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The Palaeocene-Eocene Thermal Maximum (PETM) was a significant global warming event in the Earth's history (approx. 55 Ma). The cause for this warming event has been linked to increases in greenhouse gases, specifically carbon dioxide and methane. This rapid warming took place in the presence of the existing Early Eocene warm climate. Given that projected business-as-usual levels of atmospheric carbon dioxide reach concentrations of 800-1100 ppmv by 2100, it is of interest to study past climates where atmospheric carbon dioxide was higher than present. This is especially the case given the difficulty of climate models in simulating past warm climates. This study explores the sensitivity of the simulated pre-PETM and PETM periods to change in cloud condensation nuclei (CCN) and microphysical properties of liquid water clouds. Assuming lower levels of CCN for both of these periods leads to significant warming, especially at high latitudes. The study indicates that past differences in cloud properties may be an important factor in accurately simulating past warm climates. Importantly, additional shortwave warming from such a mechanism would imply lower required atmospheric CO2 concentrations for simulated surface temperatures to be in reasonable agreement with proxy data for the Eocene.

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).

The prevailing view of the formation of porphyry copper deposits along convergent plate boundaries involves deep crustal differentiation of metal-bearing juvenile magmas derived from the mantle wedge above a subduction zone. However, many major porphyry districts formed during periods of flat-slab subduction when the mantle wedge would have been reduced or absent, leaving the source of the ore-forming magmas unclear. Here we use geochronology and thermobarometry to investigate deep crustal processes during the genesis of the Late Cretaceous–Palaeocene Laramide Porphyry Province in Arizona, which formed during flat-slab subduction of the Farallon Plate beneath North America. We show that the isotopic signatures of Laramide granitic rocks are consistent with a Proterozoic crustal source that was potentially pre-enriched in copper. This source underwent water-fluxed melting between 73 and 60 Ma, coincident with the peak of granitic magmatism (78–50 Ma), porphyry genesis (73–56 Ma) and flat-slab subduction (70–40 Ma). To explain the formation of the Laramide Porphyry Province, we propose that volatiles derived from the leading edge of the Farallon flat slab promoted melting of both mafic and felsic pre-enriched lower crust, without requiring extensive magmatic or metallogenic input from the mantle wedge. Other convergent plate boundaries with flat-slab regimes may undergo a similar mechanism of volatile-mediated lower-crustal melting. Laramide flat-slab subduction releases fluids into the overlying crust that mediate water-fluxed melting of precursor arc lower crust, ultimately forming porphyry copper deposits, according to a geochronology and thermobarometry study.

The timing and magnitude of the early Cenozoic surface uplift of the Tibetan Plateau is controversial due to a scarcity of unaltered terrestrial sediments required for palaeoaltimetry techniques. Such information is critical, however, for constraining the geodynamic and palaeoclimatic evolution of the Indian and Eurasian continents and for interpreting global climate, biodiversity and biogeochemical cycles since the Cenozoic. We find that substantial uplift occurred by 63 to 61 million years ago, before the collision of the Indian and Eurasian continental plates, based on comparison of triple oxygen isotopes of modern meteoric waters with epithermal Ag–Pb–Zn deposit quartz veins from the Palaeocene Gangdese Arc in southern Lhasa. Low δ18O and δ17O quartz values are consistent with precipitation from meteoric waters influenced by a large degree of topographic rainout. We show that by 63 to 61 Ma, the Gangdese Arc reached an elevation of ~3.5 km, suggesting that the Gangdese Arc achieved >60% of its current elevation before continent–continent collision. This uplift was probably caused by crustal shortening in response to low-angle subduction of Neo-Tethyan oceanic lithosphere. This early high palaeoelevation estimate for the Himalaya–Tibetan system challenges previous assumptions that southern Tibet uplift required continent–continent collision to achieve substantial topography.The triple oxygen isotope composition of quartz veins indicates that the southern Tibetan Plateau was already around 3.5 km high by 60 million years ago, showing that substantial surface uplift started before collision of the Eurasian and Indian plates.

