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Numerical simulations employing prognostic stable water isotopes can not only facilitate our understanding of hydrological processes and climate change but also allow for a straightforward comparison between isotope signals obtained from models and various archives. In the current work, we describe the performance and explore the potential of a new version of the Earth system model AWI-ESM, labeled AWI-ESM-wiso, in which we incorporated three isotope tracers into all relevant components of the water cycle. We present here the results of pre-industrial (PI) and mid-Holocene (MH) simulations. The model well reproduces the observed PI isotope compositions in both precipitation and sea water, and captures their major differences from the MH condition. The simulated relationship between the isotope composition in precipitation (δ18Op) and surface air temperature is very similar between the PI and MH conditions, and largely consistent with modern observations despite some regional model biases. The ratio of the MH-PI difference in δ18Op to the MH-PI difference in surface air temperature is reasonable over Greenland and Antarctica only when summertime air temperature is considered. An amount effect is evident over the North Africa monsoon domain, where a negative correlation between δ18Op and the amount of precipitation is simulated. As an example of model applications, we studied the onset date of MH West Africa summer monsoon (WASM) using daily variables. We find that defining the WASM onset based on precipitation alone may yield erroneous results due to the substantial daily variations of precipitation, which may obscure the distinction between pre-monsoon and monsoon seasons. Combining precipitation and isotope indicators, we suggest in this work a novel method for identifying the commencement of the WASM. Moreover, we do not find obvious difference between the MH and PI in terms of the mean onset date of WASM.

In previous model-data comparisons, the centennial to millennial scale variance of local climate (e.g., SST) reconstructed from proxies was significantly higher than that simulated by climate models. One possible explanation is the lack of long-term feedback mechanisms, e.g. from the representation of changes in ice-sheets in models. Additionally, proxy records are short, and sparse, and the climate signal is significantly modified during the processes of encoding, archiving, and recovery.Here we introduce new methods to infer the climate variability of the past from proxy data and compare them to new transient model simulations of the last deglaciation. This will allow us to estimate the amplitude of climate variability and to evaluate whether climate models are capable of capturing changes in climate variability between different climate states (e.g. glacial vs. interglacial periods), which is also relevant for the accuracy of future projections. We compare the variability of marine d18O reconstructed from proxies with that simulated by a state-of-the-art Earth System Model.From the proxy side, our analysis is based on a new dataset of marine oxygen isotope data from planktonic foraminifera compiled for the PALMOD project. We use new methods to first calculate power-spectra for the LGM, transition and Holocene and then to correct these spectra by fitting a Bayesian model describing the effects of bioturbation and measurement error on the reconstructed climate signal. From the model side we use marine d18O variability calculated using temperature and salinity from transient model simulations of the last deglaciation, performed within the PALMOD project, that include changes in the ice sheets.This combination of new data and methods will allow us to investigate the effect of different ice-sheet configurations and physical parametrizations in the model on their ability to characterise long-timescale climate variability and its dependence on climate state.

The climate of the past two thousand years (2k) provides context for current and future changes, and as such is vital for developing our understanding of the modern climate system. Building on previous phases of the PAGES 2k network, Phase 4 of the PAGES 2k Network paves the way for a new level of understanding of the global water cycle, including enhanced science-policy integration. Previous PAGES 2k network phases emphasised temperature reconstructions, fundamentally improving our understanding of global climate changes over the Common Era. These reconstructions received widespread recognition and were featured in the Summary for Policymakers of the IPCC’s Sixth Assessment Report. Integration of this data with state-of-the-art Earth systems models, proxy system models and data assimilation yielded a more comprehensive understanding of the associated physical drivers and climate dynamics. Phase 4 challenges our community to turn its focus towards hydroclimate. Our aim is to reconstruct hydroclimatic variability over the Common Era, from local to global spatial scales, at sub-annual to multi-centennial time scales, developing a process-level understanding of past hydroclimate events and variability. Our multi-faceted approach includes (1) developing new hydroclimate syntheses that are well-suited for data-model comparisons, (2) improving the interoperability and scope of existing data and model products, and (3) facilitating the translation of our science into evidence-based policy outcomes. In this presentation, we report on our activities and progress to date, particularly highlighting the early stages of our data synthesis efforts.

Paleoclimate proxy records from the North Atlantic region reveal substantially greater multi-centennial temperature variability during the Last Glacial Maximum (LGM) compared to the current interglacial. As there was no obvious change in external forcing, causes for the increased variability remain unknown. Here we provide a mechanism for enhanced multi-centennial North Atlantic climate variability during the LGM based on experiments with the coupled climate model CESM. The model simulates an internal mode of multi-centennial variability, which is associated with variations in the Atlantic meridional overturning circulation. In accordance with high-resolution proxy records from the glacial North Atlantic, this mode induces highest surface temperature variability in subpolar and mid latitudes and almost no variance in low latitudes. Greenland surface air temperature varies by up to 4°C, which is in line with multi-centennial variability reconstructed from ice cores. We show that this mode is based on a salt-oscillator mechanism and emerges only under full LGM climate forcing. Moderate deviations from full-glacial boundary conditions lead to its disappearance. We further argue that the multi-centennial mode has to be distinguished from millennial-scale Dansgaard-Oeschger oscillations.

