The Impact of CO2 and Climate State on Whether Dansgaard–Oeschger Type Oscillations Occur in Climate Models.

Greenland ice core records feature Dansgaard–Oeschger (D‐O) events, which are abrupt warming episodes followed by gradual cooling during ice age climate. The three climate models used in this study (CCSM4, MPI‐ESM, and HadCM3) show spontaneous self‐sustained D‐O‐like oscillations (albeit with differences in amplitude, duration, and shape) in a remarkably similar, narrow window of carbon dioxide (CO2) concentration, roughly 185–230 ppm. This range matches atmospheric CO2 during Marine Isotopic Stage 3 (MIS 3: between 27.8 and 59.4 thousand of years BP, hereafter ka), a period when D‐O events were most frequent. Insights from the three climate models point to North Atlantic (NA) sea‐ice coverage as a key ingredient behind D‐O type oscillations, which acts as a "tipping element." Other climate state properties such as Mean Atlantic Meridional Overturning Circulation strength, global mean temperature and salinity gradient in the Atlantic Ocean do not determine whether D‐O type behavior can occur in all three models. Plain Language Summary: Dansgaard‐Oeschger (D‐O) events are sudden periods of warming followed by slow cooling during ice ages, as observed in Greenland's ice core records. The study used three different climate models to analyze these events, and all models showed similar behavior within a specific range of atmospheric carbon dioxide (CO2) concentrations. This range of CO2 concentrations aligns well with the levels found during the latter half of the last ice age when D‐O events were most frequent. The study found that when CO2 levels are outside this specific range, the climate models settle into one of two stable states, either warm with high CO2 levels or cold with low CO2 levels. These states are stable until CO2 levels reach a certain "tipping point"‐ below around 185–195 ppm for the cold state and above around 217–230 ppm for the warm state. This study suggests that North Atlantic sea ice coverage is a crucial property essential for D‐O like behavior to develop in complex climate models. Key Points: Three complex climate models show internal unforced oscillations at glacial levels of atmospheric carbon dioxide concentrationNorth Atlantic (NA) sea‐ice coverage plays a crucial role behind Dansgaard–Oeschger type behavior in all three modelsThe three models reproduce oscillations in a similar range of NA sea ice coverage [ABSTRACT FROM AUTHOR]