The response of Earth’s climate system to orbital forcing has been highly state dependent over the past 66 million years.

Boron and carbon isotope data, used in an Earth system model, show that the Palaeocene–Eocene Thermal Maximum was associated with a much greater release of carbon than thought, most probably triggered by volcanism in the North Atlantic. The Palaeocene–Eocene Thermal Maximum was a surface warming event associated with ecological disruption that occurred about 56 million years ago. A large amount of carbon is thought to have been released during this event, but the total amount and the sources of carbon remain uncertain. This paper combines boron and carbon isotope data in an Earth system model and finds that the source of carbon was much larger than previously thought and that most of the carbon was probably released by volcanism associated with the North Atlantic Igneous Province. The study also suggests that the amplifying organic carbon–climate feedbacks did not have a prominent role in driving the event, but that enhanced burial of organic matter was important for sequestering the released carbon and accelerating the recovery of the climate system. The Palaeocene–Eocene Thermal Maximum1,2 (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates3. However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming1,3,4,5. Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data—a proxy for seawater pH—that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event6,7. We find strong evidence for a much larger (more than 10,000 petagrams)—and, on average, isotopically heavier—carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province10,11, rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system12.

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO ₂ ) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO ₂ beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO ₂ record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO ₂ thresholds in biological and cryosphere evolution.

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.

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).