Since I first met them, I imagined John Chruch and John Hunter riding bicycles, shirt sleeves rolled up, eyes squinting into the sun. Standing before us on the Aurora Australis in January 2002, they described their research into sea levels rising.
There are many factors that need to be taken into account when visualizing changing sea levels. Predicting sea level rise is a tricky business. Land moves upwards and downwards over geological time. In Antarctica, pressure from melting ice sheets is released, and the continent moves upwards. This complicates the readings of fixed tidal guages.
The following diagram (Church et al, 2001) takes into account many factors in its projection of sea level changes:
Looking South: Managing Technology, opportunities and the global environment, is a paper delivered by Dr John Church, to the Academy Symposium, in 2001.
The following text describe observations of sea level changes.
Observed Changes in Sea Level
Changes since the Last Glacial Maximum
Over the last several hundred thousand years, sea-level has fluctuated by over 100m as the mass of ice stored on land has changed. At the last glacial maximum, sea level was over 120m below present day levels. The majority of the ice required to lower sea level at this time was stored in the northern hemisphere ice sheets with lesser amounts stored in Antarctica and elsewhere. The most rapid rise in sea level occurred between 15,000 and 6,000 years ago at an average rate of 10 mm/yr, but with peak rates as high as 40 mm/yr. Over the past 6,000 years sea level rise has been much slower. Geological data indicate an average rate of rise over this period of 0.5 mm/yr, and perhaps 0.1 to 0.2 mm/yr over the last 3,000 years.These large transfers of mass from the ice sheets to the ocean result in vertical land movements (post-glacial rebound) that are ongoing today. In regions distant from the former ice sheets and near the center of former ice sheets the land is rising whereas in some regions surrounding the former ice sheets land is currently falling. Allowance for these vertical land motions are essential if we are to accurately infer sea-level changes from measurements of “relative” sea-level change as measured by coastal tide gauges.
Instrumental Observations of Sea-level Change
Relative sea level (ie the height of the ocean surface relative to the land) is measured by coastal and island tide gauges. Unfortunately for estimating global sea-level rise, the historical distribution of tide gauges is far from optimum. Most long records are from gauges sited along northern hemisphere (particularly European) coastlines with relatively few records at mid-ocean island sites or in the southern hemisphere. The available long records, some dating back to the 16th century, indicate an increase in the rate of sea-level rise of 0.4 to 0.9 mm/yr/century.
To estimate the absolute rate of sea-level rise, it is necessary to correct the tide gauge data sets for vertical land motion resulting from post-glacial rebound and other tectonic motions. These corrections are based on either geological observations or geophysical models. Even during the 20th century, the distributions of tide gauges necessary for measuring global average sea-level rise is far from ideal and estimates of 20th century sea-level rise depend on the selection of tide gauge sites and the method used for correcting for land motions. The IPCC assessment (Church et al, 2001) was that the rate of 20th century sea-level rise was between one and two mm/yr and that no increase in the rate of sea-level rise has been detected from the 20th century data set. In this assessment, we recognised that the spatial distribution of the gauges was potentially an issue biasing various estimates. Model results (Gregory et al, 2001) indicate that the bias introduced by the spatial sub-sampling was small while a recent analysis of estimates of ocean thermal expansion (Cabanes et al, 2001) indicated a larger bias. However, the data set used for the estimates of ocean thermal expansion is itself incomplete and the magnitude of the spatial bias inferred by Cabanes et al (2001) has been questioned (Church, 2001). The model data also indicate that it would be difficult to detect any 20th century increase in the rate of sea-level rise from the available tide gauges in the presence of spatial and temporal variability.
Satellite observations are revolutionising our ability to measure sea-level change. Satellite altimeter missions like TOPEX/POSEIDON give near global observations of sea level. Since its launch in late 1992, TOPEX/POSEIDON data indicates a rate of global-mean sea-level rise of greater than 2 mm/yr. However, the record is as yet to short to infer a recent increase in the rate of sea-level rise. Satellites have relatively short life times but a successful launch of Jason-1 on 7 December, 2001 (and subsequent missions) will ensure an ongoing record of sea-level change. It is necessary to carefully control biases within and between satellite missions using in situ tide gauges. Here again modern technology is assisting with more accurate and stable tide gauges and the ability to measure vertical land motions using the satellite based Global Positioning System.