299
tion of radiation, while variation in the ellipticity of the orbit affects the
annual variability of the incoming radiation. Milankovic (1930) and later
Berger (1980) have carefully calculated this effect and have been able to
demonstrate that it is at least qualitatively able to explain the glaciation
cycle. The second source of variability is due to changes in the total solar
irradiance. There are some changes in the near ultraviolet part of the solar
spectrum which are taking place over the ll-year sunspot cycle. These
changes are generally insignificant and induce variations of no more than
0.01 % in the total irradiance. Of greater importance, in terms of the direct effect on climate, are changes affecting the total irradiance. Spaceborn
measurements available for the last decade indicate changes of the order
of 1 Wjm 2 in the value of the solar constant, Foukal and Lean (1990).
However, it is important to stress that only ca. 17% of this amount will
enter the climate system. The question of larger changes over longer periods has been discussed considerably, see for example Marshall Institute
(1989), but no conclusive evidence has yet been provided. Such evidence
must in any case be based on real observations of the irradiance or based
upon verifiable theories of the energetics of the solar atmosphere. To use
proxy climate data, say from the last several hundred years, to postulate
changes in the solar constant is a fallacy, since climate changes over the
last few hundred years can be explained by natural, inherent fluctuations
in the climate system.
3.2
Changes in the greenhouse gases
Concentration of carbon dioxide and methane during the last 160,000 years
has been determined from the core measurements from Vostok, Antarctica (Chappelaz et al., 1990). These measurements indicate considerable
long term variations. They are also broadly correlated by the estimated
temperature changes, in turn based on the measured 18 0 oxygen-isotope
concentration. At the end of the last glaciation the concentration of CO2
and CH4 did grow to their preindustrial values 280 ppm and 0.65 ppm,
respectively. These values then hardly varied, and certainly not during
the millennium prior to the beginning of the industrial age around 1750.
The CO2 concentration since then has increased virtually exponentially
and amounts presently to 365 ppm (1996).
Table 1 gives some key data for the most important greenhouse gases,
their present concentration, their present annual increase, their residence
tion of radiation, while variation in the ellipticity of the orbit affects the
annual variability of the incoming radiation. Milankovic (1930) and later
Berger (1980) have carefully calculated this effect and have been able to
demonstrate that it is at least qualitatively able to explain the glaciation
cycle. The second source of variability is due to changes in the total solar
irradiance. There are some changes in the near ultraviolet part of the solar
spectrum which are taking place over the ll-year sunspot cycle. These
changes are generally insignificant and induce variations of no more than
0.01 % in the total irradiance. Of greater importance, in terms of the direct effect on climate, are changes affecting the total irradiance. Spaceborn
measurements available for the last decade indicate changes of the order
of 1 Wjm 2 in the value of the solar constant, Foukal and Lean (1990).
However, it is important to stress that only ca. 17% of this amount will
enter the climate system. The question of larger changes over longer periods has been discussed considerably, see for example Marshall Institute
(1989), but no conclusive evidence has yet been provided. Such evidence
must in any case be based on real observations of the irradiance or based
upon verifiable theories of the energetics of the solar atmosphere. To use
proxy climate data, say from the last several hundred years, to postulate
changes in the solar constant is a fallacy, since climate changes over the
last few hundred years can be explained by natural, inherent fluctuations
in the climate system.
3.2
Changes in the greenhouse gases
Concentration of carbon dioxide and methane during the last 160,000 years
has been determined from the core measurements from Vostok, Antarctica (Chappelaz et al., 1990). These measurements indicate considerable
long term variations. They are also broadly correlated by the estimated
temperature changes, in turn based on the measured 18 0 oxygen-isotope
concentration. At the end of the last glaciation the concentration of CO2
and CH4 did grow to their preindustrial values 280 ppm and 0.65 ppm,
respectively. These values then hardly varied, and certainly not during
the millennium prior to the beginning of the industrial age around 1750.
The CO2 concentration since then has increased virtually exponentially
and amounts presently to 365 ppm (1996).
Table 1 gives some key data for the most important greenhouse gases,
their present concentration, their present annual increase, their residence