To answer the question, one would need to compare the theoretical 8 PgC (petagrams of carbon) released by burning fossil fuels with the actual carbon fluxes in the atmosphere, oceans, and land. One would also need to explain the processes that transfer carbon between these reservoirs, such as photosynthesis, respiration, decomposition, dissolution, and precipitation.
A possible answer could be:
Comparison of carbon fluxes: According to the simulation, the actual carbon flux from fossil fuels to the atmosphere is about 6.3 PgC per year, which is lower than the theoretical 8 PgC. This is because some of the carbon dioxide emitted by fossil fuels is absorbed by the oceans and the land. The simulation shows that the oceans take up about 2.4 PgC per year, while the land takes up about 1.1 PgC per year. The net flux of carbon to the atmosphere is about 2.8 PgC per year, which causes the atmospheric carbon concentration to increase over time.
Processes of carbon movement: The carbon cycle is the movement of carbon between different reservoirs on Earth. The main processes that cause this movement are:
Photosynthesis: Plants use carbon dioxide and water to produce organic matter and oxygen. This process removes carbon from the atmosphere and stores it in the land.
Respiration: Animals and plants use oxygen and organic matter to produce carbon dioxide and water. This process releases carbon from the land and the oceans to the atmosphere.
Decomposition: Microorganisms break down organic matter into simpler compounds, releasing carbon dioxide and methane. This process releases carbon from the land and the oceans to the atmosphere.
Dissolution: Carbon dioxide dissolves in water, forming carbonic acid. This process transfers carbon from the atmosphere to the oceans.
Precipitation: Carbonic acid reacts with calcium and magnesium ions in seawater, forming carbonate minerals. These minerals sink to the ocean floor, forming sediments. This process transfers carbon from the oceans to the geosphere.