To determine which change in the carbon cycle would have the greatest effect on the amount of carbon dioxide in the atmosphere, let's evaluate each option:
A. A 20% increase in land used for agriculture: This may lead to more carbon dioxide being emitted, especially if forests or natural ecosystems are replaced by agricultural land, which generally has lower carbon storage capacity.
B. A 10% decrease in the burning of fossil fuels: This would reduce the amount of carbon dioxide released into the atmosphere from one of the largest sources of human-caused emissions.
C. A 20% decrease in the number of photosynthetic organisms: Photosynthetic organisms, such as plants and phytoplankton, absorb carbon dioxide during photosynthesis. A decrease in their numbers would likely lead to a significant increase in atmospheric carbon dioxide as less CO2 would be absorbed.
D. A 10% increase in the amount of decomposing organic matter: Decomposition releases carbon dioxide back into the atmosphere. An increase in decomposition could contribute to higher levels of atmospheric CO2.
Among these options, C (a 20% decrease in the number of photosynthetic organisms) would likely have the greatest effect on the amount of carbon dioxide in the atmosphere. This is because it would reduce the natural absorption of CO2, allowing more CO2 to accumulate in the atmosphere.