After reading the responses here, and re-reading Daly's report on the Royal Society Lecture, a couple of things are not clear. I would hope that Daly or the researchers might strive to resolve these questions. I may hope in vain, but nevertheless:
Daly says this in his report:
"Now that we have additional information to hand than existed in September last year when I first presented the story of the `Isle of the Dead', particularly the Lempriere tidal data for 1841 and 1842 (but not his earlier data) and the recent measurements by the CSIRO as reported at the lecture at the Royal Society, we can postulate more accurately what happened."
So the Lempriere tidal data for 1841-42 is "new" for his report.
Daly also says this (text in bold is my emphasis):
"They [Ross and Lempriere] then decide to calculate mean sea level from Lemprieres long term tidal records. How they do this is not known, nor what corrections they might have applied to the data, such as allowance for barometric pressure etc. Ross himself is already an expert in assessing the effect of atmospheric pressure on tide heights, having even worked out a correction factor [10]."
Here's one of the main problems. To establish mean sea level from tidal records, a record 19 years long is required! Look at the terms "National Tidal Datum Epoch" on this page: http://co-ops.nos.noaa.gov/glossary/gloss_n.html and "Mean Sea Level" on this page: http://co-ops.nos.noaa.gov/glossary/gloss_m.html. The last message in Daly's "Open Review" also refers to this period of time.
At most, Lempriere had five years of data before the benchmark was cut. That's not enough. For example, in the years 1990-93, the Pacific went into a strange El Nino phase in which El Nino conditions developed every summer and faded every winter. If something like that had happened while Lempriere was collecting data, his calculations could have been way off.
According to the CSIRO press release, the only data that they found was for 1841-42. So the predominant question is, could they use this data in combination with other techniques (see below) to establish the true mean sea level that is applicable to the position of the benchmark?
Here are the other techniques that the press release indicates were applied:
- Analysis of 19th Century sea level data
- Installing a modern tide gauge similar to those now used all over the world to measure sea level rise
- 20th Century measurements and analysis
- Site surveying using GPS (global positioning system) and modern surveying techniques
- Assessing the vertical motion of the land surface at Port Arthur using geological evidence from a shell bed at Mary Anne Bay, 42 km from Port Arthur and dated to 125,000 years ago.
- Modelling of the response of the earth since the end of the last glaciation about 6,000 years ago.
OK. Now I agree with Daly that it's not a good idea to discard the Ross account of the placing of the benchmark. At the same time, what Daly doesn't discuss is the fact that Lempriere did not have enough data to truly establish mean sea level. As he notes, how they did it using the data at hand (which was insufficient anyway) is completely unknown.
Daly overlooks or misses this point. He states in the part 2 of his "Isle of the Dead" article that Lempriere had "all the data necessary to calculate that point" (mean sea level). That's incorrect; Lempriere needed about 14 more years of data to do it.
The final possible key to the question is what Dr. Pugh says in the BBC article that Daly links to his site:
"Thomas Lempriere was a very bright environmentalist for 1841 and I think he did very good work. And when we have the CSIRO data, we'll then be able to make a direct comparison of the sea levels in 1841 and 1999."
So my feeling is, without having read the report (and it's unlikely I ever will, given that International Hydrological Review is not an online journal), that CSIRO has done the necessary work to establish what the 1841 mean sea level was at Port Arthur. Even if Ross and Lempriere did the best job of establishing mean sea level from Lempriere's data that they could, they didn't have enough data to actually do it.
The uncertainty about CSIRO's method of establishing the 1841 mean sea level is what I'd like to see cleared up. The paper probably tells how they did it -- so if anyone can get that paper and report what it says, I would be grateful.