Oxygen Cycle

Atmospheric oxygen is essential to the development of the life found on Earth. Oxygen is the most abundant element by mass in the Earth’s crust, the most abundant by mass in the oceans, and the second most abundant in the Earth’s atmosphere. Thus understanding the biogeochemistry of oxygen, its reservoirs, and the fluxes between these reservoirs is essential. Molecular oxygen maintains a nearly constant amount in the atmosphere because of the balance between emission via photosynthesis and its removal through consumption via respiration by humans, animals, and plants. A small, observed decrease in atmospheric oxygen over recent decades appears to be tied to changes in the atmospheric carbon dioxide and its feedbacks on the biosphere. The other important atmospheric gas in the oxygen cycle is ozone, O 3. Roughly 90% of atmospheric ozone is in the stratosphere, the layer of the atmosphere from about 10 to 50 km above the Earth’s surface. Ozone in the stratosphere is often called ‘good’ ozone because this layer protects life on Earth from harmful levels of ultraviolet (UV) radiation from the Sun. Human activities, particularly the production, use, and emissions of a variety of chorine- and bromine-containing halocarbons have resulted in a significant reduction in the amount of stratospheric ozone over the last several decades, with resulting increases in UV. Increases in lower atmospheric ‘bad’ ozone are also of concern because of ozone’s direct effects on humans, crops, and ecosystems.