An article on Antarctic Eocene climate

[snorkack]

An interesting article has been published, in a peer reviewed journal, on the climate of Antarctic in Eocene:
https://cp.copernicus.org/articles/20/77/2024/
I have some comments about that article. May I comment about it on this thread?

 

[berkeman]

May I comment about it on this thread?

I would think so, since the article appears to be a valid reference for PF. Just be sure to adhere to the CC guidelines here: https://www.physicsforums.com/threads/climate-change-global-warming-policy.757267/

 

[snorkack]

Thanks!
Some methodological comments.
Strict scientific references for everything is a high demand, especially for an amateur. "Remember reading it from somewhere" is often very hard to convert back into the correct reference despite being ultimately true. In the absence of privileged institutional access to paywalled sources, searches commonly run dead into links to paywalled sources for which workaround existed some years ago but not now. "Just google it" is commonly an insultingly useless advice - search terms that can be thought about yield either nothing, something known to be different despite having similar wording, or impassable (dead or paywalled) links.
I very much appreciate depth of research. Remember that it is burdensome as well, and breadth of peripheral vision also matters. When peripheral vision shows anything worth looking closer into THEN it is the time to make the effort to get scientific references, where and as much as needed.

Now, going to the sections I have comments about.

2.1 CESM simulations

A projected and interpolated version of the Antarctic topography is shown in Fig. 1 (see also Figs. S1 and S2 in the Supplement), indicating the main geographic features considered in this study.

While Figure 1 has explicit numbers on the colour scale, it is a smooth colour scale. No numbers are given for summits, so these are hard to read.
The description of the simulated Antarctic climate:

3.1 Extreme Antarctic seasonality

Average summertime temperatures over Antarctica range mostly between 20 ∘C near the coast to over 30 ∘C further inland. Only on higher terrain are cooler temperatures of 10–16 ∘C found.

3.3 Regional variation in the Antarctic climate: vegetation and ice

With elevation, we see a sharp increase in precipitation, which is probably underestimated due to the limited model resolution used here. Despite summer temperatures being well above zero, some very high precipitation amounts would allow ice to grow over these regions, especially during cooler intervals. Such ice caps would be strongly restricted to the highest elevations, as well as near the coast, where both temperatures and precipitation are conducive to ice growth. Dronning Maud Land and the Antarctic Peninsula are especially good candidates for the formation of ice caps in our simulated Eocene climate. Nevertheless, the proximity of such ice caps to the coast, in combination with cool upper-ocean temperatures and westward currents, could allow for the calving of icebergs floating along the Antarctic Peninsula.

Really?
For reference, see Fox Glacier. A link, though to Wikipedia. I hope weather data may be independently looked up and would prove similar if not identical:
https://en.wikipedia.org/wiki/Fox_Glacier_(town)
January average +14,7. I get 3 month, DJF average - the measure the authors prefer - at +14,5.
Upslope, Mount Cook was 3764 m before it collapsed in 1991.
And yet Fox Glacier does not calve icebergs into Tasman Sea. It terminates 300 m above sea.
Norway has no tidewater glaciers either. Chile and Alaska do.
By the simulation, Eocene Antarctic Peninsula and Queen Maud Land were way warmer in summer than Holocene Southern Alps.

 

[snorkack]

And now to next claim I wished to comment about...
Still section 3.3

These highly supportive conditions for plant growth are confined to a rather narrow stretch near the coast. A rapid increase in seasonality as well as overall cooling with elevation quickly makes winters too cold for most vegetation to survive.

3.6 Comparison to existing modelling work

A monsoonal climate regime dominates most of the continental interior of Antarctica, while vegetation thrives along the continental fringes.

Most of these scenarios would only allow for the existence of local ice caps, while most of the continent remains either monsoonal or vegetated.

4.1 Simulated Eocene Antarctic climate conditions

A 50–60 ∘C SAT difference is seen between the mean coldest and warmest month over central Antarctica, values only seen over parts of Siberia in the present climate. Considering warm and wet summers as well, eastern Siberia may thus provide the closest present analogue to the Eocene climate over much of Antarctica. The extreme seasonality seen in our simulations puts some question marks regarding the boundary conditions used, imposing a cool mixed forest over most of Antarctica for the Eocene cases. Our results would suggest a more lush vegetation type near the coast on one hand, and a more barren type over the continental interior. While some further testing (not shown here) does indicate that applying a tundra vegetation over most of Antarctica influences SAT, the effect is minor (localised changes of ∼1 ∘C or less) and does not alter the main results. If anything, a more barren vegetation type would act to further increase temperature seasonality over inland Antarctica in the Eocene.

