What atmospheric gas is increasing by about 0.5 percent per year?

The correct choice reflects the current trends in atmospheric science, where carbon dioxide is indeed increasing at a rate of approximately 0.5 percent per year. This rise is primarily due to human activities, particularly the burning of fossil fuels and deforestation, which release significant amounts of CO2 into the atmosphere.

Carbon dioxide is a greenhouse gas, meaning it has the ability to trap heat in the atmosphere, contributing to global warming and climate change. Its increasing concentration is a major concern for environmental scientists, as it can lead to a variety of ecological and atmospheric changes.

In contrast, ozone, nitrogen, and oxygen do not exhibit the same increasing trend as carbon dioxide. Ozone, while present in the atmosphere, is influenced by different factors such as photochemical reactions and does not have a consistent annual growth rate like CO2. Nitrogen, which makes up a significant portion of the Earth's atmosphere, is relatively stable and does not show notable increases. Oxygen levels, while they can fluctuate slightly due to changes in carbon cycling, also remain relatively constant over time.