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A reconstruction of atmospheric CO 2 concentration from boron isotopes recorded in planktonic foraminifera examines climate–carbon interactions over the past tens of millions of years and confirms a strong linkage between climate and atmospheric CO 2 . A detailed atmospheric CO 2 record Previous efforts to reconstruct atmospheric CO 2 levels prior to the observational record from ice cores have suffered from methodological problems, but here Eleni Anagnostou et al . present a new reconstruction of Eocene atmospheric CO 2 concentrations from boron isotopes stored in planktonic foraminifera that helps to plug the gap in an important period of the palaeoclimate record. Absolute values remain unclear, but were probably about 1,400 parts per million during a period of peak warmth around 52 million years ago. From then until the rise of the Antarctic Ice Sheet about 33.6 million years ago, CO 2 declined by about half, confirming a strong link between climate and atmospheric CO 2 concentrations. The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago) 1 , was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period 2 , 3 , 4 . Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO 2 ) levels during the Eocene at 500–3,000 parts per million 5 , 6 , 7 , and in the absence of tighter constraints carbon–climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments 8 , 9 , 10 , 11 to generate a new high-fidelity record of CO 2 concentrations using the boron isotope (δ 11 B) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates 6 . Although species-level uncertainties make absolute values difficult to constrain, CO 2 concentrations during the EECO were around 1,400 parts per million. The relative decline in CO 2 concentration through the Eocene is more robustly constrained at about fifty per cent, with a further decline into the Oligocene 12 . Provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the Eocene was similar to that of the late Quaternary period 13 , this CO 2 decline was sufficient to drive the well documented high- and low-latitude cooling that occurred through the Eocene 14 . Once the change in global temperature between the pre-industrial period and the Eocene caused by the action of all known slow feedbacks (apart from those associated with the carbon cycle) is removed 2 , 3 , 4 , both the EECO and the late Eocene exhibit an equilibrium climate sensitivity relative to the pre-industrial period of 2.1 to 4.6 degrees Celsius per CO 2 doubling (66 per cent confidence), which is similar to the canonical range (1.5 to 4.5 degrees Celsius 15 ), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by increased CO 2 concentrations, and that climate sensitivity was relatively constant throughout this period.

The latitudinal temperature gradient is a fundamental state parameter of the climate system tied to the dynamics of heat transport and radiative transfer. Thus, it is a primary target for temperature proxy reconstructions and global climate models. However, reconstructing the latitudinal temperature gradient in past climates remains challenging due to the scarcity of appropriate proxy records and large proxy–model disagreements. Here, we develop methods leveraging an extensive compilation of planktonic foraminifera δ18O to reconstruct a continuous record of the latitudinal sea-surface temperature (SST) gradient over the last 95 million years (My). We find that latitudinal SST gradients ranged from 26.5 to 15.3 °C over a mean global SST range of 15.3 to 32.5 °C, with the highest gradients during the coldest intervals of time. From this relationship, we calculate a polar amplification factor (PAF; the ratio of change in >60° S SST to change in global mean SST) of 1.44 ± 0.15. Our results are closer to model predictions than previous proxy-based estimates, primarily because δ18O-based high-latitude SST estimates more closely track benthic temperatures, yielding higher gradients. The consistent covariance of δ18O values in low- and high-latitude planktonic foraminifera and in benthic foraminifera, across numerous climate states, suggests a fundamental constraint on multiple aspects of the climate system, linking deep-sea temperatures, the latitudinal SST gradient, and global mean SSTs across large changes in atmospheric CO₂, continental configuration, oceanic gateways, and the extent of continental ice sheets. This implies an important underlying, internally driven predictability of the climate system in vastly different background states.

Accurate estimates of past global mean surface temperature (GMST) help to contextualise future climate change and are required to estimate the sensitivity of the climate system to CO2 forcing through Earth's history. Previous GMST estimates for the latest Paleocene and early Eocene (∼57 to 48 million years ago) span a wide range (∼9 to 23 °C higher than pre-industrial) and prevent an accurate assessment of climate sensitivity during this extreme greenhouse climate interval. Using the most recent data compilations, we employ a multi-method experimental framework to calculate GMST during the three DeepMIP target intervals: (1) the latest Paleocene (∼57 Ma), (2) the Paleocene–Eocene Thermal Maximum (PETM; 56 Ma), and (3) the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma). Using six different methodologies, we find that the average GMST estimate (66 % confidence) during the latest Paleocene, PETM, and EECO was 26.3 °C (22.3 to 28.3 °C), 31.6 °C (27.2 to 34.5 °C), and 27.0 °C (23.2 to 29.7 °C), respectively. GMST estimates from the EECO are ∼10 to 16 °C warmer than pre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (9 to 14 °C higher than pre-industrial). Leveraging the large “signal” associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM, and EECO to calculate gross estimates of the average climate sensitivity between the early Paleogene and today. We demonstrate that “bulk” equilibrium climate sensitivity (ECS; 66 % confidence) during the latest Paleocene, PETM, and EECO is 4.5 °C (2.4 to 6.8 °C), 3.6 °C (2.3 to 4.7 °C), and 3.1 °C (1.8 to 4.4 °C) per doubling of CO2. These values are generally similar to those assessed by the IPCC (1.5 to 4.5 °C per doubling CO2) but appear incompatible with low ECS values (<1.5 per doubling CO2).

