The mid-Cretaceous period was one of the warmest intervals of the past 140 million years
, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume
. In the near ...absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf-the southernmost Cretaceous record reported so far-and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian-Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120-1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The Arctic region is thought to play a key role in unraveling Mesozoic climate evolution. However, Late Cretaceous climate reconstructions in the high latitudes suffer from contradicting ...paleoclimatic interpretations. Toward the end of the Cretaceous hot-house, atmospheric CO2 concentration declined potentially enabling the formation of sea-ice in the Arctic Ocean. We use a coupled atmosphere-ocean climate model to investigate possible effects of different atmospheric CO2 levels and gateway configurations between the North proto-Atlantic Basin and the Arctic Ocean on the formation of Arctic sea-ice in the latest Cretaceous. Sensitivity tests were run with two atmospheric CO2 levels (840 and 1120 ppm, representing 3× and 4× pre-industrial concentrations, respectively) with six paleogeographic configurations. In the experiment with 840 ppm CO2, seasonal Arctic sea-ice is observed in each gateway configuration in December–June, while for 1120 ppm sea-ice in the central Arctic is either limited or absent, depending on gateway configuration. This suggests the existence of a CO2 threshold, estimated between 3× and 4× pre-industrial (PI) CO2 levels. For higher atmospheric CO2 levels sea-ice formation can only occur by the combined effect of cold winds blowing over the Arctic from continental North America during boreal winter and seawater freshening. The latter can be caused by either very limited or an absence of gateway connections between the Arctic and the open ocean. Such a configuration likely developed in the latest Cretaceous, i.e. close to the Cretaceous/Paleogene boundary interval.
•Sea-ice formation in the Arctic is mainly controlled by atmospheric CO2 concentration.•Simulations predict seasonal Arctic sea-ice formation in the latest Cretaceous.•Onset of winter/spring Arctic sea-ice ranges between 840 and 1120 ppm CO2.•Enclosed gateway configuration favors seasonal Arctic sea-ice formation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Around the early–middle Eocene boundary, the first occurrence of contourite drift sediments and widespread deep ocean erosion indicate changes in the North Atlantic ocean circulation. Interestingly, ...these changes coincide with the first steps of Cenozoic cooling from the Paleogene greenhouse climate towards the modern icehouse. The cause for this ocean circulation reorganization is poorly understood since modern water mass tracers may have worked fundamentally different in the past and the paleoceanographic proxy record is limited in both time and space. As a result, it is challenging to reliably reconstruct the climatic and tectonic boundary conditions e.g. atmospheric greenhouse gas concentration and the depth and geometry of developing and closing passages between ocean basins. In this study, we attempt to identify thresholds in tectonic gateway passages and atmospheric CO2 concentration, using the fully coupled Earth System Model COSMOS. Indeed, the simulation of Earth's past climates can unravel the physical processes driving deep-water formation in a greenhouse world. Specifically, we use COSMOS to evaluate the impact of changes in the North Atlantic gateways at the early–middle Eocene boundary on the North Atlantic Deep Western Boundary Currents under low obliquity configuration. We find that Northern Component Waters start to form when the Greenland Scotland Ridge reaches a threshold depth of deeper than 200 m, while the Arctic Ocean is still shut off from the North Atlantic. In this scenario, the relatively deep Greenland Scotland Ridge allows for sufficient inflow of warm, salty Atlantic surface waters into the Nordic Seas to initiate convection during winter cooling. Opening the seaway towards the Arctic leads to a cessation of Northern Component Water formation as it allows for inflow of brackish surface waters into the northern Nordic Seas, hindering Northern Component Water formation.
