Thread by @ncdave4life

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate 1/40> Gavin wrote, "The floating ice melting *doesn’t* set up barotropic surface gravity waves which would transmit eustatic sea level changes very rapidly across the ocean"

Thanks for acknowledging it. Too many people never admit their errors. I'm glad you aren't one of them.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate 2/40> Gavin wrote, "all local variations affect the global mean (it’s just an average of all points)."

Do not mistake how it is ESTIMATED for how it's DEFINED. "Global" also means "not merely local," and steric changes in the upper layer of the ocean are merely local.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate 3/40> Gavin wrote, "global mean sea level is the average of the sea level, not the ocean+floating ice level."

That's unconventional. Conventionally, floating ice is considered to be PART OF the ocean (though the top surface of the ice isn't usually considered local sea-level).

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate 4/40> Sea-level is more complicated than you (and 99% of other people!) might realize. Henry Reich @minutephysics does a very good job of explaining it:

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 5/40> Gavin wrote, "Therefore net melting of floating ice does affect the average salinity of the ocean (and therefore it’s volume)."

"Global" isn't just a synonym for "average." Global means not merely local. Over the 1994-2017 period salinity changes were local, not global.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 6/40> That means the claim that melting of floating ice added 1.1 mm to sea-level between 1994 & 2017 (23 years) is wrong. Even if the estimated salinity effects were right (and they weren't!), they could not have propagated beyond the Arctic & Southern Oceans by now.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 7/40> The AMOC transit time is famously about 1000 years, and even that wouldn't be sufficient for Arctic Ocean salinity changes to affect places like Hawaii.
sealevel.info/MSL_graph.php?…
sealevel.info/learnmore.html…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 8/40> Here's a diagram showing salinity in the Arctic Ocean. As you can see, the upper layer is less salty than the global 35 g/L average. But look how stratified it is, and notice how confined it is to the Arctic Ocean! It doesn't appear to be affecting North Atlantic salinity.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 9/40> And of course it certainly isn't affecting North Pacific salinity.

There's no clear indication that Arctic ice fluctuations affect salinity anywhere. Here's the source of that diagram showing distribution of salinities in the Arctic Ocean:
salinity.oceansciences.org/highlights05.h…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 10/40> In the Arctic, the main form of floating ice is sea ice, from freezing of seawater in fall & winter. Sea ice isn't continually being lost, gradually freshening the ocean. Rather, it's mostly melting in spring/summer and reforming in fall/winter, each year.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 11/40> When the sea ice melts it freshens the surface water, but when sea ice re-forms a few months later, and expels salt into the ocean beneath, the freshening effect is reversed.

From 2003-2015 Arctic sea ice declined slightly (about 3 Gt). Before 2003 it's unmeasured.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 12/40> The slight declining trend in averaged-over-a-year Arctic sea ice volume affects the average age of the. The trend decreases the amount of ice which survives the summer, so it makes the sea ice "younger," on average.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 13/40> Old, multiyear ice is low salinity, and newer, first year ice is higher salinity. So, even though the the annually averaged amount of Arctic ice is declining slightly, the average salinity of the ice is increasing slightly.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 14/40> The Arctic sea ice increases the percentage of ice which is "new ice," which contains more salt, and decreases the percentage which is old, multi-year ice. That partially offsets the effect of ice trend on Arctic Ocean salinity.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 15/40> Gavin, you calculated that 1.1 mm figure from two sources: Slater 2021 & Noerdlinger 2007. One of them is dubious, the other is just plain wrong!

The dubious source is:
Slater et al (2021). Review article: Earth's ice imbalance, The Cryosphere, 15.
tc.copernicus.org/articles/15/23…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 16/40> Slater (2021) says the Earth lost (7.6+0.9+6.5)×10³ = 15,000 Gt of floating ice between 1994 & 2017.

(Also, the Antarctic Ice Sheet is partly grounded below sea-level & thus "partly floating." But you didn't include that, and Slater's estimate is probably wrong anyhow.)

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 17/40> Slater says 7600 Gt of that was a decline on Arctic sea ice. But for that figure he mostly relies on PIOMAS:
tc.copernicus.org/articles/15/23…

PIOMAS is junk! They think whether or not measurements exist has no effect on the precision with which floating sea ice mass is known!

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 18/40>
Q: What's the difference between these two things?
A: One of them is a great big steaming pile of BS. The other is fertilizer.

