Global Warming Potential

The different types of greenhouse gases all have different properties. For example, the amount of time they reside in the atmosphere and the amount of heat that they trap can vary widely.

Many of the greenhouse gases are extremely potent—some can continue to reside in the atmosphere for thousands of years after they have been emitted. According to the U.S. Environmental Protection Agency (EPA), some greenhouse gases are 140 to 23,900 times more potent than CO2 in terms of their ability to trap and hold heat in the atmosphere over a 100-year period. It is important to note that these gases and their effects will continue to increase in the atmosphere as long as they continue to be emitted. Even though these gases represent a very small proportion of the atmosphere—less than 2 percent of the total—their enormous heat-holding potential makes them significant and represents a serious addition to global warming.

In order to understand specific greenhouse gases' potential impact, they are rated as to their global warming potential (GWP). The GWP of a greenhouse gas is the ratio of global warming—or radiative forcing— from one unit mass of a greenhouse gas to that of one unit mass of CO2 over a period of time, making the GWP a measure of the "potential for global warming per unit mass relative to carbon dioxide." In other words, greenhouse gases are rated on how potent they are compared to CO2.

GWPs take into account the absorption strength of a molecule and its atmospheric lifetime. Therefore, if methane has a GWP of 23 and carbon has a GWP of 1 (the standard), this means that methane is 23 times more powerful than CO2 as a greenhouse gas. The Intergovern-ment Panel on Climate Change (IPCC) has published reference values for GWPs of several greenhouse gases. Reference standards are also issued and supported by the United Nations Framework Convention on Climate Change (UNFCCC), as shown in the table on the next page.

The higher the GWP value, the larger the infrared absorption and the longer the atmospheric lifetime. Based on this table, even small amounts of sulfur hexafluoride and HFC-23 can contribute a significant amount to global warming.

The EPA has identified three major groups of high GWP gases: (1) hydro fluorocarbons, (2) perfluorocarbons (PFCs), and (3) sulfur hexafluoride. These represent the most potent greenhouse gases. Also, PFCs and sulfur hexafluoride have extremely long atmospheric lifetimes—up to 23,900 years. Since their lifetimes are so incredibly long, for practical purposes once they are emitted into the atmosphere, they are considered to be there permanently. According to the EPA, once they are present in the atmosphere, the result is "an essentially irreversible accumulation."

Hydrofluorocarbons are man-made chemicals. Most of them were developed as replacements for the ozone-depleting substances that were common in industrial, commercial, and consumer products. The GWP index for HFCs ranges from 140 to 11,700. Their lifetime in the atmosphere ranges from one to 260 years; the most commonly used ones have a lifetime of about 15 years and are used in automobile air-conditioning and refrigeration.

Perfluorocarbons generally originate from the production of aluminum and semiconductors. PFCs have very stable molecular structures and usually do not get broken down in the lower atmosphere. When they reach the mesosphere 37 miles (60 km) above the Earth's surface, high-energy ultraviolet electromagnetic energy destroys them, but it is

Global Warming Potential of Greenhouse Gases

GREENHOUSE GAS

LIFETIME IN THE ATMOSPHERE

GWP OVER 100 YEARS (COMPARED TO CO2)

carbon dioxide

50-200 years

1

methane

12 years

23

nitrous oxide

120 years

296

CFC 115

550 years

7,000

HFC-23

264 years

11,700

HFC-32

5.6 years

650

HFC-41

3.7 years

150

HFC-43-10mee

17.1 years

1,300

HFC-125

32.6 years

2,800

HFC-134

10.6 years

1,000

HFC-134a

14.6 years

1,300

HFC-152a

1.5 years

140

HFC-143

3.8 years

300

HFC-143a

48.3 years

3,800

HFC-227ea

36.5 years

2,900

HFC-236fa

209 years

6,300

HFC-245ca

6.6 years

560

sulfur hexafluoride

3,200 years

23,900

perfluoromethane

50,000 years

6,500

perfluoroethane

10,000 years

9,200

perfluoropropane

2,600 years

7,000

perfluorobutane

2,600 years

7,000

perfluorocyclobutane

3,200 years

8,700

perfluoropentane

4,100 years

7,500

perfluorohexane

3,200 years

7,400

Source: UNFCCC

a very slow process, which enables them to accumulate in the atmosphere for several thousand (up to 50,000) years.

Sulfur hexafluoride has a GWP of23,900, making it the most potent greenhouse gas. It is used in insulation, electric power transmission equipment, the magnesium industry, semiconductor manufacturing to create circuitry patterns on silicon wafers, and as a tracer gas for leak detection. Its accumulation in the atmosphere shows the global average concentration has increased by 7 percent per year during the 1980s and 1990s—according to the IPCC, from less than one part per trillion (ppt) in 1980 to almost four ppt in the late 1990s.

In response to global warming, the EPA is working to reduce the emission of gases with high GWP because of their potency and long lifetimes. Major emission sources of high-GWP gases are from industries such as electric power generation, magnesium production, semiconductor manufacturing, and aluminum production.

In electric power generation, sulfur hexafluoride is used in circuit breakers, gas-insulated substations, and switchgear. During magnesium metal production and casting, sulfur hexafluoride serves as a protective cover gas during processing. It improves safety and metal quality by preventing the oxidation and potential burning of molten magnesium in the presence of air. It replaced sulfur dioxide, which was more environmentally toxic. The semiconductor industry uses many high GWP gases in plasma etching and in cleaning chemical vapor deposition tool chambers. They are used to create circuitry patterns. During primary aluminum production, GWP gases are emitted as by-products of the smelting process.

The best solution found to date to ameliorate the negative impact on the environment and to combat global warming is through the EPA working with private industry to develop and implement new processes that are better for the environment. In addition, the EPA is working to limit high GWP gases through mandatory recycling programs and restrictions.

If a greenhouse gas can remain in the atmosphere for several hundred years, even though it may be in a small amount, it can do a substantial amount of damage. Some of the greenhouse effect today is due to greenhouse gases put in the atmosphere decades ago. Even trace amounts can add up significantly.

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