Forecasting is very difficult, especially when it involves the future.
Sunday, 24 June 2012
Extending the record - a sea level reconstruction for Sydney
On my reference page Sea Levels in Australia (top right sidebar) I state that Fremantle has the longest record of any Australian tide gauge, from 1897. The following is shown for Sydney:
However, it's little known that for a long period, there were two active gauges on Fort Denison, the older of the two recording from 1886, so predating the second by some 28 years. Unfortunately the two gauges used a different benchmark datum (reference level) for measurements, preventing a simple backward extension of the record. By plotting the records together, I've been able to establish a good estimate of the difference (-127mm).
Adding that offset to the earlier data to line up with the data from the second gauge, resulted in the following reconstruction.
As the 5-year (61 month centred) moving average shows, sea level was dropping between the turn of the century and the middle 1940s, then there was an upward jump to 1950. The overall rate of rise is somewhat less than that shown for the single-gauge record, which was "pulled down" by the slight dip to the late 1940s. I've produced a reconstruction for Newcastle, NSW which extends the record back from 1966 to 1925, using the same technique, and will post it very soon.