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DB2: That is because I don't take Hansen seriously here. He is an extreme outlier, as you know. I think the original 20' claim in this thread refers to 'committed sea level rise' where the CO2 in the air by 2100 is estimated to eventually (over the next 1000 years) produce a 20' sea level rise.

bjchip: There are people far more extreme than Hansen.

Well, I guess that makes me an even more extreme outlier. I'm of the opinion that the risk of substantial sea level rise is much greater than commonly assumed. I'm not trying to be an alarmist here, just a realist.

The truth is, personal experience plays a big role in how people view change. If they've experienced something in their lifetime, or to some extent their parents' lifetimes, then this becomes accepted as a possible outcome. But when faced with a situation they've never personally experienced, most people are quite reluctant to accept that kind of possible change. So it is with large changes in sea level. No-one has experienced this in their lifetimes, or their parents' lifetimes, or in recorded history. So there is a strong presumption that this kind of extreme event can not and will not happen. The financial markets provide an excellent example of this mentality. As the Great Depression receded into a hazy memory, people started to say "it can't happen this time". Of course, 2008 put an end to that -- for now. Or then there are the truisms, such as "real estate always goes up". Remember that from a decade ago? Taleb's black swans will always be with us.

And so it is in the natural world also. We tend to think -- from our short bit of personal experience on this Earth -- that things are always stable and benign. But the geologic record tells a very different story. The present epoch -- the Holocene -- is highly unusual in the stability of its climate and sea level, and is a unique period in the Vostok ice core record going back 450,000 years. Very large changes (hundreds of feet) in sea level over millenia are the norm, and that's still a large change on timescales of centuries or even decades.

Around 9500 years ago, sea levels along the shores of Haida Gwaii (the former Queen Charlotte Islands) off the west coast of Canada rose by 450 feet in just 700 years. About half of this rise was due to subsidence of the land (from glacial rebound as the continental ice sheets melted), and about half was actual sea level rise. That's 225 feet of sea level rise in 700 years, or 30 ft (10 m) per century. Ice sheet melting in response to changes in climate forcings occurs over about 500 years -- there are many instances of "meltwater pulses" in the climate record occurring on this timescale. The change in atmospheric forcings today due to the rapid, and accelerating, increase in atmospheric CO2 is exceeding those of past periods when large changes in sea level are known to have taken place. In fact, very large changes in climate (and sea level in response to this) have taken place in the past due to quite small changes in orbital forcing. Do people really believe that these changes can not happen again today?

I've said this before but it bears repeating: the last time atmospheric CO2 levels exceeded 400 pm was about 35 Myr ago at a time when there was little or no polar ice on the Earth. There's not much obviously different about the world of today than that of 35 Myr ago with regard to climate forcings. CO2 (and H2O) are similar, orbital forcings aren't very different, the continents are more or less in the same place, the sun is only slightly brighter today, and overall albedos are similar (they're a bit lower today because of polar ice, but the poles don't cover much of the earth's surface). So I can not see why the outcome shouldn't be the same: a world without *any* polar ice (and 5+ C warmer). It will take maybe 1000, 2000, or perhaps 3000 years to get there, but that still means a lot of ice melting per century. There is about 230 feet (70 m) of potential sea level presently locked up in polar ice. Melting that ice over 2000 years would mean an average sea level rises of 11.5 feet per century, but the rise will probably start more slowly than that and accelerate over 500 to 1000 years (the 500-yr melting timescale again).

I believe the sheer magnitude of a 200+ feet rise in sea level is so great, and the consequences so calamitous that people don't want to think about, and just block it out. The long timescale makes this easy -- these changes won't affect us, nor our children (much), and maybe the scientists (and their reasoning) are wrong. Maybe, but the Eemian sea level rise is reasonably well-established: a rise of about 8 m (25 ft) in response to global temperatures just 1.5 C warmer than today. We are heading for a much larger warming that that of the Eemian peak, and a correspondingly larger rise in sea level.

How we are going to come to terms with this is a conversation we need to be having.

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