If the simulation were to run for millions of years, we would likely see additional processes in the carbon cycle that occur over longer timescales. These include:
1.) Weathering of rocks: Over time, rocks exposed to the elements will break down and release carbon into the soil and atmosphere.
2.) Deep ocean circulation: The deep ocean acts as a carbon sink, absorbing carbon dioxide from the atmosphere and storing it in the deep ocean. Over millions of years, deep ocean circulation patterns can change, altering the rate at which carbon is absorbed and released.
3.) Tectonic activity: Tectonic activity, such as volcanic eruptions and mountain building, can release large amounts of carbon dioxide into the atmosphere over long timescales. Conversely, the weathering of rocks and the deposition of sediment can remove carbon dioxide from the atmosphere over similar timescales.
4.) Evolution of plant and animal life: As plant and animal life evolves over millions of years, the balance between photosynthesis and respiration can shift, altering the overall amount of carbon dioxide in the atmosphere.
Overall, running the simulation for millions of years would give a more complete picture of how the carbon cycle operates over long timescales, including the impact of geological, biological, and atmospheric processes.