Carbon dioxide—\text {CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript—from the atmosphere is taken up by photosynthetic organisms and used to make organic molecules, which travel through food chains. In the end, the carbon atoms are released as \text {CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript in respiration.
Slow geological processes, including the formation of sedimentary rock and fossil fuels, contribute to the carbon cycle over long timescales.
Some human activities, such as burning of fossil fuels and deforestation, increase atmospheric \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript and affect Earth's climate and oceans.