•Eocene simulations show sensitivity of ocean circulation to North Atlantic seaways.•Arctic restriction and GSR sill depth crucial factors for NCW formation.•The deepening of the GSR represents a threshold for the onset of NCW formation.•The onset of NCW leads to bottom water current invigoration in the North Atlantic.•Tectonic changes in the North Atlantic could have led to the onset of a bimodal AMOC.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Here, we compare the ocean overturning circulation of the early Eocene (47–56 Ma) in eight coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP) and investigate ...the causes of the observed inter‐model spread. The most common global meridional overturning circulation (MOC) feature of these simulations is the anticlockwise bottom cell, fed by sinking in the Southern Ocean. In the North Pacific, one model (GFDL) displays strong deepwater formation and one model (CESM) shows weak deepwater formation, while in the Atlantic two models show signs of weak intermediate water formation (MIROC and NorESM). The location of the Southern Ocean deepwater formation sites varies among models and relates to small differences in model geometry of the Southern Ocean gateways. Globally, convection occurs in the basins with smallest local freshwater gain from the atmosphere. The global MOC is insensitive to atmospheric CO2 concentrations from 1× (i.e., 280 ppm) to 3× (840 ppm) pre‐industrial levels. Only two models have simulations with higher CO2 (i.e., CESM and GFDL) and these show divergent responses, with a collapsed and active MOC, respectively, possibly due to differences in spin‐up conditions. Combining the multiple model results with available proxy data on abyssal ocean circulation highlights that strong Southern Hemisphere‐driven overturning is the most likely feature of the early Eocene. In the North Atlantic, unlike the present day, neither model results nor proxy data suggest deepwater formation in the open ocean during the early Eocene, while the evidence for deepwater formation in the North Pacific remains inconclusive.
Plain Language Summary
The ocean's overturning circulation refers to the replenishment of the ocean's deep water by cold dense polar surface waters and its eventual return to the surface. It affects the climate through redistribution of heat across the globe and uptake of atmosphere carbon dioxide (CO2). Here, we explore the overturning circulation of the Early Eocene, a hot period 47–56 million years ago when atmosphere CO2 levels were similar to the “worst case” projections for the end of this century, in eight climate models setup up for that time. Our results, together with available ocean circulation sediment data for the time, indicate that during the early Eocene deep water originated predominantly from cold surface waters around Antarctica. The North Atlantic source of deep water that today contributes to European's relatively mild climate for its latitude, was completely absent at the time. Interestingly, even when the carbon dioxide in the Eocene model simulations was lowered to levels similar to today and before the industrial revolution, the North Atlantic source of deep water remains absent, indicating that it is the distribution of continents and ice‐sheets, rather than CO2 that is responsible for the difference between the modern and Eocene circulation.
Key Points
This study evaluates the ocean's meridional overturning circulation during the early Eocene in eight models of the DeepMIP project
The primary region of deep‐water formation depends both on the atmospheric freshwater flux and the strait geometry in the Southern Ocean
Compatible with proxy records, six of eight models show that deep waters predominantly originated from the south
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Inconsistencies in the Eocene climates of East Asia have been revealed in both geological studies and simulations. Several earlier reconstructions showed an arid zonal band in mid‐latitude China, but ...others showed a humid climate in the same region. Moreover, previous Eocene modeling studies have demonstrated that climate models can simulate both scenarios in China. Therefore, it is essential to investigate the cause of this model spread. We conducted a series of experiments using Norwegian Earth System Model 1‐F and examined the impact of mountains in Southern China on the simulated Eocene climate. These mountains, including the Gangdese and Southeast Mountains, are located along the main path of water vapor transport to East Asia. Our results reveal that the Southeast Mountains play the dominant role in controlling the simulated precipitation in Eastern China during the Eocene. When the heights of the Southeast Mountains exceed ∼2,000 m, an arid zonal band appears in mid‐latitude China, whereas humid climates appear in Eastern China when the elevation of the Southeast Mountains is relatively low.
