• Are there other greenhouse gases?

What are greenhouse gases?

Scientists believe that rising concentrations of water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulphur hexafluoride (SF6) in the atmosphere are causing a warming of the earth's surface, which is having a destabilizing effect on our climate system. These gases, known as greenhouse gases, absorb and send infrared radiation back to the earth, causing the "greenhouse effect." (For more on this, visit U.S. EPA's Global Warming site.)

What causes global warming?

Global warming is an increase in the near surface temperature of the earth that is believed to result from a buildup of so-called greenhouse gases, which include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Although these gases are naturally occurring, particular human activities (anthropogenic sources) also generate certain greenhouse gases and contribute significantly to global warming. (For more on this, visit U.S. EPA's Global Warming site.)

What is methane?

Methane is a hydrocarbon that is a primary component of natural gas. Methane (CH4) is also a "greenhouse gas," meaning that its presence in the atmosphere affects the earth's temperature and climate system. Like carbon dioxide (CO2) and nitrous oxide (N2O), methane is a radiatively and chemically active trace gas. Methane is considered a trace gas because it is a minor atmospheric constituent (1.72 ppmv in 1990) compared to other gases such as nitrogen (N2) and oxygen (O2) (78 and 21 percent by volume, respectively). (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

Why is there concern about methane emissions?

As a contributor to global warming, methane is second only to carbon dioxide. Over the last two centuries, methane concentrations in the atmosphere have more than doubled, largely due to human-related activities. Increases in the concentration of methane in the atmosphere reduce the earth's cooling efficiency by causing more of the outgoing terrestrial radiation from the surface (i.e., of land masses) to be absorbed by the atmosphere and emitted at higher altitudes and colder temperatures, contributing to an enhanced greenhouse effect. (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

How is methane's atmospheric concentration determined?

The concentration of methane in the atmosphere is determined by the balance of the input rate and the removal rate. Input rates are increasing due to human activity. Removal rates are determined by the effectiveness of sinks. (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

Does methane have sinks?

The Intergovernmental Panel on Climate Change (IPCC) recognizes three sinks: oxidation by chemical reaction with tropospheric hydroxyl (OH), stratospheric oxidation, and microbial uptake by soils. Each of these three sinks plays a part in removing methane from the atmosphere. The strength and effectiveness of these sinks determine methane's atmospheric lifetime. Climatic conditions related to global warming and failure to adopt methane emissions reduction strategies could reduce the power of these sinks (i.e., cause a decline in the removal rate), increasing methane's lifetime. (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

Is the amount of methane in the atmosphere increasing?

Yes, but recent studies indicate that atmospheric methane concentrations may be reaching a steady state in the next 10 to 20 years if present trends continue (Dlugokencky et al., 1998). Earlier studies have shown that since 1800, atmospheric concentrations of methane have increased by 145 percent. Increases in the concentration of methane in the atmosphere roughly parallel world population growth. This rapid increase in methane concentrations also is confirmed by analyses of infrared solar spectra, which show that methane concentrations have increased by over 30 percent since 1951. Growth in methane concentrations began a slowing trend in 1992. Since 1993, methane's global growth rate has been between 2 and 8 ppbv/year. This is much slower than the 20 ppbv/year in the late 1970s and slightly less than the 9 to 13 ppbv/year during the 1980s. From 1994 to 1996, global methane concentrations have been estimated at 1,721, 1,728, and 1,730 ppbv/year, respectively. (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

Are methane emissions of worldwide concern?

Methane was recognized as a potent contributor to global warming in the Kyoto Protocol, and parties to the United Nation's Framework Convention on Climate Change acknowledged the need to stabilize methane emissions globally. The Kyoto Protocol specifically identifies methane and five other species of gas as needing to be stabilized in order to achieve the emissions targets of the industrialized world.

Industrialized countries that are members of the Organisation for Economic Co-operation and Development (OECD) account for about 20 percent of methane emissions world wide from anthropogenic sources. Developing countries and countries with economies in transition account for the other approximately 80 percent. U.S. EPA and other organizations are providing expertise to developing countries to help them develop their capacity to reduce methane emissions. (For more on this, see this site's Topic in Brief on Current Global Methane Emissions.)

