Projections of future climate and especially extreme events depend on the reliable simulation of climate variability. On timescales inaccessible to direct observations, the evaluation of simulated variability requires comparison to proxy-based reconstructions. However, simulated and proxy-based estimates of surface temperature variability disagree locally on decadal and longer timescales.

Here, we expand comparisons of surface temperature variability in simulations and observations with data covering up to the last 2 Million years. For the comparison, we compile a multi-proxy database, as well as use direct observations and an ensemble of transient and equilibrium simulations of varying complexity, including an ESM with a dynamically-coupled ice-sheet-solid earth model. Based on these, we create global and regional spectra of observed and simulated surface temperature covering daily to multi-millennial timescales. We evaluate the variability with respect to differences between land and sea, proxy type, model complexity, employed forcings and model properties such as resolution.

Our results confirm that global agreement between reconstructions and models extends to the past 2 Million years. The global composite spectrum follows a power law scaling with a break at multi-millennial timescales. The results further demonstrate that a range of models can simulate the continuum of global mean surface temperature. Regionally, we find substantial differences between simulations and observations in the tropics and subtropics, where reconstructed temperature variability surpasses simulated variability. On the model-side, the complexity of the atmosphere and representation of cloud processes seem particularly relevant for the magnitude of simulated tropical variability, however, the relationship between tropical dynamics and local temperature variability requires further investigation. In the mid and high latitudes, differences between simulations and observations are smaller, especially for complex models that include volcanic forcing. The choice between dynamic and prescribed ice sheets affects temperature variability in particular in the southern polar region, where dynamically coupled ice sheets tend to lead to better agreement with proxy-based reconstructions. We further find that forcings affect simulated variability not only on their characteristic timescales, but on both longer and shorter timescales. This highlights the importance of including long-term feedbacks and volcanic forcing to simulate the spectrum of temperature variability across timescales, a necessity for reliable projections, attribution studies and assessments of the risks of extreme events.

How to cite: Ziegler, E., Ellerhoff, B., Weitzel, N., Kapsch, M.-L., Mikolajewicz, U., and Rehfeld, K.: Differences between simulated and observed surface temperature variability in the tropics versus extratropics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16780, https://doi.org/10.5194/egusphere-egu25-16780, 2025.

Geochemical proxies are used to reconstruct changes in sea surface temperature (SST) prior to the instrumental era. However, proxy records can be influenced by non-climatic factors, including instrumental errors and sediment heterogeneity, which introduce uncertainty and further complicate the interpretation. To decipher the climate signal and noise from proxy reconstructions, here we assess the replicability of two commonly used SST proxies, foraminiferal Mg/Ca and U^K’₃₇, by analyzing records from four nearby sediment cores in the northern Okinawa Trough collected from a radius of 10 km. The results show that all records of the same proxy type display consistent glacial-interglacial trends but differ in the degree of high-frequency variability across sites in Mg/Ca records. This variability cannot be reproduced among sites, thus may reflect uncertainties in instrumental analysis or sedimentary heterogeneity rather than actual SST changes. In addition, this variability contributes to differing inter-proxy deviations across sites, demonstrating that proxy uncertainty may influence comparisons between proxies. Despite this, averaging proxy records reveals a systematic offset between Mg/Ca and U^K’₃₇. One possibility is that the discrepancy arises from different seasonal productions, as supported by modern proxy observations and the closer alignment of U^K’₃₇ with model-derived annual mean temperatures compared to Mg/Ca. However, Mg/Ca is influenced by non-thermal factors which, if taken into account, can also resolve the aforementioned discrepancy between proxies. Overall, our findings indicate that the first-order glacial-interglacial patterns in paleotemperature records are reproducible among sites, but Mg/Ca records exhibit additional high-frequency variability that may reflect proxy noise. These results may be site-specific due to the core sites being located in the depocenter of a small basin, highlighting the need for replicate studies in diverse depositional settings. Data-model comparisons can refine interpretations of proxy records. We encourage models to account for sedimentation processes, enabling more precise quantification of the impact of sedimentary heterogeneity. This approach will improve the robustness of data-model comparisons and provide a more comprehensive framework for reconstructing past climate variability.

