Other Factors Affecting The Greenhouse Effect

Aerosols, also known as particulates, are airborne particles that absorb, scatter, and reflect radiation back into space. Clouds, windblown dust, and particles that can be traced to erupting volcanoes are examples of natural aerosols. Human activities, including the burning of fossil fuels and slash-and-burn farming techniques used to clear forestland, contribute additional aerosols to the atmosphere. Although aerosols are not considered a heat-trapping greenhouse gas, they do affect the transfer of heat energy radiated from the Earth to space. The effect of aerosols on climate change is still debated, but scientists believe that light-colored aerosols cool the Earth’s surface, while dark aerosols like soot actually warm the atmosphere. The increase in global temperature in the last century is lower than many scientists predicted when only taking into account increasing levels of carbon dioxide, methane, nitrous oxide, and fluorinated compounds. Some scientists believe that aerosol cooling may be the cause of this unexpectedly reduced warming.

However, scientists do not expect that aerosols will ever play a significant role in offsetting global warming. As pollutants, aerosols typically pose a health threat, and the manufacturing or agricultural processes that produce them are subject to air-pollution control efforts. As a result, scientists do not expect aerosols to increase as fast as other greenhouse gases in the 21st century.

Understanding The Greenhouse Effect

Although concern over the effect of increasing greenhouse gases is a relatively recent development, scientists have been investigating the greenhouse effect since the early 1800s. French mathematician and physicist Jean Baptiste Joseph Fourier, while exploring how heat is conducted through different materials, was the first to compare the atmosphere to a glass vessel in 1827. Fourier recognized that the air around the planet lets in sunlight, much like a glass roof.

In the 1850s British physicist John Tyndall investigated the transmission of radiant heat through gases and vapors. Tyndall found that nitrogen and oxygen, the two most common gases in the atmosphere, had no heat-absorbing properties. He then went on to measure the absorption of infrared radiation by carbon dioxide and water vapor, publishing his findings in 1863 in a paper titled “On Radiation Through the Earth’s Atmosphere.”

Swedish chemist Svante August Arrhenius, best known for his Nobel Prize-winning work in electrochemistry, also advanced understanding of the greenhouse effect. In 1896 he calculated that doubling the natural concentrations of carbon dioxide in the atmosphere would increase global temperatures by 4 to 6 Celsius degrees (7 to 11 Fahrenheit degrees), a calculation that is not too far from today’s estimates using more sophisticated methods. Arrhenius correctly predicted that when Earth’s temperature warms, water vapor evaporation from the oceans increases. The higher concentration of water vapor in the atmosphere would then contribute to the greenhouse effect and global warming.

The predictions about carbon dioxide and its role in global warming set forth by Arrhenius were virtually ignored for over half a century, until scientists began to detect a disturbing change in atmospheric levels of carbon dioxide. In 1957 researchers at the Scripps Institution of Oceanography, based in San Diego, California, began monitoring carbon dioxide levels in the atmosphere from Hawaii’s remote Mauna Loa Observatory located 3,000 m (11,000 ft) above sea level. When the study began, carbon dioxide concentrations in the Earth’s atmosphere were 315 molecules of gas per million molecules of air (abbreviated parts per million or ppm). Each year carbon dioxide concentrations increased—to 323 ppm by 1970 and 335 ppm by 1980. By 1988 atmospheric carbon dioxide had increased to 350 ppm, an 8 percent increase in only 31 years.
As other researchers confirmed these findings, scientific interest in the accumulation of greenhouse gases and their effect on the environment slowly began to grow. In 1988 the World Meteorological Organization and the United Nations Environment Programme established the Intergovernmental Panel on Climate Change (IPCC). The IPCC was the first international collaboration of scientists to assess the scientific, technical, and socioeconomic information related to the risk of human-induced climate change. The IPCC creates periodic assessment reports on advances in scientific understanding of the causes of climate change, its potential impacts, and strategies to control greenhouse gases. The IPCC played a critical role in establishing the United Nations Framework Convention on Climate Change (UNFCCC). The UNFCCC, which provides an international policy framework for addressing climate change issues, was adopted by the United Nations General Assembly in 1992.

Today scientists around the world monitor atmospheric greenhouse gas concentrations and create forecasts about their effects on global temperatures. Air samples from sites spread across the globe are analyzed in laboratories to determine levels of individual greenhouse gases. Sources of greenhouse gases, such as automobiles, factories, and power plants, are monitored directly to determine their emissions. Scientists gather information about climate systems and use this information to create and test computer models that simulate how climate could change in response to changing conditions on the Earth and in the atmosphere. These models act as high-tech crystal balls to project what may happen in the future as greenhouse gas levels rise. Models can only provide approximations, and some of the predictions based on these models often spark controversy within the science community. Nevertheless, the basic concept of global warming is widely accepted by most climate scientists.

