The official IPCC position is:
"Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 [2.4 to 3.8] mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear" (http://www.ipcc.ch/publications_and_data/ar4/syr/en/spms1.html)
This "fast rate" from 1993 to 2003, only 10 years then, was from TOPEX and Jason measurements:
A paper by Holgate showed that the rate fluctuated over the past 110 years of known in situ measurements. The abstract reads:
Nine long and nearly continuous sea level records were chosen from around
the world to explore rates of change in sea level for 1904–2003. These records
were found to capture the variability found in a larger number of stations over
the last half century studied previously. Extending the sea level record back
over the entire century suggests that the high variability in the rates of sea
level change observed over the past 20 years were not particularly unusual. The
rate of sea level change was found to be larger in the early part of last
century (2.03 ± 0.35 mm/yr 1904–1953), in comparison with the latter part (1.45
± 0.34 mm/yr 1954–2003). The highest decadal rate of rise occurred in the decade
centred on 1980 (5.31 mm/yr) with the lowest rate of rise occurring in the
decade centred on 1964 (−1.49 mm/yr). Over the entire century the mean rate of
change was 1.74 ± 0.16 mm/yr. (Holgate, 2007)
You can see this clearly in a number of in situ stations from around the world such as
Global stations can be found here http://tidesandcurrents.noaa.gov/sltrends/sltrends_global.shtml. However, you have to be careful to pick a location that is on a stable craton (land mass), because tectonics alters the apparent rate at locations where there is subduction, glacial rebound or rifting.
In all cases, not one global station shows any acceleration in the rate of sea level rise.
In fact, this study (http://nzclimatescience.net/images/PDFs/spsl3.pdf) shows no change in many of the Pacific islands, including Tuvalu, the island that claims they are being inundated and swallowed up by the sea. Tuvalu is the poster child of sea level rise, yet this study states:
"If the depression of the 1998 cyclone is ignored there was no change is sea level at Tuvalu between 1994 and 2008; 14 years, despite 14 separate tsunami events The claim of a trend of + 6.0mm/yr is without any justification."
Maldives is another poster child, with the President of the island at Copenhagen making his claim of being swallowed up. Yet this study
(http://gsa.confex.com/gsa/inqu/finalprogram/abstract_54486.htm ) finds no increase:
"The Maldives have a uniquely position in sea level research (as discussed
in Integrated Coastal Zone Management, No. 1, 2000, p. 17-20). In the last
decade, they have attracted special attention because, in the IPCC-scenario, the
Maldives would be condemned to become flooded in the next 50-100 years. Our
research data do not lend support to any such flooding scenario, however. On the
contrary, we find no signs of any on-going sea level rise. Our
results comes from visits to numerous islands including extensive work on
Hulhudoo and Guidhoo in the north, in Viligili and Loshfuchi (the site of "the
reef woman") in the middle, and in Addu in the south. This includes coring,
levelling, sampling and dating (35 C14-dates). Present sea level was reached at
about 4500 BP. In the last 4000 years, sea level oscillated around the present
in the last 4000 years. At 3900 BP, there was a short and sharp sea level
high-stand at about +1.2 m. For the last millennium, a detailed sea level record
is established: +0 m 1000-800 BP, +60 cm 800-300 BP, 0 to just below 0 in the
18th century AD, +30 cm 1790-1970 AD, fall to 0 in ~1970 up to today. At about
1970, sea level fell by 20-30 cm (presumably due to increased evaporation). This
is recorded in storm level, high-tide level, mean sea level and in lake and
lagoon levels (from the north to the south). In the last decade, there are no
signs of any rise in sea level. Hence, we are able to free the islands from the
condemnation to become flooded in the 21st century (bold added)."
Sea level rise has two components. Steric and mass. Steric deals with the salinity and thermal components that change the volume of water, where as mass is the physical change in the volume of water. Steric is influences by the temperature of the seas, and mass is changed by the inflow of water from ice sources.
A summary of this can be found here: http://www.co2science.org/articles/V13/N4/EDIT.php
Even though TOPEX shows a 3.2mm/yr since 1993, this paper (http://ibis.grdl.noaa.gov/SAT/slr/documents/NOAA_NESDIS_Sea_Level_Rise_Budget_Report_2009.pdf) which closed the balance between the two components shows a rate of 1.8mm/yr, in close agreement with Holgate.
Thus no acceleration.
But this has not stopped AGW proponents from claiming there is an acceleration:
"Multi-century sea-level records and climate models indicate an acceleration
of sea-level rise, but no 20th century acceleration has previously been
detected. A reconstruction of global sea level using tide-gauge data from 1950
to 2000 indicates a larger rate of rise after 1993 and other periods of rapid
sea-level rise but no significant acceleration over this period. Here, we extend
the reconstruction of global mean sea level back to 1870 and find a sea-level
rise from January 1870 to December 2004 of 195 mm, a 20th century rate of
sea-level rise of 1.7 ± 0.3 mm yr−1 and a significant acceleration of sea-level
rise of 0.013 ± 0.006 mm yr−2. This acceleration is an important confirmation of
climate change simulations which show an acceleration not previously observed.
If this acceleration remained constant then the 1990 to 2100 rise would range
from 280 to 340 mm, consistent with projections in the IPCC TAR. "(Church
& White, 2006)
I fail to see how a 0.007% increase constitutes a "significant" acceleration!!
