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Hi Marc, you wrote:

"Many coasts are very gently sloped, as sea level rises water covers up
more land. In Florida a one foot rise in sea level will move the
shoreline inland by something like 100 feet. Every foot of sea level
rise increases the surface of the ocean by thousands of squre miles."

 

The effect you describe is real. But it's very small. Rather than write this up again, I'm going to quote myself from a message I wrote elsewhere (Compserve Science/Math Forum --yes, Compuserve still exists) back in April.

 

I wrote:

"Scrounging around for an envelope back...

Consider the volume of that next centimeter of sea level rise and how it compares with the previous centimeter. The volume is the total surface area of the oceans (about 70% of the Earth's surface area) multiplied by 1cm. That comes to about 3.6(10^12) cubic meters. Now the next cm has a slightly greater surface area since, as you note, the shoreline moves inland. How much additional area is there? We can estimate this if we know the total global area of the inter-tidal zone since that zone is created by a twice daily sea level cycle with a mean range of 1 meter. I've never seen that number worked up, but it's certainly less than 0.1% of the total surface area of the Earth, probably much less. And we're only looking at 1% of that number at most since we're looking at 1cm in sea level rise (vs. the 1-meter average range of the tides). So that means the additional area in question is definitely no greater than 5.1(10^9) square meters (and probably much less). That's the additional area we have to flood. It will fill to an average depth of 0.5 cm since it's a wedge of water so the total volume is only about 2.6(10^7) cubic meters. That's just about one 100,000th of the total volume of the water in the previous centimeter of sea level rise. So if we dump in exactly the same quantity of water, 3.6(10^12) m^3, instead of raising sea level by 1 cm, it will raise sea level by 0.99999cm. That's an effect that would be completely swamped in the data, pardon the pun. This is based on a very generous estimate of the additional area that is flooded. I suspect that the actual difference would be about 100 times smaller. In sum, flooding onto land cannot mask an increasing rate of sea level rise. The area flooded is far too small compared to the total area of the oceans."

Here's a link to that earlier thread, in case anyone wants to read it:

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