Comment by apo | Hacker News Reader

Comment by apo | Hacker News Reader

> The agency’s projection that the system will “no longer provide [required] risk reduction as early as 2023” illustrates the rapidly changing conditions being experienced both globally as sea levels rise faster than expected and locally as erosion wipes out protective barrier islands and marshlands in southeastern Louisiana.

The article repeatedly refers to sea level rise without noting the contribution of this factor relative to erosion. Equal? Highly tilted toward sea level rise? Does erosion account for most of the problem, and if so how does one separate this factor from seal level increase?

Neither the article nor the source document offer much insight:

https://www.eenews.net/assets/2019/04/11/document_ew_01.pdf

Also from the article:

> “I think this work is necessary. We have to protect the population of New Orleans,” Vuxton said.

I'm not so sure, especially because the full costs of doing so appear to be so uncertain.

How much money will people living in other states be willing to dump into a cause with cost increases as far as the eye can see? It's not like sea level rise is expected to suddenly stop dead in its tracks.

Ever-greater engineering efforts are one option. Mass evacuation of the most vulnerable areas is another. There may be options in-between.

This says three main factors: 1.)sediment flow changes - we changed flow of Mississippi River to benefit a lot of upstream communities (for flood control/farming/maintaining river boundaries against nature's desire) - continuing to do so robs Louisiana of land mass 2.) subsidence - pumping out water aquifers and extracting oil (without replacing fluids) cause land levels to sink 3.) sea level rise

https://www.factcheck.org/2017/03/land-loss-in-louisiana/ All three together paint a pretty grim picture of the future of New Orleans.

It's not just a few houses built by irresponsible people.

Nola has the highest port tonnage in the country. The impact on GDP is estimated in the hundreds of billions. There's also billions and billions of dollars of infrastructure here. Most goods that pass through to the midwest and many through the rest of the country come through here.

Without Nola, hundreds of billions of dollars of the economy will go away. Gas prices set a record after Katrina.

14 billion is a bargain to keep this city going.

Porque no los dos? Keep the port and relocate the rest of the city. It's not that you need hundreds of thousands people to run port facility. Damn, if you even need 500. The rest of the population are hospitality workers and ambulance chase lawyers.

Ask not whose ambulance is being chased, one day it’ll be thine with that flippant attitude. Not everyone down here is paid by the hour.

I don't know if you're trolling or just actually ignorant.

A lot of the current problem in New Orleans is that it's built on deep layers of silt which are slowly compacting. And channelizing the Mississippi has resulted in loss of protective barriers offshore because the silt now goes right out the end of the channel into deep water. Bonus the bedrock itself is slowly tilting sinking towards the gulf.

What the place needs is about three meters of compacted fill under the whole city. ALA Galveston TX and Downtown Chicago.

It's neither. Erosion and sea level changes are not at all the problem here.

The problem is that keeping the lowest point dry causes subsurface water to be removed from the area. Also, that water doesn't get replenished. Things tend to shrink when they dry out, and the land under New Orleans is no exception. It's becoming like a shriveled up raisin or sponge.

The more you pump the place dry, the more the place sinks lower.

Thanks. So is it basically the same dynamic that's causing subsidence in parts of California's Central Valley?

Yes, exactly so.

> The more you pump the place dry, the more the place sinks lower.

See also: Mexico City

I'm not an expert, but intuitively, I'd think that sea level rise would contribute to ground subsidence.

Sea level has been rising at three millimeters a year as long as detailed measurements have been made. There has been no acceleration in that rate. We are, after all, still coming out of an ice age, and the oceans are still expanding due to accumulated heat.

Locally apparent changes are primarily due to land subsidence due to sediment deposits or rebound after the ice melted.

Well, here is the data:

http://sealevel.colorado.edu/content/2018rel1-global-mean-se...

That is only a small part of the data, just 25 years worth of satellite data. There are ground measurements going back almost 150 years. Using all the data shows a clear acceleration:

https://climate.nasa.gov/vital-signs/sea-level/

The comment about coming out of an ice age is also not relevant - sea levels have indeed been rising since then, but nowhere near the current rate, so the current increase is mostly unrelated to that. For example:

https://www.giss.nasa.gov/research/briefs/gornitz_09/

Twentieth century sea level trends, however, are substantially higher that those of the last few thousand years.

Your first link has the essentially same data as what I referenced - 3.2 mm per year recently. Since 1919, they show a rise of about eight inches. The current rate amounts to 12 inches per century.

Your second link is interesting - thanks! It’s not different from what I said. We have come out of an ice age. The interglacial epochs have been historically relatively short.

Sorry, perhaps I wasn't clear. The first link I gave has two graphs. The top one indeed has the same data you linked to - the 25 year satellite record. The second link is the one I was actually referring to though - that is the almost 150 year record including tidal gauge data. There is a pretty clear acceleration there. The earlier rate is lower than the later rate - in fact you even said that too (12" vs 8"). So I am a bit unclear why you said originally there was "no acceleration in the rate"?

This article supports your claim (pdf):

Climate-change–driven accelerated sea-level rise detected in the altimeter era

https://www.pnas.org/content/pnas/115/9/2022.full.pdf

I think that fitting a quadratic to such a short run of data is premature, and that extrapolating that same acceleration for 80 years is tendentious. However, the paper is otherwise interesting, and I hope they do an update in ten years and include their previous prediction :-)

Yes, that's the article I was thinking of, good find!

Agreed the extrapolation is debatable.

What is most astounding to me is that they are able to measure not just an increase of 3mm a year in sea levels by satellite, but that the data are precise enough that they can determine the change in this rate. The precision of the instrumentation and processing required to measure to that level of precision is astounding.

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