A series of papers published shortly after the Integrated Ocean Drilling Program Arctic Coring Expedition (ACEX, 2004) on Lomonosov Ridge indicated remarkably high early Eocene sea surface temperatures (SSTs; ca. 23 to 27 ∘C) and land air temperatures (ca. 17 to 25 ∘C) based on the distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (isoGDGT and brGDGT) lipids, respectively. Here, we revisit these results using recent analytical developments – which have led to improved temperature calibrations and the discovery of new temperature-sensitive glycerol monoalkyl glycerol tetraethers (GMGTs) – and currently available proxy constraints. The isoGDGT assemblages support temperature as the dominant variable controlling TEX86 values for most samples. However, contributions of isoGDGTs from land, which we characterize in detail, complicate TEX86 paleothermometry in the late Paleocene and part of the interval between the Paleocene–Eocene Thermal Maximum (PETM; ∼ 56 Ma) and the Eocene Thermal Maximum 2 (ETM2; ∼ 54 Ma). Background early Eocene SSTs generally exceeded 20 ∘C, with peak warmth during the PETM (∼ 26 ∘C) and ETM2 (∼ 27 ∘C). We find abundant branched GMGTs, likely dominantly marine in origin, and their distribution responds to environmental change. Further modern work is required to test to what extent temperature and other environmental factors determine their distribution. Published Arctic vegetation reconstructions indicate coldest-month mean continental air temperatures of 6–13 ∘C, which reinforces the question of whether TEX86-derived SSTs in the Paleogene Arctic are skewed towards the summer season. The exact meaning of TEX86 in the Paleogene Arctic thus remains a fundamental issue, and it is one that limits our assessment of the performance of fully coupled climate models under greenhouse conditions.

The Eocene (56–34 million years ago) is characterized by declining sea surface temperatures (SSTs) in the low latitudes (∼4°C) and high southern latitudes (∼8–11°C), in accord with decreasing CO2 estimates. However, in the mid‐to‐high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non‐CO2 controls. To explore this further, we present a multi‐proxy (Mg/Ca, δ18O, TEX86) SST record from Bass River in the western North Atlantic. Our compiled multi‐proxy SST record confirms a net decline in SSTs (∼4°C) between the early Eocene Climatic Optimum (53.3–49.1 Ma) and mid‐Eocene (∼44–41 Ma), supporting declining atmospheric CO2 as the primary mechanism of Eocene cooling. However, from the mid‐Eocene onwards, east‐west North Atlantic temperature gradients exhibit different trends, which we attribute to incursion of warmer waters into the eastern North Atlantic and inception of Northern Component Water across the early‐middle Eocene transition. Plain Language Summary Over the past 541 million years, the Earth has oscillated between warm (greenhouse) and cold (icehouse) climates. The most recent transition between a greenhouse and icehouse climate state occurred during the Eocene (56–34 million years ago). This transition shows a gradual cooling, previously suggested to be driven by a decline in atmospheric carbon dioxide (CO2). However, we know little about this transition in the North Atlantic Ocean. Previous studies show limited cooling of surface waters in this region. This suggests that changes in North Atlantic temperatures are not driven by CO2. To understand how sea surface temperature changes in the western North Atlantic, we analyzed the chemistry of microscopic marine fossils in sediments. Our results show a 4°C decline in temperature from the early (∼53 Ma) to the middle Eocene (∼42 Ma). This matches computer simulations of Eocene climate and confirms CO2 was responsible for the transition. The lack of cooling observed in previous work is probably due to the development of an ancient water mass known as Northern Component Water (observed today as North Atlantic Deep Water) and changes in how the Eocene ocean transported heat. Key Points Long‐term (∼4°C) decline in North Atlantic sea surface temperatures (SSTs) between the early (∼53–49 Ma) and middle (∼44–41 Ma) Eocene This indicates that CO2 was likely responsible for the onset of long‐term Eocene cooling However, east‐west temperature gradients in the North Atlantic are decoupled, possibly due to additional non‐CO2 forcing mechanisms

Marine glycerol dialkyl glycerol tetraethers (GDGTs) are used in various proxies (such as TEX86) to reconstruct past ocean temperatures. Over 20 years of improvements in GDGT sample processing, analytical techniques, data interpretation and our understanding of proxy functioning have led to the collective development of a set of best practices in all these areas. Further, the importance of Open Science in research has increased the emphasis on the systematic documentation of data generation, reporting and archiving processes for optimal reusability of data. In this paper, we provide protocols and best practices for obtaining, interpreting and presenting GDGT data (with a focus on marine GDGTs), from sampling to data archiving. The purpose of this paper is to optimize inter-laboratory comparability of GDGT data, and to ensure published data follows modern open access principles.

