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The appearance of permanent El Niño-like conditions prior to 3 million years ago is founded on sea-surface temperature (SST) reconstructions that show invariant Pacific warm pool temperatures and negligible equatorial zonal temperature gradients. However, only a few SST records are available, and these are potentially compromised by changes in seawater chemistry, diagenesis, and calibration limitations. For this study, we establish new biomarker-SST records and show that the Pacific warm pool was ~4°C warmer 12 million years ago. Both the warm pool and cold tongue slowly cooled toward modern conditions while maintaining a zonal temperature gradient of ~3°C in the late Miocene, which increased during the Plio-Pleistocene. Our results contrast with previous temperature reconstructions that support the supposition of a permanent El Niño-like state.

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Abstract. 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 TEX 86 (tetraether index of 86 carbon atoms) sea surface temperature proxy has been increasingly applied in the reconstruction of Mesozoic and Cenozoic ocean temperatures. However, the archaeal lipids that compose TEX 86 indices, glycerol dialkyl glycerol tetraethers (GDGTs), can derive from water column production, terrestrial sources, and/or within sediments. TEX 86 -sea surface temperature estimates can also be influenced by non-temperature factors, such as growth phase and nutrient levels. Here we show that the weighted average of cyclopentane moieties, known as the Ring Index (RI), can be used to determine if TEX 86 temperature estimates are influenced by non-thermal factors and/or deviate from modern analogues. We demonstrate that RI and TEX 86 indices from the published global core top data set and mesocosm cultures are significantly correlated, as predicted through the influence of temperature on lipid biochemistry. We further show that when RI and TEX 86 indices in modern or ancient records deviate from the modern global TEX 86 -RI relationship, GDGT distributions are not solely controlled by environmental temperature and/or TEX 86 -based temperature reconstructions are questionable.

Throughout Earth's history, CO2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the major climate transition of the past 66 million years. We find close coupling between CO2 and climate throughout the Cenozoic, with peak CO2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼550 ppm in the Miocene, and falling further into ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO2 drawdown is explained by an increase in the ratio of alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO2 record highlights the truly geological scale of anthropogenic CO2 change: Current CO2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO2 levels of the Miocene or Eocene in the coming century. ▪ CO2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪ CO2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO2 system over the Cenozoic. ▪ CO2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪ Twenty-first-century carbon emissions have the potential to return CO2 to levels not seen since the much warmer climates of Earth's distant past. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.

Cyclization in glycerol dibiphytanyl glycerol tetraethers (GDGTs) results in internal cyclopentane moieties which are believed to confer thermal stability to crenarchaeal membranes. While the average number of rings per GDGT lipid (ring index) is positively correlated with temperature in many temperate environments, poor correlations are often observed in geothermal environments, suggesting that additional parameters may influence GDGT core lipid composition in these systems. However, the physical and chemical parameters likely to influence GDGT cyclization which are often difficult to decouple in geothermal systems, making it challenging to assess their influence on lipid composition. In the present study, the influence of temperature (range 65-81°C), pH (range 3.0-5.0), and ionic strength (range 10.1-55.7 mM) on GDGT core lipid composition was examined in the hyperthermoacidophile Acidilobus sulfurireducens, a crenarchaeon originally isolated from a geothermal spring in Yellowstone National Park, Wyoming. When cultivated under defined laboratory conditions, the composition of individual and total GDGTs varied significantly with temperature and to a lesser extent with the pH of the growth medium. Ionic strength over the range of values tested did not influence GDGT composition. The GDGT core lipid ring index was positively correlated with temperature and negatively correlated with pH, suggesting that A. sulfurireducens responds to increasing temperature and acidity by increasing the number of cyclopentyl rings in GDGT core membrane lipids.