Key Points
Southeast Mountains control simulated Eocene precipitation in Eastern China
When the Southeast Mountains are high, an arid zonal band appears in mid‐latitude China
The early Eocene climate in East Asia is not monsoonal climate
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Estimates of global mean near‐surface air temperature (global SAT) for the Cenozoic era rely largely on paleo‐proxy data of deep‐sea temperature (DST), with the assumption that changes in global SAT ...covary with changes in the global mean deep‐sea temperature (global DST) and global mean sea‐surface temperature (global SST). We tested the validity of this assumption by analyzing the relationship between global SST, SAT, and DST using 25 different model simulations from the Deep‐Time Model Intercomparison Project simulating the early Eocene Climatic Optimum (EECO) with varying CO2 levels. Similar to the modern situation, we find limited spatial variability in DST, indicating that local DST estimates can be regarded as a first order representative of global DST. In line with previously assumed relationships, linear regression analysis indicates that both global DST and SAT respond stronger to changes in atmospheric CO2 than global SST by a similar factor. Consequently, this model‐based analysis validates the assumption that changes in global DST can be used to estimate changes in global SAT during the early Cenozoic. Paleo‐proxy estimates of global DST, SST, and SAT during EECO show the best fit with model simulations with a 1,680 ppm atmospheric CO2 level. This matches paleo‐proxies of EECO atmospheric CO2, indicating a good fit between models and proxy‐data.
Plain Language Summary
The global mean surface temperature is a commonly used indicator to measure global climate change. Our understanding of how the global surface temperature has changed in the last 66 Myr is mainly based on the assumption that changes in the deep‐sea temperature (DST) reflect changes at the surface well. Here, we test this idea by using climate model simulations of a hothouse period, the early Eocene Climate Optimum (53–49 Myr ago). We find that changes in the global DST indeed correlate well with temperature changes at the surface. We find the best fit between the models and data from the early Eocene for atmospheric CO2 levels at around 1,680 ppm.
Key Points
In early Eocene model simulations (Deep‐Time Model Intercomparison Project DeepMIP), global mean deep‐sea, and surface temperatures are equally sensitive to atmospheric CO2 changes
Model‐simulated deep‐sea temperatures show limited spatial variability, making local estimates generally representative of the global mean
The model simulations with a CO2 forcing of 1,680 ppm match paleo‐proxies of global mean deep‐sea, sea‐surface, and surface temperature
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The early Eocene (∼56–48 Myr ago) is characterized by high CO2 estimates (1,200–2,500 ppmv) and elevated global temperatures (∼10°C–16°C higher than modern). However, the response of the hydrological ...cycle during the early Eocene is poorly constrained, especially in regions with sparse data coverage (e.g., Africa). Here, we present a study of African hydroclimate during the early Eocene, as simulated by an ensemble of state‐of‐the‐art climate models in the Deep‐time Model Intercomparison Project (DeepMIP). A comparison between the DeepMIP pre‐industrial simulations and modern observations suggests that model biases are model‐ and geographically dependent, however, these biases are reduced in the model ensemble mean. A comparison between the Eocene simulations and the pre‐industrial suggests that there is no obvious wetting or drying trend as the CO2 increases. The results suggest that changes to the land sea mask (relative to modern) in the models may be responsible for the simulated increases in precipitation to the north of Eocene Africa. There is an increase in precipitation over equatorial and West Africa and associated drying over northern Africa as CO2 rises. There are also important dynamical changes, with evidence that anticyclonic low‐level circulation is replaced by increased south‐westerly flow at high CO2 levels. Lastly, a model‐data comparison using newly compiled quantitative climate estimates from paleobotanical proxy data suggests a marginally better fit with the reconstructions at lower levels of CO2.
Plain Language Summary
Approximately 50 Myr ago, a period known as the early Eocene, atmospheric carbon dioxide levels were significantly higher than today, and were more similar to what they could be in the future, if efforts to reduce human greenhouse gas emissions are unsuccessful. However, rainfall changes during this period are less well understood, especially over data‐sparse regions such as Africa. Here, a collection of state‐of‐the‐art climate models are used to study African rainfall during this period, comparing the simulations first to present‐day African rainfall (to validate the models), second to varying levels of atmospheric carbon dioxide, and lastly to newly compiled reconstructions of early Eocene rainfall (from plant fossils). The main findings are that although the models can reproduce present‐day rainfall over Africa, and compare reasonably well with the reconstructions, there is no clear rainfall signal when atmospheric carbon dioxide is increased. Nevertheless, the combination of a different continental configuration, vegetation, topography, and atmospheric carbon dioxide leads to changing rainfall patterns, connected to temperature and low‐level wind changes.
