Friday, March 30, 2007

Fergus Brown asks a sensible question:

in the comments about Worry,

I want to know what we are meant to understand by the term 'collapse'. It conjurs images of a Larsen-type crack and float-off, but I don't think this is what is expected of the WAIS, for example. Does the term refer to a point at which sudden and massive changes occur almost immediately, or to a system-state which represents a tipping-point, beyond which certain (relatively slow) processes are irrevocable?
IOW, what does a 'collapse' of the ice sheets mean?
UPDATE: Fergus has a blog

Steve Bloom, ankh and the mice do a fine job, and Eli adds a piece, but the Rabett would just like to add something from Hansen's A Slippery Slope: How Much Global Warming Constitutes “Dangerous Anthropogenic Interference”?
Time constants: the slippery slope. Three time constants play critical roles in creating a slippery slope for human society: T1, the time required for climate, specifically ocean surface temperature, to respond to a forced change of planetary energy balance; T2, the time it would take human society to change its energy systems enough to reverse the growth of greenhouse gases; T3, the time required for ice sheets to respond substantially to a large relentless positive planetary energy imbalance. I define “substantially” to mean a total sea level rise of at least two meters, because that would be sufficient to flood large portions of Bangladesh, the Nile Delta, Florida, and many island nations, causing forced migration of tens to hundreds of millions of people. That criterion requires an ice melt contribution from Greenland and Antarctica of at least 1.5 meters, given the approximate half meter contribution expected this century from ocean thermal expansion and alpine glaciers.

T1, the climate response time, is 50-100 years, as a result of the large thermal inertia of the ocean. T2, the energy infrastructure time constant, also is perhaps 50-100 years. Although new technologies that reduce or eliminate greenhouse gases might be developed rapidly, these need to replace a huge fossil fuel infrastructure, and this technologic task is preceded by the time required to achieve world-wide agreement on the need for replacement.

T3, the ice sheet response time, is the time constant of issue. I argue that T3 is of the order of centuries, not millennia, as commonly assumed. Growth of ice sheets requires millennia, as growth is a dry process limited by the snowfall rate. Ice sheet disintegration, on the other hand, is a wet process that can proceed more rapidly, as evidenced by the saw-toothed shape of glacial-interglacial temperature and sea level records. For example, I referred above to the 20-meter sea level rise that occurred in about 400 years during deglaciation 14,000 years ago.
The take home is that ice lost in centuries stays lost for millenia or more. Once the ice starts to go, it is gone. Hansen believes that T3 ~ T1 + T2 and is of the order of 1-3 centuries for reasons set forth in the Arxiv manuscript, the reference given above and elsewhere.
The likelihood that T3 is comparable to T1 + T2 has a staggering practical implication. T3 >> T1 + T2 would permit a relatively complacent “wait and see” attitude toward ice sheet health. If, in the happy situation T3 >> T1 + T2, we should confirm that human forcings were large enough to eventually alter the ice sheets, we would have plenty of time to reverse human forcings before the ice sheets responded.

Unfortunately, T3 ~ T1 + T2 implies that once ice sheet changes pass a critical point, it will be impossible to avoid substantial ice sheet disintegration. The reason for this is evident in the definition of the time constants. The comparability of these time constants, together with the planetary energy imbalance, make the ice sheets a ticking time bomb.
Is this correct? Wanna bet the planet friend? (OK, but we do need to concentrate on this issue until we better understand it, not to try and hand wave it away).

8 comments:

Anonymous said...

The way I like to look at this whole ice sheet melt-dynamics issue is as an erosion process.

As with rock, with ice, flowing water is a very powerful eroding force which greatly accelerates the speed with which ice melts and disintegrates.

The fact that this factor has been left out completely from the latest IPCC report means that the sea level rise estimate that they came up with is almost certainly too conservative (perhaps by a large margin).

Hank Roberts said...

Okay, someone's got to know the scientists in the Andrill group; last I saw was they had a shipload (that's a 'p') of cores on a boat headed for the research lab. Papers should be coming out. Their website is awfully empty, unless I'm missing something.

Last news is a month old:

SPIEGEL ONLINE - March 2, 2007, 12:35 PM

http://www.spiegel.de/international/0,1518,469495,00.html

ANTARCTIC ICE STUDY
Refrigeration System for the Earth's Oceans Threatens to Break Down

By Stefan Schmitt

"A surprising discovery in the Antarctic has scientists alarmed. According to an analysis of core samples, the southern ice shelf reacts far more sensitively to warming temperatures than scientists had previously believed. The message: sea levels may rise even further than feared......"

Much more at the article, link above.

I know it takes time to work on this material, it has to be done carefully and documented well. But .... anyone?

Steve Bloom said...

Good eye, Hank. I had completely missed that one. It's also the first I've heard of an extended warm period 5 million years ago. Paleoclimate sure is interesting.

Down in the article it said the cores are on a boat headed for Miami, and the team scientists will meet there in May to divvy them up. That sounds like no papers before the end of the year at best. OTOH they should be in a good position to feed information to Hansen's NAS panel (the formation of which I'm assuming is a done deal).

Fergus Brown said...

Fergus brown is humbled by the Rabett's reference; thanks, Eli. To add to your material and the stuff on the RC post, here are three new abstracts which deal with the issues. Being a new blogger, I have no shame at all in directing interested parties to my own site; http://fergusbrown.wordpress.com/2007/03/30/sea-ice-sheets-levels/

The way things are going, my money's on a metre by 2100.
Regards,

Hank Roberts said...

I mentioned the ANDrill story in that thread at Stoat, among several others I thought interesting on the lines of rapid change under old ice.

> Flowing water ... powerful erosive

No kidding. And water loaded with silt may be what formed a drumlin almost overnight under ice; water picks up to its capacity while flowing; drops silt when it slows, then can pick up/erode more.

Look at the Antarctic as ice-free; a central sea, lots of sediment. Build an ice cap on that; you get water and silt being extruded out of the basins, even flowing 'uphill' under that pressure; look at the radar maps, the ice may have pushed a lot of silt out to its edges over time, you can see the pattern of river drainage channels in the radar imagery.

Have a look for more links to info I found along those lines, posted in latter comments here: http://scienceblogs.com/stoat/2007/02/why_do_science_in_antarctica.php#comments

(I suppose I'm trying to be a 'Giant Cowbird' blogger; I add to other bloggers' nests rather than building my own, and hope to contribute by eating bugs there ... but there are other categories .... )
http://birding.about.com/od/behavior/a/broodparasites.htm

EliRabett said...

Well, you could always go back to being ankh, an honorary anonymouse. While as a Rabett we see little point in abandoning my nest for the blorg model like seed or grist, Eli would be happy to post the occasional post on request. Some editorial judgement allowed. EliRabett2003 at yahoo.

Anonymous said...

Hank said: "I'm trying to be a 'Giant Cowbird' blogger; I add to other bloggers' nests rather than building my own"

In the case of Eli's Blog, perhaps "Cowbunny" (related to the Jackalope) might be a btter term.

Adam said...

That page (scroll down) fails to mention that Rosebud mothered some Jackabasselopes with Portnoy being the father.