Curt Stager has some reassurance for us regarding Greenland's ice sheet:
Another way to estimate the durability of Greenland's ice is to look to the distant past. Ice cores and marine sediments show that dozens of cyclic natural warmings have punctuated the last 2 to 3 million years without totally deglaciating the poles. The one before the last ice age, the Eemian Interglacial, kept Arctic summers several degrees warmer than now between 130,000 and 117,000 years ago, but at least half of Greenland remained glaciated even after 13,000 years of Eemian heating.Oh, good. As long as it doesn't get any warmer than it was in the Eemian, we shouldn't lose more than half of the Greenland ice sheet. But of course, if you lose ice from Greenland you will probably lose ice from elsewhere, too, so we should probably look at what sea levels did in the Eemian, rather than just one source of what. They were about 13 to 20 feet higher than today.
Now one might object that Stager says this took place over 13,000 years. But that's not exactly what he said. He said that after 13,000 years, about half the ice was gone. That's not quite the same thing. Glaciers persist because of a balance of accumulation of new ice and melt. A given amount of warming pushes the equilibrium of the glacier towards a reduced size, but unless the reduced size starts to compromise the accumulation zone, the new equilibrium can be stable. Meaning that Greenland could have lost all that ice in a couple of centuries following the warming, and then remained in a stable half-melted state for the next 12,800 years. We don't know. Our proxies are not detailed enough to tell us. We know half the ice was there after twelve thousand years, but we don't know if that represents a rapid melt and a new equilibrium, or a gradual 13,000-years melting.
But, still. Twenty feet. We can do twenty feet, surely? I see the Bangladeshi delegation have their hands up. Please hold your questions until the end. Thank you.
The trouble with the reassuring anecdote of the Eemian is that it was not really all that much warmer than the present. Some parts of Europe were 1-2C cooler than today. The Arctic may have been "several degrees" warmer -- due to Arctic amplification, which we already observe with regards to the present warming.
|SRES A1B, warming per century, from Real Climate|
So what were the sea levels like the last time the world was +4C warmer (+8C at the poles) (the warming expected over the next century or so)?
If you look at the +2C line (which is probably the best case scenario for 2100, with aggressive mitigation; we are +0.8C now) you'll see the world has not seen temperatures that warm in the last ten million years, including the Eemian. The last time we were at +4C was in the Eocene, some 40 million years ago. What were the sea levels then? That turns out to be a tricky question to answer precisely, because over tens of millions of years, the land as well as the seas are moving up and down. But there was little ice in Antarctica, or in Greenland. Which, if it happened today, would equate to a 80 meter sea level rise -- 260 feet.
This is what the United States looks like after an 80 meter rise:
The Eastern seaboard is gone.
The Gulf is gone.
The principle cities of the West Coast are gone.
I don't know how long we would have to remain at +4C for this to happen. A very long time, I fervently hope. But if you are looking for analogies to the coming warming, mine seems closer to the coming centuries than his. Yet he's a note paleoclimatologist with a book out on this very subject:
So why what seems to be a falsely reassuring analogy? Perhaps I'm missing something basic. He's the climate scientist, not me. To my layman's understanding, he seems to be avoiding the uneasy truth of just how far out of the realm of anything we know we are pushing the world's climate. He's striving to sound reasonable, but the threat is objectively so much bigger than the discourse is prepared to accept, that to sound reasonable you have to downplay the logical implications of the paleoclimate record.