The hottest climate intervals in Earth’s more recent geological history (especially the Phanerozoic Eon, the last ~540 million years of complex life) reached global mean surface temperatures (GMST) of up to about 36 °C (97 °F).

This is based on a 2024 reconstruction (Judd et al. in Science) that combined extensive proxy data with climate modeling for the past 485 million years. It shows GMST ranging from a low of ~11 °C to a high of ~36 °C—hotter and more variable than many earlier estimates. Today’s GMST is roughly 15 °C.

The hottest epochs

Cretaceous Hot Greenhouse / Cretaceous Thermal Maximum (roughly 90–100 million years ago, peaking in the Turonian ~92 Ma):

Frequently identified as the hottest period since the evolution of complex (non-microbial) life. Global average surface temperatures reached ~36 °C. Polar regions were ice-free, with high-latitude ocean temperatures sometimes exceeding 20–27 °C, and tropical temperatures potentially reaching 40+ °C in places. No major mass extinction is associated with this prolonged warmth.

Early Eocene Climatic Optimum (EECO) and related Hothouse state (~50–47 million years ago), including the Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma):

These were among the hottest of the last 66 million years. The broader Hothouse interval had temperatures more than 10 °C warmer than today. The PETM itself involved a rapid rise of ~5–8 °C (global mean estimates often put peak GMST in the low-to-mid 30s °C range, e.g., around 34 °C in some reconstructions). Polar regions supported subtropical vegetation (palms, crocodiles).

Earlier intervals (e.g., parts of the Paleozoic or the Neoproterozoic) also featured extreme warmth, and the Hadean Eon (Earth’s first ~500 million years) involved a largely molten surface far hotter than any later climate. However, the Cretaceous peak is the standout for well-documented conditions with complex ecosystems.

Earlier intervals (e.g., parts of the Paleozoic or the Neoproterozoic) also featured extreme warmth, and the Hadean Eon (Earth’s first ~500 million years) involved a largely molten surface far hotter than any later climate. However, the Cretaceous peak is the standout for well-documented conditions with complex ecosystems.

Atmospheric CO₂ was the dominant long-term control on these temperatures (with a strong correlation across the Phanerozoic).

• Cretaceous Thermal Maximum: Commonly estimated well above 1,000 ppm (sometimes several times higher in various proxies and models; pre-industrial levels were ~280 ppm).
• Early Eocene / PETM peak: Background levels were already high (hundreds to over 1,000 ppm); during the PETM spike, estimates reach as high as ~1,600–2,000+ ppm (some reconstructions go higher).

These ancient high-CO₂ greenhouse states were sustained over long geological timescales and supported diverse (if different) life.

The Phanerozoic Eon is the current and most recent geologic eon, spanning from approximately 538.8 million years ago to the present.

It represents only about the last one-eighth of Earth’s history but contains nearly all the fossil evidence of complex, macroscopic life (the name comes from Greek words meaning “visible life”).

It is divided into three eras based primarily on major changes in life forms:

Era	Time Span (approx.)	Key Features
Paleozoic (“Ancient Life”)	539–252 Ma	Cambrian explosion of animal diversity; colonization of land by plants, arthropods, and vertebrates; ends with the largest mass extinction (Permian–Triassic)
Mesozoic (“Middle Life”)	252–66 Ma	Age of reptiles/dinosaurs; appearance of mammals, birds, and flowering plants; ends with the Cretaceous–Paleogene extinction
Cenozoic (“New Life”)	66 Ma–present	Age of mammals; rise of modern ecosystems, grasses, and eventually humans; ongoing icehouse conditions

These eras are further subdivided into 12 periods (Cambrian through Quaternary).

During the Phanerozoic, Earth’s climate has oscillated between long greenhouse (ice-free poles, high sea levels) and icehouse (polar ice sheets present) states. Global mean surface temperatures ranged from roughly 11 °C to 36 °C according to recent reconstructions, with a strong overall correlation to atmospheric CO₂ levels.

