Monday, October 26, 2015

A Paper From Hansen et al. Is Now Open For Discussion - 2

Fig. 1
In the first post of this series, that Dredd Blog post linked to the Hansen et al. paper (A Paper From Hansen et al. Is Now Open For Discussion).

The Hansen team's choice of venue in which to air the paper is turning out to be a useful choice, in the sense that the comments and responses are not only helpful and instructive, but they also seem to take place in less time than some peer review processes do.

Today, I want to consider one comment made concerning the paper, as well as the
Fig. 2 Changes only
comprehensive response which the team provided about that comment.

One reason is that it gives us a good framework and opportunity to discuss the hypothesis about why the  jagged edge tide gauge graphs depict considerable fluctuations in sea level (fluctuations which take place as a matter of course in relatively very short time scales).

Fig. 3 Hansen 2015 paper @Fig. 29
Take a look at Fig. 1 and Fig. 2, which are graphs of PSMSL historical records only.

That is, there are no future projections included in those two graphs made from actual official records at the New York tide gauge station #12 ("The Battery").

The Fig. 1 graph has two boxes showing the year and the sea level at that year so as to emphasize the degree of fluctuation.

The first box shows one fluctuation that took place 1870-1878 which created a "W" shape.

The 1870 value is 6.769 RLR meters (6769 millimeters), which drops to 6.674m (6674mm) by 1874 (95mm decrease)  only 4 years later, then it jumps back up to 6.792m, 6792mm (118mm increase), 4 years later in 1878.

This is remarkable in the sense that Fig. 3 shows the mean at 3.3 mm yr (4 x 3.3mm = 13.2mm), so a 95mm and 118mm fluctuation are clearly inordinate.

This phenomenon is not something that only happens in the long-ago past, as the second box on Fig. 1 shows (1989 - 1997 jump of 132mm; 1997 @ 7133mm − 1989 @ 7001mm = 132mm).

Recall that NOAA folks were a bit astounded when a similar fluctuation took place north of the Fig. 1 and Fig. 2 location, which is PSMSL Station #12:
Coastal sea levels along continental margins often show significant year-to-year upward and downward fluctuations. These fluctuations are superimposed on a longer term upward trend associated with the rise in global mean sea level, with global mean sea level rising at roughly 3 mm per year during the recent 20 years of accurate satellite measures. For society, it is the regional changes along any particular coastal zone that are most important. Our analysis of multi-decadal tide gauge records along the North American east coast identified an extreme sea-level rise event during 2009–2010. Within this relatively brief two-year period, coastal sea level north of New York City jumped by up to 128 mm. This magnitude of inter-annual sea level rise is unprecedented in the tide gauge records, with statistical methods suggesting that it was a 1-in-850 year event.
(Agnotology: The Surge - 16, quoting NOAA). Their exclamation "1-in-850 year event" indicates that they are evidently not watching closely enough, because fluctuations are to be expected as ice sheet disintegration accelerates.

My hypothesis is that both Greenland and Antarctic ice sheet influence is primarily at play in these fluctuations, but there are some other factors too.

One example:
There are a number of implicit predictions, but most revealing, via contrast with IPCC models, is strong cooling of the Southern Ocean surface and in the North Atlantic (Fig. 1, from our paper). These coolings result mainly from the stratification effect induced by injection of meltwater into upper layers of the ocean. Lesser density of fresh meltwater, compared to salty ocean water, reduces sinking of surface water to the deep ocean. Reduced Antarctic Bottom Water formation reduces the amount of relatively warm deep water rising to the surface, where it increases heat flux to the atmosphere and space. Instead heat is kept at depth, raising deep water temperature and melting ice shelves (see diagram in Fig. 22 of our ACPD paper).
(Hansen Team Response). The Hansen team sees ocean surface cooling north of the PSMSL Station #12, which is closer to Greenland than NYC is.

That will cause some thermal sea level fall (SLF), as will Greenland ice sheet mass loss (The Gravity of Sea Level Change).

Then, the counter influence of Antarctica ice sheet mass loss will cause sea level rise (SLR) on the east coast (thus, both rise and fall of varying amounts at varying times), as fully explained in the video at the bottom of today's post.

There will be more on the saw-tooth fingerprints at PSMSL tide gauge stations in future posts of this series.

The next post in this series is here, the previous post in this series is here.

Professor Jerry Mitrovica, Harvard University, comes to D.C. to 'splain:

1 comment:

  1. Dr. Mitrovica brings the best out in sea level science.