11.4 Can 20th Century Sea Level Changes be Explained?
In order to have confidence in our ability to predict future changes in sea
level, we need to confirm that the relevant processes (Section
11.2) have been correctly identified and evaluated. We attempt this by seeing
how well we can account for the current rate of change (Section
11.3). We note that:
- some processes affecting sea level have long (centuries and longer) time-scales,
so that current sea level change is also related to past climate change,
- some relevant processes are not determined solely by climate,
- fairly long records (at least 50 years according to Douglas, 1992) are needed
to detect a significant trend in local sea level, because of the influence
of natural variability in the climate system, and
- the network of tide gauges with records of this length gives only a limited
coverage of the worlds continental coastline and almost no coverage
of the mid-ocean.
| Table 11.14: Sea level rise 1990 to 2100
due to climate change derived from AOGCM experiments following the IS92a
scenario, including the direct effect of sulphate aerosols. See Tables
8.1 and 9.1 for further details of models
and experiments. Results were extrapolated to 2100 for experiments ending
at earlier dates. The uncertainties shown in the land ice terms are those
discussed in this section. For comparison the projection of Warrick et al.
(1996) (in the SAR) is also included. Note that the minimum of the sum of
the components is not identical with the sum of the minima because the smallest
values of the components do not all come from the same AOGCM, and because
for each model the land ice uncertainties have been combined in quadrature;
similarly for the maxima, which also include non-zero contributions from
smaller terms. |
 |
| Experiment |
Sea level rise (m) 1990 to 2100 |
| |
Expansion |
Glaciers
|
Greenland
|
Antarctica a
|
Sum b
|
|
min
|
max
|
min
|
max
|
min
|
max
|
min
|
max
|
min
|
max
|
| CGCM1 GS |
0.43
|
0.03
|
0.23
|
0.00
|
0.07
|
0.07
|
0.02
|
0.45
|
0.77
|
| CSIRO Mk2 GS |
0.33
|
0.02
|
0.22
|
0.01
|
0.08
|
0.12
|
0.04
|
0.29
|
0.60
|
| ECHAM4/OPYC3 GS |
0.30
|
0.02
|
0.18
|
0.02
|
0.03
|
0.17
|
0.06
|
0.19
|
0.48
|
| GFDL_R15_a GS |
0.38
|
0.02
|
0.19
|
0.01
|
0.09
|
0.09
|
0.01
|
0.37
|
0.67
|
| HadCM2 GS |
0.23
|
0.02
|
0.17
|
0.01
|
0.05
|
0.09
|
0.00
|
0.21
|
0.48
|
| HadCM3 GSIO |
0.24
|
0.02
|
0.18
|
0.00
|
0.05
|
0.13
|
0.03
|
0.18
|
0.46
|
| MRI2 GS |
0.11
|
0.01
|
0.11
|
0.00
|
0.03
|
0.04
|
0.00
|
0.11
|
0.31
|
| DOE PCM GS |
0.19
|
0.01
|
0.13
|
0.01
|
0.06
|
0.13
|
0.04
|
0.12
|
0.37
|
| Range |
0.11
|
0.43
|
0.01
|
0.23
|
0.02
|
0.09
|
0.17
|
0.02
|
0.11
|
0.77
|
| Central value |
0.27
|
0.12
|
+0.04
|
-0.08
|
0.44
|
| SAR |
Best estimate |
0.28
|
0.16
|
+0.06
|
-0.01
|
0.49
|
| 7.5.2.4 |
Range |
|
0.20
|
0.86
|
|
Figure 11.11: Global average sea level rise 1990 to 2100 for the IS92a
scenario, including the direct effect of sulphate aerosols. Thermal expansion
and land ice changes were calculated from AOGCM experiments, and contributions
from changes in permafrost, the effect of sediment deposition and the long-term
adjustment of the ice sheets to past climate change were added. For the
models that project the largest (CGCM1) and the smallest (MRI2) sea level
change, the shaded region shows the bounds of uncertainty associated with
land ice changes, permafrost changes and sediment deposition. Uncertainties
are not shown for the other models, but can be found in Table
11.14. The outermost limits of the shaded regions indicate our range
of uncertainty in projecting sea level change for the IS92a scenario. |
The estimated contributions from the various components
of sea level rise during the 20th century (Table 11.10,
Figure 11.9) were constructed using the results
from Section 11.2. The sum of these contributions
for the 20th century ranges from 0.8 mm/yr to 2.2 mm/yr, with a central
value of 0.7 mm/yr. The upper bound is close to the observational upper bound
(2.0 mm/yr), but the central value is less than the observational lower bound
(1.0 mm/yr), and the lower bound is negative i.e. the sum of components is biased
low compared to the observational estimates. Nonetheless, the range is narrower
than the range given by Warrick et al. (1996), as a result of greater constraints
on all the contributions, with the exception of the terrestrial storage terms.
