A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Jan 23, 12:39 -0800
Mark Coady, you wrote:
"I am wary of newspaper science, as the forum of front page journalism does not always lend itself to technical precision."
Sure. And there are lots of other factors at work here, too. The baseline claim that sea level rise in the northeast suddenly started accelerating forty years ago doesn't hold up. We've got a long dataset from the tide gauge at "The Battery", see below, at the southern tip of Manhattan, and there's no significant change apparent during the 125-year run from 1893 to 2018 (there's some earlier data with a gap, but the continuous dataset begins in 1893). In that period sea level here on the east coast has risen at a rate of about 11 inches per century. Sources that claim evidence of accelerating sea level rise are usually combining data from different sources, like tide gauge data in earlier decades and satellite data in later decades. Unfortunately, there are calibration issues.
"The simple minded part of me goes back to digging a large fish pond using a full hose from corner to corner to keep the sides level. My gut level, which is of course overly simplistic, says that if the mass of ocean water were already dispersed based on the mass and gravity of this third rock and our other gravitational impacts like moon and sun.....why would it unevenly change the height of sea level if you added more water? Isn’t ultimately all one connected big puddle that would relevel at the end...?"
That's exactly right. Of course, there can be periodic motions. In your fish pond, we could set up a number of distinct resonant, long-lastiing oscillations that would create steady waves rocking back and forth down the pond (in lakes, these are known as "seiches"). But if the oscillations are not driven by some external force, they eventually die out, and, as you say, it's back to even levels on both ends. We can also modify levels through currents. Suppose you place a turbine at the bottom of your fish pond that forces water towards one end. The pressure from the turbine will create a modest increase in water level at that end that will last as long as the turbine is running. In the oceans, the "turbines" are replaced by the heat engines that maintain major ocean currents. These lead to measurable, long-lasting differences in ocean height across oceans. In addition, hot water expands. So anywhere that has permanently warmer water, even if the pattern is complex and internally changing like in the Gulf Stream, is elevated, literally higher, than neighboring areas. It's these small differences in height that are observed by ocean survey satellites that allow us to map ocean currents. We see the "bumps" in the ocean surface (after a very difficult mathematical process that removes all other phenomena, especially the tides), and those bumps correspond to higher temperatures, and those match well with the ocean currents.
In the North Atlantic, the ocean currents have long-term stability. That could change, and it may be changing right now (depending on how fast the Greenland ice cap is melting). In the Pacific long-term and cyclic changes in ocean currents are substantial and well-known. They're a major component of the ENSO (El Niño Southern Oscillation) cycle. There are cycles in local sea level all around the Pacific that are directly connected to the ENSO pattern. For example, if we look at the sea level data from the tide gauge in San Diego, we can see several periods lasting a few years where sea level jumps up. In the data below, we can see that sea level jumps up in the early 1980s and again around 2013-2015 as well as other dates. These are both well-understood temporary level changes that are part of the ENSO effects.
"Of course we do have all that melting ice...what of that......that’s relocating a substantial once solid mass while changing it to a liquid state..."
Right. Just remember floating ice doesn't impact sea level. Ice from the Greenland ice cap and the (non-floating) Antarctic ice cap matters. If that melts, sea level rises. But melting Arctic sea ice doesn't affect sea level.
"What of our “wobble”.......uh...rotational speed, precession, nutation, and regional perception of the vertical?"
Yes, the Earth's rotational speed is affected and so is the exact orientation of the Earth's axis. Suppose we melt the polar ice caps. That moves mass from a mean position that is close to the Earth's axis to a mean position that is uniform over the Earth's surface and therefore further from the Earth's axis on average. So, in order to conserve angular momentum (an absolute here), the Earth's rotation slows down very slightly. This, plus other factors (generally larger factors), is why that quantity delta-T can't be predicted years in advance. And that's why we can't provide a future list of leap seconds. And it's why The Nautical Almanac isn't published a decade in advance.
Make no mistake. Climate change impacts the Earth's rotation. We already see this in long-term changes believed to be due to the glacial rebound occurring in Canada (centered on Hudson's Bay) and in Scandinavia. Those areas are rising (so relative sea level is falling). This redistribution of the Earth's rocky mass is changing the Earth's rotation.
"Are we going to have to correct our CNAV texts because earth is changing its distribution of mass and we’ll have lots more non uniformly distributed water sloshing about in funny places....or is it just too many decimal places right of zero."
It's in the small numbers, yes, and it's already part of the process, as well. Truth is, we could easily publish The Nautical Almanac or equivalent a decade in advance since we can make good estimates of the behavior of delta-T and the rotation axis, and since these are tiny effects, below the worries of normal, manual celestial navigation. The only issue would be some small inaccuracy in the Moon's position (leap seconds take care of the bigger mis-alignments). Publishing that data a century ahead would be much more problematic, but a decade really wouldn't matter in a practical sense.
You mentioned gravity changes, too. What would be the effect of an extra six inches of water off the eastern US coast (hypothetically maintained by some strange change in ocean currents)? I haven't worked this out in detail, but my estimate is "not much". The map of the deflections of the vertical would change a little but not at a level of concern even for exacting celestial sights.
One of my favorite "mad scientist" plans is to dig a great cave near a physics laboratory and fill it with liquid Mercury metal. Make it, let's say, 100 meters in radius, and connect it to a second cave ten miles away. Now pump the Mercury back and forth at the press of a button. When the cave beneath the laboratory is fillled, the gravitational field of all the metal will be enough to change the deflection of gravity by several seconds of arc, which is quite measurable and would even show up in precise astronomical observations (e.g. with a theodolite). It would also increase the local gravitational acceleration, g, by a small but measurable amount. Then I call in the media and announce that I can control gravity. And the money starts pouring in... The small "catch" in this mad scientist scheme is the cost of constructing the pair of underground caverns and the pumping system. It may also be somewhat expensive to purchase that much Mercury. Somewhat....
By the way, when looking at the attached graphs, try to ignore the linear trends that are drawn in. They can "guide the eye" too much. At least consider the possibility that there is non-linearity in there. For more details and more graphics, visit here: https://tidesandcurrents.noaa.gov/sltrends/sltrends_us.html
Location: Secret Underground Headquarters of NavList
Coordinates: x: 519.2512, y:3140.8172, z: -2338.4067