A proxy compatible model for the YD and the subsequent greening of the Sahara
Bibliographic record
Abstract
Throughout the Quaternary, northern Africa has experienced recurring periods of intensified precipitation, known as African Humid Periods. The most recent such period began after the termination of the Younger Dryas (YD), which was a dramatic reversion to ice-age temperatures during the deglaciation. One intriguing explanation for the timing of this most recent greening of the Sahara is that the rapid recovery of the Atlantic Meridional Overturning Circulation (AMOC) after the YD caused a northward shift of the Inter-Tropical Convergence Zone (ITCZ), resulting in increased precipitation in Northern Africa. In previous attempts to model the YD (Peltier et al., 2006; Peltier, 2007), and the subsequent transition to a Green Sahara, (Menviel et al. 2021, 2011), the total volume of freshwater forcing applied to the Arctic Ocean was quite large. The Eustatic Sea Level (ESL) increase associated with the freshwater influx in these studies is not compatible with proxy inferred ESL constraints. Furthermore, the increase in precipitation at the end of the YD was not nearly as abrupt as that which was simulated for the Green Sahara period at the end of the penultimate deglaciation, a fact that the authors attributed to the misalignment of the timing of AMOC maximum and the maximum in insolation forcing at the YD. Here we present a model of the YD, in which only 0.15 Sv forcing for 100 years, applied to the Beaufort Gyre, was needed to collapse the AMOC and keep it in a collapsed state for nearly 1000 additional years. As the YD is approximately 1000 years long, we are able to achieve this interval of AMOC shutdown without continuously hosing the model with freshwater. As a result, the ESL rise in our model is physically plausible. Furthermore, unlike the results of Menviel et al. (2021, 2011), the simulated precipitation over North Africa in our model increases abruptly, in step with the abrupt resumption of the AMOC. The AMOC itself recovers due to a reinvigorated ocean-atmosphere flux exchange that occurs following the opening of a polynya in the Irminger Sea.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.002 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 0.001 |
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".