How to cite: Tung, R.-Y., Ho, S. L., Kubota, Y., Yamamoto, M., Hefter, J., and Shen, C.-C.: Unraveling proxy noise and climate signal in paleotemperature records: a replicate study in the northwest Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5444, https://doi.org/10.5194/egusphere-egu25-5444, 2025.

            Pollen data are among the most abundant and spatially-temporally resolved proxies for quantitatively reconstructing European climate evolution since the Last Glacial Maximum (LGM, ~19-26 kyr, [1]). This period of near-climatic stability, characterized by a climate drastically different from the present, serves as a key reference point for evaluating the reliability/performance of climate models used to project anthropogenic climate change, especially in Paleoclimate Modelling Intercomparison Project (PMIP) ([2]) Pollen assemblages are particularly useful for studying climatic spatial gradients, the proximity effects of ice sheets, seasonality, and environmental changes (e.g., [3]). However, the number of available fossil sites, the quality of the records (i.e., diversity and taxonomic resolution), and the age constraints remain limited. In Europe, various pollen-based transfer functions have been used to provide climate reconstructions (e.g., Modern Analogue Technique (MAT) and Weighted Averaging Partial Least Squares (WAPLs); [4]), each depending differently on sampled modern analog climates. The lack of agreement between inverse methods necessitates the development of new transfer approaches to enable more robust and reliable reconstructions.

            Here, we present a new compilation of pollen sequences and revised age models for Europe ([5]) as well as a synthesis of reconstructions based on the most comprehensive European calibration dataset available to date (~8700 spectra): EMPD2 (Eurasian Modern Pollen Database, [6]). Temperatures and precipitation reconstructed using MAT and WAPLs are compared with outputs from the probabilistic CREST method ([7]), which is applied for the first time in Europe. In the studied areas, we demonstrate the value of the Plant Functional Type (PFT) biomization method ([8]) and so-called megabiomization approach ([9]) for quantifying coherent and large-scale environmental and climatic changes. By comparing outputs from these different approaches, we show that CREST (i) is particularly sensitive to detailed pollen (i.e., species), (ii) performs better with more taxonomically detailed pollen data, and (iii) is less dependent on the availability of modern analogues than MAT and WAPLs. Furthermore, reconstructions from CREST exhibit less abrupt variability than those from the other two methods.

To assess the significance, sensitivity, and robustness of the cooling and drying trends inferred from pollen data, we present some inter-proxy comparisons and compare these with simulation outputs from the intermediate-complexity climate model iLOVECLIM.

[1] - Tarasov, P. E., et al., (2013). https://doi.org/10.1016/j.quaint.2012.04.007

[2] - Harrison, S. P., et al., (2014).  https://doi.org/10.1007/s00382- 013-1922-3

[3] - Brewer.S, et al., (2008). https://doi.org/10.1016/j.quascirev.2008.08.029

[4] - Overpeck, J., Webb, T. et Prentice, I. (1985). https://doi.org/10.1016/0033-5894(85)90074-2

[5] - Blaauw.M, (2010). https://doi.org/10.1016/j.quageo.2010.01.002

[6] - Davis, B. A. S., et al., (2020). https://doi.org/10.5194/essd-12-2423-2020

[7] - Chevalier.M, (2022). https://doi:10.5194/cp-18-821-2022

[8] - Prentice, C., et al., (1996). https://doi.org/10.1007/BF00211617

[9] - Li, C., et al., (2024). https://doi.org/10.5194/egusphere-2024-1862

How to cite: Fénisse, G., Chevalier, M., Peyron, O., Bekaert, D. V., and Blard, P.-H.: Paleoclimate reconstructions based on European pollen data since the Last Glacial Maximum: probabilistic inversion, megabiomization, and multi-method approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17475, https://doi.org/10.5194/egusphere-egu25-17475, 2025.