The South Pacific Convergence Zone is a band of intense austral summer rainfall in the tropical Pacific. Changes in the South Pacific Convergence Zone are linked to Pacific sea surface temperatures on decadal timescales, but its behaviour and impacts over longer timescales remain poorly understood due to limited proxy records and model uncertainties. We combine new plant wax hydroclimate records with existing proxy evidence and climate model simulations to investigate South Pacific Convergence Zone changes over the past 1500 years. Our findings indicate that between 1000 and 200 years ago, the eastern South Pacific Convergence Zone became wetter while the western part became drier. Model simulations suggest that these centennial-scale changes were driven by Pacific sea surface temperature gradients. This eastward shift coincides with Polynesian colonisation, implying hydroclimate shifts both ‘pushed’ migration eastward and ‘pulled’ successful eastern settlement. Changes in the Pacific sea surface temperature gradient caused precipitation to shift eastward for centuries in the South Pacific, and probably influenced Polynesian migration and settlement patterns, according to hydroclimate reconstructions from Tahiti and Nuku Hiva, French Polynesia.

The paleoclimate record provides a test-bed in which climate models can be evaluated under conditions of substantial CO 2 change; however, these data are typically under-used in the process of model development and evaluation. Here, we use a set of metrics based on paleoclimate proxy observations to evaluate climate models under three past time periods. We find that the latest CMIP6/PMIP4 ensemble mean does a remarkably good job of simulating the global mean surface air temperatures of these past periods, and is improved on CMIP5/PMIP3, implying that the modern climate sensitivity of the CMIP6/PMIP4 model ensemble mean is consistent with the paleoclimate record. However, some models, in particular those with very high or very low climate sensitivity, simulate paleo temperatures that are outside the uncertainty range of the paleo proxy temperature data; in this regard, the paleo data can provide a more stringent constraint than data from the historical record. There is also consistency between models and data in terms of polar amplification, with amplification increasing with increasing global mean temperature across all three time periods. The work highlights the benefits of using the paleoclimate record in the model development and evaluation cycle, in particular for screening models with too-high or too-low climate sensitivity across a range of CO 2 concentrations.

Paleoclimate model simulations provide reference data to help interpret the geological record and offer a unique opportunity to evaluate the performance of current models under diverse boundary conditions. Here, we present a dataset of 35 climate model simulations of the warm early Eocene Climatic Optimum (EECO; ~ 50 million years ago) and corresponding preindustrial reference experiments. To streamline the use of the data, we apply standardised naming conventions and quality checks across eight modelling groups that have carried out coordinated simulations as part of the Deep-Time Model Intercomparison Project (DeepMIP). Gridded model fields can be downloaded from an online repository or accessed through a new web application that provides interactive data exploration. Local model data can be extracted in CSV format or visualised online for streamlined model-data comparisons. Additionally, processing and visualisation code templates may serve as a starting point for advanced analysis. The dataset and online platform aim to simplify accessing and handling complex data, prevent common processing issues, and facilitate the sharing of climate model data across disciplines.

The early Eocene (56 to 48 million years ago) is inferred to have been the most recent time that Earth's atmospheric CO2 concentrations exceeded 1000 ppm. Global mean temperatures were also substantially warmer than those of the present day. As such, the study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to evaluate climate models under conditions of high greenhouse gas forcing. The Deep Time Model Intercomparison Project (DeepMIP) is a systematic model–model and model–data intercomparison of three early Paleogene time slices: latest Paleocene, Paleocene–Eocene thermal maximum (PETM) and early Eocene climatic optimum (EECO). A previous article outlined the model experimental design for climate model simulations. In this article, we outline the methodologies to be used for the compilation and analysis of climate proxy data, primarily proxies for temperature and CO2. This paper establishes the protocols for a concerted and coordinated effort to compile the climate proxy records across a wide geographic range. The resulting climate “atlas” will be used to constrain and evaluate climate models for the three selected time intervals and provide insights into the mechanisms that control these warm climate states. We provide version 0.1 of this database, in anticipation that this will be expanded in subsequent publications.