In particular, the long-term contribution from the ice sheets has been narrowed
substantially from those given in Warrick et al. (1996) by the use of additional
constraints (geological data and models of the ice sheets) (Section
11.3.1).
The reason for the remaining discrepancy is not clear. However, the largest
uncertainty (by a factor of more than two) is in the terrestrial storage terms.
Several of the components of the terrestrial storage term are poorly determined
and the quoted limits require several of the contributions simultaneously to
lie at the extremes of their ranges. This coincidence is improbable unless the
systematic errors affecting the estimates are correlated. Furthermore, while
coupled models have improved considerably in recent years, and there is general
agreement between the observed and modelled thermal expansion contribution,
the models ability to quantitatively simulate decadal changes in ocean
temperatures and thus thermal expansion has not been adequately tested. Given
the poor global coverage of high quality tide gauge records and the uncertainty
in the corrections for land motions, the observationally based rate of sea level
rise this century should also be questioned.
In the models, at least a third of 20th century anthropogenic eustatic sea
level rise is caused by thermal expansion, which has a geographically non-uniform
signal in sea level change. AOGCMs do not agree in detail about the patterns
of geographical variation (see Section 11.5.2). They
all give a geographical spread of 20th century trends at individual grid points
which is characterised by a standard deviation of 0.2 to 0.5 mm/yr (Gregory
et al., 2001). This spread is a result of a combination of spatial non-uniformity
of trends and the uncertainty in local trend estimates arising from temporal
variability. As yet no published study has revealed a stable pattern of observed
non-uniform sea level change. Such a pattern would provide a critical test of
models. If there is significant non-uniformity, a trend from a single location
would be an inaccurate estimate of the global average. For example, Douglas
(1997) averaged nine regions and found a standard deviation of about 0.3 mm/yr
(quoted by Douglas as a standard error), similar to the range expected from
AOGCMs.
A common perception is that the rate of sea level rise should have accelerated
during the latter half of the 20th century. The tide gauge data for the 20th
century show no significant acceleration (e.g., Douglas, 1992). We have obtained
estimates based on AOGCMs for the terms directly related to anthropogenic climate
change in the 20th century, i.e., thermal expansion (Section
11.2.1.2), ice sheets (Section 11.2.3.3), glaciers
and ice caps (Section 11.5.1.1) (Figure
11.10a). The estimated rate of sea level rise from anthropogenic climate
change ranges from 0.3 to 0.8 mm/yr (Figure 11.10b).
These terms do show an acceleration through the 20th century (Figure
11.10a,b). If the terrestrial storage terms have a negative sum (Section
11.2.5), they may offset some of the acceleration in recent decades. The
total computed rise (Figure 11.10c) indicates
an acceleration of only 0.2 mm/yr/century, with a range from -1.1 to +0.7 mm/yr/century,
consistent with observational finding of no acceleration in sea level rise during
the 20th century (Section 11.3.2.2). The sum of terms
not related to recent climate change is -1.1 to +0.9 mm/yr (i.e., excluding
thermal expansion, glaciers and ice caps, and changes in the ice sheets due
to 20th century climate change). This range is less than the observational lower
bound of sea level rise. Hence it is very likely that these terms alone are
an insufficient explanation, implying that 20th century climate change has made
a contribution to 20th century sea level rise.
Recent studies (see Sections 2.3.3, 2.3.4)
suggest that the 19th century was unusually cold on the global average, and
that an increase in solar output may have had a moderate influence on warming
in the early 20th century (Section 12.4.3.3). This warming
might have produced some thermal expansion and could have been responsible for
the onset of glacier recession in the early 20th century (e.g., Dowdeswell et
al., 1997), thus providing a possible explanation of an acceleration in sea
level rise commencing before major industrialisation.
