I probably shouldn't have said "substantial" since what I really meant was "not cancelled out in the way mwengler describes".
I don't think I can actually do the calculation without an estimate of the typical gradient of coastal land in the US (i.e., the conversion factor from sea level rise to shrinkage) but let's make a crude guess and see what happens. So, North America has an area of about 25M km^2 so our square is about 5000km on a side. Expected sea level rise by 2100 is about 0.5m (I've seen wildly inconsistent figures for this, though). Let's suppose that sea-level land has a typical gradient of 1 in 50, so that a 0.5m rise means a 25m shrinkage in the usable land. Then the total amount of land lost would be about 20000km x 25m = 20km x 25km = 500 km^2, roughly comparable to the area of San Francisco.
This is probably an underestimate: North America is wigglier than our square model (so more coast relative to its area) and I suspect that actually coastal land is flatter than 1 in 50.
So it's a small fraction of the total area (as of course was obvious from the outset) but personally I'd consider it a substantial loss if an area the size of San Francisco fell into the sea.
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).