What reservoir of the carbon cycle changes the most and the quickest?

The atmosphere is the reservoir of the carbon cycle that changes the most and the quickest. It can experience rapid fluctuations in carbon dioxide levels due to human activities, such as fossil fuel combustion and deforestation, as well as natural processes like respiration and photosynthesis. These changes can occur on short timescales, ranging from days to years, making the atmospheric carbon pool highly dynamic compared to other reservoirs like oceans and geological formations.