[jdontillman]

The NOAA Sea Level Trends data and visualizations are fascinating:

https://tidesandcurrents.noaa.gov/sltrends/

Sea levels have been monitored for over 100 years... because of commerce.

East coast US seems to be going up about 3mm/year, or a foot per century. That's on top of 7 foot tides.

West coast US is very steady, barely moving at all.

The sea level is dropping in Alaska by 10mm/year. Dropping in Scandinavia also.

Some cities show rising sea levels, but really the cities are sinking due to collapsing aquifers; New Orleans and Bangkok. And measurements made on river deltas are always going to be wild due to silt and all.

Most notably, I can't find an example in the NOAA data of the rate of sea level rise increasing due to industrialization. Anybody?

[martinpw]

> Most notably, I can't find an example in the NOAA data of the rate of sea level rise increasing due to industrialization. Anybody?

You can look at the graph on the NOAA site here and see the acceleration visually: https://www.climate.gov/news-features/understanding-climate/...

The notes to the left of the table say 1.4mm/yr for much of 20th Century to 3.6mm/yr now.

There is also evidence of acceleration of sea level rise just from satellite data, 0.08mm/y^2:

https://www.sciencedaily.com/releases/2018/02/180212150739.h...

[Trumpi]

Back in 2006, our expectations were set when An Inconveient Truth contemplated what would happen if sea levels rose by six meters (in our lifetime). 3.6mm a year seems a bit meh in that context.

[TaylorAlexander]

Something I see a lot is people looking at worst case predictions as if they were presented as the most likely predictions. A lot of the IPCC reports say things like “if we continue to increase emissions, this will happen” (a worst case scenario) and also “if we make moderate improvements, this will happen” (a slightly better scenario). Then people who don’t want to talk about climate change mitigation will highlight the worst case scenario and say “see they were wrong and hysterical!” even when the moderate prediction, which they will ignore, ended up being essentially totally accurate.

[Trumpi]

Well, I'll go a step further and say that 3.6mm/yr is not evidence of a crisis.

[adgjlsfhk1]

that's not a problem in the decade timeline but it is problematic over a century or two. it's especially problematic since it's accelerating and has a couple decades of inertia. the CO2 we release today will raise sea levels for the next 50 years or so.

[southernplaces7]

>that's not a problem in the decade timeline but it is problematic over a century or two.

So after roughly 100 years, we can look forward to the terrible catastrophe of sea levels (in some places only, not in others) rising by.. just over a foot. A problem for many really flat coastal areas, sure, but hardly the picture of global coastal flooding much of the alarmism has put forward. And if that one-foot rise happens across a full century, there will be many measures that can be taken to counter it even if the rise itself is unstoppable.

I'm not arguing against human-caused climate change, but some of the hyperbole i've seen said with deep certainty goes well beyond the scope of known evidence, realized events or even many scientific assessments.

With such things, it's not hard to see why some people find good reasons for being skeptical of yet another worst-case prediction.

[arethuza]

Isn't that relative sea level changes in places like Scandinavia - the sea isn't dropping, the land is rising?

https://en.wikipedia.org/wiki/Relative_sea_level

[jdontillman]

Exactly.

Because the sensors are mounted on that very land.

So tectonic movement will certainly have an effect. And you can see remnants of earthquakes in some of the data.

[AnimalMuppet]

In Scandinavia, isn't it glacial rebound rather than tectonic?

[bell-cot]

> 3. (geology) Of, relating to, or caused by large-scale movements of the Earth's (or a similar planet's) lithosphere

- https://en.wiktionary.org/wiki/tectonic

[LeifCarrotson]

I expect the parent is referring to human-caused melting of glaciers as the root cause of that tectonic movement, rather than the expected natural motion of the tectonic plates.

[Scarblac]

It's the glaciers melting at the end of the last Ice Age that Scandinavia is still rising from.

https://en.wikipedia.org/wiki/Last_Glacial_Period#Weichselia...

> As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland, where the land is rising at a rate of as much as 8–9 mm per year, or 1 m in 100 years. This is important for archaeologists, since a site that was coastal in the Nordic Stone Age now is inland and can be dated by its relative distance from the present shore.

