not appear to be directly\nassociated with decreasing boreal insolation or increasing greenhouse gas concentration (Fig.\n4a), because such global-scale forcings could induce spatially consistent temperature variations.”\nIt is not obvious these two mechanisms should be discarded. Is it possible different mechanism is\ndominated in different regions?\nL196: “…might be broadly comparable to recent Arctic amplification and Eurasian cold winters”.\nThis does not seem to me a good analogy. Recent arctic amplification is thought to be caused by\nthe rising CO2, and the warming tends to occur uniformly. The Eurasian cooling is also for winter,\nopposite to the warm season proxy here. How are these consistent?\n203: “As ice sheet-induced anticyclone might have persisted over northern mid-high latitudes (Fig.\n4d), we suggest that large-scale changes in land-sea temperature and pressure gradients could\ninduce more meridional, or wavy patterns of westerlies, and southward deflection of the polar front\nto continental Eurasia”. What cause this southward shift, orbital, which season? CO2? Ice sheet?\nShould Ref 7 be Erb et al., 2022?\nThis ref. 7 (Osama) is for Last Glacial Maximum Reanalysis, and shows a warming trend in the\nHolocene. Erb et al. is on the Holocene with multiple models and does not show warming trend.\nErb, M. P. et al. Reconstructing Holocene temperatures in time and space using paleoclimate data\nassimilation. Clim. Past 18, 2599–2629 (2022).\nFig.2 and 3, each curve, include the lake name for each label, e.g. a) Lake…..b)Lake……This helps\nreaders…not have to go back to the caption.\nThere is no Supplementary Table 2, referred in the caption of Fig.3. There is only supplementary\ndata set 2. But that is not what a reviewer would like to see. A reviewer just needs the type,\nlocation et al of each record used.", "reviewer_2": "In this manuscript, the authors present alkenone based temperature reconstructions for the\nHolocene: one from the monsoonal marginal zone and three from the continental/westerlies realm.\nBy contextualization of the data they shape out two different spatial patterns of temperature\nchanges throughout the study area and covered time period.\nThe overall aim of the manuscript ist o solve question connected to the „Holocene conundrum“.\nA recent study (Herzschuh et al 2023; https://cp.copernicus.org/articles/19/1481/2023/) highlighted\nthe importancy of focussing on spatial Holocene temperature patterns and their regional drivers,\nfor the understanding of this proxy-model mismatch. By further investigating the situation in Asia,\nthis manuscript by Jiang et al, is an important and interesting contribution to the debate.\nDespite these aspects, I´m not convinced that this study is suitable for NCOMM for the following\nreasons:\n1. I miss a more critical discussion of the alkenone proxy which is often problematic in lakes.\nI´m not familiar with all of the cited original studies including those of which data from other lakes\nwere plotted, but it appears that many factors can influence the different kinds of uk37-proxies.\nSalinity changes, translocation of alkenone producers, glacial meltwater influence, community\nchanges of alkenone producers,… are just some of the factors which are mentioned in some of the\ncited papers. Partially, this is solved in the manuscript by comparison with independent proxies,\nbut I still miss a discussion on how reliable alkenone-based temperature proxies are in the specific\ncase, not only in the methods but in the main part of the manuscript.\n2. Connected with these aspects, I find the recent CP-paper by Herzschuh et al is more convincing,\nbecause solely focussing on pollen transfer functions, which have been proven to work well in\ncontext of precipation and temperature reconstructions. Unfortunately, this also takes away a bit of\nnovelty from this new manuscript by Jiang et al, especially since Herzschuh et al already showed\nsome spatial heterogenity of Holocene temperature in Asia.\nConcluding, I think this new manuscript is definetly be worth to be published after revisions, but\nmore likely in a journal such as QSR or CP or maybe COMM EARTH ENV.\nMinor comments:\n- The above mentioned paper by Herzschuh needs to be cited\n- Maps in Fig 3a and Ext Fig 5a would benefit from labelling the dots (as in Fig. 1)\n- There is a lot of reference to figures in the appendix. I would recommend to reconsider if some of\nthose figures could be shifted to the main manuscript, in case this manuscript is re-arranged to a\nless short-format version."}, "author_response": "We have carefully followed the editor's and reviewers' comments revising our manuscript. In\norder to clearly highlight the changes made in the revision, we have copied all these comments\nbelow in black and inserted our replies in blue. Line numbers mentioned in our response letter\nrefer to the ones in our revised manuscript.\nReviewer #1 (Remarks to the Author):\nThis paper presents the Holocene temperature reconstruction based on alkenone UK37 in four\nlakes in northern Eurasia. The temperatures show opposite trends between southwestern\nSiberia (three sites in warming trend) and northeastern China (one site in cooling trend). This\nopposite trend seems to be also consistent with some other temperature proxies of\nreconstructions. Therefore, the new reconstructions highlight the spatial pattern of the\ntemperature trend in the Holocene. The paper also propose that the contrasting temperature\ntrend is caused by the impact of residual ice on atmospheric circulation. Personally, I think the\ndata analysis itself (including the synthesis with other proxies) is interesting. The argument that\nthe same proxy should represent the same seasonal temperature change is also legitimated\n(although the comparison with other proxies makes this seasonality messy, because the\nunknown seasonality of other proxies). The mechanism part is, however, too speculative and\nconfusing, at best. Overall, the paper is poorly presented as it stands. Therefore, I recommend\na major revision before it can be published. The major point of the paper, seems to me, is that\nthey proposed a regional pattern of temperature trend in northern Eurasia region and this pattern,\nif true, remains to be explained physically.