Greenhouse Gases and Global Warming Potential

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The Global Warming Potential of Key Greenhouse Gases

Consumption of our fossil fuels[1] has allowed society to prosper, but burning fossil fuels is not a harmless, cost-free activity.[2] Some of the pollutants created by burning fossil fuels are inherently harmful and impose external costs on society.  For instance, sulfur in fossil fuels, when burned, is emitted as sulfur dioxide (SO2), which causes adverse respiratory effects and can be converted into acidic compounds that fall to the earth as acid precipitation.  High temperature combustion results in the creation of nitrogen oxides (NOx), which can be noxious in their own right, and when combined with volatile organic compounds, humidity, and sunlight can result in ground level (tropospheric) ozone (O3), the major component of smog, with its adverse health effects.  Burning fossil fuels can also release soot and fine particulates, which pose a health risk to people with asthma, and which can carry heavy metals, SO2, mercury and carcinogens into human lungs.  These pollutants also have adverse effects on human health and ecosystems worldwide.

Other emissions from fossil fuel combustion, such as carbon dioxide (CO2), are themselves benign (unless present in a high enough concentration to cause asphyxiation.)  However, in the atmosphere, CO2, together with water vapor, methane (CH4), nitrous oxide (N2O) and other trace gases, such hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), have the ability to trap heat in the atmosphere.

Water vapor is the largest natural contributor to the greenhouse effect. Human activity increases atmospheric water vapor concentration indirectly by the emission of other greenhouse gases, such as carbon dioxide that warm the atmosphere, thereby increasing the rate of evaporation; this increased evaporation increases the amount of water vapor in the atmosphere, which further accelerates global warming.

Methane (CH4), the major component of natural gas, is anthropogenically released into the atmosphere from coal mining, leaking natural gas pipelines, ruminant livestock such as cows, rice paddies, and solid waste facilities. Nitrous oxide (N2O) is produced both naturally in soil and water, and by human activity in agriculture, energy, industrial and waste management activities.

HFCs are non-stratospheric ozone depleting chemicals that are used as a replacement for stratospheric ozone depleting chemicals known as chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), methyl chloroform, carbon tetrachloride, bromine halons, methyl bromine and hydrobromofluorocarbons, that are regulated under the Montreal Protocol on Substances that Deplete the Stratospheric Ozone Layer and its Amendments. PFCs and SF6 are emitted by aluminum smelting, semiconductor manufacturing, electric power transmission and magnesium casting.  Taken together HFCs. PFCs, and SF6, only account for a small portion of global warming; however, these powerful greenhouse gases have extremely long lifetimes in the atmosphere and are being emitted in growing quantities.

Each greenhouse gas has a different ability to absorb infrared radiation. This heat trapping capacity is generally expressed in terms of “global warming potential,” which “is an index, based upon radiative properties of well-mixed greenhouse gases, measuring the radiative forcing of a unit of mass of a given well-mixed greenhouse gas in the present day atmosphere integrated over a chosen time horizon, relative to carbon dioxide.” Intergovernmental Panel on Climate Change Fourth Assessment Report (FAR), Climate Change 2007: The Physical Science Basis (2007) 946.  (IPCC FAR) Radiative forcing is calculated in terms of watts of solar energy per square meter. A positive forcing, such as that produced by increasing concentrations of greenhouse gases, such as carbon dioxide, methane and nitrous oxide, tends to warm the earth’s surface.  A negative forcing, which can arise from an increase in aerosols such as sulfur dioxide that reflect light from the sun back out into space, or volcanic ash, soot, or carbon black that block sun light, tends to cool the earth.  IPCC FAR (Science) at 31-35.

Greenhouse gases become well-mixed in the atmosphere with weeks of being emitted.  Therefore, for mitigation of global warming purposes, it does not matter where or by whom the gas is emitted or where and by whom the gases are sequestered (permanently removed from the atmosphere).  What matters is the total radiative forcing attributable to the greenhouse gases in the atmosphere.  By creating a global warming potential (GWP) scale the relative contribution of each greenhouse gas can be compared.  The scale sets CO2 as 1.  All other GHGs are then rated in their radiative forcing relative to CO2.   Thus, we can say that one ton of methane as the equivalent of 21 tons of carbon dioxide. We express this relationship using the term carbon dioxide equivalence or CO2e. CO2e is the foundation for creating a market-based cap and trade system that could include all GHGs.

