Fact check: What were the global temperatures during the Paleocene-Eocene Thermal Maximum?

1. Why the PETM reads like a climate wake-up call

Multiple lines of evidence indicate the PETM was a geologically rapid pulse of warming driven by a large input of isotopically light carbon, producing a negative carbon isotope excursion and major shifts in ocean chemistry and ecosystems ^{[4] [2]}. Studies synthesize deep-sea sediments, terrestrial records, and geochemical proxies to show a quasi-uniform global warming of 5–8 °C, polar amplification, and loss of seasonal snow, with the event lasting on the order of ~200,000 years ^{[2] [4]}. These features establish the PETM as a clear analog for understanding carbon-forced climate system responses.

2. How much warmer were temperatures in absolute terms?

Proxy-model comparisons yield two complementary portrayals: one focuses on anomalous warming (temperature change relative to preceding levels), while another reports absolute global mean surface temperatures for the interval. A 2020 reconstruction places global mean surface temperature during the PETM around 31.6 °C (range 27.2–34.5 °C), which is substantially warmer than pre‑industrial climates and implies a hot background state into which the PETM warming was superimposed ^[3]. This matters because absolute warmth affects baseline climate feedbacks and habitability patterns.

3. Convergence around a ~5.6 °C warming signal

A 2022 spatial synthesis reported a global mean temperature change of ~5.6 °C with a tight 95% confidence interval (5.4–5.9 °C), arguing for robust, spatially consistent warming patterns across ocean and land records ^[1]. Earlier reviews also characterize the PETM as a 5–8 °C event, reflecting agreement across decades of work even as methods improved ^{[2] [4]}. The repeated convergence on a mid-range warming near 5–6 °C strengthens confidence in the magnitude of the transient temperature rise itself.

4. Tropical sea-surface records tell a consistent, sobering story

Tropical proxy records demonstrate pronounced regional change: a 2003 study estimated tropical Pacific sea-surface temperatures rose by ~4–5 °C during the PETM, consistent with an atmospheric CO2 increase severalfold above late Paleocene levels ^[5]. These tropical ocean changes are important because they control heat and moisture delivery to the atmosphere and drive ecological shifts, and they corroborate global anomaly estimates by showing large, biologically consequential ocean warming in low latitudes.

5. What the PETM implies about climate sensitivity

Analyses that combine warming magnitude and carbon perturbation estimates infer a high equilibrium climate sensitivity for the PETM interval; the 2022 study reported an equilibrium sensitivity of about 6.5 °C per CO2 doubling, higher than many modern estimates ^[1]. This elevated sensitivity may reflect state-dependence—climate feedbacks that operated differently in a warmer early Paleogene world—or uncertainties in carbon release and proxy calibration. The implication is that feedback strength and baseline climate state significantly shape temperature outcomes.

6. Sources of disagreement: proxies, baselines, and interpretation

Differences among studies stem from proxy type (e.g., isotopes vs. biomarkers), spatial sampling, and whether studies report absolute temperatures or relative anomalies ^{[3] [1] [4]}. Absolute-temperature reconstructions depend on calibration choices and baseline estimates; anomaly-focused work is less sensitive to baseline offsets but depends on spatial coverage. These methodological distinctions explain why some studies emphasize a ~31.6 °C absolute mean while others emphasize a ~5.6 °C anomaly ^{[3] [1]}.

7. Takeaway for the big picture and what’s left unresolved

The PETM unequivocally involved a rapid, multi-degree global warming event tied to massive carbon release, with robust evidence for a ~5–8 °C transient warming and tropical SST increases of 4–5 °C, set against a warm early Paleogene background that some reconstructions place near ~30–32 °C ^{[2] [5] [3]}. Remaining uncertainties center on the precise absolute baseline temperatures, the full carbon source budget, and the degree to which climate sensitivity in that state applies to today ^{[1] [4]}. These are active research frontiers because they determine how closely the PETM serves as an analog for modern anthropogenic change.

8. How to read the literature responsibly

Readers should note that the literature combines proxy records and model interpretations that are mutually informative but methodologically distinct; agreement on the magnitude of warming strengthens confidence, while lingering gaps about baseline warmth and sensitivity reflect real scientific uncertainty ^{[3] [1]}. For policy and communication, the clearest PETM message is that Earth’s climate system responded to large carbon inputs with sustained, multi-degree warming and major ecological upheaval—facts supported across decades of studies and summarized here to show both consensus and the limits of current quantification ^{[2] [4]}.