Key Points
State‐of‐the‐art climate models are used to study African hydroclimate during the early Eocene (approximately 50 Myr ago)
With increasing levels of CO2, there are changes to African precipitation, due to dynamical changes such as low‐level circulation
A comparison between the models and newly compiled climate estimates shows a marginally better match at lower levels of CO2
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present results from an ensemble of eight climate models, each
of which has carried out simulations
of the early Eocene climate optimum (EECO, ∼ 50 million years
ago). These simulations have been ...carried out in the framework of the Deep-Time Model Intercomparison Project
(DeepMIP; http://www.deepmip.org, last access: 10 January 2021); thus, all models have been configured with the same
paleogeographic and vegetation boundary conditions. The results indicate that
these non-CO2 boundary conditions contribute between 3 and
5 ∘C to Eocene warmth. Compared with
results from previous studies, the DeepMIP simulations generally show a reduced spread of the global mean surface temperature response across the ensemble for a given atmospheric CO2 concentration as well as an increased climate sensitivity on average. An energy balance analysis of the model ensemble indicates that global mean warming in the Eocene compared with the preindustrial period mostly arises from decreases in emissivity due to the elevated CO2 concentration (and associated water vapour and long-wave cloud feedbacks), whereas the reduction in the Eocene in terms of the meridional temperature gradient is primarily due to emissivity and albedo changes owing to the non-CO2 boundary conditions (i.e. the removal of the Antarctic ice sheet and changes in vegetation). Three of the models (the Community Earth System Model, CESM; the Geophysical Fluid Dynamics Laboratory, GFDL, model; and the Norwegian Earth System Model, NorESM) show results that are consistent with the proxies in terms of the global mean temperature, meridional SST gradient, and CO2, without prescribing changes to model parameters. In addition, many of the models agree well with the first-order spatial patterns in the SST proxies. However, at a more regional scale, the models lack skill. In particular, the modelled anomalies are substantially lower than those indicated by the proxies in the southwest Pacific; here, modelled continental surface air temperature anomalies are more consistent with surface air temperature proxies, implying a possible inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large-scale features and model–data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean
circulation, hydrological cycle, and modes of variability, and encourage sensitivity studies to aspects such as paleogeography, orbital configuration, and aerosols.
We investigate the impact of different CO2 levels and different subarctic gateway configurations on the surface temperatures during the latest Cretaceous using the Earth System Model COSMOS. The ...simulated temperatures are compared with the surface temperature reconstructions based on a recent compilation of the latest Cretaceous proxies. In our numerical experiments, the CO2 level ranges from 1 to 6 times the preindustrial (PI) CO2 level of 280 ppm. On a global scale, the most reasonable match between modeling and proxy data is obtained for the experiments with 3 to 5 × PI CO2 concentrations. However, the simulated low‐ (high‐) latitude temperatures are too high (low) as compared to the proxy data. The moderate CO2 levels scenarios might be more realistic, if we take into account proxy data and the dead zone effect criterion. Furthermore, we test if the model‐data discrepancies can be caused by too simplistic proxy‐data interpretations. This is distinctly seen at high latitudes, where most proxies are biased toward summer temperatures. Additional sensitivity experiments with different ocean gateway configurations and constant CO2 level indicate only minor surface temperatures changes (<~1°C) on a global scale, with higher values (up to ~8°C) on a regional scale. These findings imply that modeled and reconstructed temperature gradients are to a large degree only qualitatively comparable, providing challenges for the interpretation of proxy data and/or model sensitivity. With respect to the latter, our results suggest that an assessment of greenhouse worlds is best constrained by temperatures in the midlatitudes.
Key Points
Simulated surface temperatures show best fit to proxy‐based temperature estimates in the midlatitudes
In the high latitudes the data‐model match can be improved by considering simulated summer rather than annual mean temperatures
Gateway alterations improve the fit with the data only locally, at high altitudes in the North America
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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