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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L01602, doi:10.1029/2005GL024826, 2006
A 20th century acceleration in global sea-level rise
John A. Church
and Neil J. White
Received 6 October 2005; revised 22 November 2005; accepted 1 December 2005; published 6 January 2006.
] Multi-century sea-level records and climate models
2. Reconstructing Monthly Sea Levels
indicate an acceleration of sea-level rise, but no
] We use the same techniques as in our earlier study
century acceleration has previously been detected. A
[Church et al., 2004] of using tide-gauge data to determine
reconstruction of global sea level using tide-gauge data
the changes in amplitude between consecutive months of a
from 1950 to 2000 indicates a larger rate of rise after 1993
selected number of these EOFs. These techniques were
and other periods of rapid sea-level rise but no significant
developed to estimate historical values of surface atmo-
acceleration over this period. Here, we extend the
spheric pressure and sea surface temperatures [Kaplan et al.,
reconstruction of global mean sea level back to 1870 and
2000; Rayner et al., 2003]. First differences of the tide-
find a sea-level rise from January 1870 to December 2004
gauge data are used because it is not possible to relate all of
of 195 mm, a 20th century rate of sea-level rise of 1.7 Â±
the separate records to a single vertical datum. The first
0.3 mm yr
and a significant acceleration of sea-level rise
differences of the EOF amplitudes are integrated backward
of 0.013 Â± 0.006 mm yr
. This acceleration is an important
in time to estimate sea-level fields and hence GMSL each
confirmation of climate change simulations which show an
month. A revised scaling of the EOFs results in realistic
acceleration not previously observed. If this acceleration
formal error estimates.
remained constant then the 1990 to 2100 rise would range
] We calculate the EOFs from 12 years (compared with
from 280 to 340 mm, consistent with projections in the
9 years in our earlier study) of satellite altimeter data (T/P
Citation: Church, J. A., and N. J. White (2006), A
and Jason-1) from January 1993 to December 2004. All
20th century acceleration in global sea-level rise, Geophys. Res.
standard corrections except the inverse barometer correction
Lett., 33, L01602, doi:10.1029/2005GL024826.
are applied, including corrections for the drift of the water-
vapour measurements [Keihm et al., 2000; MacMillan et al.,
2004] for both T/P and Jason-1 and for the drift of the T/P
[Mitchum, 2000]. We map the
] Most estimates of 20th century sea-level rise have
altimeter data to a 1
1 month grid. We remove
depended on averaging the rates of rise from the few, long,
the seasonal signal and a linear trend in GMSL, as we will
high-quality tide-gauge records that are available [Douglas,
use the EOFs to model variability about the time-varying
1991, 2001; Peltier, 2001]. However, these records contain
An additional spatially uniform field is included in
significant decadal variability, obscuring any acceleration
the reconstruction to represent changes in GMSL. Omitting
[Woodworth, 1990; Douglas, 1992]. Even when a global-
this field results in a much smaller rate of GMSL rise,
mean sea level (GMSL) record is known to contain an
inconsistent with tide-gauge data
(in the mean and at
acceleration (as in numerical models), an acceleration is
individual sites) and earlier studies [e.g., Douglas, 1991],
difficult to detect in an average of a small number of records
and results in unrealistically large spatial variability in
[Gregory et al., 2001].
regional trends as a finite number of EOFs cannot
] The TOPEX/Poseidon (T/P) and Jason-1 satellite
adequately represent a substantial change in mean sea level.
altimeters have produced high quality measurements of near
Trends from our reconstructed time series agree well with
global (66 S to 66 N) sea level from 1993. The spatial
trends from long tide-gauge records. The EOFs provide
correlations from this data set, expressed as Empirical
information on global correlations of sea-level variability.
(EOFs), together with the
There is no assumption that the spatial pattern of sea-level
longer but sparse tide-gauge data set, have been used to
rise for 1993 to 2004 is maintained over a longer period. We
produce estimates of reconstructed global sea-level variabil-
have not tried to detect the regional pattern of sea-level rise
ity [Chambers et al., 2002] and rise [Church et al., 2004]
resulting from the elastic response of the earth to present
for 1950 to 2000. As these estimates explicitly account for
day contributions from glaciers and ice sheets [Mitrovica et
the spatial redistribution of sea level, the temporal variabil-
ity is at least an order of magnitude lower than that present
] We use monthly sea-level data downloaded from the
in individual records. The estimates also allow for the
Permanent Service for Mean Sea Level
possibility of spatial variability in the rate of sea-level rise
[Woodworth and Player, 2003]) web site (
[Nakiboglu and Lambeck, 1991].
ac.uk/psmsl/) in February 2003. Careful selection and edit-
ing criteria as given by Church et al.  are employed.
CSIRO Marine and Atmospheric Research, Hobart, Tasmania,
Where there are multiple records near a single satellite grid
point, the changes in height at each time step were averaged.
