C.5 Observed and Modelled Changes in Solar and Volcanic Activity
Radiative forcing of the climate system due to solar irradiance change is estimated
to be 0.3 ± 0.2 Wm-2 for the period 1750 to the present (Figure
8), and most of the change is estimated to have occurred during the first
half of the 20th century. The fundamental source of all energy in the Earth's
climate system is radiation from the Sun. Therefore, variation in solar output
is a radiative forcing agent. The absolute value of the spectrally integrated
total solar irradiance (TSI) incident on the Earth is not known to better than
about 4 Wm-2, but satellite observations since the late 1970s show
relative variations over the past two solar 11-year activity cycles of about 0.1%,
which is equivalent to a variation in radiative forcing of about 0.2 Wm-2.
Prior to these satellite observations, reliable direct measurements of solar irradiance
are not available. Variations over longer periods may have been larger, but the
techniques used to reconstruct historical values of TSI from proxy observations
(e.g., sunspots) have not been adequately verified. Solar variation varies more
substantially in the ultraviolet region, and studies with climate models suggest
that inclusion of spectrally resolved solar irradiance variations and solar-induced
stratospheric ozone changes may improve the realism of model simulations of the
impact of solar variability on climate. Other mechanisms for the amplification
of solar effects on climate have been proposed, but do not have a rigorous theoretical
or observational basis.
Stratospheric aerosols from explosive volcanic eruptions lead to negative forcing
that lasts a few years. Several explosive eruptions occurred in the periods 1880
to 1920 and 1960 to 1991, and no explosive eruptions since 1991. Enhanced stratospheric
aerosol content due to volcanic eruptions, together with the small solar irradiance
variations, result in a net negative natural radiative forcing over the past two,
and possibly even the past four, decades.
C.6 Global Warming Potentials
Radiative forcings and Global Warming Potentials (GWPs) are presented in Table
3 for an expanded set of gases. GWPs are a measure of the relative radiative
effect of a given substance compared to CO2, integrated over a chosen
time horizon. New categories of gases in Table 3 include
fluorinated organic molecules, many of which are ethers that are proposed as halocarbon
substitutes. Some of the GWPs have larger uncertainties than that of others, particularly
for those gases where detailed laboratory data on lifetimes are not yet available.
The direct GWPs have been calculated relative to CO2 using an improved
calculation of the CO2 radiative forcing, the SAR response function
for a CO2 pulse, and new values for the radiative forcing and lifetimes
for a number of halocarbons. Indirect GWPs, resulting from indirect radiative
forcing effects, are also estimated for some new gases, including carbon monoxide.
The direct GWPs for those species whose lifetimes are well characterised are estimated
to be accurate within ±35%, but the indirect GWPs are less certain.
| Table 3: Direct Global Warming Potentials (GWPs)
relative to carbon dioxide (for gases for which the lifetimes have been
adequately characterised). GWPs are an index for estimating relative global
warming contribution due to atmospheric emission of a kg of a particular
greenhouse gas compared to emission of a kg of carbon dioxide. GWPs calculated
for different time horizons show the effects of atmospheric lifetimes of
the different gases. [Based upon Table
6.7] |
 |
| Gas |
|
Lifetime
(years)
|
Global Warming Potential
(Time Horizon in years)
|
|
| |
20 yrs
|
100 yrs
|
500 yrs
|
|
| Carbon dioxide |
CO2 |
|
1
|
1
|
1
|
| Methanea |
CH4 |
12.0 b
|
62
|
23
|
7
|
| Nitrous oxide |
N2O |
114 b
|
275
|
296
|
156
|
| Hydrofluorocarbons |
|
|
|
|
|
| HFC-23 |
CHF3 |
260
|
9400
|
12000
|
10000
|
| HFC-32 |
CH2F2 |
5.0
|
1800
|
550
|
170
|
| HFC-41 |
CH3F |
2.6
|
330
|
97
|
30
|
| HFC-125 |
CHF2CF3 |
29
|
5900
|
3400
|
1100
|
| HFC-134 |
CHF2CHF2 |
9.6
|
3200
|
1100
|
330
|
| HFC-134a |
CH2FCF3 |
13.8
|
3300
|
1300
|
400
|
| HFC-143 |
CHF2CH2F |
3.4
|
1100
|
330
|
100
|
| HFC-143a |
CF3CH3 |
52
|
5500
|
4300
|
1600
|
| HFC-152 |
CH2FCH2F |
0.5
|
140
|
43
|
13
|
| HFC-152a |
CH3CHF2 |
1.4
|
410
|
120
|
37
|
| HFC-161 |
CH3CH2F |
0.3
|
40
|
12
|
4
|
| HFC-227ea |
CF3CHFCF3 |
33
|
5600
|
3500
|
1100
|
| HFC-236cb |
CH2FCF2CF3 |
13.2
|
3300
|
1300
|
390
|
| HFC-236ea |
CHF2CHFCF3 |
10
|
3600
|
1200
|
390
|
| HFC-236fa |
CF3CH2CF3 |
220
|
7500
|
9400
|
7100
|
| HFC-245ca |
CH2FCF2CHF2 |
5.9
|
2100
|
640
|
200
|
| HFC-245fa |
CHF2CH2CF3 |
7.2
|
3000
|
950
|
300
|
| HFC-365mfc |
CF3CH2CF2CH3 |
9.9
|
2600
|
890
|
280
|
| HFC-43-10mee |
CF3CHFCHFCF2CF3 |
15
|
3700
|
1500
|
470
|
| Fully fluorinated species |
|
|
|
|
|
| SF6 |
|
3200
|
15100
|
22200
|
32400
|
| CF4 |
|
50000
|
3900
|
5700
|
8900
|
| C2F6 |
|
10000
|
8000
|
11900
|
18000
|
| C3F8 |
|
2600
|
5900
|
8600
|
12400
|
| C4F10 |
|
2600
|
5900
|
8600
|
12400
|
| C4F8 |
|
3200
|
6800
|
10000
|
14500
|
| C5F12 |
|
4100
|
6000
|
8900
|
13200
|
| C6F14 |
|
3200
|
6100
|
9000
|
13200
|
| Ethers and Halogenated Ethers |
|
|
|
|
|
| CH3OCH3 |
|
0.015
|
1
|
1
|
<<1
|
| HFE-125 |
CF3OCHF2 |
150
|
12900
|
14900
|
9200
|
| HFE-134 |
CHF2OCHF2 |
26.2
|
10500
|
6100
|
2000
|
| HFE-143a |
CH3OCF3 |
4.4
|
2500
|
750
|
230
|
| HCFE-235da2 |
CF3CHClOCHF2 |
2.6
|
1100
|
340
|
110
|
| HFE-245fa2 |
CF3CH2OCHF2 |
4.4
|
1900
|
570
|
180
|
| HFE-254cb2 |
CHF2CF2OCH3 |
0.22
|
99
|
30
|
9
|
| HFE-7100 |
C4F9OCH3 |
5.0
|
1300
|
390
|
120
|
| HFE-7200 |
C4F9OC2H5 |
0.77
|
190
|
55
|
17
|
| H-Galden 1040x |
CHF2OCF2OC2F4OCHF2 |
6.3
|
5900
|
1800
|
560
|
| HG-10 |
CHF2OCF2OCHF2 |
12.1
|
7500
|
2700
|
850
|
| HG-01 |
CHF2OCF2CF2OCHF2 |
6.2
|
4700
|
1500
|
450
|
|
|