The Holocene temperature history and its forcing mechanisms remain elusive due to the conflict between proxy reconstructions and model simulations. Recently, this model-data inconsistency has been partly attributed to the seasonal bias in the proxy indicators. This study attempts to assess changes in the seasonal variability of sea surface temperature in the low-latitude western Pacific since the Last Glacial Maximum (LGM), based on the standard deviation of oxygen isotope (δ¹⁸O) measured from individual planktonic foraminifera at four sites. The reconstructed temperature seasonality shows interhemispheric trends since the LGM, increasing in the Northern Hemisphere and decreasing in the Southern Hemisphere from the LGM to the early Holocene, then reversing towards the late Holocene. And we find that the meridional gradient in the amplitudes of temperature seasonality was similar to today, which is consistent with the orbital insolation forcing. Combining with the model outputs from transient simulations (Trace-21ka), we suggest that temperature seasonality of the west Pacific warm pool appears to be controlled solely by precession, and shows no evident response to changes in global ice volume and atmospheric CO₂ levels. According to our reconstruction, if proxies from tropical-subtropical oceans tended to record warm season temperature changes as proposed by previous studies, they would cause the ‘Thermal Maximum’ phenomenon during the early-mid Holocene.

How to cite: Yuan, Z. and Huang, E.: Precession Control of Temperature Seasonality Changes in the West Pacific Warm Pool since the Last Glacial Maximum, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-521, https://doi.org/10.5194/egusphere-egu25-521, 2025.

The El Niño-Southern Oscillation (ENSO) events that persist and develop for a second year or beyond (multi-year) are relatively rare. Compared to single-year events they create higher cumulative impacts and are linked to extended periods of extreme weather events worldwide. Past ENSO variations help us to better understand and anticipate how multi-year ENSO may change in the future. Here we combine proxy data reconstructions with a multi-model ensemble of climate simulations to investigate the evolution of multi-year ENSO during the Holocene (about 11,700 years ago to the present day), when the global annual mean climate was relatively stable and mainly driven by seasonal distribution of insolation. We find that the ratio of multi-year El Niño and La Niña to single-year events increased by a factor of 5, associated with a longer ENSO period (from 3.5 to 4.1 years) over the past ~7,000 years in monthly-resolved fossil coral oxygen isotope reconstructions from the equatorial central Pacific. This change is verified qualitatively by comparison with a subset of transient Holocene model simulations with a more realistic representation of ENSO periodicity. More frequent multi-year ENSO and prolonged ENSO period are caused by a shallower thermocline and stronger upper ocean stratification in the tropical Eastern Pacific towards the present day. The sensitivity of ENSO duration to orbital forcing provides a warning signal highlighting the urgency of minimising anthropogenic influence which could accelerate the long-term trend towards more persistent ENSO damages.

How to cite: Lu, Z., Schultze, A., Carré, M., Brierley, C. M., Hopcroft, P. O., Zhao, D., Zheng, M., Bracconot, P., Yin, Q., Jungclaus, J., Shi, X., Yang, H., and Zhang, Q.: Enhanced frequency of multi-year El Niño-Southern Oscillation across the Holocene, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5133, https://doi.org/10.5194/egusphere-egu25-5133, 2025.