Hot greenhouse peaks occurred in the early Paleozoic, parts of the Mesozoic (especially the Cretaceous Thermal Maximum), and the early Cenozoic (Early Eocene Climatic Optimum and PETM).

Major icehouse intervals include the late Paleozoic (Carboniferous–Permian) and the current late Cenozoic icehouse (starting ~34 million years ago with Antarctic glaciation).

The Phanerozoic is the eon during which continents drifted into their modern configuration, oxygen levels became suitable for complex animal life, and the biosphere diversified dramatically—despite multiple mass extinctions.

Phanerozoic Eon Extinctions

Phanerozoic Eon mass extinctions (spanning the last ~539 million years) include several major events that dramatically reduced biodiversity. The most prominent are the traditional “Big Five”, first highlighted by Raup and Sepkoski in the 1980s as statistically elevated extinction rates above background levels. Modern analyses show a continuum of extinction intensities rather than sharply distinct categories, but these five remain the largest post-Cambrian biodiversity crises.

There are also many secondary or “second-order” events (e.g., end-Capitanian, Toarcian, Cenomanian–Turonian). Estimates of species/genera loss vary by dataset and taxonomic level, but the figures below reflect widely cited approximations (mostly marine invertebrates, as the fossil record is strongest there).

The Big Five Mass Extinctions

Rank (by severity)	Event	Timing	Approximate Losses	Primary Causes	Key Impacts
1 (most severe)	End-Permian (“Great Dying” / Permian–Triassic)	~251.9 Ma	~81–96% marine species; ~70% terrestrial vertebrates; ~57% families	Siberian Traps large igneous province (LIP) volcanism → massive CO₂/SO₂ release, global warming, ocean anoxia/acidification, euxinia	Ended the Paleozoic Era; wiped out trilobites, many corals, most synapsids; longest recovery (~5–10+ Myr)
2	Late Ordovician (Ordovician–Silurian)	~445–443 Ma	~85% marine species; ~57% genera; ~27% families	Glaciation of Gondwana → sea-level drop + cooling, followed by rapid warming/anoxia	Mostly marine (brachiopods, trilobites, graptolites, etc.); two pulses
3	Late Devonian (mainly Frasnian–Famennian)	~372–359 Ma	~75% species; ~35–50% genera	Complex: possible ocean anoxia, cooling, sea-level changes, nutrient runoff from early forests, volcanism	Prolonged series of pulses; heavy impact on reef builders, placoderms, trilobites
4	End-Triassic (Triassic–Jurassic)	~201.3 Ma	~70–80% species; ~48% genera; ~23% families	Central Atlantic Magmatic Province (CAMP) LIP volcanism → warming, ocean acidification	Cleared competitors for dinosaurs; ended many archosauromorphs, conodonts
5	End-Cretaceous (Cretaceous–Paleogene / K–Pg)	66 Ma	~75% species; ~40–50% genera	Chicxulub asteroid impact (+ possible contribution from Deccan Traps volcanism)	Non-avian dinosaurs, pterosaurs, ammonites, many marine reptiles extinct; started Cenozoic mammal radiation

Other notable Phanerozoic events

• End-Capitanian (~260 Ma) — Often linked to Emeishan Traps; significant but smaller than the main end-Permian pulse.
• Toarcian (Early Jurassic, ~183 Ma) and Cenomanian–Turonian (~94 Ma) — Oceanic anoxic events tied to LIP volcanism.
• Eocene–Oligocene (~34 Ma) — Cooling and Antarctic glaciation; more of a turnover than a classic mass extinction.
• Ongoing Holocene/Anthropocene biodiversity crisis — Sometimes called a potential “sixth mass extinction” due to human activities, though it has not yet reached the taxonomic severity of the Big Five.

