Air pollution, release into the atmosphere of various gases, finely divided solids, or finely dispersed liquid aerosols at rates that exceed the natural capacity of the environment to dissipate and dilute or absorb them. These substances may reach concentrations in the air that cause undesirable health, economic, or aesthetic effects.
Plants occur over Earth’s surface in well-defined patterns that are closely correlated with both climate and the history of the planet. Forests are the most important of these natural communities from the standpoint of area, carbon content, annual carbon fixation, the cycling of nutrient elements, and influence on energy and water budgets, as well as being the principal reservoir of biodiversity on land. The most extensive forests are the boreal coniferous forests of North America, Scandinavia, northern Europe, and northern Asia. The moist forests of the tropics are the most diverse, often containing as many as 100 species of tree.