And the reviewers don´t seem to catch it either:

Inland areas seem to present a paradoxical scenario: too cold during the winters to permit the growth of vegetation and yet too warm during the summers to facilitate glacial inception, unless we consider the presence of formidable high mountain ranges.

L176 and L245: The authors conclude that the prevailing temperature and precipitation conditions are conducive to the growth of temperate forests along the coast but not in the inner continent. It's worth considering whether the vegetation prescribed in the simulations (Baatsen et al. 2020) as a cool/warm mixed forest remains consistent despite the observed seasonality. Another question arises regarding the representation of the inner Antarctica continent. Given the simulations, it appears that a bare soil with a higher albedo might provide a more accurate representation of this region. It can be useful to explore the potential impact of such a change in representation on the overall conclusions of the study.

Really?
Look at Siberia. And the other example. Inland North America. Holocene.
For the standard, 4x PIC, see figure 2. Winter, JJA average -16 over large area in inland Antarctica, -20 over a small spot.
And now compare East Siberia and Manchuria. And North America.
https://en.wikipedia.org/wiki/Jiamusi
January average -17,9, DJF mean -15,6. July average +22,8, JJA mean +21,2
https://en.wikipedia.org/wiki/Chita,_Zabaykalsky_Krai
January average -24,6, DJF mean -21,7, July average +19,5, JJA mean +17,7
https://en.wikipedia.org/wiki/Winnipeg
January average -16,3, DJF mean -14,4, July average +19,5, JJA mean +18,4
https://en.wikipedia.org/wiki/Yellowknife
January average -25,5, DJF mean -23,3, July average +17,1, JJA mean +15,1
All of these places are largely forested, with heavy vegetation coverage and biomass and reasonable diversity. None of them is "tundra", let alone "barren ground".

Does any of my critizism make any sense?

 

[snorkack]

Another comparison I spot:

4.2 Agreement with available proxies

Our model results show mild and perennially wet conditions in Antarctic coastal regions, which would support a cool temperate (Nothofagus) forest or sub-tropical vegetation.

We have another observation about genus Nothofagus... from Pliocene Antarctic. Meyer Desert Nothofagus beardmorensis.
https://www.researchgate.net/public...ius_Group_Transantarctic_Mountains_Antarctica
The fossils indicate creeping shrubs, with branches of not over 2 cm diametre and no main stem. And deciduous leaves.
And comparison with the northernmost dwarf shrub of the world... Salix arctica. Which occurs on Peary Land. A bit more south, in Svalbard, there are also massive ice caps and three species of Salix and one of Betula.
The climate of Longyearbyen, in the favourable Isfjorden region where Betula nana occurs at favourable spots:
https://en.wikipedia.org/wiki/Longyearbyen
January average -10,9, DJF mean -10,6, July average +7,0, JJA mean +5,5
The fossil evidence of Nothofagus beardmorensis, for which fossils are extant and accessible, suggest that genus Nothofagus once evolved species which was able to tolerate cold summers and harsh winters as a tundra shrub and has since gone extinct when the niche vanished. Nothofagus is poor at spreading.
It makes sense that genus Nothofagus may also have once evolved, presumably deciduous, species which were capable of tolerating cold winters given warm summers with some rainfall, and forming temperate forests in interior Antarctica, that also went extinct with their niche.

 

[snorkack]

a cool temperate (Nothofagus) forest

Nothofagus is a cool temperate genus but Holocene Antarctic flora includes some conifers
Chile:
https://www.actahort.org/books/615/615_14.htm
9 species of 8 genera in 3 families:
Araucaria araucana (Araucariaceae),
Austrocedrus chilensis,
Fitzroya cupressoides,
Pilgerodendron uviferum (Cupressaceae), and
Lepidothamnus fonkii,
Podocarpus nubigenus,
P. salignus,
Prumnopitys andina and
Saxegothaea conspicua (Podocarpaceae)
New Zealand:
https://www.conifers.org/topics/nztrees.php
20 species of 10 genera in 3 families, the same 3 as in Chile:

