ONSET OF 20TH CENTURY SEA-LEVEL RISE: EVIDENCE FROM PROXY RECORDS

The geographical coverage of instrumental sea-level observations is not uniform. Large gaps exist in the southern hemisphere, while the longest records all come from northern Europe. Geological and archaeological records of sea-level change, on the other hand, are often imprecise and lack sufficient temporal resolution in recent centuries. This paper presents high-resolution proxy sea-level reconstructions from recent salt-marsh sediments. It is shown that these can offer useful supplements to the small data base of long observational records.

Proxy records presented in this paper were obtained from salt marshes in Atlantic Canada, Iceland, the British Isles, and New Zealand. In each location the record is derived by using salt-marsh microfossils as sea-level indicators. Former sea-level positions are dated by a range of radiometric techniques (AMS¹⁴C, ²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am) and by specific stratigraphic markers (e.g., pollen, tephra, Pb concentrations, Pb isotopic ratios, palaeomagnetism). The resolution of the salt-marsh records is generally 5 to 10 years (age) and ±5 to ±20 cm (altitude) depending on site characteristics, especially tidal range. Comparison with nearby tide-gauge records demonstrate that the proxy records filter out much of the interannual variability of sea-level change but are capable of accurately reproducing the (multi-)decadal trends of relative sea-level change.

It appears that high rates of sea-level rise were experienced during the 20^th century in both northern and southern hemispheres. These rates commenced between the middle 19^th and the early 20^th century, coincident with global temperature rise. It is concluded that proxy records from salt-marsh sediments are of great value for documenting the spatial and temporal patterns of sea-level rise in the past 200 years.