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Images
figure 7
Decadal contributions to radiative forcing due to increases in greenhouse gas concentrations for periods
between 1765 and 1990. Houghton, J.T., G.J. Jenkins, J.J. Ephraums, eds, 1990: 1990 Intergovernment Panel on
Climate Change, Cambridge University Press.Figure 2.3, page 55.
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The insidious characteristic of most greenhouse gases is that they
have long lifetimes in the atmosphere, as measured by their "half life"
(the time for half of an initial amount released to be removed from the
atmosphere by natural processes). Carbon dioxide has a half life of about
120 years, methane 10.5 years, nitrous oxide 132 years, and the CFCs 16 to
more than 500 years, as shown in the accompanying table. So, for instance,
of the 20,000 kg of carbon dioxide you put into the atmosphere in 1996,
10,000 kg will still be contributing to enhanced greenhouse warming in the
year 2116, 5,000 kg will be remaining in 2236, 2,500 kg in 2356, ..., 1 kg
in the year 3676. At least 1 kg of carbon dioxide you put into the
atmosphere this past year will contribute to enhanced greenhouse warming
for the next 1,680 years!
From these examples, you can see that there are two factors that
combine to determine the "global warming potential" (GWP) of a greenhouse
gas: (1) radiation absorbing capacity, and (2) lifetime in the atmosphere.
We will come back to GWP in a later lecture.
Calculations based on these considerations are used to produce figure 7 that takes into account the effect of greenhouse gases
that humans have put into the atmosphere since the Industrial Revolution.
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Decadal contributions to radiative forcing due to increases in greenhouse gas concentrations for periods
between 1765 and 1990. Houghton, J.T., G.J. Jenkins, J.J. Ephraums, eds, 1990: 1990 Intergovernment Panel on
Climate Change, Cambridge University Press.Figure 2.3, page 55.