GREENHOUSE GASES - DIGGING A LITTLE DEEPER
Atmospheric greenhouse gases play a critical role in shaping our global climate. Naturally-occurring trace gases in the atmosphere include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Human activities also impact the concentrations of these gases in the atmosphere. In addition, many commonly used industrial products, including solvents, adhesives, and pesticides, contain halocarbons which can also impact the climate. The full range of sources of greenhouses gases – both natural and anthropogenic – is not yet fully understood and continues to be the subject of both research and debate.
Water vapor is the most abundant of the greenhouse gases, and is the dominant contributor to the natural greenhouse effect. Human activity has little direct impact on the concentration of water vapor in the atmosphere; however, changes in its concentration are an indirect result of climate feedbacks related to the warming of the atmosphere.
As temperatures rise, more water evaporates from ground sources – rivers, oceans, etc. Because the air is warmer, the relative humidity can also be higher, leading to more water vapor in the atmosphere. Higher concentrations of water vapor are able to absorb more thermal infrared radiation from the Earth, further warming the atmosphere. The warmer atmosphere can then hold more water vapor, and the cycle continues. This is cycle is considered a positive feedback loop. However, uncertainty exists in both the extent and importance of this feedback loop. As water vapor increases in the atmosphere, more of it will also condense into clouds, which reflect incoming solar radiation away from the Earth’s surface thereby becoming a cooling force.
Carbon dioxide is released into the atmosphere through both natural and human processes. Natural production and absorption of carbon dioxide is primarily through the biosphere and the oceans via the carbon cycle. Human activities such as fuel burning (coal, oil, natural gas, and wood), cement production, and changes in land use have altered the natural carbon cycle by increasing the concentration of carbon dioxide in the atmosphere.
Carbon dioxide was the first greenhouse gas found to be increasing in atmospheric concentration with conclusive measurements made in the last half of the 20th century. Carbon dioxide levels as a component of the atmosphere have increased nearly 30 percent beginning from the late 18th century to the present time, and is now at approximately 370 parts per million (ppm) and rising. Prior to industrialization, carbon dioxide levels fluctuated near 280ppm, with seasonal variations as vegetation drew down carbon dioxide in the spring and summer for photosynthesis, releasing it in the fall and winter through decomposition.
Methane, which comes from both natural and human sources, is an extremely powerful warming agent – even more effective than carbon dioxide – however its lifetime in the atmosphere is brief, only about 12 years.
In nature, methane is released through biological processes in low oxygen environments, such as swamplands. Human activities, including growing rice, raising cattle, using natural gas and coal mining, are increasingly adding to the level of methane in the atmosphere. Since the beginning of the 19th century, methane levels have risen 150 percent, though the pattern of methane emissions is highly irregular and, since 1990, has leveled off for reasons that are unclear.
Nitrous Oxide, otherwise known as “laughing gas,” is a long-lived warming gas, persisting in the atmosphere for approximately 120 years. It is produced naturally from a wide variety of biological sources in both soil and water, particularly microbial action in wet tropical forests. Human-related sources of nitrous oxide include agricultural soil management, animal manure management, sewage treatment, combustion of fossil fuel, and the production of a variety of acids.
It is also important to account for the various interactions between natural processes and human influences in the nitrogen cycle, since human impacts can significantly enhance the natural processes that lead to N2O formation. For example, fertilizer use and nitrogen-loaded runoff into waterways can enhance nitrous oxide emissions from natural sources. Concentrations of nitrous oxide began to rise at the beginning of the industrial revolution, although emissions have been somewhat difficult to quantify on a global scale, primarily because it is one of the least studied greenhouse gases to date.
Ozone is a highly reactive molecule composed of three atoms of oxygen. Ozone concentrations vary by both geographic location and altitude. At lower levels in the trophosphere, ozone exerts a warming force upon the atmosphere, primarily due to human processes. Automobile emissions, industrial pollution, and the burning of vegetation increase the levels of carbon and nitrogen molecules which – when reacting to sunlight – produce ozone, an important contributor to photchemical smog. Levels of ozone have nearly doubled since the 1800s, and have increased nearly 30 percent since the industrial revolution.
In the stratosphere, a decrease in ozone concentration exerts a cooling force upon the atmosphere. Much of the decline in this stratospheric ozone can be attributed to the destructive action of CFCs (see below). As ozone contintues to contribute both the the warming and cooling of the atmosphere, its role in the overall enhancement of the greenhouse effect will continue to be difficult to determine.
Halocarbons are compounds of human origins used primarily as cooling agents, propellants, and cleaning solvents in a broad range of applications. The most familiar type of halocarbons are the chlorofluorocarbons (CFCs); however, since it was discovered that they destroy stratospheric ozone, they are continuing to be phased out under the terms of the Montreal Protocol. Although levels of CFCs are declining, their long atmospheric lifetimes assure that they will continue to contribute to the greenhouse effect for some time.
Another set of synthesized halocarbon compounds – created as substitutes to replace CFCs – are called HFCs (hydrofluorcarbons). While they are also greenhouse gases, they are less stable in the atmosphere and therefore have a shorter lifetime and less of an impact as a greenhouse gas. Also, at lower altitudes, halocarbons function as a warming gas; however, in the upper atmosphere, they exert a cooling impact through their interaction with ozone. Therefore, the ultimate impact of halocarbons on the greenhouse effect is highly uncertain.