\nWe appreciate Reviewer’s comments and have revised our manuscript accordingly. We\nunderstand Reviewer’s concerns regarding the seasonality of proxies and the comparison of\ndifferent proxies, which are hotly debated in paleoclimatic studies. Temperature records from\nthe interior of the mid-latitude Eurasian continent inferred from different proxies suggested to\nreflect seasonal/annual mean temperature changes, e.g., oxygen isotope records from\nKinderlinskaya Cave and Lena River Delta (revised Fig. 3h, 3i, winter temperature), GDGTs\nrecord from Sahara Sand Peatland (revised Fig. 3g, annual mean temperature), and several\npollen records from this region (revised Fig. 4a, annual mean temperature), display similar\nHolocene warming trends as our alkenone-based warm season temperature records from\nSiberian lakes (revised Fig. 3g). Our results, together with published temperature records,\nsuggest that the spatial patterns of Holocene temperature changes over the mid-latitude\nEurasian continent appear to persist throughout the year, rather than being limited to a particular\nseason.\nMoreover, ~75% of the temperature records presented in revised Fig. 4a are inferred from\n(re)calibrated pollen reconstructions, which do not show substantial differences in seasonal and\nannual mean Holocene temperature trends. Initially, we plotted summer (inferred from pollen,\nalkenone, and chironomid records) and annual mean (inferred from pollen and GDGTs records)\ntemperature records separately, but the spatial patterns were broadly similar. This again\nindicates that the spatial patterns inferred from proxy-based records are not limited to a\nparticular season. Therefore, we presented annual mean temperature changes inferred from\npollen and several GDGTs records, and seasonal temperature changes inferred from other\nproxies, in revised Fig. 4a.\nWe have included seasonality information of the records discussed in this study in\nSupplementary Table 2. We have made the following modifications to better address\nReviewer’s concern about seasonality:\n(1) We have added seasonality information of records to the caption of revised Fig. 3 and\nrevised main text (Line 148-150, 152-154).\n(2) We have modified previous Fig. 3a (revised Fig. 4a) to display records representing\ndifferent seasons with different symbols.\n(3) We have described the reasons for presenting temperature records inferred from different\nproxies in revised Fig. 4a (Line 383-388).\n(4) We have compared seasonal/annual temperature changes inferred from models with\nproxy-based records/spatial patterns in revised main text (Line 206-220) and revised Fig.\n4-6.\nModification of the mechanism section is referred to our response to Major Comment #3.\nHere are my major concerns.\n1. There are many parts of the paper ambiguous about the seasonality. Since the -\nwarming/cooling involves potentially seasonality, it is important to be very clear when\nwarming/cooling is stated for both data and model, is it annual mean, or a seasonal change\n(which season)? In addition, since the Holocene temperature conundrum is for global mean\nand annual mean temperature, it should also be clear if the temperature is regional or global\nmean. Here is one example of the ambiguity. (There are too many to list in the paper!)\nIn the introduction section,\nL41: “However, the long-term Holocene warming trend observed in transient climate model\nsimulations is suggested to be forced by retreating Northern Hemisphere ice sheets and rising\ngreenhouse gas concentrations”. Is this annual mean, or seasonal, it is global mean or regional?\nThe long-term Holocene warming trend observed in transient climate model simulations is for\nglobal annual temperature.\nWe appreciate Reviewer’s comments and have carefully clarified our description about\nseasonality of records and regional/global mean of temperature in Abstract (Line 23, 27),\nIntroduction (Line 44, 59, 60, 62, 64, 66), and Results and discussion (Line 141, 148, 149, 152,\n153, 156, 158, 202, 203, 208, 209, 214, 216).\n2. The last section on the mechanism of the regional pattern is, at best, confusing. Indeed, I\ndon’t see any physical sensible physical argument, except the speculation of the role of residual\nice sheet. For example,\nL178: “Remnant ice sheets appear to have exerted a continuous influence on atmospheric\ncirculation dynamics over the mid-latitude Eurasian continent during the early to mid-Holocene\nas a coupled atmosphere-ocean model simulation shows that they could have induced pressure\nand anticyclonic winds anomalies over northern mid-high latitudes, and a large-scale\natmospheric and oceanic reorganization could have occurred in response to the demise of\nLaurentide ice sheet and reduction of meltwater flux. Moreover, a blocking index\nreconstruction (Fig. 4d) suggests persisted atmospheric blocking over Scandinavia associated\nwith ice sheet forcing during the early to mid-Holocene, causing relaxation of polar jet stream\nand depressed surface temperature in the interior of the mid- to high-latitude Eurasian continent.\nBoth colder conditions in central to eastern Europe and western to central Siberia “\nI don’t quite understand these arguments. First, do you mean local ice sheet over Eurasia or\nremote ice sheet from North America? Either way, how much residual ice sheet is left in the\nHolocene in Eurasia and North America? Second, what is the evidence that the residual ice\nsheet from North America can cause the contrasting temperature response pattern in northern\nEurasia? What caused the change of the blocking index (assuming the reconstruction is correct)?\nIs it produced by models or your speculation? In either case, it is not obvious why the jet shifts\nsouthward in response to the retreating of ice sheet. This southward migration is opposite to\nthe robust results of model experiments on the effect of Laurentide ice sheet at LGM: the retreat\nof LGM Laurentide ice sheet causes the jet moving northward (from more a southern more\nzonal position at LGM northward to a northeast-southwest orientation as in the present day),\ninstead of southward. Third, what is the evidence of “a large-scale atmospheric and oceanic\nreorganization” in response to the residual ice sheet and the associated (small) flux of meltwater?\nThe reorganization in the coupled system is usually discussed for the early deglacial period,\nwhen both the ice sheet and the meltwater flux are large.\nThe spatial patterns of Holocene temperature changes over mid-latitude Eurasia, as inferred\nfrom our alkenone results and published temperature records, are not captured by the transient\nmodel simulations (e.g., Liu et al., 2014; Bader et al., 2020), although model results (Liu et al.,\n2014) do suggest that retreating ice sheets contributed to the global warming trend over the\nHolocene. Furthermore, we do not find evidence to suggest that orbital forcing and atmospheric\ngreenhouse gases play different roles in temperature changes in subregions of the extratropical\nEurasian continent. Therefore, it seems unlikely that changes in global-scale forcings included\nin current models can explain the spatial patterns of Holocene temperature changes. Additional\nclimate forcings or feedbacks are required to explain the spatial patterns of Holocene\ntemperature changes over mid-latitude Eurasia. To some degree, we agree with Reviewer’s\nassessment that the spatial pattern, which we believe is clearly identified in our present study,\nremains to be explained physically. However, we still wish to offer some speculations.