From S.J. Holgate, in GRL:
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).
On the decadal rates of sea level change during the twentieth century, GRL, Vol. 34, L01602, doi:10.1029/2006GL028492, 2007
Of late it’s been about 3 mm/yr, still a tiny number compared to what we would get if substantial ice sheet melt gets going. In that regard, these are still early days. IIRC there’s some conflict between gauges (what this paper apparently used) and satellite altimeters.
The rate of sea level rise as estimated from gauges has large decadal variability. For example, see Figure 2 in Holgate (2006) or Figure 2c in Church & White (2006). Both figures show the trend (linear trends for overlapping 10-year periods) varying between -1 and +5 mm/yr or so. This means that an estimate of the trend over, say, a 50-year period is sensitive to the choice of end points.
Church and White (2006) analysed data from many stations (many of them not operating continuously over the period) back to 1870 and found an acceleration of 0.013 ±
0.006 mm/yr2 (95%) for 1870 to 2001. (This acceleration might be easier to understand if you express it as 1.3 mm/yr per century, i.e. it’s reasonably large compared with the mean trend of ~1.7 mm/yr.) They note that there was an apparent change in trend around 1930 so they compare the trends for the 2 halves of their record and find slopes of 0.71 ± 0.40 mm/yr (1870–1935) and 1.84 ± 0.19 mm/yr (1936–2001)
Holgate (2006) uses fewer, higher-quality data, apparently to avoid inhomogeneities in the coverage. He is therefore restricted to a shorter period than Church and White (2006). He compares different periods from C & W (1904-1953 vs 1954-2003) and gets a result that sounds rather different. The difference probably arises from the difference in end points (though their datasets are not identical).
What’s not clear to me is how much of the large decadal variability is real (i.e it’s a property of the global ocean) and how much is a result of the sparsity of the tide gauge data. Eg. see Figure 1 in Holgate (2006)–there are large parts of the world not covered, even allowing for the fact that sea level trends tend to have large spatial scales. I guess we’ll know the answer to that question once we have a few decades of satellite altimeter data!
Ref:
Church J. A., White N. J. 2006. A 20th century acceleration in global sea-level rise. Geophysical Research Letters 33: L01602, doi:10.1029/2005GL024826.
Re the question implied by the title of your posting, I’d also like to draw your attention to the following (Holgate 2006 paragraph 20):
Although the mean rate of change of global mean sea level is found to be greater in the first half of the twentieth century, the two rates are consistent with being the same at the 95% confidence level, given their individual standard errors. However, a greater rate of rise in the early part of the record is consistent with previous analyses of tide gauge records which suggested a general deceleration in sea level rise during the 20th century [Woodworth, 1990; Douglas, 1992; Jevrejeva et al., 2006]. A twentieth century deceleration is consistent with the work of Church and White [2006] who, although finding evidence for a post-1870 acceleration based on an EOF reconstruction of global sea level, found that much of the overall acceleration occurred in the first half of the 20th century. Church and White [2006] suggested that the greater rate of sea level rise observed in the first half of last century was due to reduced volcanic emissions (and hence also lower variability in sea level) during the 1930s to 1960s. This idea is supported by results from the HadCM3 model which suggest that the simulated global mean sea level did not accelerate through the twentieth century due to the offsetting of anthropogenic warming by reduced natural forcing [Gregory et al., 2006].
Mark –
Thanks much for the thoughtful explanation.
Would the increased volume required to rise levels further (think of a sloping sided bath filling with water) have much affect on rates of sea-level rise? Or does that analogy hold erm, hold water with the Earth?
I would also like to thank Mark.
Aw shucks 🙂
Adam, the effect you describe is real, but negligible. Think of a map of the world with current coastlines. Now think of a map of the world with coastlines adjusted for any sea level increase you wish to assume (say 0.9 m, the upper end of the IPCC TAR range for 2100). How much larger is the area of the ocean in the latter case? Not much.
Thanks Mark, I was wondering if the increase in volume was larger than might initially be considered as world-wide it would add up (and then you multiply it by the depth). I’m not sure which is counter-intuitive…that it would be negligible or not negligible, when comparing it to the volume of sourc water (the ice). That said, I do realise that some rise is due to expansion which complicates it. Thanks for resolving it for me.