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6.5.4. Greenhouse Radiative Forcing 1765 to 1990

From the modelled DF-DC relationships, the increase in radiative forcing due to the enhanced greenhouse gas concentrations can be calculated. Instrumental records exist for the most recent decades, whilst proxy data are used to calculate greenhouse gas concentrations earlier in the study period. Table 6.4 gives the concentration changes for five of the commonly known greenhouse gases, whilst Table 6.5 details their contribution to radiative forcing for a number of time intervals. Here, 1765 has been regarded as the onset of the Industrial Revolution.

Table 6.4. Changes in atmospheric concentration of the major greenhouse gases since 1750

 

Year

CO2 (ppmv)

CH4 (ppbv)

N2O (ppbv)

CFC11 (pptv)

CFC12 (pptv)

1765

279

790

275

0

0

1900

296

974

292

0

0

1960

316

1272

297

18

30

1970

325

1421

299

70

121

1980

337

1569

303

158

273

1992

355

1714

311

270

504



Table 6.5. Change in radiative forcing (Wm-2) due to concentration changes in greenhouse gases

 

Time period

CO2

CH4

N2O

CFC11

CFC12

Total

1765-1900

0.37

0.1

0.027

0.0

0.0

0.53

1765-1960

0.79

0.24

0.045

0.004

0.008

1.17

1765-1970

0.96

0.30

0.054

0.014

0.034

1.48

1765-1980

1.20

0.36

0.068

0.035

0.076

1.91

1765-1990

1.50

0.42

0.10

0.062

0.14

2.45

Direct radiative forcing greenhouse gases (i.e. excludes radiative effects of ozone loss).


Changes in CO2 concentration over the last two centuries have contributed most to the greenhouse radiative forcing. Over the period 1765 to 1990, CO2 forcing has accounted for 61% of the total enhanced greenhouse forcing. Nevertheless, other greenhouse gases, in particular the halocarbons, are now accounting for an increasing proportion of the total greenhouse forcing, due to their relatively larger GWPs. The total increase in radiative forcing since 1765 is shown in Figure 6.7.

The total increase in direct radiative greenhouse forcing is approximately 2.5Wm-2 (Table 6.5). This value should be compared to the solar constant, 1368Wm-2, the amount of total solar radiation intercepted by the Earth. The average amount of radiation arriving at the top of the troposphere is about 270Wm-2. This figure is lower than the solar constant, for it takes into account the latitudinal and temporal variations in insolation.