Who are the biggest methane emitters?

In order of importance, China, the former Soviet Union, India, the United States, and Brazil are estimated to be responsible for almost half of all anthropogenic methane emissions. The key sources of methane emissions for these five emitters varies greatly. For example, the two key sources of methane emissions in China are coal and rice production, whereas the former Soviet Union (mainly Russia) emits most of its methane from natural gas and oil systems. India's primary sources are rice and livestock production, whereas landfills are the largest source of U.S. methane emissions. (For more on this, see this site's Topic in Brief on Current Global Methane Emissions.)

How does methane compare to carbon dioxide as a contributor to global warming?

The concept of global warming potential (GWP) has been developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. This measurement of GWP relies on carbon dioxide as the reference gas. Thus, the GWP of a greenhouse gas is the ratio of global warming (both direct and indirect), also known as radiative forcing, from one unit mass of a greenhouse gas to one unit mass of carbon dioxide over a period of time. Methane has a global warming potential of 21 over a 100-year period. This means that on a kilogram for kilogram basis, methane is 21 times more potent than carbon dioxide during this time period. (For more on this, see this site's Topic in Brief on Methane in the Atmosphere.)

Where does methane come from?

Methane is naturally occurring, but human-related (anthropogenic) activities such as fossil fuel production, animal husbandry (digestive processes of ruminant livestock and manure), rice cultivation, biomass burning, and waste management release significant quantities of methane, which have fundamentally altered the composition of the earth's atmosphere. Methane's natural sources include wetlands, gas hydrates, and permafrost. (For more on this, see this site's Topic in Brief on Sources of Methane.)

What efforts are being made to reduce methane emissions?

Around the world, industry, working in conjunction with government, has implemented emissions reduction strategies that have improved operational efficiency. In the United States, many companies are working with the U.S. Environmental Protection Agency in voluntary efforts to help achieve emissions reductions by implementing cost-effective management methods and technologies. (For more on this, see this site's Topic in Brief on Voluntary Programs.)

Why concentrate on actions to reduce methane now?

Because of its potency on a ton-by-ton basis, methane reductions have a larger impact on climate change than reductions in carbon dioxide. Additionally, methane has a relatively short lifetime--about 12 years compared to 120 years for carbon dioxide. For these reasons, programs and policies that target reductions in methane emissions can help mitigate the rate of climate change at a faster rate than those that target reductions in emissions of carbon dioxide and other longer-lived greenhouse gases. Another reason for acting now is that methane emissions are often associated with wasted energy. Capturing these emissions will improve operational efficiency, often saving firms money. (For more on this, see this site's Topic in Brief on Reducing Methane Emissions.)

Is there any beneficial use for methane?

Because methane is the primary component of natural gas, in many cases methane from anthropogenic sources can be captured and used for fuel. Thus, by capturing methane lost during normal operations, firms can reap the benefits of using this fuel source onsite, selling it to utilities, or selling it directly to end users while also benefiting the environment. For these reasons, many U.S. industries are participating in voluntary programs with U.S. EPA to target such emissions. EPA works closely with participating industries, providing technical information and helping them overcome permitting or regulatory barriers to making their operations more cost effective. (For more on this, see this site's Topic in Brief on Voluntary Programs.)

Are there other greenhouse gases?

Methane (CH4) and carbon dioxide (CO2) are only two of the six species of gases that need to be stabilized in order to mitigate the economic and environmental damages associated with global warming. Nitrous oxide (N2O) is another potent greenhouse gas that has both natural and anthropogenic sources. Several classes of halocarbons that contain fluorine and chlorine are also greenhouse gases. Fluorine-containing halocarbons include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Unlike methane, these greenhouse gases are generated exclusively by anthropogenic (human-related) activities. Currently, atmospheric concentrations of these gases are small relative to other greenhouse gases. These compounds, however, have the potential to greatly impact global warming due to their potency and extremely long atmospheric lifetimes. Because they remain in the atmosphere almost indefinitely, concentrations of these gases will increase as long as emissions continue. (For more on this, see this site's Topic in Brief on Other Greenhouse Gases.)