\nExamination of climate forcings or feedbacks inferred from model simulations and climatic\nrecords could provide a plausible interpretation of the spatial patterns observed from proxy\nreconstructions that have not been captured by current transient models. Model studies\n(Harrison et al., 1992; Siegert and Dowdeswell, 2004) have reported the presence of\nanticyclonic circulation over the Fennoscandian sector of the Scandinavian ice sheet during\nexpanded ice extent, which depressed surface temperatures over western Eurasia before the\nHolocene. Results from the GENMOM coupled atmosphere-ocean model suggest that an ice-\nsheet-induced perturbation of atmospheric circulation dynamics was still present during the\nearly to mid-Holocene, producing positive/negative 500-hPa geopotential height and sea-level\npressure anomalies at northern mid-high latitudes, despite the reduced Eurasia ice sheets (\nAlder and Hostetler, 2015). Therefore, we suggest that the atmospheric circulation dynamics over the\nmid-latitude Eurasian continent appears to be still influenced by remnant Eurasia ice sheet over\nScandinavian during the early to mid-Holocene, although, based on current knowledge, the\namount of residual ice sheet at that period remains unclear. We insert here (below) the figure\nby Alder and Hostetler (2015) to show the GENMOM-simulated seasonal 500 hPa geopotential\nheight and wind anomalies relative to PI.\nRedacted\n,3/:B5683# ?53 08;176:4 6:23C 0D */73= 3? /8$ !'%&(\" 6> =31;:>?=@1?32 0/>32 ;: ?53 E18O\ngradient between DYE-3 and Renland ice cores, because atmospheric blocking in the North\n)?8/:?61 >31?;= /:2 -1/:26:/A6/ <=;2@13> <;>6?6A3 E18O anomalies in the eastern\npercipitation\n=38/?6A3 ?; >;@?53=: +=33:8/:2$ .53 08;176:4 6:23C 1;==38/?3> >?=;:48D B6?5 ?53 E18O record\nfrom Kinderlinskaya Cave, which is interpreted as a winter temperature indicator. The larger\nE18O gradient during the early Holocene is explained by the persistence of blocking near\nScandinavia that formed in response to the ice-sheet forcing, and therefore depressed surface\ntemperatures over western Eurasia (Baker et al., 2017).\nBased on evidence presented above, and resemblances in atmospheric circulation dynamics\nand temperature conditions over the mid-latitude Eurasian continent during current winters and\nthe early to mid-Holocene, we suggest that enhanced wavy pattern of westerlies associated with\nthe presence of high-pressure system and frequent atmospheric blocking events over the mid-\nlatitude Eurasian continent before ~6,000 a BP appears to be the most plausible interpretation\nfor the proposed spatial patterns of Holocene temperature changes. The wavy pattern of\nwesterlies suggested in our study is not equivalent to the southward migration of jet, but rather\na more meridional pattern of westerlies that brings cold (sub)polar airmass reaching the interior\nof the mid-high latitude Eurasian continent, and thus depressed surface temperatures over this\nregion during the early to mid-Holocene. However, regions outside the influence of cold air\nmasses should have followed the decreasing boreal insolation, displaying Holocene long-term\ncooling trends. We insert here (below) the figure by Siegert and Dowdeswell (2004) to show\nthe wavy pattern of westerlies generated by an Atmospheric General Circulation Model.\nRedacted\nWassenburg et al. (2016) compared Holocene rainfall record from northwest Africa based on\n><383;?539E18O with a speleothem-based rainfall record from Europe. They suggested that the\ntwo records are positively correlated during the early Holocene, followed by a shift to an anti-\ncorrelation during the mid-Holocene which is similar to the modern condition. Based on\nsimulation results generated by an Earth system model, they proposed that the shift to the anti-\ncorrelation reflects a large-scale atmospheric and oceanic reorganization in response to the\ndemise of the Laurentide ice sheet and a strong reduction of meltwater flux. That study showed\nthat atmospheric circulation dynamics during the early to mid-Holocene could have been\ndifferent from the late Holocene. Stager and Mayewski (1997) also suggested that atmospheric\ncirculation dynamics changed abruptly during the early to mid-Holocene, based on abrupt\nchanges in paleoclimatic records from equatorial East Africa, Antarctica, and Greenland which\nindicates a large-scale climatic reorganization to full postglacial conditions. Based on results\nfrom simulations and reconstructions, we consider that residual ice sheet and meltwater,\ntogether with changes in orbital forcing and land-sea thermal differences, could have\ninfluenced the atmospheric circulation dynamics during the early to mid-Holocene, and the\natmospheric and climatic reorganization occurred after the demise of ice sheet and reduced\nmeltwater.\nWe understand Reviewer’s concern about the physical argument of our suggested/speculated\nmechanism. The spatial patterns of Holocene temperature changes over mid-latitude Eurasia\nhave not been captured by current transient model simulations, thereby limiting our ability to\nverify the proposed mechanism through modeling. By integrating anomalies of climate\nforcings or feedbacks inferred from model simulations and climatic records, and comparing\ncirculation and temperature conditions during the early to mid-Holocene with modern winter\nconditions, we could offer a plausible interpretation for the spatial patterns of Holocene\ntemperature changes. We agree with Reviewer that the proposed mechanism needs to be\nphysically validated in future studies. At present, our objective is to identify the spatial patterns\nof Holocene temperature changes over mid-latitude Eurasia, argue against the seasonality\neffects on such patterns (at least in the study region), and present a plausible interpretation for\nfurther studies.\nWe have revised the relevant section to incorporate Reviewer’s comments. We have clarified\nthat the spatial patterns of Holocene temperature changes over mid-latitude Eurasia, not\ncaptured by current simulations, need to be explained with additional forcings/feedbacks (Line\n239-240, 242-248). We have softened our wording about the suggested mechanism (Line 31,\n34, 251-253, 280-281, 305-306). We have modified our wording regarding cold airmasses\nreaching the interior of the mid-high latitude Eurasian continent (Line 263-264, 287-288).