With ongoing warming and sea ice loss, the Arctic Ocean and its marginal seas as a habitat for pelagic calcifiers are changing, possibly resulting in modifications of the regional carbonate cycle and the composition of the seafloor sediment. A substantial part of the pelagic carbonate production in the Arctic is due to the calcification of the dominant planktonic foraminifera species Neogloboquadrina pachyderma. To quantify carbonate production and loss in the upper water layer by this important Arctic calcifier, we compile and analyse data from vertical profiles in the upper water column of shell number concentration, sizes and weights of this species across the Arctic region during summer. Our data are inconclusive on whether the species performs ontogenetic vertical migration throughout its life cycle or whether individual specimens calcify at a fixed depth within the vertical habitat. The base of the productive zone of the species is on average located below 100 m and at maximum at 300 m and is regionally highly variable. The calcite flux immediately below the productive zone (export flux) is on average 8 mg CaCO3 m−2 d−1, and we observe that this flux is attenuated until at least 300 m below the base of the productive zone by a mean rate of 6.6 % per 100 m. Regionally, the summer export flux of N. pachyderma calcite varies by more than 2 orders of magnitude, and the estimated mean export flux below the twilight zone is sufficient to account for about a quarter of the total pelagic carbonate flux in the region. These results indicate that estimates of the Arctic pelagic carbonate budget will have to account for large regional differences in the export flux of the major pelagic calcifiers and confirm that substantial attenuation of the export flux occurs in the twilight zone.

The Last Glacial Maximum (23,000 – 19,000 years ago; LGM) is the most recent time when Earth’s climate was fundamentally different from today. The LGM hence remains a prime target to evaluate climate models outside current boundary conditions. Evaluation of paleoclimate simulations is usually done using proxy-based reconstructions. However, such reconstructions are indirect and associated with marked uncertainty, which often renders model-data comparison equivocal. Here we take a different approach and use macro-ecological patterns preserved in fossil marine zooplankton to evaluate simulations of LGM near-surface ocean temperature. We utilise the distance-decay pattern in planktonic foraminifera to evaluate modelled temperature gradients. Distance decay emerges because of differences in habitat preferences among species that cause the compositional similarity between assemblages to decrease the further apart they are from each other in environmental space. Distance decay is a fundamental concept in ecology and is observed in many different taxa and ecosystems, including planktonic foraminifera that show a monotonous decrease in similarity with increasing difference in temperature. Because the ecological niches of planktonic foraminifera are unlikely to have changed since the LGM, the distance-decay relationship based on simulated LGM temperatures and LGM assemblages should in principle be identical to the modern distance decay pattern. Thus we can use fossil planktonic foraminifera species assemblages to evaluate climate model simulations based on ecological principles. Our analysis is based on an extended new LGM planktonic foraminifera database (2,085 assemblages from 647 unique sites) and a suite of 10 simulations from state-of-the-art climate models (PMIP3 and 4). We find markedly different planktonic foraminifera distributions during the LGM, primarily due to the equatorward expansion of polar assemblages at the expense of transitional assemblages. The distance-decay pattern that emerges when the LGM assemblages are combined with simulated ocean temperatures is different from the modern pattern. All simulations suggest large thermal gradients between regions where the planktonic foraminifera indicate no, or only weak, gradients. This pattern arises from the pronounced shift to polar species assemblages in the North Atlantic where the simulations predict only moderate cooling. In general, the models predict spatially rather uniform cooling, whereas the microfossil evidence suggests more pronounced regional differences in the temperature change. The difference between reconstructions and the simulations reaches up to 10 K in the North Atlantic. Importantly, simulations with a reduced AMOC and hence lower North Atlantic near sea surface temperatures, yield a distance-decay pattern that is much more similar to the modern pattern. The planktonic foraminifera assemblages thus question the view of the LGM ocean as an equilibrium response to external forcing.