Last decade has seen greatly intensified interest in understanding the temperature evolution in the Holocene (~last 10,000 years), which provides the background climate for our ongoing anthropogenic global warming.  Much of the effort so far has focused on the mean annual temperature (MAT). The so called Holocene temperature conundrum still remains unresolved: has the global MAT exhibited a cooling trend as indicated in most proxy reconstructions, or a warming trend in response to increased concentration of greenhouse gases and retreating ice sheet as in most climate models. Here, we further point out a conundrum on the Holocene seasonal temperatures in the Northern Hemisphere extra-tropics: in comparison with a simple analogue model that predicts the seasonal cycle of temperature from insolation based on present observations, most available seasonal temperature reconstructions severely underestimate the decrease of seasonal cycle amplitude in the Holocene. Meanwhile, a newly developed set of summer and annual temperature reconstructions based on soil bacteria’s membrane lipids (branched glycerol dialkyl glycerol tetraethers (brGDGT)) exhibits an evolution pattern similar to the analogue model. Our study highlights the current uncertainty in seasonal temperature reconstructions, with implications to the MAT.

How to cite: Liu, Z., Cheng, J., Zheng, Y., Zhang, W., Liu, H., Wu, H., Zhu, J., and Xie, S.: The Holocene Seasonal Temperature Conundrum, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1812, https://doi.org/10.5194/egusphere-egu25-1812, 2025.

Global databases of Holocene paleoclimates have been assembled, but these contain few data from the north American Arctic, especially from the High- and Mid-Arctic zones. A series of lake sediment cores from across the North American Arctic as well as data on treeline variations have been analyzed for several different proxy-climate data. The results show longitudinal differences in the timing of the maximum temperatures, with transitions synchronous across the North American Arctic, although not necessarily in the same direction. For example, at 8.2ka, the western and central Arctic cooled, but eastern Arctic and northern Greenland warmed. This space-time pattern of the Holocene Thermal Maximum (HTM) can be attributed to changes in the atmospheric circulation in response to the melting ice sheet, changes in the local energy balance in response to orbital insolation changes and other forcing.

The impacts of these changes on Arctic ecosystems are subtle but noticeable. Multiple proxies from the same core or from nearby lakes sometimes show coherent changes but at other times differences. For terrestrial ecosystems, biodiversity seems less affected by warmer conditions than biological production, which increased during local HTM. Periods of warm conditions and high terrestrial plant production were associated with a decrease in diatom production (as measured by accumulation rates) in some sites, and in some cases, with an absence of diatoms in the sediments (diatom-free zones), for reasons not yet clear. Secondary production of chironomid communities living in the lake sediments was sometimes coherent with diatom production, but not at other times.

How to cite: Gajewski, K. and Tamo, C.: The Holocene thermal maximum in the North American Arctic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1547, https://doi.org/10.5194/egusphere-egu25-1547, 2025.

The Holocene history of Australia’s hydroclimate is surprisingly poorly understood. This is, in part, because of the relatively weak forcing of Holocene climate versus that of the late Pleistocene. However, it is commonly suggested that eastern Australian climates dried in the late-Holocene and that this was in response to increased activity in the El Niño-Southern Oscillation (ENSO), in particular, the intensification of the El Niño phase of the ENSO cycle. While this has been inferred from numerous locations, data from K'gari (a World Heritage-listed subtropical east coast sand island once known Fraser Island) was amongst the first used to develop this hypothesis and features heavily in discussion of the causes and effects of ENSO intensification. K’gari’s lake systems have significant cultural, environmental and economic value and are a key aspect of the island’s World Heritage status. This study examines published and new radiocarbon dates from three >4.5 m deep K'gari lakes. There was a hiatus in sedimentation in the lakes representing a marked drying event during the middle Holocene (7,640 to 5,600 a BP), followed by wetter late Holocene climate, which contrasts with previous arguments about K’gari’s history. It will also be argued that this phenomenon is observed in other lakes on the eastern Australian margin in a manner previously unrecognised. Hence, there is a need to re-evaluate the notion of ENSO intensification driving late-Holocene drying on the eastern Australian coastal margin. It also indicates that even the deepest K’gari lakes are vulnerable to drying and the risks associated with drying should be considered in their management.