\n3. It will be helpful to clarify the point of the paper by presenting some model results. If model\nshows homogenous warming/cooling for seasonal and/or annual mean, it presents a clear\nmodel-data inconsistency of the regional temperature response. The simplest is to check some\nregional temperature trend in the available model experiments or data assimilation reanalysis\nproducts. Are all of the models generating a uniform temperature change? It will be more\nfruitful if the authors collaborate with some modelers for the model-data comparison. The\nmodel result of Extended Data Fig.5b seems to me not much relevant to the point of the paper.\nThis model is used, not because it explains the physical mechanism, but because it looks like\nwhat the authors want.\nWe agree with Reviewer that model-data comparison helps to clarify our statement. This is also\nthe reason we presented our previous Supplementary Fig. 5, which we meant to show that the\nsimulated pattern (Bader et al., 2020) is OPPOSITE to the one identified in our present study.\nMeanwhile, we have invited Dr Jun Cheng, who is now our coauthor, to help model-data\ncomparison.\nAs suggested, we have compared our alkenone-based warm season temperature records with\nCCSM3-simulated Holocene JJA temperature changes (Liu et al., 2009) in northeastern China\nand southwestern Siberia (revised Fig. 6e, f). Model results show summer cooling trends in\nboth regions, contrasting with our alkenone records. We have also compared CCSM3-\nsimulated JJA, DJF, and annual temperature anomalies (Liu et al., 2009, revised Fig. 5a, d)\nwith pollen-based seasonal temperature records (Zhang et al., 2022, revised Fig. 5b, c). Model\nresults again indicate similar seasonal/annual mean temperature trends in northeastern China\nand southwestern Siberia, i.e., long-term winter/annual warming and summer cooling trends.\nThe spatial pattern inferred from pollen synthesis, i.e., long-term seasonal/annual cooling\ntrends in 0–50°N of Asia but warming trends in 50–75°N of Asia, have not been captured by\nmodel simulations.\nSeasonal surface air temperature anomalies generated by GENMOM, PMIP3, and PMIP4\nmodels all exhibit consistent spatial patterns, indicating higher summer temperatures but lower\nwinter temperatures over the extratropical Eurasian continent at 6,000 a BP compared to the\npre-industrial era (Alder and Hostetler, 2015; Brierley et al., 2020). Recent data assimilations\n(Erb et al., 2022; Kaufman et al., 2023, revised Fig. 4b) show spatially unanimous higher\nannual mean temperatures over the extratropical Eurasian continent at 6,000 a BP than 500 a\nBP, failing to capture the substantial temperature discrepancies over this region during the mid-\nHolocene. Annual mean surface air temperature changes generated by the PMIP4 model\nindicate lower temperatures over the mid-latitude Asia at 6,000 a BP compared to the pre-\nindustrial era (Brierley et al., 2020, revised Fig. 4c). Such model-based spatial pattern is in\ncontrast to results derived from data assimilations and proxy reconstructions. We recognize that\nconducting model–data comparisons could help us gain a better understanding of spatial\npatterns of Holocene temperature changes and associated mechanisms. We also plan to\ncollaborate with modelers to conduct model-based climate sensitivity experiments to test the\nspeculated mechanisms, but it is expected to be a long-term effort.\nWe have collaborated with a paleomodeler and expanded the model-data comparison section\nin the revised main text to better address Reviewer’s concerns. We have incorporated CCSM3-\nsimulated regional temperature changes into our discussion (Line 213-216) and compared\nproxy-based records with CCSM3-simulated results in revised Fig. 5, 6. Additionally, we have\nincorporated more data assimilation and model results into our discussion (Line 209-216, 216-\n220). We have included annual mean surface air temperature conditions at 6,000 a BP compared\nto the pre-industrial period generated by data assimilation (Erb et al., 2022) and PMIP4 model\n(Brierley et al., 2020) in revised Fig. 4, which highlights the contrasting spatial patterns\nbetween proxy-based reconstructions and data assimilation/model results.\nMinor points:\n“A spatiotemporal analysis of annual temperature variability using data from model\nsimulations suggests a Holocene cooling mode over the Arctic Ocean and mid-to-high latitude\nEurasia. A data assimilation reconstruction combining proxy records and simulation results\nalso yields warmer conditions across continental Eurasia during the mid-Holocene relative to\nthe preindustrial era”\nBe consistent, or at least very careful, in specifying the trend of cooling/warming against the\ncolder/warmer condition in the mid-Holocene. They are of the opposite signs. For example, the\nsentence above is unambiguous. The first part “cooling” mode seems to refer to the cooling\ntowards late Holocene (so it is warmer condition in the mid-Holocene). The second part\n“warmer condition during the mid-Holocene” also implies a cooling towards late Holocene.\nAre they the same or not? It may be more clear to use either the trend or the mid-Holocene\ncondition relative to the present throughout the paper.\nThe “cooling mode” indicates long-term Holocene cooling trends. The “warmer condition\nduring the mid-Holocene”, inferred from the comparison of temperatures at 6,000 a BP with\nthe preindustrial era, indicates cooling trends since the mid-Holocene. We have modified our\ndescription of temperature trends here (Line 55-60) and throughout the paper.\n“Yet, a recent synthesis of pollen records indicates a warm early to mid-Holocene followed by\ngenerally cooling trends in southern Asia and Europe, but long-term Holocene warming trends\nin northern high-latitude Asia. Given the uncertainties in regional temperature changes and\npoorly constrained climate responses to various forcings, the Holocene temperature conundrum\nremains highly contentious, and our understanding of Holocene hydroclimatic changes and\nassociated mechanisms remains largely speculative”\nIt appears the authors try to say models show the same sign of cooling towards late Holocene,\nbut a recent synthesis shows opposite trend in different regions in Eurasia continent. That is\nfine, but i) this point should be clarified explicitly, ii) this is not much relevant to the conundrum,\nwhich is about global mean and annual mean.\nThanks for this suggestion and we have modified this part accordingly (Line 60-70).\nExtended Data Fig.4: What does it mean by “southern Asia” here? Lake Yihesariwusu is in\nnortheastern China, which is NOT the southern Asia we usually refer to (or I never heard this\nregion is called southern Asia). Be careful on your regional reference. In the region of your\ninterest “northern Eurasia”, this is the “southern region”, while southern Siberia is the “northern\nregion”. You can predefine these regions first (give approximate latitude/longitude ranges…).