First Assessment Report - IPCC

2 Radiatn e Foi c mg of Climate 61
Table 2.9: Example of use of Global Waiminq Potentials
The table shows the mtegiated effects o\ei a 100 yeai time
hoiizon of total emissions in 1990 i>i\en as a fiaction of
the total effec t
Trace Gas
co 2
CH4
N20
CFC-11
CFC 12
HCFC-22
CFC 113
CFC-114
CFC 115
CCI4
CH3CCI3
CO
NO x
NMHCs
Current Man
Made Emissions
Tgyr-1
26000
300
6
03
04
01
0 15
0015
0 005
0 09
081
200
66
20
Proportion of
total effects
%
61
15
4
2
7
04
15
02
01
03
02
1
6
05
Carbon dioxide emissions given on CO2 basis,
equivalent to 7 GtC yr"' Nitrous oxide
emissions given on N2O basis, equivalent to 4
MtN yr ' NO x emissions given on NO2 basis
equivalent to 20 MtN yr '
negative indirect effect of NO x emissions which would
oppose the forcing due to increased troposphenc oiont
formation
As an example of the use of the Global Warming
Potentials, Table 2 9 shows the integrated effects over a
100 year time horizon for the estimated human-related
greenhouse gas emissions in 1990 The derived cumulative
effects, derived by multiplying the appropriate GWP by the
1990 emissions rate, indicates that CO2 will account for
61% of the radiative forcing over this time period
Emissions of NO x , whose effect is entirely indirect is
calculated to contribute 6% to the total forcing
2.3 Other Radiative Forcing Agents
231 Solar Radiation
The Sun is the primary source of energy tor the Earth s
climate system Variations in the amount of solar iadiation
received by the I arth can affect our climate There are two
distinct sources of this variability The first which acts
with greatest impact on time-scales of 10,000 to 100,000
years is caused by changes in the Sun-Earth orbital
parameters The second comes from physical changes on
the Sun itself, such changes occur on almost all time-scales
2 3 11 Vai /ability due to 01 bital chanqes
Variations in climate on time-scales ranging from 10,000 to
100,000 years, including the major glacial/interglacial
cycles during the Quaternary period, are believed to be
initiated by variations in the Earth s orbital parameters
which in turn influence the latitudinal and seasonal
variation of solar energy received by the Earth (the
Milankovitch Effect) Although the covariation of these
orbital parameters and the Earth s climate provides a
compelling argument in favour of this theory, internal
feedback processes have to be invoked to explain the
observed climatic variations, in particular the amplitude of
the dominating 100,000 year period one such feedback
could be the changes to the carbon cycle and the
greenhouse effect of atmospheric CO2 (see Section 1)
The radiative forcing associated with the Milankovitch
Effect can be given for particular latitudes and months to
illustrate that the rate of change of forcing is small
compared to radiative forcing due to the enhanced
greenhouse effect, of course, the climatic impact of the
Milankovitch Effect results from the redistribution of solar
energy, latitudinally and seasonally, so that a comparison is
necessarily rather rough As an example, in the past 10,000
years, the incident solar radiation at 60°N in July has
decreased by about 35 Wm-2 ( e g , Rind et al , 1989), the
average change in one decade is -0 035 Wm-2, compared
with the estimate, in Section 2 2 5, that the greenhouse
forcing over the most recent decade increased by 0 6
Wm-2 more than 15 times higher than the Milankovitch
forcing
2 3 12 Vai lability due to chaises in total sola/ madiance
Variations in the short-wave and radio-frequency outputs of
the Sun respond to changes in the surface activity of the
star and follow in phase with the 11-year sunspot cycle
The greatest changes, in terms of total energy, occur in the
short-wave region, and particularly the near ultraviolet At
0 3 u.m, the solar cycle variation is less than 1%, since only
about 1% of the Suns radiation lies at this or shorter
wavelengths solar-cycle variations in the ultraviolet will by
themselves induce variations of no more than 0 01% in
total irradiance, although these may be important for
atmospheric chemistry in the middle atmosphere
Of greater potential importance, in terms of direct affects
on climate are changes integrated over all wavelengths the
total solai irradiance or the so-called solar constant
Continuous spaceborne measurements of total irradiance
have been made since 197S These have shown that on
time-scales of days to a decade there are irradiance