213

Chapter 2 Changes in Atmospheric Constituents and in Radiative Forcing

Notes:

a The CO2 response function used in this report is based on the revised version of the Bern Carbon cycle model used in Chapter 10 of this report (Bern2.5CC; Joos et 

al. 2001) using a background CO2 concentration value of 378 ppm. The decay of a pulse of CO2 with time t is given by

 Where a0 = 0.217, a1 = 0.259, a2 = 0.338, a3 = 0.186, τ1 = 172.9 years, τ2 = 18.51 years, and τ3 = 1.186 years.

b The radiative effi ciency of CO2 is calculated using the IPCC (1990) simplifi ed expression as revised in the TAR, with an updated background concentration value of 

378 ppm and a perturbation of +1 ppm (see Section 2.10.2). 

c The perturbation lifetime for methane is 12 years as in the TAR (see also Section 7.4). The GWP for methane includes indirect effects from enhancements of ozone 

and stratospheric water vapour (see Section 2.10.3.1). 

d Shine et al. (2005c), updated by the revised AGWP for CO2. The assumed lifetime of 1,000 years is a lower limit.

e Hurley et al. (2005)

f

 Robson et al. (2006)

g Young et al. (2006)

i = 1

3

 a0 + Σ ai • e -t/τi

Table 2.14 (continued)

 

 Global Warming Potential for

 Given Time Horizon Industrial Designation Radiative

or Common Name Lifetime Effi ciency SAR‡

(years) Chemical Formula (years) (W m–2 ppb–1) (100-yr) 20-yr 100-yr 500-yr

Perfl uorinated compounds (continued) 

PFC-218 C3F8 2,600 0.26 7,000 6,310 8,830 12,500

PFC-318 c-C4F8 3,200 0.32 8,700 7,310 10,300 14,700

PFC-3-1-10 C4F10 2,600 0.33 7,000 6,330 8,860 12,500

PFC-4-1-12 C5F12 4,100 0.41 6,510 9,160 13,300

PFC-5-1-14 C6F14 3,200 0.49 7,400 6,600 9,300 13,300

PFC-9-1-18 C10F18 >1,000d 0.56 >5,500 >7,500 >9,500

trifl uoromethyl SF5CF3 800 0.57 13,200 17,700 21,200

sulphur pentafl uoride

Fluorinated ethers 

HFE-125 CHF2OCF3 136 0.44 13,800 14,900 8,490

HFE-134 CHF2OCHF2 26 0.45 12,200 6,320 1,960

HFE-143a CH3OCF3 4.3 0.27 2,630 756 230

HCFE-235da2 CHF2OCHClCF3 2.6 0.38 1,230 350 106

HFE-245cb2 CH3OCF2CHF2 5.1 0.32 2,440 708 215

HFE-245fa2 CHF2OCH2CF3 4.9 0.31 2,280 659 200

HFE-254cb2 CH3OCF2CHF2 2.6 0.28 1,260 359 109

HFE-347mcc3 CH3OCF2CF2CF3 5.2 0.34 1,980 575 175

HFE-347pcf2 CHF2CF2OCH2CF3 7.1 0.25 1,900 580 175

HFE-356pcc3 CH3OCF2CF2CHF2 0.33 0.93 386 110 33

HFE-449sl 

(HFE-7100) C4F9OCH3 3.8 0.31 1,040 297 90

HFE-569sf2 C4F9OC2H5 0.77 0.3 207 59 18

(HFE-7200) 

HFE-43-10pccc124 CHF2OCF2OC2F4OCHF2 6.3 1.37 6,320 1,870 569

(H-Galden 1040x) 

HFE-236ca12 CHF2OCF2OCHF2 12.1 0.66 8,000 2,800 860

(HG-10)

HFE-338pcc13 CHF2OCF2CF2OCHF2 6.2 0.87 5,100 1,500 460

(HG-01) 

Perfl uoropolyethers 

PFPMIE CF3OCF(CF3)CF2OCF2OCF3 800 0.65 7,620 10,300 12,400

Hydrocarbons and other compounds – Direct Effects 

Dimethylether CH3OCH3 0.015 0.02 1 1 <<1

Methylene chloride CH2Cl2 0.38 0.03 31 8.7 2.7

Methyl chloride CH3Cl 1.0 0.01 45 13 4
214

Changes in Atmospheric Constituents and in Radiative Forcing Chapter 2

observed or a clear potential for future emissions has been 

identifi ed. The uncertainties of these direct GWPs are taken to 

be ±35% for the 5 to 95% (90%) confi dence range. 

2.10.3 Indirect GWPs

Indirect radiative effects include the direct effects of 

degradation products or the radiative effects of changes in 

concentrations of greenhouse gases caused by the presence of 

the emitted gas or its degradation products. Direct effects of 

degradation products for the greenhouse gases are not considered 

to be signifi cant (WMO, 2003). The indirect effects discussed 

here are linked to ozone formation or destruction, enhancement 

of stratospheric water vapour, changes in concentrations of the 

OH radical with the main effect of changing the lifetime of CH4, 

and secondary aerosol formation. Uncertainties for the indirect 

GWPs are generally much higher than for the direct GWPs. 

