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?

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.

> 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 ).

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.

> 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.

Fascinating; thanks.

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

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