ARAUCARIACEAE
Agathis australis	New Zealand kauri
CUPRESSACEAE
Libocedrus bidwillii	Pahautea, mountain cedar
Libocedrus plumosa	Kawaka
PODOCARPACEAE
Dacrycarpus dacrydioides	Kahikatea
Dacrydium cupressinum	Rimu
Halocarpus bidwillii	Mountain pine, bog pine
Halocarpus biformis	Pink pine
Halocarpus kirkii	Monoao
Lepidothamnus intermedius	Yellow silver pine
Lepidothamnus laxifolius	Mountain rimu, pygmy pine
Manoao colensoi	Silver pine
Pectinopitys ferruginea	Miro
Phyllocladus alpinus	Mountain toatoa
Phyllocladus toatoa	Toatoa
Phyllocladus trichomanoides	Tanekaha
Podocarpus acutifolius	Westland totara
Podocarpus laetus	montane totara, Hall's totara
Podocarpus nivalis	Snow totara
Podocarpus totara	Totara
Prumnopitys taxifolia	Matai

For Tasmania, 10 species in 8 genera:
https://www.utas.edu.au/dicotkey/dicotkey/CONIFERS/conifer_spp.htm

Athrotaxis cupressoides Athrotaxis selaginoides Callitris oblonga Callitris rhomboidea

Diselma archeri Lagarostrobos franklinii

Microcachrys tetragona Pherosphaera hookeriana Phyllocladus aspleniifolius

Podocarpus lawrencei

In total, between the 8 genera of Chile, 10 of New Zealand and 8 of Tasmania, I get the following extant genera:

1. Agathis (Araucariaceae, NZ)
2. Araucaria (Araucariaceae, Chile)
3. Athrotaxis (Cupressaceae, Tasmania)
4. Austrocedrus (Cupressaceae, Chile)
5. Callitris (Cupressaceae, Tasmania)
6. Dacrycarpus (Podocarpaceae, NZ)
7. Dacrydium (Podocarpaceae, NZ)
8. Diselma (Cupressaceae, Tasmania)
9. Fitzroya (Cupressaceae, Chile)
10. Halocarpus (Podocarpaceae, NZ)
11. Lagarostrobos (Podocarpaceae, Tasmania)
12. Lepidothamnus (Podocarpaceae, Chile and NZ)
13. Libocedrus (Cupressaceae, NZ)
14. Manoao (Podocarpaceae, NZ)
15. Microcachrys (Podocarpaceae, Tasmania)
16. Pectinopitys (Podocarpaceae, NZ)
17. Pherosphaera (Podocarpaceae, Tasmania)
18. Phyllocladus (Podocarpaceae, NZ and Tasmania)
19. Pilgerodendron (Cupressaceae, Chile)
20. Podocarpus (Podocarpaceae, Chile, NZ and Tasmania)
21. Prumnopitys (Podocarpaceae, Chile and NZ)
22. Saxegothaea (Podocarpaceae, Chile)

Does it make sense if any genera (extant, extinct or endemic and therefore by definition extinct) from these three families grew in Antarctic in Eocene?

 

[snorkack]

A vital fact about Holocene climate is that except for Antarctica, which does not have warm summers, the other lands do not have cold winters. The coldest place in southern winter I could find is Tolhuin, in inland Tierra del Fuego:
https://en.wikipedia.org/wiki/Tolhuin
July average -1,3, JJA mean -0,5, January average +9,4, DJF mean +8,9.
Note the trees in streetview, too.
Of course high mountain areas above treeline are colder in winter (and tend not to have weather stations) but they are colder in summer and are not supportive of tree growth. Recent Southern Hemisphere tree genera do not have sites where they have enough summer warmth to grow at all, but harsh winter frosts to adapt to.

 

[Baluncore]

July average -1,3, JJA mean -0,5, January average +9,4, DJF mean +8,9.

Since this is a scientific forum, and with the European convention of using commas as decimal points, those temperatures must be assumed to be specified in degrees Celsius.
That is quite warm, it gets colder in winter in Tasmania.

 

[snorkack]

Since this is a scientific forum, and with the European convention of using commas as decimal points, those temperatures must be assumed to be specified in degrees Celsius.
That is quite warm, it gets colder in winter in Tasmania.

