One way to model ice ages is using a general circulation model (GCM) or Earth System Model of Intermediate Complexity (EMIC) to simulate the global climate representative of a particular time in history. Many early modeling studies attempted to simulate the most recent glacial, the Last Glacial Maximum (LGM). Since those early studies, a major project was established called the Paleoclimate Modelling Inter-comparison Project (PMIP), providing a rich source of modeling studies of the LGM.
The PMIP is designed much like the Atmospheric Model Intercomparison Project (AMIP) or the Coupled Model Intercomparison Project (CMIP), with a range of modeling groups running their GCMs and EMICs with a set suite of forcing factors. Key epochs simulated as part of PMIP were the climatic optimum at 6,000 years ago and the Last Glacial Maximum (LGM), about 20,000 years ago. These times were chosen as end-members of the range of global climate in the recent geological past. They were compared with a present climate with pre-industrial greenhouse gas levels. The project has several aims. It aims to test the ability of models to simulate climates quite distinct from today, and the differences between models. Modeling extreme climates will help to understand the drivers of climate change and the climate's response to different forcing. Finally, the PMIP will provide many results to help fill in the gaps in knowledge of past climates and understand proxy evidence.
The first phase of PMIP included GCMs where full ocean dynamics were not considered, and which used low resolution EMICs. In most models, the ocean surface temperatures were prescribed from a map derived from proxy evidence. Some models also included interaction with the ocean surface. The main step to the second phase of PMIP was the inclusion of full ocean models. Some models also included interactive vegetation.
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