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QuoteThe historic trend is assumed to be the [global eustatic SLR] plus or minus [local factors] such as subsidence or uplift or other local meteorological conditions that could cause the difference. If you are not using a local SLR scenario (running with a eustatic SLR scenario) then those [local factors] are assumed to remain consistent throughout the projection.
Therefore if the local historical trend is 2mm/year greater than the eustatic trend due to subsidence then [local factors] is calculated as 2mm/year and the future SLR is predicted to be 2mm/year greater than eustatic SLR projections.
Historic Eustatic Trend = Global eustatic SLR. (According to technical documentation, use rate of 1.7 mm/yr from 1900 to 2000)
What time period would be best to use for the Historic Trend?
The reason that parameter is there at all is to match the time period of the local historic trend. Therefore if your local data is from 1970-2000 you would probably want to use a higher historic trend than 1.7 mm/year.
And is it correct that the Historic Eustatic Trend is subtracted from the Historic Trend to identify the isostatic/local trend in SLR?
Yes, that is the whole point of those two parameters. That is why their time periods should match if possible.
So if you had a local trend of 4 mm/year from 1960-2010 and you have data showing that the historic eustatic trend from 1960-2010 is 2.5 mm/year (rough estimate off the top of my head). Then SLAMM will assume that the 1.5 mm/year difference is due to local factors and should be added to any eustatic SLR scenarios.
On the other hand if you have local SLR projections, like many states are producing, then these two parameters should be set to the same value to indicate that you do not want to adjust the future SLR projections as local factors have already been accounted for. (e.g. using NY specific SLR scenarios, we set both the historic trend and historic eustatic trend to 1.7mm/year.)
Hope this is clear. -- Jonathan