Climate Change 2001:
Working Group I: The Scientific Basis
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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
c-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
a The methane GWPs include an indirect contribution from stratospheric H2O and O3 production.
b The values for methane and nitrous oxide are adjustment times, which incorporate the indirect effects of emission of each gas on its own lifetime.


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