[BurningFrog]

Yeah, the sea must be "level" in some sense across the planet.

Don't know if local variations in gravity may have a measurable impact?

[cwillu]

PDF of gravitational correction factors by geographical region for use in theoretical weighscale calibration (vs calibrating with actual test weights): https://hardyinst.custhelp.com/app/answers/detail/a_id/1588/...

[CorrectHorseBat]

The melting of glaciers themselves can have an impact on gravity, causing water levels to drop around them. https://www.theguardian.com/environment/ng-interactive/2018/...

[beerandt]

The absolute value of gravity is (to oversimplify) the sea-level datum for current generation datums, like the delayed NGS (nominal year) 2022 datums.

They've (NGS) been flying absolute gravity measurement equipment in planes with precision GPS and lidar since ~2005 as part of the GRAV-D program.

Additionally, slight 'variations' (ie deviations from the expected circular/elliptical) in the orbits of GPS satellites can be used to infer slight variations in the gravity below.

Separately, the military likely has had all of this measured and modeled for decades as part of ballistic missile targeting, but remains mostly classified at useful resolutions.

The transitions from NGVD27 to NAVD88 to the work-in-progress 22 datum is exactly because the idea of a simple "universal" sea-level doesn't exist in the manor you would expect it to.

But even with the GRAV-D based datums, the actual observed water levels will occur at slightly different datum heights in different locations, for various geophysical reasons.

[margalabargala]

> East coast US seems to be going up about 3mm/year, or a foot per century. That's on top of 7 foot tides.

That's the average rise it you average all data going back to the 1950s.

Unfortunately, looking back to the 50s is only really useful if interested in how much sea levels have already risen. Otherwise it presents an unrealistically optimistic view, as the rate has accelerated since.

Per the article, the east coast (North Carolina) is currently experiencing an increase of about a third of an inch per year, which is an increase of one foot per 36 years, or close to a meter per century.

If current trends continue, this will accelerate, not decelerate.

[jessewmc]

Your unit mixtures hurt my brain

[amelius]

What I wonder about is: rising wrt. what? Did they take into the account the possibly vertical movement of tectonic plates?

[revelio]

They do take that into account. They calculate that the ocean floor is sinking but then ADD that displacement to their reported measurement of "sea level", which is backwards. When the ocean floor sinks sea level goes down, but they report it as going up instead, using this justification:

https://sealevel.colorado.edu/index.php/presentation/what-gl...

currently some land surfaces are rising and some ocean bottoms are falling relative to the center of the Earth (the center of the reference frame of the satellite altimeter).

since the ocean basins are getting larger due to GIA, this will reduce by a very small amount the relative sea level rise that is seen along the coasts.

We apply a correction for GIA because we want our sea level time series to reflect purely oceanographic phenomena. In essence, we would like our GMSL time series to be a proxy for ocean water volume changes. This is what is needed for comparisons to global climate models, for example

This is nonsense, of course. Volume isn't measured in millimeters, that's a measurement of distance. GMSL when talking about satellites is defined as the average distance of the surface of the ocean from the center of the Earth, and the reason people care about it is because if it gets too high then things we care about end up flooded. Changing the definition of GMSL half way through from sea level to sea depth is the kind of slippyness that pervades this space.

[mc32]

I think the arctic region is affected by glacial rebound: the ground slowly recovers from past glacial compression.

Subsidence is a thing in Silicon Valley. San Jose has sunk some feet since the aquifer was drawn, fist for agriculture and now urbanization.

[twawaaay]

There is only one reason the sea can rise currently due to weather and that is glaciers getting melted.

Another reason (but not due to weather) is that when land somewhere goes up, it will displace water everywhere else. And so, for example, if the land is still recovering from the ice age we should see ocean levels going up everywhere except for the pieces of land that are recovering from the weight of the glacier that is no longer there.