Antarctic Climate and Ecosystems Cooperative Research Centre,
The error estimates of first differences of
50 mm (the
Hobart, Tasmania, Australia.
solution is not sensitive to the value used) was computed
Copyright 2006 by the American Geophysical Union.
from the rms of the differences between the few sets of
nearby (within about 100 km) sea-level records. The impact
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CHURCH AND WHITE: AN ACCELERATION IN GLOBAL SEA-LEVEL RISE
corrections (0.09 mm yr
from the rms difference between
GMSL trends calculated using three different GIA models
[Church et al.,
2004]) and uncertainties in the EOFs
(0.1 mm yr
, see below). For 1950 to 2000, the linear
least squares trend is 1.75 mm yr
, consistent with earlier
estimates [Church et al., 2004; Douglas et al., 1991, 2001;
2001]. The yearly-averaged reconstructed GMSL
agrees with the T/P/Jason-1 satellite altimeter data from
1993 to within the error estimates (Figure 2). It also agrees
well with an average estimated directly from the tide
gauges, but has much smaller error bars.
] Fitting a quadratic to the GMSL time series gives an
(twice the quadratic coefficient) of
0.006 mm yr
(95%) for 1870 to 2001. The differences
between the quadratic and the GMSL time series have an
rms value of only 7.5 mm (Figure 2b), less than the error
estimates for most of the record. For the 20th century alone,
Figure 1. The number and distribution of sea-level records
the acceleration is smaller at 0.008 Â± 0.008 mm yr
available for the reconstruction. (a) The number of locations
Another approach, given the clear change of slope at
with sea-level data.
(d) the distribution of
1930, is to do linear regressions on the two halves
gauges in the 1980s, 1950s and 1900s.
(1870 - 1935 and 1936 - 2001) of the record. The slopes
are 0.71 Â± 0.40 and 1.84 Â± 0.19 mm yr
implying an acceleration of 0.017 Â± 0.007 mm yr
on measured sea level of the ongoing response of the earth
to changes in surface loading following the last glacial
maximum was removed using the same estimate of glacial
(GIA) as in our earlier study, as
calculated by Mitrovica and colleagues
Mitrovica, 1996; Milne et al., 2001].
] As the analysis technique allows us to ingest data
from a time-varying array of tide gauges in a consistent way,
we can use many more gauges to estimate GMSL than the
traditional approach of estimating sea-level rise [Douglas,
1991, 1992, 2001; Peltier, 2001]. This leads to better spatial
coverage and reduced errors in the estimate of GMSL as the
few very long records available are clustered in small
regions (mostly NW Europe, North America and a few in
Australia and New Zealand). The number of sea-level
records available for each year is a maximum in the
1980s (Figure 1) but decreases rapidly in the 1990s as a
result of late submission of data sets. Running back in time,
the number of gauges drops to about 140 in 1950, to just
under 50 in 1900, with the majority of gauges in the
northern hemisphere. By the 1870s, there are only 10 gauges
(none in the southern hemisphere) and by 1860 there are
only five gauges available, too few to extend the recon-
struction back past 1870. The number of records limits our
ability to reconstruct GMSL to January1870 to December
2001. To ensure that we always solve an over-determined
problem we use 5 EOFs prior to 1900 and 10 after 1900, but
the number of EOFs has virtually no impact on the
Figure 2. Global mean sea level from the reconstruction
dominant EOF amplitudes or on GMSL.
for January 1870 to December 2001. (a) The monthly global
average, the yearly average with the quadratic fit to the
yearly values and the yearly averages with the satellite
3. Global-Mean Sea Level From 1870 to 2004
altimeter data superimposed are offset by 150 mm. The one
] From the start of the reconstruction in January 1870
(dark shading) and two (light shading) standard deviation
to the end of the altimeter data in December
error estimates are shown. (b) Departures of the GMSL
(135 years), the total GMSL rise is 195 mm (Figure 2),
from the quadratic fit to the data. (c) Linear trends in sea
an average of 1.44 mm yr
. For the 20th century, the rise is
level from the reconstruction for overlapping
about 160 mm and the linear least-squares trend is 1.7 Â±
periods. The trend for each period is plotted at the centre
0.3 mm yr
(95% confidence limits). This error includes
time of the period. The error estimates of GMSL are a
allowance for the serial correlation of the time series, (four
minimum of about 5 mm in the 1980s rising to about 22 mm
years of data per degree of freedom), uncertainties in GIA
2 of 4
CHURCH AND WHITE: AN ACCELERATION IN GLOBAL SEA-LEVEL RISE
The rms residual to the linear fits is lower at
short-term (years to a decade or so) reductions in GMSL
mm), consistent with much of the acceleration
following major volcanic eruptions. The post-1960 major
occurring in the first half of the 20th century rather than a
volcanic eruptions of Mt. Agung (1963), El Chichon (1982)
smooth acceleration over the whole period. Recent esti-
and Mt Pinatubo (1991) offset about 0.005 mm yr
mates of regional- and global-MSL constructed using very
acceleration that is otherwise present, perhaps explaining
different techniques are qualitatively similar to ours, includ-
why little acceleration has been detected over the second
ing a significant acceleration in the first half of the 20th
half of the
20th century. The 1930s acceleration occurs
century (S. A. Jevrejeva et al., Nonlinear trends and multi-
during a period of little volcanic activity.
