Declining atmospheric CO2 during the late Middle Eocene climate transition
Bibliographic record
Abstract
The transition from the extreme greenhouse of the early Paleogene (∼52 Ma) to the present-day icehouse is the most prominent change in Earth's Cenozoic climate history. During the late Middle Eocene climate transition (42-38 Ma), which preceded the onset of long-lived, continental-scale ice sheets, there is concordant evidence for brief pulses (\<1 m.y. in length) of global warmth and ice sheet growth but few constraints on atmospheric CO~2~. Here we estimate the concentration of atmospheric CO~2~ during this critical interval using stomatal indices of fossil *Metasequoia* needles from ten levels in an exceptionally well-preserved core from the Giraffe kimberlite locality in northwestern Canada. Reconstructed CO~2~ concentrations are mainly between 700 to 1000 ppm, but include a secular decline to 450 ppm towards the top of the investigated section. Because the CO~2~ threshold for nucleating continental ice sheets at this time was ∼500 to 750 ppm, the CO~2~ decline is compatible with a rapid (\<10^4^ yrs) transition from warm, largely ice-free conditions to cooler climates with ice sheets. These fossils provide direct evidence that high-latitude deciduous forests thrived in the geological past under CO~2~ concentrations that will likely be reached within the 21^st^ century (500-1000 ppm).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".