NOTE: I’ve rewritten this post and redone the graphic. The original map on which I based the reconstruction, provided by the USGS, is distinctly different than the one the USGS provides today. The difference is, in fact, rather dramatic. In comparing the older and newer versions of the maps, I have decided to assume the later, more recent, version is more correct. I admit to being a little annoyed at the USGS providing a truly bogus map on their web site, but that is water under the bridge, as it were. So, the following post is edited a bit and a new graphic is provided. Thanks to wehappyfew for pointing out the likely error on the map.

There have been times in the past when there was very little ice trapped in glaciers. During this time, sea levels were higher because that water was in the ocean (most of it, anyway). It has been a long time since then. However, with global warming, more and more glacial ice is returning to the sea and this contributes to sea level rise.

The amount of fossil carbon that needs to be released into the atmosphere to cause most of the glacial ice to melt is not known. We can’t directly use ancient time periods to assess modern sea level rise by measuring the sea levels from those periods because there has been too much other stuff going on in ocean basins and along current coast lines. But, we can estimate that there was very little glacial ice during, for example, the early Eocene, and the transition of Carbon in the atmosphere to the formation of glaciers might be under 800 ppm. So, if we double the current amount of CO2 in the atmosphere, maybe that would melt all the glaciers. There was more methane in the air at that time as well, but we are releasing plenty of methane as we also release Carbon, so that’s not much of a problem. The biggest factor is probably this: The configuration of continents have changed since that time to increase the likelihood of glacial formation at the poles, so returning to some Eocene (or other) atmospheric CO2 value may result in much less melting. But that’s OK, because we can certainly increase the amount of carbon to more than around 800 ppm!

If we release CO2 at approximately modern rates (baed on population size), and have population increase up to a point, thus increasing CO2 release (in other words, do nothing significant to mitigate Carbon release, increase the number of people actively releasing it, and population goes up towards 8 or so billion) we can reach over 1000 ppm by 2300 AD, or sooner. That’s surely enough to melt most of the glaciers except bits and pieces in the coldest regions of Antarctica.

It is estimated (see this web page.) that there is about 80 meters of ocean trapped in glacial ice. There are plenty of web sites out there that allow you to add ocean height to see how coastal regions would change, but the ones I know about don’t go to 80 meters. So, to find out what North America would look like, I found a map that has pixels to indicate altitude, with different colors representing topography, at a fine enough level to work with.

The USGS has a map with color coded topography. There is a color break at 60 meters, which is much less than the maximum possible sea level rise. The next break is at 114 meters. That is higher than sea levels will rise. However, if sea level rises to about 80 meters, it will do so unevenly (it may, for example, be much higher in the Carolinas). Then, as sea level rises, the land will be pushed down various amounts by the weight of the water, so 80 meters might be considered a minimum estimate of rise in some areas. Even more important, I suspect, erosion would cause important changes. If you look at, say, a 60 meter topo line in a region made of something other than hard rock, you have to assume that transgression of the sea including the effects of erosion would move way inland in some cases, beyond that topo line.

So, since we are at present looking for an 80 meter contour line easily located on the right scale map, and we only have 60 and 114, but the real contour line we are probably looking for is higher than 80, we could round UP to 114. But that would almost certainly depict inundation of areas that won’t actually be inundated. So, what I’ve decided to do is to put the ocean at 60 meters, then make a grey area (to reflect, well, this being a grey area!) between 60 meters and 114 meters. With ALL of the ice melted, the shoreline will likely be somewhere in this grey area, probably covering all of it (and more?) along the south coast and probably much less in Maine. Either way, Florida is toast. Wet soggy toast.

Also, I decided to focus in on this map a bit and depict the US east of the Rockies. At this scale, the west coast is fairly uninteresting using this method (the continental margin is right at the coast, so it is very steep). And, the transgression effect, the sea moving laterally across the land after a rise, is probably very locally variable and unpredictable there anyway.

One of the interesting things I discovered is that when defining the zone between 60 and 114 meters, that turns out to be a fairly narrow strip along much of the coast. This is what one would expect if somewhere in that zone is the original high strandline from the last time sea levels were that high (a few million years ago or so). So that’s cool.

This is a VERY ROUGH approximation. Just for fun.