year cycles in sea level records, submitted to Journal of
] The quadratic implies that the rate of rise was zero in
Geophysical Research, 2005).
about 1820 when GMSL was about 200 mm below present
] While the GIA corrections are essentially constant
day values. This level is consistent with estimates from
from 1870 to 2000, it is possible that a temporally varying
bench marks carved in rock in Tasmania in 1840 [Hunter et
tide-gauge array may combine with errors in the GIA
2003] and the height of ancient Roman fish tanks
corrections to give an error in the computed acceleration.
[Lambeck et al., 2004], which implies virtually no long-
Tests, including assuming a 100% error in the GIA, indicate
term average change in GMSL from the first century AD to
negligible impact on the computed acceleration. Using
EOFs obtained from the recent altimeter data is another
] The 19th century commencement of the acceleration
potential source of error. Tests using EOFs determined from
is consistent with geological data [Donelly et al., 2004] and
different subsets of the 12 year altimeter record lead to 1-
long tide-gauge records [Woodworth, 1990, 1999; Maul and
uncertainty in GMSL trends of about 0.06 mm yr
Martin, 1993] but this is the first time a post 1870 (and 20th
negligible effects on the estimates of the acceleration. El
century) acceleration of GMSL has been detected. This
Nino-Southern Oscillation variability (ENSO, the dominant
acceleration is an important confirmation of climate simu-
signal in the low order EOFs) has been present for millen-
lations [Gregory et al., 2001] which show an acceleration
nia. However, over time, changes in ENSO patterns could
not previously detected in observations. Sea-level rise from
occur and we therefore almost double our uncertainty
20th century climate simulations [Church et al., 2001] is
estimate, assigning a total
error from uncertainty in
somewhat less than that inferred from observations, perhaps
EOFs of 0.1 mm yr
. Note also that any non-stationarity
because the acceleration of sea-level rise commenced during
of patterns will also be reflected in the error estimates of
the 19th century and by 1870, at the start of our recon-
GMSL. This conclusion of the relatively minor impact of
struction, was already rising at a rate of about 0.6 mm yr
the assumption of stationarity of EOFs on GMSL trends is
Both the rate of rise and the observed acceleration should be
consistent with previous work
[Chambers et al.,
valuable constraints to test the next round of climate
Church et al., 2004] and the application of these techniques
to the estimate of sea surface temperature variations [Rayner
et al., 2003].
Acknowledgments. This paper is a contribution to the CSIRO
Climate Change Research Program and the CSIRO Wealth from Oceans
Flagship and was supported by the Australian Governmentâs Cooperative
4. Implications and Conclusions
Research Centres Programme through the Antarctic Climate and Ecosys-
tems Cooperative Research Centre. T/P and Jason-1 data were obtained
] If this acceleration was maintained through the 21st
from the NASA Physical Oceanography DAAC at the Jet Propulsion
century, sea level in 2100 would be 310 Â± 30 mm higher
Laboratory/California Institute of Technology. Sea-level data is from the
Permanent Service for Mean Sea Level.
than in 1990, overlapping with the central range of projec-
tions in the Intergovernmental Panel on Climate Change
Third Assessment Report (IPCC TAR) [Church et al.,
Chambers, D. P., C. A. Melhaff, T. J. Urban, D. Fuji, and R. S. Nerem
2001]. For 1910 to 1990, the acceleration in ocean thermal
(2002), Low-frequency variations in global mean sea level: 1950 - 2000,
expansion (only) in these climate models range from 0.005 Â±
J. Geophys. Res., 107(C4), 3026, doi:10.1029/2001JC001089.
0.003 mm yr
to 0.014 Â± 0.004 mm yr
(Table 11.2 of the
Church, J. A., et al. (2001), Changes in sea level, in Climate Change 2001:
IPCC TAR), consistent with the present estimates of
The Scientific Basis, edited by J. T. Houghton et al., pp. 639 - 694, Cam-
bridge Univ. Press, New York.
0.013 mm yr
and 0.017 mm yr
for the 132 year period
Church, J. A., N. J. White, R. Coleman, K. Lambeck, and J. X. Mitrovica
and the 0.008 mm yr
for the 20th century.
(2004), Estimates of the regional distribution of sea-level rise over the
] Between 1930 and 1960, GMSL rises faster than the
1950 to 2000 period, J. Clim., 17, 2609 - 2625.
Church, J. A., N. J. White, and J. M. Arblaster (2005), Significant decadal-
quadratic curve at a rate of about 2.5 mm yr
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