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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.