\nZhang et al. (2022) used southern/northern Asia to refer 0–50°N/50–75°N of Asia. We have\nprovided latitude ranges of these regions to make our description clearer (Line 60-63, caption\nof revised Fig. 5).\nL99- “warm conditions prevailed in northern China, northern and western Europe (Fig. 3a)”.\nThis description is confusing. To my eye, the warming is dominant in central Europe, while\ncooling occurs in the surrounding area. Certainly, northern and western Europe are dominated\nby cooling (blue), instead of warming. It seems the authors claimed this pattern in Europe\nbecause they want to show a coherent pattern. But, the European pattern does not seem to me\nrelated to the pattern of their lake sites. Again, are these temperature change of seasonal (which\nseason if yes) or annual mean?\nThe sentence Reviewer mentioned is “warm conditions prevailed in northern China, northern\nand western Europe during the early to mid-Holocene”, which refers to records showing\nrelatively high temperatures during the early to mid-Holocene and long-term Holocene cooling\ntrends, i.e., blue dots in previous Fig. 3a (revised Fig. 4a). It seems that our understanding of\nthe cooling trends in northern and western Europe aligns with what Reviewer has observed.\nWe have modified our wording regarding “warming/cooling conditions during the early to mid-\nHolocene” and “Holocene warming/cooling trends” here (Line 155-159) and throughout the\npaper.\nWe also note that several records from eastern Europe show Holocene cooling trends (blue\ndots), not consistent with our proposed spatial pattern. However, records indicating Holocene\nwarming trends concentrate within the interior of the mid-latitude Eurasian continent, and more\nrecords showing lower temperatures before ~6,000 a BP to the south of Scandinavia than north.\nGiven the uncertainties in proxy reconstructions and temperature (re)calibrations, our\nassessment regarding cold conditions over the interior of the mid-latitude Eurasian continent\nduring the early to mid-Holocene appears to be reasonable. Meanwhile, we have softened our\nwording in the revised main text (Line 158-159).\nWe have presented records representing different seasons with different symbols in revised Fig.\n4a.\nL142 “However, the spatial patterns of Holocene temperature changes observed in reanalyzed\nmodel data , which show marked long-term cooling trends over the extratropical Eurasian\ncontinent (Fig. 4g, Extended Data Fig. 5b), suggested to respond to increased Arctic sea ice\ncoverage, are opposite to what the proxy-based results suggest (Fig. 2, 3).” Again, is Fig.3g for\nannual mean or seasonal? If your data is seasonal, why don’t you compare the seasonal\ntemperature? Same when other models/data assimilation products are mentioned. Refer to my\nmajor comment 3 on this.\nOur previous Fig. 3g (revised Fig. 3f) shows alkenone-based warm season temperature records\nfrom Siberian lakes. However, the spatial patterns of Holocene temperature changes over mid-\nlatitude Asia appear to persist throughout the year, rather than being limited to the warm season,\nas mentioned at the beginning of this letter. Therefore, we compared proxy-based records\ninferred from different proxies with model-simulated annual mean temperature changes in\nrevised Fig. 4. Meanwhile, we also compared simulated seasonal temperature records with\nproxy-based records in revised Fig. 5, 6.\nL170 “The spatial patterns of Holocene temperature changes do not appear to be directly\nassociated with decreasing boreal insolation or increasing greenhouse gas concentration (Fig.\n4a), because such global-scale forcings could induce spatially consistent temperature\nvariations.”\nIt is not obvious these two mechanisms should be discarded. Is it possible different mechanism\nis dominated in different regions?\nThe spatial patterns of Holocene temperature changes over the extratropical Eurasian continent\nsuggested in our study, not captured by model simulations or data assimilation studies, appear\nto be difficult to explain with changes in boreal insolation or greenhouse gas concentration, as\nboth factors have been included in current models. Currently, it is difficult to find evidence that\ninsolation and greenhouse gas concentration play different roles in temperature changes in\nsubregions of extratropical Eurasia.\nBased on the ice-sheet influence on atmospheric circulation dynamics suggested in model\nsimulation and reconstruction studies, in addition to the resemblances in atmospheric\ncirculation dynamics and temperature conditions over the mid-latitude Eurasian continent\nduring current winters and the early to mid-Holocene, we consider that the enhanced wavy\npattern of westerlies, associated with the persistent anticyclone system perhaps induced by the\nremnant ice sheets before ~6,000 a BP, appear to be the most plausible mechanistic control on\nspatial patterns of Holocene temperature changes over this region. We suggest that cold\nconditions in the interior of the mid- to high-latitude Eurasian continent, in contrast to the\nhigher temperatures in surrounding regions, during the early to mid-Holocene, shaped the\nspatial patterns of long-term Holocene temperature changes over the mid-latitude Eurasian\ncontinent. Meanwhile, we do not deny the influence of insolation and greenhouse gas\nconcentration on Holocene temperature changes, which could play decisive roles in\ntemperature changes over the extratropical Eurasia continent during the mid- to late Holocene.\nAlso, regions outside the influence of cold air masses during the early to mid-Holocene, e.g.,\nLake Yihesariwusu, should have followed the decreasing boreal insolation, displaying\nHolocene long-term cooling trends.\nWe have modified the text in this section (Line 244-248, 281-284) to make our statement clearer.\nL196: “…might be broadly comparable to recent Arctic amplification and Eurasian cold\nwinters”. This does not seem to me a good analogy. Recent arctic amplification is thought to\nbe caused by the rising CO2, and the warming tends to occur uniformly. The Eurasian cooling\nis also for winter, opposite to the warm season proxy here. How are these consistent?\nWe understand that the forcings of Arctic warming during current winters and the early to mid-\nHolocene appear to be different. However, the spatial pattern of temperature conditions during\nthe early to mid-Holocene, i.e., overall warm conditions in the Arctic Ocean and low\ntemperatures over the central Eurasian continent, broadly resemble current winter conditions.