Geologically abrupt carbon perturbations such as the Palaeocene-Eocene Thermal Maximum (PETM, approx. 56 Ma) are the closest geological points of comparison to current anthropogenic carbon emissions. Associated with the rapid carbon release during this event are profound environmental changes in the oceans including warming, deoxygenation and acidification. To evaluate the global extent of surface ocean acidification during the PETM, we present a compilation of new and published surface ocean carbonate chemistry and pH reconstructions from various palaeoceanographic settings. We use boron to calcium ratios (B/Ca) and boron isotopes (δ11B) in surface- and thermocline-dwelling planktonic foraminifera to reconstruct ocean carbonate chemistry and pH. Our records exhibit a B/Ca reduction of 30-40% and a δ11B decline of 1.0-1.2‰ coeval with the carbon isotope excursion. The tight coupling between boron proxies and carbon isotope records is consistent with the interpretation that oceanic absorption of the carbon released at the onset of the PETM resulted in widespread surface ocean acidification. The remarkable similarity among records from different ocean regions suggests that the degree of ocean carbonate change was globally near uniform. We attribute the global extent of surface ocean acidification to elevated atmospheric carbon dioxide levels during the main phase of the PETM. This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

The Palaeocene-Eocene Thermal Maximum (PETM) was a significant global warming event in Earth's deep past (56 Mya). The warming across the PETM boundary was driven by a rapid rise in greenhouse gases. The event also coincided with a time of maximum insolation in Northern Hemisphere summer. There is increased evidence that the mean warming was accompanied by enhanced seasonality and/or extremes in precipitation (and flooding) and drought. A high horizontal resolution (50 km) global climate model is used to explore changes in the seasonal cycle of surface temperature, precipitation, evaporation minus precipitation and river run-off for regions where proxy data are available. Comparison for the regions indicates the model accurately simulates the observed changes in these climatic characteristics with North American interior warming and drying, and warming and increased river run-off at other regions. The addition of maximum insolation in Northern Hemisphere summer leads to a drier North America, but wetter conditions at most other locations. Long-range transport of atmospheric moisture plays a critical role in explaining regional changes in the water cycle. Such high-frequency variations in precipitation might also help explain discrepancies or misinterpretation of some climate proxies from the same locations, especially where sampling is coarse, i.e. at or greater than the frequency of precession. This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

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.

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO2 and at the onset of the PETM.Widespread shallow-water hydrothermal venting in the North Atlantic, probably a source of methane, coincided with the onset of the Palaeocene–Eocene Thermal Maximum, according to borehole proxy records and seismic imaging.

Whole-genome duplications (WGDs) are widespread and prevalent in vascular plants and frequently coincide with major episodes of global and climatic upheaval, including the mass extinction at the Cretaceous–Tertiary boundary (c. 65 Ma) and during more recent periods of global aridification in the Miocene (c. 10–5 Ma). Here, we explore WGDs in the diverse flowering plant clade Malpighiales. Using transcriptomes and complete genomes from 42 species, we applied a multipronged phylogenomic pipeline to identify, locate, and determine the age of WGDs in Malpighiales using three means of inference: distributions of synonymous substitutions per synonymous site (K s) among paralogs, phylogenomic (gene tree) reconciliation, and a likelihood-based gene-count method. We conservatively identify 22 ancient WGDs, widely distributed across Malpighiales subclades. Importantly, these events are clustered around the Eocene–Paleocene transition (c. 54 Ma), during which time the planet was warmer and wetter than any period in the Cenozoic. These results establish that the Eocene Climatic Optimum likely represents a previously unrecognized period of prolific WGDs in plants, and lends further support to the hypothesis that polyploidization promotes adaptation and enhances plant survival during episodes of global change, especially for tropical organisms like Malpighiales, which have tight thermal tolerances.

Despite decades of research, hundreds of peer‐reviewed papers, and considerable relevance to Earth's future, the rapid global warming and tremendous CO2 input at the start of the Paleocene‐Eocene Thermal Maximum (PETM) remain highly debated. Cael and Foster (2026, ) present a statistical treatment of Cenozoic benthic foraminifera (BF) stable isotope data, which confirms and confounds PETM knowledge. They show that Cenozoic BF stable isotope anomalies, after removing long‐term trends, generally conform to Laplace distributions. They then demonstrate deviations in BF stable isotopes across the PETM lie outside this distribution, statistically signifying an extreme. Cael and Foster (2026, ) reason this necessitates a special mechanism, propelling an idea of “super‐volcanism” and external carbon injection. But neither their statistics nor recent papers support such inference. The paper highlights basic issues regarding the PETM and ultimately an urgency for a fuller understanding of Earth's carbon cycle.