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Global Warming Potentials

Although there are a number of ways of measuring the strength of different greenhouse gases in the atmosphere, the Global Warming Potential (GWP) is perhaps the most useful, particularly as a policy instrument. GWPs measure the influence greenhouse gases have on the natural greenhouse effect, including the ability of greenhouse gas molecules to absorb or trap heat and the length of time greenhouse gas molecules remain in the atmosphere before being removed or broken down, the atmospheric lifetime. In this way, the contribution that each greenhouse gas has towards global warming can be assessed.

GWPs can also be used to define the impact greenhouse gases will have on global warming over different time periods or time horizons. These are usually 20 years, 100 years and 500 years. For most greenhouse gases, the GWP declines as the time horizon increases. This is because the greenhouse gas is gradually removed from the atmosphere through natural removal mechanisms, and its influence on the greenhouse effect declines. Some of the CFCs however, have long atmospheric lifetimes, and the 100-year GWP may be greater than the 20 year GWP.

Conventionally, the GWP of carbon dioxide, measured across all time horizons, is 1. The GWPs of other greenhouse gases are then measured relative to the GWP of carbon dioxide. Other greenhouse gases have much higher GWPs than carbon dioxide, but because their concentration in the atmosphere is much lower, carbon dioxide is still the most important greenhouse gas, contributing about 60% to the enhancement of the greenhouse effect.

Greenhouse gas

Global Warming Potential (GWP)

Time Horizon

20 years

100 years

500 years

Carbon dioxide

1

1

1

Methane

62

23

7

Nitrous oxide

275

296

156

CFC-12

7900

8500

4200

HCFC-22

4300

1700

520