It has also not stopped them from claiming future increases away from this average. We have seen all these wild speculations, 20 feet in 100 years, 50 feet in 100 years, etc. Anything to make the scariest claims. But how realistic are any of these? It’s simple to show. On only has to plot the current rate over that past 110 years of in situ measurements and then extrapolate the various scenarios.
As you can see, to get to a linear run up to 2 meters in 100 years would require an instant change in the current rate of rise from 1.74mm/year to 20 mm/year. The longer this snap up trick doesn’t happen the steeper that rate to the 2 meters must be.
But of course that is not what these guys claim. Their claim is a gradual increase as in the curved red line. But what they don’t tell you is this is a growth curve. It is simple enough to show what the growth rate must be to reach 2 meters in 100 years. You simply put the data into a spreadsheet and have it calculate what the growth would have to be to meet that target. It’s 4.3% per year. At this rate the doubling time is around 9 years.
This means that every 9 years the rate of sea level rise would double from the previous 9 years. This is the only way to get from here to 2 meters in 100 years. This means by the last year, 2099, the rate of sea level rise would have to be a staggering 84mm/year! That’s more in one year than in all of the previous 110 years. Sorry that is just not credible.
In Sept of 2008, 3 researchers, and supporters of AGW, dropped a bombshell paper where they calculated the maximum possible rise in sea level up to 2100.
"On the basis of climate modeling and analogies with past conditions, the
potential for multimeter increases in sea level by the end of the 21st century
has been proposed. We consider glaciological conditions required for large
sea-level rise to occur by 2100 and conclude that increases in excess of 2
meters are physically untenable. We find that a total sea-level rise of about 2
meters by 2100 could occur under physically possible glaciological conditions
but only if all variables are quickly accelerated to extremely high limits. More
plausible but still accelerated conditions lead to total sea-level rise by 2100
of about 0.8 meter. These roughly constrained scenarios provide a "most likely"
starting point for refinements in sea-level forecasts that include ice flow
dynamics."(Pfeffer et al., 2008)
This sent off a scurry of activity at RealClimate who were not too happy with the paper. (http://www.realclimate.org/index.php/archives/2008/09/how-much-will-sea-level-rise/)
Pfeffer even had to come to RC to defend the paper.
But of course, we all know projections of what MIGHT be are a far cry from what WILL be. The current TOPEX of 3.2mm/year is just one increase of several that Holgate noted has occurred in the past 110 years. Until that rate goes beyond that normal variation, there is no scientific justification for claiming this shows a true acceleration.
Here is a case in point. This paper claims acceleration at Brest France:
"Three linear trend periods can indeed be distinguished in the Brest MTL
time series over the period 1807–2004:
(a) 1807–1890, over which the sea level rate is estimated at −0.09±0.15 mm/year
(b) 1890–1980, at +1.3±0.15 mm/year
(c) 1980–2004, at 3.0±0.5 mm/year"( Wöppelmann, et al, 2006).
So let’s have a look at these "accelerations" on the graph. (I emailed Wöppelmann questioning this "acceleration" and asking for his data. He did not reply, sound familiar?)
It is clear from this graph that there have been other times in the past where the rate was higher in short periods than it is today. It’s a huge leap of faith to claim these three periods are any kind of significant acceleration.
They note that this "acceleration" is "…estimated at 0.0071±0.0008 mm/year2 over the period 1807–2004 using a simple quadratic least-squares adjustment." With an over all rate of 1.00 for Brest, an "acceleration" of .0071 is 0.71% growth. At that rate of growth the highest sea level can get in 100 years is 23cm, or 1 foot. The upper end of the IPCC prediction. That’s a far cry from the 80cm even for the max that Pfeffer et al claim.
Suffice it to say, there is no credible evidence for any kind acceleration in sea level rise, which is required by AGW theory to happen. It’s just not there. Those such as Church and Wöppelmann are grasping at straws. This is not only unscientific, it is disgraceful to try and deceive the public and policy makers. It is clear to anyone that there is no acceleration.
"there is no discernible divergence in the rate of sea-level rise over the past
two centuries to suggest a connection with the documented increase in
atmospheric CO2 concentration… the rate of sea-level rise has been linear over
this time period and shows no indication of the pronounced mid-20th-century
increase" (Larsen and Clark, 2006)
Church, J. A., and N. J. White (2006), A 20th century acceleration in global sea-level rise, Geophys. Res. Lett., 33, L01602, doi:10.1029/2005GL024826.
Holgate, S. J. (2007), On the decadal rates of sea level change during the twentieth century, Geophys. Res. Lett., 34, L01602, doi:10.1029/2006GL028492.
Larsen, C.E. and I. Clark. 2006. A search for scale in sea-level studies. Journal of Coastal Research, 22(4) ,788–800.
Wöppelmann . G, N. Pouvreau, B. Simon (2006) Brest sea level record: a time series construction back to the early eighteenth century, Ocean Dynamics 56: 487–497
Wöppelmann, G., B. Martin Miguez, M.-N. Bouin, and Z. Altamimi. (2007) Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide. Global and Planetary Change, 57, 396–406.
W. T. Pfeffer, J. T. Harper, S. O'Neel (2008) Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise, Science: Vol. 321. no. 5894, pp. 1340 - 1343