\nWe consider that the atmospheric circulation pattern during these two periods appears to be\ncomparable, based on the resemblances in the spatial patterns of temperatures over the mid-\nlatitude Eurasian during current winters and the early to mid-Holocene.\nWe also realized that using “Arctic amplification” might cause ambiguity, so we have modified\nthis sentence to make our statement clearer (Line 276).\n203: “As ice sheet-induced anticyclone might have persisted over northern mid-high latitudes\n(Fig. 4d), we suggest that large-scale changes in land-sea temperature and pressure gradients\ncould induce more meridional, or wavy patterns of westerlies, and southward deflection of the\npolar front to continental Eurasia”. What cause this southward shift, orbital, which season?\nCO2? Ice sheet?\nHere we meant that cold (sub)polar airmass reached the interior of the Eurasian continent due\nto the wavy patterns of westerlies. We have modified this sentence to clarify our statement\n(Line 287-288).\nShould Ref 7 be Erb et al., 2022?\nThis ref. 7 (Osama) is for Last Glacial Maximum Reanalysis, and shows a warming trend in\nthe Holocene. Erb et al. is on the Holocene with multiple models and does not show warming\ntrend.\nErb, M. P. et al. Reconstructing Holocene temperatures in time and space using paleoclimate\ndata assimilation. Clim. Past 18, 2599–2629 (2022).\nKaufman and Broadman (2023) show blended proxy data and simulated temperature at 6 ka\nrelative to 0.5 ka using data from LGMR data assimilation by Osman et al. (2021), which\nindicates higher annual mean temperature over the extratropical Eurasian continent at 6 ka\nrelative to 0.5 ka. We have removed the citation of Osman et al. (2021) from Line 60, 206,\nremained the citation of Kaufman and Broadman (2023), and added Erb et al. (2022) to our\nreference list.\nFig.2 and 3, each curve, include the lake name for each label, e.g. a) Lake…..b)Lake……This\nhelps readers…not have to go back to the caption.\nWe thank for this suggestion and have added site names to revised Fig. 2 and 3.\nThere is no Supplementary Table 2, referred in the caption of Fig.3. There is only\nsupplementary data set 2. But that is not what a reviewer would like to see. A reviewer just\nneeds the type, location et al of each record used.\nWe have modified filenames of Supplementary Tables.\nThanks again for carefully reading our manuscript, figures, and captions.\nReviewer #2 (Remarks to the Author):\nIn this manuscript, the authors present alkenone based temperature reconstructions for the\nHolocene: one from the monsoonal marginal zone and three from the continental/westerlies\nrealm. By contextualization of the data they shape out two different spatial patterns of\ntemperature changes throughout the study area and covered time period.\nThe overall aim of the manuscript is to solve question connected to the „Holocene conundrum“.\nA recent study (Herzschuh et al 2023; https://cp.copernicus.org/articles/19/1481/2023/) \nhighlighted\nthe importancy of focussing on spatial Holocene temperature patterns and their regional drivers,\nfor the understanding of this proxy-model mismatch. By further investigating the situation in Asia,\nthis manuscript by Jiang et al, is an important and interesting contribution to the debate.\nDespite these aspects, I´m not convinced that this study is suitable for NCOMM for the following\nreasons:\n1. I miss a more critical discussion of the alkenone proxy which is often problematic in lakes.\nI´m not familiar with all of the cited original studies including those of which data from\nother lakes were plotted, but it appears that many factors can influence the different kinds\nof uk37-proxies. Salinity changes, translocation of alkenone producers, glacial meltwater\ninfluence, community changes of alkenone producers,… are just some of the factors which\nare mentioned in some of the cited papers. Partially, this is solved in the manuscript by\ncomparison with independent proxies, but I still miss a discussion on how reliable\nalkenone-based temperature proxies are in the specific case, not only in the methods but in\nthe main part of the manuscript.\nWe would like to clarify that we put relevant discussion in Methods previously. Meanwhile,\nwe understand your concern and concur with you that alkenone proxies, like many other\npaleoclimatic indicators, are affected by various factors. It is important to consider the effects\nof different influencing factors when interpreting paleoclimatic records. We believe that the\nalkenone-based temperature records used in this study are reliable terrestrial temperature\nindicators in mid-latitude Asia for the following reasons:\n(1) The applicability of\nU#$ index as a temperature indicator in mid-latitude Asian lakes has\nbeen examined through both regional calibrations and paleoclimate reconstructions. The\nU#$ -temperature calibration based on lake surface sediment samples from mid-latitude Asia\n(Chu et al., 2005) is largely consistent with a culture experiment conducted in this region\n(Sun et al., 2007), indicating that\nU#$ index could record the growing season temperature\nof alkenone-producing haptophytes in mid-latitude Asian lakes. Meanwhile, the\nU#$ index has been successfully employed to reconstruct Holocene temperature changes over\nthis region, after careful consideration of its suitability for different lakes (e.g., He et al.,\n2013; Jiang et al., 2022; Zhao et al., 2013).\n(2) Alkenone C isomer has not been identified from all four investigated mid-latitude Asian\n37\nlakes, indicating that alkenones in these lakes are produced only by Group II haptophytes\nduring the Holocene (Longo et al., 2013; Yao et al., 2022). In the absence of Group I\nhaptophytes, the alkenone-based records in our study are not affected by phylogenetic effect.\nWe have added an explanation of phylogenetic effect to revised main text (Line 87-92).\n(3) The similar variation trends of alkenone records from three Siberian lakes, combined with\nspatial patterns of\nU#$ and %C records observed in investigated mid-latitude Asian\nlakes, led us to believe that variations in alkenone-based records in our study preliminary\nresponse to climatic changes rather than in-lake factors.\n(4) Alkenone %C records from investigated lakes could successfully indicate lake salinity\nvariations and thus hydroclimatic changes during the Holocene. The\nU# records from\nthese lakes, following the structure of %C variations, could be largely confounded by\nsalinity changes, while lake salinity has little effect on the\nU#$ index. Therefore, we use\nU#$ index as a reliable temperature indicator in our study.\nTo incorporate Reviewer’s comment, we have moved the alkenone interpretation from the\nprevious Methods section to the main text and also modified the text in this section (Line 81-\n125), which could provide a fair assessment of the reliability of alkenone records used in this\nstudy.