sealevel.info/pile_of_steami…

psc.apl.uw.edu/wordpress/wp-c…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 19/40> Slater also reports 6500 Gt of reduced floating ice in the form of shrinking Antarctic ice shelves. That estimate is problematic, too.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 20/40> For one thing, when ice shelves break off, they don't necessary melt immmediately. "B22" famously broke off (reducing Antarctic ice shelf mass by 500 Gt) in 2002, and it STILL has not fully melted.
newsweek.com/huge-iceberg-b…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 21/40> Ice shelves form when ice slides into the sea from glaciers, so the ice shelves are entirely fresh water (unlike sea ice). They are part of a larger water circulation process, in which moisture evaporates from the ocean (making it saltier), precipitates out as snowfall…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 22/40> …on ice sheets and glaciers, slides downhill into the Southern Ocean forming ice shelves, and then melts (or breaks off into icebergs which then melt) into the ocean (making it fresher). I think it is a mistake to focus on just one part of that cycle (the melting of…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 23/40> …ice shelves), and ignore the rest. It's a bit more complicated than that, e.g., sublimation + precipitation also moves Antarctic ice mass into the ocean. But the net effect of that entire process on ocean salinity is not far from zero.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 24/40> What matters w/r/t sea-level is grounded (not floating) ice mass trends. In Antarctica, the processes adding & removing ice mass are in almost perfect balance. Some studies say it's lost ice, others say it's gained ice.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 25/40> A source of the disagreement is uncertainty about modeling vertical land motion beneath the ice. But all studies agree that the Antarctic ice mass trend, whether positive or negative, is negligible: < 3 inches of global sea-level change / century.

https://twitter.com/ncdave4life/status/1368541864635346944

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 26/40> The best and most comprehensive study to date of Antarctic grounded ice mass is this one, from NASA:
cambridge.org/core/journals/…

They reported a trend of -12 ±64 Gt/yr.

That's ≅ zero. The CI is 5× the trend, and 12 Gt/yr = only 0.1 inch of sea-level change per century.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 27/40> Ice shelves are just a transition stage for ice, as it flows downhill from the land into the ocean. Ice shelves are continually replenished by ice from Antarctic glaciers, and continually eroded by melting from the bottom, and calving icebergs (which also gradually melt).

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 28/40> If the flow of ice from the glaciers were to accelerate, it would suggest that Antarctica was beginning to contribute more to global sea-level rise — and it would also be reflected by growing ice shelves. If anything, shrinking ice shelves suggest the opposite.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 29/40> Slater's numbers for ice mass loss are at least plausible, though inadequately supported.

Noerdlinger's paper is much worse:
academic.oup.com/gji/article/17…

Noerdlinger's mostly easily refuted claim is this (from the Abstract):

"The sliding of grounded ice into the…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 30/12>
"…sea, however, produces a mean water level rise in two parts; some of the rise is delayed. The first part, while the ice floats, is equal to the volume of displaced sea water. The second part, equal to 2.6% of the first, is contributed as it melts."

That's wrong, 2×.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 31/40> In the first place, if ice falls into the Arctic or Southern Ocean, its displacement immediately affects global sea-level (and globally averaged sea-level), as gravity rebalances mass. But Noerdlinger's salinity effects are NOT "contributed as it melts."

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 32/40> Meltwater's effect on ocean salinity is still local to the immediate area of Arctic or Southern Ocean. It could only contribute to sea-level elsewhere as currents mix the oceans, on timescales of hundreds or thousands of years. (AMOC transit time is about 1000 years.)

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 33/40> Contrary to Noerdlinger's claim, there's no difference in effect from adding freshwater to the sea, and adding ice which melts. In both cases, the displacement (mass) has the ONLY non-local effect on sea-level. In both cases salinity effects are local until the oceans mix.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 34/40> The long mixing process does not change the amount of water, amount of salt, or average salinity of the oceans. The floating freshwater "bulge" in the ocean, before it mixes with the rest of the ocean, is as much a part of average sea-level as any other part of the ocean.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 35/40> In the second place, the 2.6% figures is wrong.

Gavin, you relied on this claim from Noerdlinger's Abstract:

"the melting of ice floating on the ocean will introduce a volume of water about 2.6 per cent greater than that of the originally displaced sea water. The…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 36/12>
"…melting of floating ice in a global warming will cause the ocean to rise."

The prose is oddly awkward ("in a global warming"), but the gist is that loss of floating ice reduces ocean salinity & density, thus increasing seawater volume, globally.

That is wrong.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 37/40> The immediate effect of adding freshwater to the ocean, whether directly, or by melting of floating ice, is a locally-fresher, less-dense water "bulge," floating on the denser ocean beneath. When that fresher water mixes with the rest of the world's oceans, it slightly…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 38/40> …freshens them, which must cause sea-levels elsewhere to rise slightly.

But it simultaneously lowers sea-level locally, where the bulge used to be. Inexcusably, Noerdlinger ignores that part!

You SHOULD be wondering about the relative sizes of those opposite effects!

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 39/40> Wonder no more.

Seawater's density is an almost perfectly linear function of its salinity. So the increase in sea-level elsewhere from reduced salinity is exactly offset by the local sea-level decrease.

So there is NO effect at all on global sea-level.

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics 40/40> That means Noerlinger is wrong. The correct figure is not 2.6%, it is 0.00%. The melting of floating sea-ice and ice shelves does not affect global sea-level, at all.

And "by about half" is obviously wrong, too:
sealevel.nasa.gov/news/261/melti…

@ClimateOfGavin @Theranello @EytanPs89 @NASAClimate @minutephysics @threadreaderapp @rattibha @threaddotblue unroll