Most Phanerozoic extinctions involved rapid climate and environmental change:

• Large Igneous Province (LIP) volcanism — Dominant trigger for end-Permian, end-Triassic, and parts of others (releases greenhouse gases, causes warming/acidification/anoxia).
• Bolide impacts — Clearly primary for the end-Cretaceous (Chicxulub); debated or secondary elsewhere.
• Glaciation and sea-level change — Key for Late Ordovician and parts of Late Devonian.
• Proximate kill mechanisms often included ocean anoxia, acidification, temperature extremes, and habitat loss.

These events repeatedly reset evolutionary trajectories, eliminating dominant groups and allowing survivors (and new lineages) to radiate into emptied ecological niches. The end-Permian was by far the most catastrophic, nearly ending complex life as we know it.

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A 485-million-year history of Earth’s surface temperature

“A 485-million-year history of Earth’s surface temperature” is a major 2024 research paper published in Science (20 September 2024; DOI: 10.1126/science.adk3705).

Led by Emily J. Judd (Smithsonian National Museum of Natural History / University of Arizona), with co-authors including Jessica E. Tierney, Daniel J. Lunt, Isabel P. Montañez, Brian T. Huber, Scott L. Wing, and Paul J. Valdes.

It produced PhanDA (Phanerozoic Data Assimilation), a new reconstruction of global mean surface temperature (GMST) spanning most of the Phanerozoic Eon (the last 485 million years). The team used data assimilation — a statistical method that combines:

• Geological proxy data (e.g., oxygen isotopes from fossils and other paleoclimate indicators)
• Climate model simulations

This approach creates a more complete and consistent global temperature record than proxies or models alone.

Key findings

• Temperature range: GMST varied between 11 °C and 36 °C (52–97 °F).
  • Lowest: ~11 °C (Late Pleistocene glacial periods)
  • Highest: ~36 °C (Turonian stage of the Late Cretaceous, ~90–94 million years ago)
• This range is larger than most previous reconstructions (which often suggested roughly 14–26 °C).
• Earth spent more time in warmer climate states than colder ones.
• Tropical temperatures ranged from 22 °C to 42 °C, challenging the idea of a fixed upper limit on tropical heat.
• Strong polar amplification (larger temperature changes at high latitudes) and a shallowing of the pole-to-equator temperature gradient as global temperatures rose.
• Strong correlation with atmospheric CO₂: CO₂ is identified as the dominant long-term control on Phanerozoic climate.
• Apparent Earth system sensitivity of approximately ~8 °C per doubling of CO₂ (higher than typical modern estimates that focus on shorter timescales).

The reconstruction aligns well with independent Cenozoic temperature estimates, increasing confidence in the results for earlier periods.

This study provides one of the most detailed and rigorous long-term temperature curves available and is frequently cited in discussions of past greenhouse climates, climate sensitivity, and the role of CO₂ in Earth’s history.

Published: Science (20 September 2024)

DOI: 10.1126/science.adk3705

Authors: Emily J. Judd (Smithsonian National Museum of Natural History / University of Arizona), with co-authors including Jessica E. Tierney, Daniel J. Lunt, Isabel P. Montañez, Brian T. Huber, Scott L. Wing, and Paul J. Valdes.

Abstract

A long-term record of global mean surface temperature (GMST) provides critical insight into the dynamical limits of Earth’s climate and the complex feedbacks between temperature and the broader Earth system. Here, we present PhanDA, a reconstruction of GMST over the past 485 million years, generated by statistically integrating proxy data with climate model simulations. PhanDA exhibits a large range of GMST, spanning 11° to 36°C. Partitioning the reconstruction into climate states indicates that more time was spent in warmer rather than colder climates and reveals consistent latitudinal temperature gradients within each state. There is a strong correlation between atmospheric carbon dioxide (CO₂) concentrations and GMST, identifying CO₂ as the dominant control on variations in Phanerozoic global climate and suggesting an apparent Earth system sensitivity of ~8°C.