Hothouse Earth during the Cretaceous‒Paleogene period: an overview

As anthropogenic carbon emissions continue to rise, global temperatures are experiencing unprecedented increases. In response to the looming threats of climate change, the Paris Agreement proposed the goal of limiting global warming to 1.5–2.0°C and advocated for global emission reductions. However, recent studies suggest that even if these targets are met, internal feedback mechanisms within the climate system could still propel global warming beyond critical thresholds, potentially shifting Earth’s climate from cyclical glacial-interglacial alternation to a hothouse state. Notably, 2024 was the hottest year on record, with the global average surface temperature 1.55°C higher than pre-industrial levels, making it the first calendar year since the Industrial Revolution to exceed 1.5°C of warming. If the current warming trend observed in recent decades continues, the Earth’s average temperature could reach 20°C by ~2300, resulting in a permanent hothouse state. This alarming prospect raises concerns within both the scientific community and the general public about the potential for catastrophic outcomes.The Cretaceous‒Paleogene period represents Earth’s most recent prolonged hothouse state, characterized by sustained high temperatures and elevated atmospheric CO2 concentrations. Understanding this period offers critical insights into future climate scenarios. This study synthesizes current knowledge of the Cretaceous‒Paleogene hothouse Earth, exploring its driving mechanisms, environmental characteristics, ecological responses, and ultimate termination. Key findings include: (1) through integrated analysis of carbon emission patterns from mid-ocean ridges, continental rifts, large igneous provinces, and continental arcs, coupled with paleoclimatic records, we propose that continental arc magmatism was likely the primary driver of the Cretaceous‒Paleogene hothouse conditions. (2) Multiple episodes of carbon cycle perturbations, lasting 104–105 years, characterized the hothouse climate regime, driving rapid climatic warming events. These include the Cretaceous Oceanic Anoxic Events (OAEs) and the Paleogene hyperthermal events (e.g., the Paleocene–Eocene Thermal Maximum (PETM)). The OAEs are characterized by extensive black shale deposition, with distinct positive carbon isotope excursions observed during OAE1a, OAE1d, and OAE2, while OAE1b is marked by a notable negative excursion. In contrast, the Paleogene hyperthermal events consistently exhibit negative carbon isotope excursions with limited black shale deposition. This pronounced dichotomy in geochemical signatures and depositional patterns between these events can be primarily explained by fundamental differences in both the nature of carbon sources and the underlying perturbation mechanisms. (3) Hyperthermal events, characterized by pronounced negative carbon isotope excursions, occurred during prolonged periods of warming, indicating increased vulnerability of Earth’s surface organic carbon reservoirs to increases in global temperature. OAE1b, PETM, and subsequent Eocene hyperthermal events were likely triggered by perturbations in these reservoirs, highlighting the necessity for comparative studies on their respective carbon emission fluxes and associated environmental impacts. (4) The hyperthermal events were associated with intensified hydrological cycles, characterized by enhanced high-latitude precipitation and complex spatial variability in mid- to low-latitude rainfall patterns. (5) The hothouse climate facilitated the expansion and diversification of thermophilic plant groups, promoting the spread of forest from low to middle and high latitudes and enhancing terrestrial plant diversity. In marine environments, there was also an increase in the diversity of dinoflagellate cysts, calcareous nannofossils, and planktic foraminifera.This synthesis highlights that the Cretaceous‒Paleogene hothouse state was fundamentally maintained by deep Earth carbon emissions, while its termination was governed by carbon sequestration through enhanced chemical weathering and organic matter burial. However, significant uncertainties remain regarding quantitative carbon fluxes, spatial patterns of hydrological changes, and ecosystem responses to rapid warming. Future research directions should emphasize integrated Earth system approaches to better constrain these critical aspects of hothouse Earth dynamics.