Global Warming Potentials

(selected greenhouse gases)

Greenhouse Gas

Chemical formula

Lifetime (years)

Global Warming Potential (Time Horizon – years)

Major GHGs

100  years (UNFCCC)#

20 years

100  years

500 years

Carbon dioxide

CO2

variable§

1

1

1

1

Methane *

CH4

12

21

72

25

7.6

Nitrous oxide

N2O

310

310

289

298

153

Hydrofluorocarbons

HFC-23

CHF3

264

11,700

12,000

14,800

12,200

HFC-32

CH2F2

5.6

650

2,330

675

205

HFC-125

C2HF5

32.6

2,800

6,350

3500

1,00

HFC-134a

CH2FCF3

14

1,300

3,830

1,430

435

HFC-143a

C2H3F3

52

3,800

5,890

4,470

1,590

HFC-152a

C2H4F2

1.4

140

437

124

38

HFC-236fa

C3H2F6

240

6,300

8,100

9,810

7,660

Perflourinated compounds

Sulphur hexafluoride

SF6

3200

23900

16300

22,800

32,600

Nitrogen triflouride

NF3

740

12,300

17,200

20,700

Perfluoromethane

CF4

50,000

6500

5,210

7,390

11,200

Perfluoroethane

C2F6

10,000

9200

8,630

12,200

18,200

Other

Methyl chloride

CH3Cl

1

45

13

4

Source: IPCC FAR (Science) 33-34, Table TS.2  The full table also includes substances controlled by the Montreal Protocol, other hydrofluorocarbons, perfluorinated compounds, fluorinated ethers, perfluoropolyethers, and a few other compounds.  § Derived from the Bern carbon cycle model. See IPCC FAR (Science) Ch. 10.  CO2 atmospheric lifetimes depend on the rate at which oceans and plants remove it from the atmosphere.  If not removed by natural processes a molecule of CO2 can remain in the atmosphere for centuries. For more detailed information about GWP see, http://cdiac.ornl.gov/pns/current_ghg.html * The GWP for methane includes indirect effects of tropospheric ozone production and stratospheric water vapor production.  # National greenhouse gas emission reports made under the UNFCCC use the global warming potentials contained in the IPCC’s Second Assessment Report (SAR), IPCC, The Science of Climate Change: Summary for Policymakers and Technical Summary of the Working Group I Report (1995).

Each GWP varies based on the time horizon that is selected.  This is because each gas has its own average lifetime in the atmosphere.  Although water vapor is a powerful GHG, it is not given a GWP because it is not emitted—water vapor levels are directly related to temperature, which varies based upon the concentration of other GHGs in the atmosphere.

The UNFCCC national reporting system currently uses the GWP established in 1995, as does the Kyoto Protocol.  In the intervening decade science has improved its understanding of the atmospheric characteristics of each gas.  The IPCC FAR (2007) presents different lifetimes based on these deeper understandings of these gases and improved atmospheric modeling.  Generally, the 2007 GWPs are higher than the 1995 numbers.  This means each unit of GHG is warming the earth more than was thought in 1995, which implies that greater reductions will be needed to meet goals established using the 1995 GWP.

The table also shows that the time horizon chosen can make a substantial difference in the GWP of a particular gas.  The UNFCCC uses a 100 year time horizon.? The IPCC has just begun a review of GWPs and other possible metrics as part of its preparation of AR5.  See IPCC Working Group I Technical Support Unit, Meeting Report of the Expert Meeting on the Science of Alternative Metrics (2009) at http://www.ipcc.ch/pdf/supporting-material/expert-meeting-metrics-oslo.pdf


[1] Humans also consume other carbon-based sources of energy, especially wood.  Large portions of developing countries rely on wood for fuel, either directly, or after converted into charcoal.  In those regions, so much wood is used so inefficiently as fuel that demand for wood far exceeds the rate that forests can be regenerated.  However, compared to fossil fuels, forest can be regrown in a relatively short time (decades to a century for forests compared to tens of millions of years for fossil fuels).  United Nations Development Programme, World Energy Assessment: Energy and the Challenge of Sustainability (Jose Goldemberg et al, eds. 2000) 65-68, 370 (World Energy Assessment.).

[2] Nor is burning wood or charcoal harmless.  The indoor pollution from using wood for heating and cooking and the increasing shortage of locally available of wood increases poverty and diminishes public health. Id. at 68-69.

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