Final answer:
GCM's used in the IPCC Fourth Assessment Report (AR4) simulate climate scenarios and help scientists understand how different factors affect the Earth's climate. They make projections about future climate change.
Step-by-step explanation:
GCM's used in the IPCC Fourth Assessment Report (AR4) simulate climate scenarios. They are computer models that utilize complex mathematical equations to represent various components of the climate system, such as the atmosphere, oceans, and land surface. These models can help scientists understand how different factors, such as greenhouse gas emissions, affect the Earth's climate and make projections about future climate change.