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TABLE 13-1
Changing Atmospheric Composition

Species

Mean global concentration

Annual rate of increase during 1980s

 

Pre-industrial

Circa 1987

CO2

˜280 ppm

348 ppm

0.5%

CH4

˜600 ppb

1680 ppb

0.8%

N2O

˜ 285 ppb

307 ppb

0.2%

CFCl3

0

240 ppt

4%

CF2C12

0

415 ppt

4%

CCl4

0

140 ppt

1.5%

CH3CC13

0

150 ppt

4%

CH3Cl

600 ppt?

600 ppt

˜0%

CO

?

90 ppb

˜1% (northern hemisphere) <1% (southern hemisphere)

Source: WMO (1990)

These gases act as greenhouse gases that contribute to the radiative forcing of the atmosphere, increasing the radiative forcing at the tropopause by about 0.5 watts/meter2 over the past decade. The record of global mean surface temperature exhibits fluctuations, but with an apparent increasing trend (Figure 13-1). Global mean surface air temperatures have increased by as much as 0.3ºC to 0.5ºC this century (Hansen and Lebedeff, 1988; Jones, 1988). The temperature trend for the United States is more ambiguous because of the smaller sampling area, but also shows a temperature increase, albeit smaller, about 0.1ºC to 0.3ºC (Hansen et al., 1989).

Column ozone has been decreasing over the past 2 decades in both hemispheres (see UNEP/WMO, 1990).2 The decrease in stratospheric ozone

2Column ozone is the abundance of ozone, predominantly stratospheric, that is obtained by integrating the amount of atmospheric ozone in the vertical direction.



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