It does not.
https://en.wikipedia.org/wiki/Liawenee
July average +2,1, JJA mean +2,6, January average +12,4, DJF mean +11,5
https://en.wikipedia.org/wiki/Mount_Wellington_(Tasmania)
July average +0,7, JJA mean +1,0, January average +9,5, DJF mean +8,8

Also something, now that anyone is responding...
Since I am critizising the article in public, would it be polite to notify the corresponding author?

 

[Baluncore]

Since I am critizising the article in public, would it be polite to notify the corresponding author?

Not until you realise that, "numbers without units, are meaningless".

 

[snorkack]

Not until you realise that, "numbers without units, are meaningless".

The units are, as you correctly guessed, in degrees Celsius, and in this case they are actually supported by links which do give the unit.
Refer to the argument about not unnecessarily sacrificing breadth for depth. Also, in this forum, editing of posts has time limit.

 

[snorkack]

Now, note the diversity of genus Pinus.
Pinus includes Pinus caribaea, which occurs only in tropical lowlands, at the coldest in Bahamas and Cuba. It does not encounter frosts in its native range, and does not tolerate them:
https://www.conifers.org/pi/Pinus_caribaea.php
Pinus also includes Pinus sylvestris. Which has a broad range... into East Siberia.
https://en.wikipedia.org/wiki/Pinus_sylvestris#/media/File:Pinus_sylvestris_range-01.png
It does not spread as far north in East Siberia as Larix gmelinii does, but Central Yakutia is pretty extremely continental and the pines there look pretty treeish... Lower Bestyakh:
https://en.wikipedia.org/wiki/Nizhny_Bestyakh#/media/File:Посёлок_Нижний_Бестях,_Якутия.JPG
and the climate right across the river, in Yakutsk:
https://en.wikipedia.org/wiki/Nizhny_Bestyakh
January average -38,6, DJF mean -36,7, July average +19,5, JJA mean +17,0

For the extant Holocene Palearctic conifer species pool, Yakutsk (DJF mean -36,7) has treeish evergreen Pinus present alongside deciduous Larix. Chita (DJF mean -21,7) also has Pinus and Larix, and IIRC some others. Yellowknife (DJF mean -23,3) has Pinus and Larix present, but IIRC, they are not prevalent - evergreen Picea and Abies are more prevalent there.
Consider that by late Eocene, Antarctic continent had been roughly at its then position for tens of millions of years, with only slow drift. It had been a continental mass all that time. There had been some climatic changes, but these had apparently been gradual, not as sudden as stadial/interstadial oscillations of Pleistocene. No major disruptions since the start of Cenozoic.
Plants had had tens of millions of years to adapt to their local climate.

I suggest that there might have been conifers that had adapted to cold winters of interior Antarctic continent. These conifers might have belonged to genera that had widespread range of adaptations (like Pinus) and for which the cold intolerant species still exist, like Podocarpus, or to genera that only existed in Antarctica, like 18 out of 22 conifer genera that exist in Chile, NZ or Tasmania exist in only one of these.

 

[snorkack]

As for relative importance of broadleaved vs. coniferous trees, the analogies of Holocene temperate oceanic climates are inconclusive.
Western European oceanic climate vegetation is notoriously dominated by deciduous broad-leaved trees like Quercus and other. Pinus, Picea and Abies are totally missing, and Taxus and Juniperus are present but restricted to underbrush and marginal soils.
Western North American oceanic climate vegetation is notoriously dominated by evergreen conifers like Picea, Pseudotsuga, Tsuga, Abies. Deciduous broad-leaved trees like Alnus, Acer and Quercus are present but on restricted sites.
In Southern oceanic climates, the dominant trees include conifers and evergreen broad-leaved trees (not sure about ratio of prevalence). Deciduous broad-leaved trees are either rare (two species of Nothofagus in Chile, one in Tasmania and that scrubby rather than tall-growing) or altogether missing (New Zealand).

 

[snorkack]

However, there is some pattern noted in Northern subpolar treeline.
In the oceanic climate of Northwest Europe, like Scandinavia, treeline is formed by deciduous (though often called "narrow-leaved") Betula. Where Pinus and Picea are present, they do not reach as high or as far on coast/islands as Betula.
In the more continental climate of East Europe east of White Sea, Picea reaches to timberline. Betula trees stay further south. Betula genus reaches further, but as bushy species, not tall trees.
In North America, in oceanic climate of Southwest Alaska between Gulf of Alaska and Bering Sea, timberline again consists of Betula. Picea stays further east. In the more continental inland Alaska and all the way east to Labrador, it is Picea that forms treeline.
In East Asia, in Kamchatka coastal areas, again the vegetation is dominated by Betula. Picea occurs in more continental interior of Kamchatka peninsula, and on west coast of Okhota sea.