Efforts To Control Greenhouse Gases

Due to overwhelming scientific evidence and growing political interest, global warming is currently recognized as an important national and international issue. Since 1992 representatives from over 160 countries have met regularly to discuss how to reduce worldwide greenhouse gas emissions. In 1997 representatives met in Kyōto, Japan, and produced an agreement, known as the Kyōto Protocol, which requires industrialized countries to reduce their emissions by 2012 to an average of 5 percent below 1990 levels. To help countries meet this agreement cost-effectively, negotiators are trying to develop a system in which nations that have no obligations or that have successfully met their reduced emissions obligations could profit by selling or trading their extra emissions quotas to other countries that are struggling to reduce their emissions. Negotiating such detailed emissions trading rules has been a contentious task for the world community since the signing of the Kyōto Protocol. A ratified agreement is still not yet in force, and ratification received a setback in 2001 when newly elected U.S. president George W. Bush renounced the treaty on the grounds that the required carbon-dioxide reductions in the United States would be too costly. He also objected that developing nations would not be bound by similar carbon-dioxide reducing obligations. However, many experts expect that as the scientific evidence about the dangers of global warming continues to mount, nations will be motivated to cooperate more effectively to reduce the risks of climate change.

Ozone Layer

Ozone Layer, a region of the atmosphere from 19 to 48 km (12 to 30 mi) above the earth's surface. Ozone concentrations of up to 10 parts per million occur in the ozone layer. The ozone forms there by the action of sunlight on oxygen. This action has been taking place for many millions of years, but naturally occurring nitrogen compounds in the atmosphere apparently have kept the ozone concentration at a fairly stable level. Concentrations this great at ground level are dangerous to breathe and can damage the lungs. However, because the ozone layer of the atmosphere protects life on earth from the full force of the sun's cancer-causing ultraviolet radiation, it is critically important. Thus, scientists were concerned when they discovered in the 1970s that chemicals called chlorofluorocarbons, or CFCs (see Fluorine)—long used as refrigerants and as aerosol spray propellants—posed a possible threat to the ozone layer. Released into the atmosphere, these chlorine-containing chemicals rise and are broken down by sunlight, whereupon the chlorine reacts with and destroys ozone molecules—up to 100,000 per CFC molecule. For this reason, the use of CFCs in aerosols has been banned in the United States and elsewhere. Other chemicals, such as bromine halocarbons, as well as nitrous oxides from fertilizers, may also attack the ozone layer. Destruction of the ozone layer is predicted to cause increases in skin cancer and cataracts, damage to certain crops and to plankton and the marine food web, and an increase in carbon dioxide (see Global Warming) due to the decrease in plants and plankton.

Beginning in the early 1980s, research scientists working in Antarctica have detected a periodic loss of ozone in the atmosphere high above that continent. The so-called ozone “hole,” a thinned region of the ozone layer, develops in the Antarctic spring and continues for several months before thickening again. Studies conducted with high-altitude balloons and weather satellites indicated that the overall percentage of ozone in the Antarctic ozone layer is actually declining. Flights over the Arctic regions found a similar problem developing there.

In 1987 the Montréal Protocol, a treaty for the protection of the ozone layer, was signed and later ratified by 36 nations, including the United States. A total ban on the use of CFCs during the 1990s was proposed by the European Community (now called the European Union) in 1989, a move endorsed by U.S. President George Bush. In December 1995 over 100 nations agreed to phase out developed countries' production of the pesticide methyl bromide, predicted to cause about 15 percent of ozone depletion by the year 2000. Production of CFCs in developed countries ceased at the end of 1995 and will be phased out in developing countries by 2010. Hydrochlorofluorocarbons, or HCFCs, which cause less damage to the ozone layer than CFCs do, are being used as substitutes for CFCs on an interim basis, until 2020 in developed countries and until 2016 in developing countries. To monitor ozone depletion on a global level, in 1991 the National Aeronautics and Space Administration (NASA) launched the 7-ton Upper Atmosphere Research Satellite. Orbiting earth at an altitude of 600 km (372 mi), the spacecraft measures ozone variations at different altitudes and is providing the first complete picture of upper atmosphere chemistry.

The World Meteorological Organization observed a 45 percent depletion of the ozone layer over one-third of the northern hemisphere, from Greenland to western Siberia, for several days during the winter of 1995-1996. The deficiency was believed to have been caused by chlorine and bromine compounds combined with polar stratospheric clouds formed under unusually low temperatures.

Ozone: Environmental Effects

Ozone at ground level is a health hazard, causing respiratory ailments such as bronchitis and asthma. It also damages vegetation and causes rubber and some plastics to deteriorate. Nitrogen oxides and volatile organic gases emitted by automobiles and industrial sources combine to form ozone. In 1998, the United States Environmental Protection Agency (EPA) implemented a new air rule designed to curb nitrogen oxides released by coal-fired electric power plants. Many cities issue public air quality warnings when ozone levels rise to dangerous levels. See also Pollution.

Ozone in the upper atmosphere, however, is vital to life. This ozone forms by the action of ultraviolet light from the Sun on molecules of ordinary oxygen. The ozone layer absorbs ultraviolet radiation so that much of it never reaches the ground. Certain industrial compounds cause ozone to break down, opening holes in the ozone layer and exposing life on the ground to dangerous levels of ultraviolet radiation. A single atom of chlorine, for example, floating about in the upper atmosphere, can destroy hundreds of thousands of molecules of ozone because the chlorine acts as a catalyst and is not itself altered in the process. See also Chlorofluorocarbons; Ozone Layer.