CO2 is probably pretty closely related to global temperatures, all else being equal, but the distribution of heat can determine sea level (really, glacial ice mass at the poles) in a way that varies by a few meters, at least with a given global average temperature. Putting this another way, the average global temperature can be the same during two eras, but the average temperature in temperate and arctic regions can be a couple of degrees different C. Thus, the same CO2 produces a range of sea levels.

Over the full range of sea level changes (hundreds of feet) CO2 will correlate pretty well. But when we speak of 10, 20, 30 feet (less than 10% of the total range of variation during the last million years)…well, that is, I think, the expected variation due to other causes (i.e., the exact nature of the distribution of heat north-southwise).

What this means is that water on the top of an ice sheet can propagate down to the bottom as liquid water.  When that liquid encounters ice that is below the freezing point the liquid freezes and deposits its heat of fusion warming that ice up to the melting point.  At the melting point, ice has essentially zero strength. 

The only thing that keeps the bottom of the ice sheet frozen is the geothermal heat being conducted away through the ice sheet and dissipated into space during winter. Heat flow only occurs when there is a temperature gradient. If the temperature gradient in the ice sheet goes to zero (because it is all at the melting point), the heat flow goes to zero too and the ice at the bottom melts due to geothermal heat.

Km thick ice sheets don’t melt like an ice cube. When they get too warm, they fail catastrophically and flow into the sea like a river. This is a highly non-linear process that once it starts cannot be stopped.

Some of earlier altimeter data of Greenland showed the ice cap to be getting thicker. The gravity data now shows that it is losing mass. What might have happened (but the data is too sparse to be sure), is that the ice sheet got thicker because it got warmer in depth via thermal expansion.

At present CO2 levels (~400 ppm), the Greenland ice sheet is unstable and will melt. That is not something that any knowledgeable ice sheet modeler disputes. The only issue is over the timing.

When Greenland does melt, sea level will go up ~7 meters. What is the value of the real estate that will be flooded when that happens? If we could stave off that melting for one year by spending the equivalent of the net present value of one years continued use of that property every year, it would be cost-neutral to do so. Right now it would probably take less than one year’s NPV. If we wait long enough, the cost to prevent Greenland from melting will increase beyond the point where it is even possible.

I’ve never really lived any place snowy… just upstate New York and Minnesota. No glaciers. Well, in the ADK’s there was permanent ice but that is probably gone now.

Nature has never forced the ice sheets as hard as we are doing now, and it’s hard to predict how exactly they will fail or how long it will take. But these are huge things, and they won’t melt like a popsicle on a hot sidewalk overnight.

No, they won’t. What I’m saying here, though, is different. The models thus far have been outpaced on arctic melting for the last several years. The current models may allow for but don’t well accommodate Eemian sea levels. To put it a slightly different way (though I do say this above but I probably didn’t highlight it sufficiently): There is too much ice right now, if we take the Eemian as a standard (which we could do just as well as take the present); this combined with melting outpacing the models leads me to suggest that the transfer of H2O from glacial ice to oceanic liquid could be much faster than otherwise predicted (the alternative to “30 years” for some value is not “Tomorrow” though) and possibly even shift metastatically.

I do agree with you that things can shift in a big way, but that big way is exactly those exceptional regions you leave as an exercise. The exact favored wobbles of the main features can in fact change. These changes may be small on the globe but they are huge on the ground. Whence weird seasons.

And Greenland is having a weird season now. I don’t know if you’ve ever lived in a snowy place though. A little surface melt is a long way from substantial runoff. The water will mostly soak into the ice pack, compressing it a little.

Nature has never forced the ice sheets as hard as we are doing now, and it’s hard to predict how exactly they will fail or how long it will take. But these are huge things, and they won’t melt like a popsicle on a hot sidewalk overnight.

The worst case for the first meter seems longer than 50 years. Maybe 100, maybe 200; the general consensus is weakly around a century. After that, though, coastlines that aren’t steep are going to be a big mess for a very long time.

But it won’t be this year or next.

That was true at one time. Other than the Tibetan glaciers, most other glacial fields are rapidly melting.

Glacier Nationa Park went from 150 glaciers at the beginning of the 20th century to about 25 now, and many have melted in the last couple of years.

Here’s an interesting test of the models: The prevailing model predicts that the last of Glacier’s glaciers will be gone by 2030. Good luck with that!