Figure 5.13: Anthropogenic aerosol emissions projected for the SRES scenarios (Nakicenovic et al., 2000).
|
Figure 5.13: Anthropogenic aerosol emissions projected for the SRES scenarios (Nakicenovic et al., 2000). |
The production of sea salt aerosol is also a strong function
of wind speed. The semi-empirical formulation of Monahan et al. (1986) was used
to produce global monthly sea salt fluxes for eight size intervals (dry diameter
of 0.06 to 16 µm) using procedures discussed in Gong et al. (1997a,b). In order
to project sea salt emissions for the workshop, the ratio of daily average wind
speed in the ten years prior to 2100 and 2000 was used to scale the 2000 daily
average sea salt flux to the one in 2100. Because this method may overestimate
emissions if the product of the daily average ratio with the daily average winds
in 2000 produces high wind speeds with a high frequency, the calculation was
checked using the ratio of the monthly average wind speeds. This produced a
total sea salt flux that was 13% smaller than the projections given in the workshop
specifications.
Predicted sea salt emissions were 3,340 Tg in 2000 and increased to 5,880
Tg in 2100. These increases point to a potentially important negative climate
feedback. For example, the present day direct radiative impact of sea salt is
estimated to be between -0.75 and -2.5 Wm–2 using the clear-sky estimates
from Haywood et al. (1999) or -0.34 Wm–2 using the whole-sky estimates
from Jacobson (2001). Assuming the ratio of whole-sky to clear-sky forcing from
Jacobson (2000b), we project that these changes in sea salt emissions might
lead to a radiative feedback in 2100 of up to -0.8 Wm–2. If we assume
that the near-doubling of the sea salt mass flux would result in a proportional
increase in the number flux, a significant increase in reflected radiation may
also result from the indirect effect. The LLNL/Umich model was used to evaluate
the possible impact of these emissions. It was found that the changes in natural
emissions in 2100 result in a radiative feedback of -1.16 Wm–2 in
the 2100 A2 scenario. These projected climate changes rely on the projected
wind speed changes from the NCAR CSM model. Because projections from other models
may not be as large, we also calculated the projected sea salt emissions from
three other climate models using the ratio of monthly average winds for the
time period from 2090 to 2100 to that for the time period 1990 to 2000 to scale
the 2000 sea salt fluxes. Compared to the projections from the NCAR CSM, these
models resulted in annual average fluxes that were 37% higher (GFDL model),
13% smaller (Max Planck model), and 9% smaller (Hadley Centre UK Met Office
model). The monthly average temperature change associated with these wind speed
and sea salt projections was 2.8°K (GFDL model; Knutson et al. 1999), 2.8°K
(Max Planck model; Roeckner et al. 1999) and 2.15°K (Hadley Centre UK Met
Office model; Gordon et al., 2000) compared to the temperature projection of
1.8°K from the NCAR CSM model (Dai et al., 2001).
In order to project future aerosol concentrations, we formed the average burdens
from the models that gave reasonable agreement with observations for the 2000
scenario. As noted above, the differences in the burdens calculated by the different
models point to substantial uncertainties in the prediction of current burdens
and these translate into similar uncertainties in projecting future burdens.
Except for sulphate and black carbon, however, the future projected global average
burdens scaled approximately linearly with emissions. Thus, we may assume that
the projected uncertainty in future burdens is mainly determined by the uncertainty
in the emissions themselves together with the uncertainty in the burdens associated
with different model treatments. For SO42-, future anthropogenic
concentrations were not linear in the emissions. For example, some models projected
increases in burden relative to emissions while others projected decreases.
The range of projected changes in anthropogenic burden relative to emissions
was -14% to +25% depending on the scenario. Use of the average of the models
for the projection of anthropogenic SO42- and total SO42-
may therefore bias the results somewhat, but the uncertainties in the projected
SO42- concentrations are smaller than those introduced
from the range of estimates for the 2000 scenario itself. Table
5.14 gives our projected average burdens for each draft SRES scenario. Results
for the burdens associated with the final SRES scenarios are reproduced in Appendix
II and are shown in Figure 5.13 (see Chapter
9 and Nakic´enovic´ et al. (2000) for scenario definitions).
| Table 5.14: Projected future aerosol burden for draft SRES scenarios. The range predicted from the models that participated in the workshop are given for 2000. | ||||||
 |
||||||
|
2000
(SC1) |
A2 2030
(SC2) |
A2 2100
(SC3) |
B1 2100
(SC4) |
A2 2100 with
natural aerosols (SC5) |
B1 2100 with
natural aerosols (SC8) |
|
|
||||||
| Sulphate Natural (TgS) |
0.26
0.15 - 0.36 |
0.26
|
0.26
|
0.26
|
0.28
|
0.28
|
| Sulphate Anthr. (TgS) |
0.52
0.35 - 0.75 |
0.90
|
0.55
|
0.28
|
0.54
|
0.26
|
| Nitrate Natural (TgN) |
0.02
|
0.02
|
||||
| Nitrate Anthr. (TgN) |
0.07
|
0.38
|
||||
| Ammonium Nat. (TgN) |
0.09
|
0.09
|
||||
| Ammonium Anthr. (TgN) |
0.33
|
0.72
|
||||
| BC (Tg) |
0.26
0.22 - 0.32 |
0.33
|
0.61
|
0.25
|
0.61
|
0.25
|
| OC Natural (Tg) |
0.15
0.08 - 0.28 |
0.15
|
0.15
|
0.15
|
0.22
|
0.22
|
| OC Anthr. (Tg) |
1.52
1.05 - 2.21 |
1.95
|
2.30
|
0.90
|
2.30
|
0.90
|
| Dust (D<2 µm) (Tg) |
12.98
6.24 - 17.73 |
13.52
|
13.52
|
13.52
|
13.54
|
13.54
|
| Dust (D>2 µm) (Tg) |
19.58
7.39 - 33.15 |
19.58
|
19.58
|
19.58
|
20.91
|
20.91
|
| Sea salt (D<2 µm) (Tg-Na) |
2.74
1.29 - 7.81 |
2.74
|
2.74
|
2.74
|
4.77
|
4.77
|
| Sea salt (D>2 µm) (Tg-Na) |
3.86
1.22 - 6.51 |
3.86
|
3.86
|
3.86
|
6.68
|
6.68
|
|
||||||
Continues on next page
|
Other reports in this collection |
|
