A multi-model assessment of the early last deglaciation (PMIP4 LDv1)

At the onset of the last deglaciation, beginning ~19 thousand years ago, ice sheets that covered the Northern Hemisphere at the Last Glacial Maximum started to melt, Earth began to warm, and sea levels rose. This time period is defined by major long-term, millennial-scale, climate transitions from the cold glacial to warm interglacial state, as well as many short-term, centennial- to decadal-scale warmings and coolings of more than 5 °C, sudden reorganisations of basin-wide circulations, and jumps in sea level of tons of meters. Long transient simulations of the deglaciation have been increasingly performed to better understand the long and short term processes, examine different possible scenarios, and compare model output to observable records. The Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an international coordinated effort in simulating the last deglaciation whilst encompassing a broad range of models and model complexities. This study is a multi-model intercomparison of 17 simulations of the last deglaciation from nine different climate models. Unlike other multi-model intercomparison projects, these simulations do not follow one particular experimental design but follow an intentionally flexible protocol suitable for all participants. The design of the protocol provides the opportunity to compare results from models using different forcings and examine a variety of scenarios, hence, representing the range of uncertainty in climate predictions of the time period. One particularly challenging choice to make in the experimental design is how to incorporate the resultant freshwater flux from the melting ice sheets. This research focusses on the divergence between climate trajectories in the simulations as a result of the meltwater scenario preferred by the modelling groups as well as other experimental design choices and their impact on the onset of the deglaciation. These results provide a better understanding of modelling this time period as well as model biases and uncertainty with respect to deglacial forcings and the observable proxy records.