How to cite: Tibby, J. and Haidee Cadd, H.: Mid-Holocene drying of K'gari lakes (subtropical eastern Australia) necessitates re-evaluation of links to the El Niño-Southern Oscillation and future drying risk, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21086, https://doi.org/10.5194/egusphere-egu25-21086, 2025.

The relatively short observational record limits our ability to understand the long-term variability of key climate factors like temperature and hydroclimate. Climate models and paleoclimate proxies appear to have long-term temperature variabilities that diverge from each other at local and long time scales. But it is unclear whether these divergences also apply to hydroclimate and whether long-term hydroclimate variability is fundamentally different than temperature variability.

Here we evaluate the long-term variability over the Common Era of temperature and hydroclimate using a climate model (the Community Earth System Model-Last Millennium Ensemble, CESM-LME) and a paleoclimate reconstruction based on this model (the Paleo Hydrodynamics Data Assimilation product, PHYDA); this framework allows us to see how a model’s long-term climate variability is affected by informing it with proxy data. We specifically focus our analyses on the continuum of temperature (tas) and the Palmer Drought Severity Index (PDSI) in four regions of low reconstruction uncertainty: the Western USA, the Eastern USA, Central Europe, and Scandinavia.

Using the power-scaling exponents β from the relationship S(τ) ∼ τ^β, where S denotes the power spectral density (PSD) and τ the period, we find universally higher values of β in PHYDA (except for tas globally); in these four regions PHYDA’s β values are 0.30 to 0.63 higher than CESM-LME. Thus, long range dependence behavior is more pronounced in PHYDA than in the CESM-LME model. We find that PHYDA has different spatial distributions of β than CESM-LME. We also find that hydroclimate is spectrally flatter than temperature in CESM-LME, whereas temperature and hydroclimate β-values are comparable in PHYDA. These results show that CESM-LME’s hydroclimate and temperature is less dominated by long timescales compared to PHYDA’s. The robustness of the low-frequency variability signal in PHYDA was verified by performing pseudoproxy experiments. Furthermore, preliminary results of other temperature DA reconstructions over the Holocene and since the Last Glacial Maximum also reveal spectral divergencies with model data. In particular, for the PSD of the global mean temperatures, higher beta values were obtained for the reconstructions compared to the model data, indicating a deficit in simulated low-frequency.

How to cite: Pliemon, T., Steiger, N., Laepple, T., Rehfeld, K., and Hébert, R.: Spectral divergence in hydroclimate and temperature between models and reconstructions , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6713, https://doi.org/10.5194/egusphere-egu25-6713, 2025.

Querying, cleaning, and annotating paleoclimate data can be a cumbersome task, as is reproducing a published reconstruction method. Most of this work can now either be automated or avoided by utilizing the LiPD ecosystem.

With Pages2k, Temp12k, and other compilations, the linked paleodata (LiPD) format has solved the problems of data cleaning and annotation. These compilations and others are stored in the LiPDverse, with all metadata, paleo data, and chronology data. Packages in R and Python support querying the database (lipdR, pylipd). And a set of paleoclimate analysis packages allow for changepoint detection, age-modeling, and various other analyses (actR, geochronR, pyleoclim).

A new web platform called PReSto (the paleoclimate reconstruction storehouse) provides GUIs for easier querying of the LiPDverse. Additionally, several published climate reconstructions are available for visualization and comparison. And the creation of custom reconstructions from several of these published methods is now enabled in a streamlined web interface, code free. 

With a new partnership (FREE SODA, Dutch Research Council) the LiPDverse database continues to grow. We are currently adding a collection of records from the Southern Ocean. Several new web features are also being developed, such as map overlays and projections, as well as interactive data comparison tools.

How to cite: Edge, D., Thöle, L., and Nick, M.: The expanding LiPD (Linked Paleo Data) ecosystem: improving your paleoclimate research workflow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13533, https://doi.org/10.5194/egusphere-egu25-13533, 2025.