Mind that Arctic is not causing sea level rise. Any ice that is floating on water will not cause any water level change when it melts. (I know this is somewhat unintuitive but it comes directly from Archimedes principle)

So we are talking basically Antarctic ice and Greenland because these are by far the largest bodies of frozen water that are supported by land rather than floating on the ocean.

I think it should be pretty easy to observe how much of that water melted or slipped into the sea.

I also think that currently, coastal erosion is mostly caused by changing weather patterns. Basically this comes down to wind blowing in different directions, speed and variety and these changing patterns mean coasts are eroding in different places than before.

[locopati]

Also, warming oceans causes expansion which causes sea level rise.

https://www.jpl.nasa.gov/edu/learn/project/how-warming-water....

[twawaaay]

Not true.

First of all, most of the temperature rise only happening close to the surface with average surface temperature rise being only about 1.5F or 1C since 1901.

This article says deep ocean water is projected to warm up by only 0.2C during next 50 years: https://www.openaccessgovernment.org/deep-ocean-warming-subt...

Furthermore, at around 4C which is what deep ocean water is close to (everything below 200m is essentially 4C), thermal expansion is almost nil. For colder water thermal expansion is actually negative.

4C is when water is at its densest. It is not an accident that all oceans are 4C, because 4C water sinks to the bottom and anything colder or hotter than 4C floats up. This remarkable property of water is what causes even shallow water to be fantastically stable in temperature -- a lake that has more than couple tens of meters in depth is likely to be 4C at the bottom throughout the year whether it is frosty winter or hot summer above it, unless some kind of powerful event is able to mix the water in the lake.

Now, the small temperature differences will definitely have outsize effects on water circulation, ocean currents, life and weather. But I doubt they will cause meaningful sea rise unless somebody can calculate otherwise?

[Retric]

That’s mostly accurate, but the nuances are significant and lead to different conclusions. For example the hypolimnion may be much warmer than 4C in lakes in warmer areas. More importantly tropical ocean water is above 4C down to roughly 2km and not only is that depth expected to increase, but also the depth of warm water as you go north. https://en.wikipedia.org/wiki/Thermocline

The important thing to remember is even a 1 part in 1,000 decrease in density * 2km of depth = 2m of expansion. Ballpark estimates aren’t enough you really need fairly detailed simulations to get any significant accuracy. Actually doing such simulations shows meaningful sea level rise from thermal expansion at ~0.07 inches per year or roughly half the current rate of increase. This might not sound like much, but consider that volume of sand you need to replace to maintain beaches etc etc.

[kergonath]

> That’s mostly accurate

No, that is mostly inaccurate. Thermal expansion is small, but there is an awful lot of water. As you point out yourself, thermal expansion contributes about half the sea level rise. Oceans absorb energy just like the atmosphere does and this effect has been known for quite a while (e.g. https://www.nature.com/articles/330127a0 ).

[mcguire]

The average ocean surface temperature is about 20°C and the thermal expansion coefficient is 0.000207/°C (https://www.engineeringtoolbox.com/water-density-specific-we...). If I have my google-fu and math right, that's about 1cm/° for a 50m deep water column.

Thermal expansion of surface water is not negligible.

[Tuna-Fish]

> But I doubt they will cause meaningful sea rise unless somebody can calculate otherwise?

Plenty of people have, in fact, calculated otherwise.

Roughly half of the current, ongoing sea level rise is from expansion of water. The effect is small, but there is a lot of water that is expanding.

[jdontillman]

Fascinating; thanks.

Question: How does the salinity and pressure of deep sea water affect that 4C point?

[atlantic]

Not necessarily. The density of water peaks around 4ºC. So as oceans warm between 0º and 4º, they actually contract in volume.

[twawaaay]

Except water in oceans is at 4C (at least almost everything deeper than ca 200m). And in the vicinity of 4C the thermal expansion is very negligible. This graph should explain why: https://images.app.goo.gl/FXzvTkPvE9dYxoUA7

[atlantic]

Which supports the broader point that thermal expansion of water does not contribute to rising sea levels.

[withinboredom]

Umm. I’m pretty sure if water that exists above the water line is melted into the water line, the overall water line will rise. You can directly observe this in a glass of ice that starts with no liquid water will melt into a glass of liquid water.

