Tuesday, April 21, 2009

Catching up on sea level

Bit of a place holder while I finish two other sea level things -- faq update, and comments on the Nature paper.

One important bit, which I was reminded of in email:
A good decade before the Noerdlinger and Brouwer paper, Phil Hays had mentioned to me that the ice shelves were a good 10 times the sea level effect of sea ice.

The sea level FAQ of mine I mentioned is quite old. What you see at the moment is the 1997 version, which is only minorly different from the 1991 version. I leave the 1997 available because as it's been cited in the professional literature, I do want that exact version available. Still, a fair amount has changed in the last 15 years. Not the main emphasis, which is largely time-independant. But many details are now in need of revision. Among them are: glaciers are more significant than my casual writing there suggested, Greenland is much less stable than indicated (recent discoveries), West Antarctica may be rather more stable.

Vernon: Maybe you could do some rewriting of your note(s)? I still can't figure out why you think my FAQ is misleading. My main point is that the sea level effect of sea ice is not zero, though it is a small number. Your conclusion is that the effect is not zero, though it's a small number. Note that the faq predates the Rothrock paper you mention. Also note that a) sea water's density dependance on temperature is somewhat different than fresh -- enough to matter for this situation b) global mean ocean water temperature is 3.5 C, not 20 (and at colder temperatures, the density depends less on temperature) c) you've assumed that all energy to melt the ice comes from the ocean, which is physically unlikely. As, to be sure, is it unlikely to be entirely from the atmosphere (which was effectively my take). The reason for my taking it all from the atmosphere was to isolate the haline effects of melting the ice from the thermal effects. Plus, the sea ice is usually in water colder than global average, i.e., very near freezing (by which time, the density dependance of sea water on temperature is very much smaller, about 16x if I remember right, than on salinity).

One feature of the sea level business is that once you try to attain accuracy, it gets very messy very quickly. c.f. Jay Alt's comment:

Even the 'simplest' calculations of sea level rise from earlier decades contain at least 10 components that influence the result. Tides, temps, winds, . . .

Here is a very nice resource from James Titus, who examined these issues in depth for EPA.
http://users.rcn.com/jtitus/
http://www.climatesciencewatch.org/index.php/csw/details/epa-titus/

I've looked at the links and they're nice -- readable by nonprofessionals without losing the science.

The Nature paper http://www.nature.com/nature/journal/v458/n7240/full/nature07933.html is indeed interesting. It's also just far enough from my areas that it's difficult for me to be sure that something was missed, as opposed to being too obvious to mention in a professional setting. In short, though, it looks good and interesting, but may not mean quite what some of the press stories are suggesting. More to come.

1 comment:

Philip H. said...

And, for us oceanographers, there's all sorts of issues of which datum you are measuring sea level against, and hence how accurate that datum is.