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Historic Trend and Historic Eustatic Trend

Started by SAM, April 09, 2020, 10:56:47 PM

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SAM

I am using SLAMM 6.7 version. I am confused to give values in Historic Trend (mm/yr) and Historic Eustatic Trend (mm/yr) in edit sites parameters. In my region, the historical rate of sea-level rise is 5.2 mm/yr. What value I will assign in the historical eustatic trend?

Jonathan S. Clough

SLAMM often uses global future SLR estimates and then applies these locally based on the difference between local (RSLR) and global (eustatic) SLR.  [If the future SLR scenarios you are planning to use are localized (RSLR) then set these two parameters to the same value (e.g. 5.2 mm/year each) and move on.]

To estimate future RSLR from global SLR, SLAMM uses the difference between the local trend and the eustatic trend and assumes that that remains consistent into the future. 

However, the eustatic trend value can depend on the time of the measurement.  You want to match the eustatic historic trend time period with the local historic trend time period.

What time period is the 5.2 mm/year based on?

The eustatic rate of SLR is estimated at 1.7 mm/year from 1900 to 2000, but would be greater from 1970-2015 for example.

Here is another time I tried to explain these parameters:

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

SAM

Thank you Jonathan for your informative reply.

The time period in my region given by the tide gauge station is from 1948 to 2015.

Jonathan S. Clough

Here are some references:

Start Date End Date Rate Source N
1900 2000 1.7 mm/year +-0.5 IPCC 2007a §5.5.2.1 http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch5s5-5-2.html
1961 2003 1.8 mm/year +- 0.5 IPCC 2007a §5.5.2.1 http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch5s5-5-2.html
1993 2003 3.1 mm/year Grinsted et al. (2009).  Historical Data
1993 2003 3.3 mm/year Grinsted et al. (2009).  Satelite Altimetry

Also see http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch5s5-5-2-5.html

So maybe assume 1.8 mm/year from 1948-2003 and then 3.3 mm/year from 2003-2015?  So something like 2.1 mm/year for 1948-2015 perhaps?

There may be some newer references-- I haven't done a literature search recently.

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