Understanding the climate of the past two millennia (2k) remains vital for developing our wider comprehension of the climate system, including the nature and scale of recent and future anthropogenic change. Phase 4 of the PAGES 2k network will build on previous phases and take us to a new level of understanding and science-policy integration.During previous phases, PAGES 2k members compiled global networks of proxy measurements, extending records beyond the instrumental period by more than an order of magnitude, reconstructing past climate and developing new knowledge of past variability and processes. Through data-model integration with state-of-the-art Earth systems models, proxy system modelling and data assimilation, we took key steps towards a more comprehensive understanding of climate dynamics.Phase 4 will take us even further, challenging our community to turn its focus primarily towards the hydroclimate of the Common Era: performing new reconstructions and improving the interoperability, extent and scope of our data and model products. In doing so, we also seek to facilitate the translation of our science into evidence-based policy outcomes. Our overarching aim is to reconstruct hydroclimate variability over the Common Era from local to global spatial scales, at sub-annual to multi-centennial time scales. We propose to achieve this through new community-led data curation efforts and the development of new data-driven tools and practices to maximise the interoperability of convenient, efficient and widespread model/data products. We will aim for a process-level understanding of past hydroclimate events and variability by evaluating and constraining Earth system models and through data assimilation.Our coordination team places a strong emphasis on respect and inclusion, aiming to foster a diverse and equitable community. Through a ‘hub and spoke’ structure, our team will provide a facilitation, coordination and support role (the hub) for Pages 2k working groups (the spokes). We are actively seeking participation of those engaging in climate policy and climate services. Welcome to Phase 4! We warmly invite your collaborations and contributions!

Planktonic foraminifera, marine protists and calcifiers, are globally responsible for about one quarter of the global pelagic calcite flux, but their contribution to it in the subarctic and Arctic realm has not been fully resolved. With ongoing warming and sea ice loss, the Arctic Ocean and its marginal seas are becoming more hospitable to pelagic calcifiers like planktonic foraminifera, resulting in modifications of the regional carbonate cycle and the composition of the seafloor sediment. To quantify planktonic foraminifera Arctic calcite production and loss in the upper water layer, we compile and analyze data on the dominant planktonic foraminifera species of the Arctic realm, Neogloboquadrina pachyderma, from vertical profiles in the upper water column. Using summertime observations of shell concentrations, sizes and weights of this species across the Arctic, we calculate the base of the productive zone of N. pachyderma to be regionally highly variable, on average located at 125 m, at maximum at 300 m. The calcite flux immediately below the productive zone (production flux) is on average 8 mg CaCO3 m-2 d-1, and we observe that this flux is attenuated until at least 300 m below the base of the productive zone. Across the Arctic realm, the summer production flux of N. pachyderma calcite varies by more than two orders of magnitude and the estimated mean export flux below the twilight zone is sufficient to account for about a quarter of the total pelagic carbonate flux in the region. These results indicate that estimates of the Arctic pelagic carbonate budget will have to account for large regional differences in production flux of the major pelagic calcifiers and confirm that substantial attenuation of the production flux occurs in the twilight zone.

An increasing number of climate model simulations is becoming available for the transition from the Last Glacial Maximum to the Holocene. Assessing the simulations’ reliability requires benchmarking against environmental proxy records. To date, no established method exists to compare these two data sources in space and time over a period with changing background conditions. Here, we develop a new algorithm to rank simulations according to their deviation from reconstructed magnitudes and temporal patterns of orbital- as well as millennial-scale temperature variations. The use of proxy forward modeling avoids the need to reconstruct gridded or regional mean temperatures from sparse and uncertain proxy data. First, we test the reliability and robustness of our algorithm in idealized experiments with prescribed deglacial temperature histories. We quantify the influence of limited temporal resolution, chronological uncertainties, and non-climatic processes by constructing noisy pseudo-proxies. While model-data comparison results become less reliable with increasing uncertainties, we find that the algorithm discriminates well between simulations under realistic non-climatic noise levels. To obtain reliable and robust rankings, we advise spatial averaging of the results for individual proxy records. Second, we demonstrate our method by quantifying the deviations between an ensemble of transient deglacial simulations and a global compilation of sea surface temperature reconstructions. The ranking of the simulations differs substantially between the considered regions and timescales. We attribute this diversity in the rankings to more regionally confined temperature variations in reconstructions than in simulations, which could be the result of uncertainties in boundary conditions, shortcomings in models, or regionally varying characteristics of reconstructions such as recording seasons and depths. Future work towards disentangling these potential reasons can leverage the flexible design of our algorithm and its demonstrated ability to identify varying levels of model-data agreement.

Biodiversity is expected to change in response to future global warming. However, it is difficult to predict how species will track the ongoing climate change. Here we use the fossil record of planktonic foraminifera to assess how biodiversity responded to climate change with a magnitude comparable to future anthropogenic warming. We compiled time series of planktonic foraminifera assemblages, covering the time from the last ice age across the deglaciation to the current warm period. Planktonic foraminifera assemblages shifted immediately when temperature began to rise at the end of the last ice age and continued to change until approximately 5,000 years ago, even though global temperature remained relatively stable during the last 11,000 years. The biotic response was largest in the mid latitudes and dominated by range expansion, which resulted in the emergence of new assemblages without analogues in the glacial ocean. Our results indicate that the plankton response to global warming was spatially heterogeneous and did not track temperature change uniformly over the past 24,000 years. Climate change led to the establishment of new assemblages and possibly new ecological interactions, which suggests that current anthropogenic warming may lead to new, different plankton community composition.