\n2. Connected with these aspects, I find the recent CP-paper by Herzschuh et al is more\nconvincing, because solely focussing on pollen transfer functions, which have been proven\nto work well in context of precipation and temperature reconstructions. Unfortunately, this\nalso takes away a bit of novelty from this new manuscript by Jiang et al, especially since\nHerzschuh et al already showed some spatial heterogenity of Holocene temperature in Asia.\nConcluding, I think this new manuscript is definetly be worth to be published after revisions,\nbut more likely in a journal such as QSR or CP or maybe COMM EARTH ENV.\nWe did not cite Herzschuh et al. (2023) in our original manuscript, as it was formally published\nafter our submission. However, we did cite Zhang et al. (2022), which also reports spatial\nheterogeneity in this region. We have now cited this recent study in our revised manuscript\n(Ref. 13 in revised manuscript), as it provides valuable insights into regional temperature\nchanges and offers a significant pollen database for paleoclimatic studies. While we\nacknowledge the importance of pollen-based temperature reconstructions in paleoclimatic\nstudies, we strongly believe that combining records inferred from different proxies could\nenhance our better understanding of the Holocene temperature changes, particularly regarding\nthe suggested seasonality bias in temperature proxies. Meanwhile, we would like to clarify that\nthe spatial patterns of Holocene temperature changes over mid-latitude Asia proposed in our\nstudy differ substantially from the latitudinal patterns discussed in Zhang et al. (2022) cited in\nour original manuscript and Herzschuh et al. (2023).\nThe study conducted by Herzschuh et al. (2023) investigated pollen-based latitudinal patterns\nof Holocene temperature changes from the Northern Hemisphere. Additionally, a recent\nsynthesis of pollen records by Zhang et al. (2022), as cited in our original manuscript, also\ndiscussed the latitudinal pattern of Holocene temperature changes over the Northern\nHemisphere. Pollen-based annual temperature records from Herzschuh et al. (2023) show long-\nterm Holocene warming trends in 40-50°N of Asia, while cooling trends are observed in 60-\n70°N of Asia. Yet, pollen-based seasonal temperature records from Zhang et al. (2022) reveal\nHolocene long-term cooling trends in 0-50°N of Asia but warming trends in 50-75°N of Asia\nin both summer and winter seasons. Also, in Herzschuh et al. (2023) seasonal temperature\ntrends could differ from annual trends, while in Zhang et al. (2022) seasonal trends are\nessentially the same as annual ones. Different pollen-based spatial patterns of Holocene\ntemperature changes over mid-latitude Asia proposed by Zhang et al. (2022) and Herzschuh et\nal. (2023) indicate that Holocene temperature changes over this region may not conform to a\nlatitudinal pattern. Therefore, conducting further studies that integrate temperature records\ninferred from various proxies and consider spatial patterns beyond latitudinal binning may\nprovide new insights into Holocene temperature changes. We inset here (below) the figures by\nHerzschuh et al. (2023) and Zhang et al. (2022) to show different pollen-based latitudinal\npatterns of Holocene temperature changes over mid-latitude Asia.\ntemperature anomalies. Starting with alkenone records, we identified different temperature\nvariation trends from mid-latitude Asian lakes. We then compared the spatial patterns inferred\nfrom alkenone results with other temperature records inferred from various proxies, models,\nand data assimilations, and proposed that colder airmass appears to have prevailed in the\ninterior of the mid-latitude Eurasian continent during the early to mid-Holocene. Such spatial\npattern is unlikely to be attributed to seasonality bias in proxies and is independent of latitude\npositions.\nOverall, we are confident that our study presents sufficient novelty. The spatial patterns we\nsuggest, which differ from pollen-based latitudinal patterns, might account for the incoherent\npollen-based spatial patterns proposed by Zhang et al. (2022) and Herzschuh et al. (2023). Our\nstudy challenges the proposed seasonality bias in proxies and modeled spatial patterns,\nhighlighting that spatial patterns of Holocene temperature changes should be re-considered in\nboth record integrations and model simulations, with important implications for terrestrial\nhydroclimate changes. Our results also challenge current approaches addressing the Holocene\ntemperature conundrum (at least for this region), implying that several current views\nconcerning the Holocene temperature conundrum need to be re-considered.\nTo address Reviewer’s comment, we have included Herzschuh et al. (2023) in the revised main\ntext (Line 63-67) and reference list (Ref. 13 in revised manuscript). We have also emphasized\nthat the spatial pattens proposed in our study differ from the pollen-based latitudinal patterns,\n(revised Fig. 5, Line 162-164, 175-176, 194-196).\nMinor comments:\n- The above mentioned paper by Herzschuh needs to be cited\nWe have included Herzschuh et al. (2023) in reference list (Ref. 13 in revised manuscript).\n- Maps in Fig 3a and Ext Fig 5a would benefit from labelling the dots (as in Fig. 1)\nWe appreciate the suggestion. It is worth noting that previous Fig. 3a (revised Fig. 4a) contain\n~160 records. Labeling each dot would make the figures complicated and challenging to read.\nOur primary objective is to highlight the spatial patterns of temperature changes over mid-\nlatitude Eurasia in these figures. Actually, we have provided a list of records presented in both\nfigures in Supplementary Table 2, which includes detailed site locations and references. To\nincorporate Reviewer’s comment while maintaining the readability of the figures, we have\nadded site location names to revised caption of Fig. 1, and added site numbers same as Fig. 1\nto revised Fig. 4a.\n- There is a lot of reference to figures in the appendix. I would recommend to reconsider if\nsome of those figures could be shifted to the main manuscript, in case this manuscript is re-\narranged to a less short-format version.\nWe very much appreciate this suggestion. We have moved the alkenone interpretation section\nfrom Methods to revised main text (Line 81-125), and now most of references cited in this\nstudy have been included in revised main text. We have modified and moved two\nsupplementary figures to the main manuscript (revised Fig. 4, 5) to better address our statement.\nReferences\n1. Alder, J. R. & Hostetler, S. W. Global climate simulations at 3000-year intervals for the last\n21000 years with the GENMOM coupled atmosphere–ocean model. Clim. Past 11, 449–\n471 (2015).