The indirect GWP will in many cases depend on the location 

and time of the emissions. For some species (e.g., NOx) the 

indirect effects can be of opposite sign, further increasing the 

uncertainty of the net GWP. This can be because background 

levels of reactive species (e.g., NOx) can affect the chemical 

response nonlinearly, and/or because the lifetime or the 

radiative effects of short-lived secondary species formed can be 

regionally dependent. Thus, the usefulness of the global mean 

GWPs to inform policy decisions can be limited. However, they 

are readily calculable and give an indication of the total potential 

of mitigating climate change by including a certain forcing 

agent in climate policy. Following the approach taken by the 

SAR and the TAR, the CO2 produced from oxidation of CH4, 

CO and NMVOCs of fossil origin is not included in the GWP 

estimates since this carbon has been included in the national 

CO2 inventories. This issue may need to be reconsidered as 

inventory guidelines are revised.

2.10.3.1 Methane

Four indirect radiative effects of CH4 emissions have been 

identifi ed (see Prather et al., 2001; Ramaswamy et al., 2001). 

Methane enhances its own lifetime through changes in the 

OH concentration: it leads to changes in tropospheric ozone, 

enhances stratospheric water vapour levels, and produces CO2. 

The GWP given in Table 2.14 includes the fi rst three of these 

effects. The lifetime effect is included by adopting a perturbation 

lifetime of 12 years (see Section 7.4). The effect of ozone 

production is still uncertain, and as in the TAR, it is included 

by enhancing the net of the direct and the lifetime effect by 

25%. The estimate of RF caused by an increase in stratospheric 

water vapour has been increased signifi cantly since the TAR 

(see Section 2.3.7). This has also been taken into account in the 

GWP estimate for CH4 by increasing the enhancement factor 

from 5% (TAR) to 15%. As a result, the 100-year GWP for CH4

has increased from 23 in the TAR to 25. 

2.10.3.2 Carbon Monoxide

The indirect effects of CO occur through reduced OH levels 

(leading to enhanced concentrations of CH4) and enhancement 

of ozone. The TAR gave a range of 1.0 to 3.0 for the 100-year 

GWP. Since the TAR, Collins et al. (2002) and Berntsen et al. 

(2005) have calculated GWPs for CO emissions that range 

between 1.6 and 2.0, depending on the location of the emissions. 

Berntsen et al. (2005) found that emissions of CO from Asia 

had a 25% higher GWP compared to European emissions. 

Averaging over the TAR values and the new estimates give a 

mean of 1.9 for the 100-year GWP for CO. 

2.10.3.3 Non-methane Volatile Organic Compounds

Collins et al. (2002) calculated indirect GWPs for 10 

NMVOCs with a global three-dimensional Lagrangian 

chemistry-transport model. Impacts on tropospheric ozone, 

CH4 (through changes in OH) and CO2 have been considered, 

using either an ‘anthropogenic’ emission distribution or a 

‘natural’ emission distribution depending on the main sources 

for each gas. The indirect GWP values are given in Table 2.15. 

Weighting these GWPs by the emissions of the respective 

compounds gives a weighted average 100-year GWP of 3.4. Due 

to their short lifetimes and the nonlinear chemistry involved in 

ozone and OH chemistry, there are signifi cant uncertainties in 

the calculated GWP values. Collins et al. (2002) estimated an 

uncertainty range of –50% to +100%. 

2.10.3.4 Nitrogen Oxides

The short lifetime and complex nonlinear chemistry, which 

cause two opposing indirect effects through ozone enhancements 

and CH4 reductions, make calculations of GWP for NOx

emissions very uncertain (Shine et al., 2005a). In addition, the 

effect of nitrate aerosol formation (see Section 2.4.4.5), which 

has not yet been included in model studies calculating GWPs 

for NOx, can be signifi cant. Due to the nonlinear chemistry, the 

net RF of NOx emissions will depend strongly on the location 

of emission and, with a strict defi nition of a pulse emission 

for the GWP, also on timing (daily, seasonal) of the emissions 

(Fuglestvedt et al., 1999; Derwent et al., 2001; Wild et al., 2001; 

Stevenson et al., 2004; Berntsen et al., 2005, 2006). Due to the 

lack of agreement even on the sign of the global mean GWP for 

NOx among the different studies and the omission of the nitrate 

aerosol effect, a central estimate for the 100-year GWP for NOx

is not presented. 

2.10.3.5 Halocarbons

Chlorine- and bromine-containing halocarbons lead to ozone 

depletion when the halocarbon molecules are broken down in 

the stratosphere and chlorine or bromine atoms are released. 

Indirect GWPs for ozone-depleting halocarbons are estimated 

in Velders et al. (2005; their Table 2.7). These are based on