[nrclark]

I could be wrong about this (it's been awhile since I took chemistry), but I think the ice has to be floating for the Archimedes principle to apply.

You can fill a glass of ice-water right up to the brim, and it won't spill over as the ice melts. But only if the ice is floating in the water. It's because the ice's mass pushes down on the liquid water, displacing a fixed amount relative to the weight of the ice.

[withinboredom]

Ah. I missed the “floating” part. That makes sense.

[brilee]

This is untrue.

Ice is about 91% the density of water. If you have water at 0 height, and you put in water equivalent to +1 unit of height. If it's in liquid form, obviously height goes up by +1. If it's in ice form, there is +1.09 height worth of ice (because it expands when it freezes), but it only displaces water up to +1 unit of height in order to support its weight through buoyancy. The overall change in height is +1 unit regardless of whether it's liquid or ice

[trompetenaccoun]

No, it's not just due to weather (you probably mean climate anyway). For example, ice melting in one place, say Antarctica, will affect sea levels elsewhere on the planet because of weaker gravitational forces where the ice used to be. Geoscience is complex and measuring changes on such a global scale is not "pretty easy", even if it's only sea level changes. It's nothing like a bathtub or elementary school physics.

Not to rant but this is one of those threads again. The majority of comments contain misinformation.

[WillPostForFood]

We shouldn't be calling subsidence sea level rise. They are two distinct, measurable, phenomena that happen to have the same impact.

[sp332]

I don't think it was possible to measure them separately until we had good satellite data from the last two decades. If we're extending trends back to the beginning of the industrial revolution, they're going to be mixed.

[revelio]

Satellite data isn't that good unfortunately. Tide gauges say 1.5mm/year of rise, satellites say 3mm/yr, and they estimate satellite error to be +/- 0.5mm year, so the error bars are a significant fraction of the size of the overall change.

Also that assumes the scientists are completely accurate and don't make any mistakes. As recently as 5 years ago they discovered that the measured rise for almost all of the 90s was wrong and revised it by 3mm/year +- 1.7mm/year - the error was the same amount as the imputed level of rise!

This is no knock on the scientists, because measuring the height of a moving ocean from orbit to a level of accuracy that lets you see mm level changes is inevitably going to be very hard. But we should bear in mind that they're heavily biased towards wanting to believe the data is accurate. Their decade long inability to get the measurements correct didn't seem to have any impact on their confidence that they're currently getting it right, and why would it? Those who seize the data, seize the day.

[stochtastic]

> Also that assumes the scientists are completely accurate and don't make any mistakes. As recently as 5 years ago they discovered that the measured rise for almost all of the 90s was wrong and revised it by 3mm/year +- 1.7mm/year - the error was the same amount as the imputed level of rise!

Citation for this? There are always improvements to our understanding of past data, but you seem to be implying that the uncertainty exceeds the signal. That's simply not the case [1].

[1] — https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6... pp.1291-1292

[revelio]

Ablain, "Uncertainty in Satellite estimate of Global Mean Sea Level changes, trend and acceleration", p4

They have shown that there was a drift in the GMSL record over the period 1993-1998. This drift is caused by an erroneous on-board calibration correction on TOPEX altimeter side-A (noted TOPEX-A). TOPEX-A was operated from launch in October 1992 to the end of January 1999. Then TOPEX side-B altimeter (noted TOPEX-B) took over in February 1999 (Beckley et al., 2017). The impact on the GMSL changes is -1.0 mm/yr between January 1993 and July 1995, 120 and +3.0 mm/yr between August 1995 and February 1999, with an uncertainty of ±1.7 mm/yr (within a 90%CL, (Ablain, 2017)).

[stochtastic]

Figure 1 from that paper didn't convince you that it was a minor issue? Their correction is about 7% of the 1993-2018 signal, and it resolves the discrepancy with respect to other estimates, including non-satellite ones. Nobody relies on estimates from a single satellite alone.

https://essd.copernicus.org/articles/11/1189/2019/essd-11-11...