Thursday, September 27, 2018

Hot, Warm, & Cold Thermal Facts: Tidewater-Glaciers - 5

Fig. 1 Area A-F Melting
In this series  we talk about the temperature of tidewaters that melt tidewater glaciers (Hot, Warm, & Cold Thermal Facts: Tidewater-Glaciers, 2, 3, 4).

As pointed out in this series, the seawater that melts the very cold and dense ice at the calving edge of such glaciers is very, very cold.

But less cold seawater can melt colder glacial ice that is submerged in that seawater, especially when it is being brought to the glacial ice by an ocean current that has more flowing water volume than all of the rivers of the earth combined (Mysterious Zones of Antarctica - 3, 4).
Fig. 2. Amundsen Sea

The TEOS-10 function that finalizes the several functions that calculate what temperature of seawater will cause the glacial melt is gsw_melting_ice_into_seawater (or gsw_melting_seaice_into_seawater for an ice shelf or other ice mass floating on the ocean surface).

In this series we are only concerned with tidewater glaciers, not sea ice, so I use the gsw_melting_ice_into_seawater function, which is sufficient:
"When the output ice mass fraction w_Ih_final is zero, the final state is pure seawater that is warmer than the freezing temperature and which contains no frazil ice component."
(Notes on the function, emphasis added). The water deep beneath the surface all around Antarctica where there are tidewater glaciers is less cold than it was "back in the day."

Fig. 3 Bellingshausen Sea
I mean back in the day when the precipitation falling on Antarctica was enough to keep up with the melt taking place.

The deeper waters are less cold (a.k.a. "warmer") than they were then, so the melt rate is way, way more now.

It has been said that the melt rate has tripled in the past not-so-many years.

The precipitation amount is still classified as a desert amount (the Sahara gets more precipitation), so the net result is a loss of the amount of the mass of the ice sheet.

Fig. 4 East Indian Ocean
The graphs today (Fig. 2 - Fig. 7) show why.

The melt temperature of the tidewater glacial ice is much lower than the temperature of the less cold (a.k.a. "warmer") seawater making continual contact with it.
Fig. 5 Ross Sea

The use of the term "warm seawater" is a misuse of the reality down in the deeps, because it generates and perpetuates a myth in the mind of many people.
Fig. 6 Weddell Sea

Fig. 7 West Indian Ocean
The Antarctic tidewater glaciers are melting at various depths because the waters are less cold than they need to be to keep the melt down to an amount that is offset by precipitation on the ice sheet.

That balance was lost further back than we are accustomed to thinking it did.

It began a couple of decades after the Industrial Revolution began circa 1750.

That melting began with the Greenland Ice Sheet (Proof of Concept - 5).

The Antarctic will catch up and pass Greenland in due time, which is closer than commonly expected.

A final word about the today's graphs.

The straighter lines at the bottom of the graph is the general temperature required to melt the ice and the general temperature of the seawater mix at that depth due to the melting.

The less-straight lines at the top of the graphs are the temperatures of the seawater at several depths.

Even though that seawater is cold, it is less cold by a thermodynamically significant amount.

It is that increasing difference in temperature which determines the speed of the melt.

The previous post in this series is here.

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