On the enigmatic similarity in Greenland δ¹⁸O between the Oldest and Younger Dryas

Greenland ice core δ¹⁸O temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO₂, while modeling studies suggest that the YD was approximately 4-5°C warmer than the OD.

This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture-tracing module to investigate atmospheric processes that may have been responsible for the similar δ¹⁸O values during the OD and YD. Our results show that the summer-to-winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the δ¹⁸O signal toward warmer (summer) temperatures (enriched δ¹⁸O values and it accounts for ~45% of the expected YD-OD δ¹⁸O difference).