Think about it: Since hard-shelled organisms evolved about 550 million years ago, billions of tons of limestone rock have been produced from their shells. Limestone is made of calcium carbonate, with the formula CaCO3. Based on this, how do you think the amount of atmospheric CO2 has changed in the last 550 million years, and how has this affected Earth's climate? Explain your answer.

Hard-shelled organisms absorbed calcium and carbonate iron and use them in making their hard shells which are calcium carbonate. When these organisms die, there shells are used in the formation of limestone rock. Limestone rocks are a huge store carbon. When acid rain reacts with limestone, these trapped carbon are released to the atmosphere thereby causing an Increase in atmospheric carbon.

Conclusively, the accumulation of limestone rock and subsequent reactions with acids such as acid rain has resulted in an increased atmospheric carbon throughout the years.