Global Warming

Global warming is caused by the increase of greenhouse gases in the atmosphere due to human activities. Greenhouse gases trap solar radiation (heat) near Earth's surface, causing Earth's temperature to rise.   Greenhouse gases are released mainly through three processes: burning fossil fuels, land use changes, and industrial processes. Fossil fuel combustion produces carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and hydrofluorocarbons (HFCs). Land use change includes deforestation, agricultural expansion, urbanization, and industrialization. Industrial processes produce various types of pollutants including sulfur oxides (SOx), nitrogen oxides (NOx), mercury, lead, cadmium, polychlorinated biphenyls (PCBs), and persistent pesticides.

Carbon dioxide is the primary greenhouse gas emitted from human activity. Methane is produced primarily from agriculture and rice cultivation, while nitrous oxide is formed from fertilizer production and manure management. Other significant greenhouse gases include HFCs, halons, perfluorocarbons, and SF6.  Greenhouse gases are classified according to their atmospheric lifetime. Short-lived greenhouse gases have lifetimes less than 10 years; intermediate-lifetime gases have lifetimes between 10 and 100 years; and long-lived gases have lifetimes greater than 100 years. Greenhouse gases are categorized as either anthropogenic (human generated) or natural. Anthropogenic greenhouse gases are those that are created directly by humans, such as CO2 and CH4. Natural greenhouse gases are those that occur naturally in the environment, such as water vapor and ozone.   There are two major categories of greenhouse gases: radiatively active and inactive. Radiative greenhouse gases are those that affect climate via direct heating of the earth's surface. Inactive greenhouse gases do not contribute to climate change. Examples of radiative greenhouse gases include carbon dioxide, methane, nitrous oxide, and water vapor. Radiative greenhouse gas concentrations vary over time, depending on how much sunlight reaches the earth's surface.

Greenhouse gases are measured in parts per million (ppm). One ppm equals 1 part per billion.  Greenhouse gases are regulated under the Clean Air Act Amendments of 1990. These amendments require the Environmental Protection Agency (EPA) to set national ambient air quality standards (NAAQS) for six greenhouse gases: carbon monoxide, non-methane volatile organic compounds (NMVOCs), methane, nitrogen oxides, tropospheric ozone, and fluorine.  Greenhouse gases are monitored at monitoring stations located throughout the United States. The EPA maintains a list of these stations on its website.  Greenhouse gases are also monitored at the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring Network. NOAA monitors greenhouse gases using instruments aboard ships and aircraft.

Greenhouse gases are continuously increasing in concentration in the atmosphere. Since 1980, the amount of carbon dioxide in the atmosphere has increased by 30%. The amount of methane in the atmosphere has increased approximately 50% since 2000. Greenhouse gases are transported around the world by winds. As greenhouse gases move across the ocean, they warm the upper layers of the ocean. Warm water then rises toward the surface, where it releases some of its heat back into the atmosphere. This cycle of rising and falling temperatures is called the thermohaline circulation.  Greenhouse gases are absorbed by the oceans and land surfaces. When greenhouse gases enter the oceans, they dissolve into seawater and become part of the marine biosphere. When greenhouse gases enter terrestrial ecosystems, they are converted into biomass and stored in soils.  Greenhouse gases are removed from the atmosphere by chemical reactions. Plants take in carbon dioxide and release oxygen. Microorganisms break down organic matter, releasing carbon dioxide.