Final answer:
Carbon dioxide sinks in air due to its greater density compared to air and behaves cyclically in the atmosphere with increased levels from fossil fuel burning contributing to global warming. The increase and decrease in levels are not linear due to complex atmospheric interactions.
Step-by-step explanation:
Carbon dioxide does not rise and fall in a straight line because of the physical properties governing its behavior in the atmosphere, its interaction with solar radiation, and the impact of human activities. Unlike lighter gases such as hydrogen, carbon dioxide has a density greater than air, causing it to sink when released. The phase diagram for carbon dioxide shows that it cannot exist as a liquid under ambient pressure conditions and sublimes directly from solid to gas at 1 atm.
Furthermore, atmospheric carbon dioxide levels fluctuate cyclically due to processes such as photosynthesis and respiration. However, the burning of fossil fuels has led to an unprecedented increase in CO₂ levels, contributing to global warming. The addition of CO₂ raises the altitude at which the atmosphere emits radiation to space, and to maintain the balance of incoming and outgoing radiation, the Earth's surface temperature must increase.
Finally, measurements such as those from the Keeling Curve also show the increase is not linear but logarithmic, with evidence pointing to rising carbon dioxide concentrations being closely tied to increasing global temperatures.