\n2. Bader, J. et al. Global temperature modes shed light on the Holocene temperature\nconundrum. Nat. Commun.11, 1–8 (2020).\n3. Brierley, C. M. et al. Large-scale features and evaluation of the PMIP4-CMIP6 mid\nHolocene simulations. Clim. Past 16, 1847–1872 (2020).\n4. Chu, G. et al. Long-chain alkenone distributions and temperature dependence in lacustrine\nsurface sediments from China. Geochim. Cosmochim. Acta 69, 4985–5003 (2005).\n5. Erb, M. P. et al. Reconstructing Holocene temperatures in time and space using\npaleoclimate data assimilation. Clim. Past 18, 2599–2629 (2022).\n6. Harrison, S. P., Prentice, I. C., & Bartlein, P. J. Influence of insolation and glaciation on\natmospheric circulation in the North Atlantic sector: implications of general circulation\nmodel experiments for the Late Quaternary climatology of Europe. Quat. Sci. Rev.11, 283–\n299 (1992).\n7. He, Y. et al. Late Holocene coupled moisture and temperature changes on the northern\nTibetan Plateau. Quat. Sci. Rev. 80, 47–57 (2013).\n8. Herzschuh, U. et al. Regional pollen-based Holocene temperature and precipitation patterns\ndepart from the Northern Hemisphere mean trends. Clim. Past 19, 1481–1506 (2023).\n9. Jiang, Q. et al. Exceptional terrestrial warmth around 4200–2800 years ago in Northwest\nChina. Sci. Bull. 67, 427–436 (2022).\n10.Kaufman, D. S. & Broadman, E. Revisiting the Holocene global temperature conundrum.\nNature614, 425–435 (2023).\n11.Liu, Z. et al. Transient simulation of last deglaciation with a new mechanism for Bølling-\nAllerød warming. Science325, 310–314 (2009).\n12.Longo, W. M., Dillon, J. T., Tarozo, R., Salacup, J. M. & Huang, Y. Unprecedented\nseparation of long chain alkenones from gas chromatography with a poly\n(trifluoropropylmethylsiloxane) stationary. Org. Geochem.65, 94–102 (2013).\n13.Osman, M. B. et al. Globally resolved surface temperatures since the Last Glacial\nMaximum. Nature599, 239–244 (2021).\n14.Siegert, M. J. & Dowdeswell, J. A. Numerical reconstructions of the Eurasian Ice Sheet and\nclimate during the Late Weichselian. Quat. Sci. Rev.23, 1273–1283 (2004).\n15.Sun, Q., Chu, G., Liu, G., Li, S. & Wang, X. Calibration of alkenone unsaturation index\nwith growth temperature for a lacustrine species, Chrysotila lamellosa (Haptophyceae).\nOrg. Geochem.38, 1226–1234 (2007).\n16.Wassenburg, J. A. et al. Reorganization of the North Atlantic Oscillation during early\nHolocene deglaciation. Nat. Geosci.9, 602–605 (2016).\n17.Yao, Y. et al. Phylogeny, alkenone profiles and ecology of Isochrysidales subclades in saline\nlakes: Implications for paleosalinity and paleotemperature reconstructions. Geochim.\nCosmochim. Acta 317, 472–487 (2022).\n18.Zhao, C. et al. Holocene temperature fluctuations in the northern Tibetan Plateau. Quat.\nRes. 80, 55–65 (2013).\n19.Zhang, W. et al. Holocene seasonal temperature evolution and spatial variability over the\nNorthern Hemisphere landmass. Nat. Commun.13, 5334 (2022)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed most of my questions reasonably well. Therefore, I recommend\npublication. However, there are two comments that they have to clarify before the acceptance.\n1. Since seasonality is a crucial factor here, it is important to specify the warming for annual mean\nor some season. For example, in the abstract, for line 27, 28, 29,30, warming or cooling are used,\nbut without specification of its seasonality/annual mean. This should be corrected! Also in the text.\n2. Fig.4b, c", "reviewer_d": "Is it the wrong sign? The caption says it is the trend from 6ka to 0ka. But I think\nthis is the wrong sign. Please check!\nAs a check, and also useful for readers, the authors can also plot on Fig.6e, f, the annual mean\ntrend of model, say, in a dotted line.", "reviewer_2": "This is the revised version of a manuscript submitted to NCOMMS by Jiang and co-authors.\nA notify and appreciate that the authors extensively responded to all of the reviewers comments\nand made substantial changes that lead to significant improvement of the manuscript. The critical\ndiscussion about value of alkenone based temperature proxies and potential bias by in-lake effects\nwas shifted to the main part of the manuscript, which I consider as important, given the\ncomplications in proxy interpretation that often arise due to those effects. Further, discussion on\npotential seasonality effects of different proxy groups comes out more clearly in the revised\nversion. Finally, the model-proxy comparison aspect was much strenghthened in the new version,\nwhich significantly improves the discussion on spatial trends in temperature change in the studied\nregion.\nSome scepticism about bias on alkenone based temperature proxies remain. However, in context\nof the compiled data, the model-proxy comparison, and the arguments given by the authors, the\npresented data, their contextualisation, and inferred conclusions contain sufficient novelty and\nquality to push the debate about the Holocene conundrum forward.\nI therefore recomment this manuscript for publication in NCOMMS."}, "author_response": "We have carefully followed Editor's and reviewers' comments revising our manuscript. In order\nto clearly highlight the changes made in the revision, we have copied all these comments below\nin black and inserted our replies in blue. Line numbers mentioned in our response letter refer\nto the ones in our revised manuscript.\nReviewer #1 (Remarks to the Author):\nThe authors have addressed most of my questions reasonably well. Therefore, I recommend\npublication. However, there are two comments that they have to clarify before the acceptance.\n1. Since seasonality is a crucial factor here, it is important to specify the warming for annual\nmean or some season. For example, in the abstract, for line 27, 28, 29,30, warming or\ncooling are used, but without specification of its seasonality/annual mean. This should be\ncorrected! Also in the text.\nWe used “long-term cooling trend in warm season temperatures” in Line 27 to specify the\nseasonality of alkenone records. Existing temperature records from surrounding regions, in\nLine 29, refer to records inferred from different proxies which might indicate temperature\nchanges in different seasons, so we do not point out the seasonality here (although they are\nspecified in the main text). We have also checked our text to specify the seasonality. Meanwhile,\nwe understand that defining the seasonality of certain temperature reconstructions based on\nmultiproxy data can be challenging, while the seasonality of temperatures from model\nsimulations is clearly stated. Therefore, given these circumstances, we would like to retain the\noriginal description in Abstract as it stands.\n2. Fig.4b, c"}]

Decision letter