11.5.1.2 Projections for SRES scenarios
Few AOGCM experiments have been done with any of the SRES emissions scenarios.
Therefore to establish the range of sea level rise resulting from the choice
of different SRES scenarios, we use results for thermal expansion and global-average
temperature change from a simple climate model based on that of Raper et al.
(1996) and calibrated individually for seven AOGCMs (CSIRO Mk2, CSM 1.3, ECHAM4/
OPYC3, GFDL_R15_a, HadCM2, HadCM3, DOE PCM). The calibration is discussed in
Chapter 9, Section 9.3.3 and
the Appendix to Chapter 9. The AOGCMs used have a range
of effective climate sensitivity of 1.7 to 4.2°C (Table
9.1). We calculate land-ice changes using the global average temperature
change from the simple model and the global average mass balance sensitivities
estimated from the AOGCM IS92a experiments in Section
11.5.1.1 (Tables 11.12 and 11.13).
We add contributions from the continuing evolution of the ice sheets in response
to past climate change, thawing of permafrost, and the effect of sedimentation
(the same as in Section 11.5.1.1). The methods used
to make the sea level projections are documented in detail in the Appendix to
this chapter.
| Table 11.16: Sea level rise due to thermal
expansion in 2xCO2 and 4xCO2 experiments. See Chapter
8, Table 8.1 for further details of models. |
 |
| |
Sea level rise (m)
in 2 xCO2 experiment
|
 h2
/ h1 |
Final sea level rise (m)
|
| |
at 2 xCO2
(h1 )
|
500 yr later
(h2 )
|
|
2 xCO2
experiment
|
4CO2
experiment
|
| CLIMBER |
0.16
|
0.67
|
4.2
|
0.78
|
1.44
|
| ECHAM3/LSG |
0.06
|
0.57
|
9.2
|
1.53 a
|
2.56 a
|
| GFDL_R15_a |
0.13
|
1.10
|
8.5
|
1.96
|
3.46
|
| HadCM2 |
0.09
|
0.70
|
7.8
|
–
|
–
|
| BERN2D GM |
0.23
|
1.12
|
4.9
|
1.93
|
3.73
|
| BERN2D HOR |
0.22
|
0.92
|
4.2
|
1.28
|
4.30
|
| UVic GM |
0.11
|
0.44
|
3.9
|
0.53
|
1.24
|
| UVic H |
0.13
|
0.71
|
5.6
|
1.19
|
2.62
|
| UVic HBL |
0.10
|
0.44
|
4.3
|
0.65
|
1.78
|
|
Figure 11.15: Global average sea level rise from thermal expansion in
model experiments with CO2 (a) increasing at 1%/yr for 70 years
and then held constant at 2x its initial (preindustrial) concentration;
(b) increasing at 1%/yr for 140 years and then held constant at 4x its
initial (preindustrial) concentration. |
For the complete range of AOGCMs and SRES scenarios and including uncertainties
in land-ice changes, permafrost changes and sediment deposition, global average
sea level is projected to rise by 0.09 to 0.88 m over 1990 to 2100, with a central
value of 0.48 m (Figure 11.12). The central value
gives an average rate of 2.2 to 4.4 times the rate over the 20th century.
The corresponding range reported by Warrick et al. (1996) (representing scenario
uncertainty by using all the IS92 scenarios with time-dependent sulphate aerosol)
was 0.13 to 0.94 m, obtained using a simple model with climate sensitivities
of 1.5 to 4.5°C. Their upper bound is larger than ours. Ice sheet mass balance
sensitivities derived from AOGCMs (see Section 11.5.1.1)
are smaller (less positive or more negative) than those used by Warrick et al.,
while the method we have employed for calculating glacier mass loss (Sections
11.2.2 and 11.5.1.1) gives a smaller sea level
contribution for similar scenarios than the heuristic model of Wigley and Raper
(1995) employed by Warrick et al.
In addition, Warrick et al. included an allowance for ice-dynamical changes
in the WAIS. The range we have given does not include such changes. The contribution
of the WAIS is potentially important on the longer term, but it is now widely
agreed that major loss of grounded ice from the WAIS and consequent accelerated
sea-level rise are very unlikely during the 21st century. Allowing for the possible
effects of processes not adequately represented in present models, two risk
assessment studies involving panels of experts concluded that there was a 5%
chance that by 2100 the WAIS could make a substantial contribution to sea level
rise, of 0.16 m (Titus and Narayanan, 1996) or 0.5 m (Vaughan and Spouge, 2001).
These studies also noted a 5% chance of WAIS contributing a sea level fall of
0.18 m or 0.4 m respectively. (See Section 11.5.4.3
for a full discussion.)
The range we have given also does not take account of uncertainty in modelling
of radiative forcing, the carbon cycle, atmospheric chemistry, or storage of
water in the terrestrial environment. The recent publications by Gornitz et
al. (1997) and Sahagian (2000) indicate that this last term could be significant
(Section 11.2.5). Future changes in terrestrial storage
depend on societal decisions on the use of ground water, the building of reservoirs
and other factors. We are not currently in a position to make projections incorporating
future changes in these factors, although we note that the assumptions behind
the construction of the SRES scenarios imply increasing water consumption, which
may entail both more ground water extraction and more reservoir capacity. Continued
anthropogenic water storage on land at its current rate could change the projected
sea level rise 1990 to 2100 by between –0.21 and +0.11 m. We emphasise that
estimates of the relevant factors are highly uncertain (see Sections
11.2.5 and 11.4).
The evolution of sea level rise for the average of the seven AOGCMs for each
of the six illustrative SRES scenarios is shown in Figure
11.12, and the shading shows the range for all 35 SRES scenarios. It is
apparent that the variation due to the choice of scenario alone is relatively
small over the next few decades. The range spanned by the SRES scenarios by
2040 is only 0.02 m or less. By 2100, the scenario range has increased to 0.18
m, about 50% of the central value. All the AOGCMs have a similar range at 2100
expressed as a fraction of their central value. Of the six illustrative scenarios,
A1FI gives the largest sea level rise and B1 the smallest.
The average-AOGCM range for all 35 scenarios (dark shading in Figure
11.12) covers about one third of the all-AOGCM range (light shading). That
is, for sea level rise 1990 to 2100, the uncertainty in climate sensitivity
and heat uptake, represented by the spread of AOGCMs, is more important than
the uncertainty from choice of emissions scenario. This is different for three
reasons from the case of global average temperature change (Section
9.3.2.1), where the scenario and modelling uncertainties are comparable.
First, the compensation between climate sensitivity and heat uptake does not
apply to thermal expansion. Second, models with large climate sensitivity and
temperature change consequently have a large land-ice melt contribution to sea
level. Third, both thermal expansion and land-ice melt depend on past climate
change, being approximately proportional to the time-integral of temperature
change; the SRES scenarios differ by less in respect of the time-integral of
temperature change over the interval 1990 to 2100 than they do in respect of
the temperature change at 2100.