 

[snorkack]

Observed fossil vegetation of greenhouse polar areas tends to be deciduous:
https://eprints.whiterose.ac.uk/id/eprint/58/1/osbornecp1.pdf
Note a qualification at bottom right of page 61 of magazine/3 of pdf, referring to references 7, 21 and 22:

Moreover, it has been recognized that Antarctic polar forests appear to have contained a higher proportion of evergreens than their Arctic counterparts^7,21,22

 

[snorkack]

For an analysis of Pliocene Arctic climate see
https://www.researchgate.net/public..._River_Delta_northern_Yakutia_Eastern_Siberia
Note how some of the vegetation indicators diverge from the climate models (Discussion, left column of page 10).

 

[snorkack]

An issue I see with fossil record of Pliocene vegetation and climate proxies is as follows:
Preserved and accessible fossils tend to come from regions that are now and were then coastal. Like the coastal areas of Antarctic and Arctic seas.
For physical climate reasons, more continental climate and colder winters might be expected in inland areas - interior Antarctica, interior Greenland, middle Lena valley, middle Mackenzie valley.
Interior Antarctica and Greenland are now covered by ice so the preglacial deposits there, even if preserved, are hard to access.
Middle Lena and Mackenzie valleys may have been inland in Pliocene. Were they erosion areas in Pliocene? Or were any deposits laid down there, preserving fossils and climate proxies?

 

[snorkack]

The example of Holocene vegetation suggests that Palearctic continental interior moderately cold winters, in the range of -5 to -25, favour a bunch of evergreen conifers. Picea, Abies, Pinus, in North America also Thuja, Tsuga.
So could evergreen conifers (different genera) be expected for Eocene Antarctic interior?

 

[snorkack]

Something that the article hints at... Section 4.1:

A 50–60 ∘C SAT difference is seen between the mean coldest and warmest month over central Antarctica, values only seen over parts of Siberia in the present climate. Considering warm and wet summers as well, eastern Siberia may thus provide the closest present analogue to the Eocene climate over much of Antarctica.

...but does not outright state, is that the Antarctic interior Eocene climate as modelled is a no-analogue climate compared to the modern one.
Consider Figure 5, the 2xPIC case.
Sadly, while the longitudes are expressly labelled on the images, the latitudes are not. Nor are the comparative positions of x axis lines on the left graphs.
The coldest winters for the line are about -30 JJA (a downward jog of the blue line). Giving summer of about +25...+26 DJF
Compare the cross-section from East Siberia south:
Yakutsk
https://en.wikipedia.org/wiki/Nizhny_Bestyakh
January average -38,6, DJF mean -36,7, July average +19,5, JJA mean +17,0
Chita
https://en.wikipedia.org/wiki/Chita,_Zabaykalsky_Krai
January average -24,6, DJF mean -21,7, July average +19,5, JJA mean +17,7
Jiamusi
https://en.wikipedia.org/wiki/Jiamusi
January average -17,9, DJF mean -15,6. July average +22,8, JJA mean +21,2
Shenyang
https://en.wikipedia.org/wiki/Shenyang
January average -11,4, DJF mean -8,7, July average +24,9, JJA mean +23,7
Beijing
https://en.wikipedia.org/wiki/Beijing
January average -2,7, DJF mean -1,0, July average +27,2, JJA mean +26,2
The point indicated seems to be a saddle separating Transantarctic Mountains from Gamburtsev Mountains and Wilkes Land lowlands from Weddell Sea lowlands. Judging by Figure 8 that shows 500 m lines approaching but not meeting there, maybe 400 m altitude? Approximately the region where South Pole drifted in the 38 million years since.
And the modelled climate is a no analogue one. Winter almost but not quite as cold as in Yakutsk - summer far hotter than in Yakutsk, about as hot as in Beijing, but Beijing has a far milder winter.

 

[snorkack]

How would that no-analogue climate in terms of temperature amplitude by itself affect the vegetation of interior Antarctica?
I suspect that most plants would not be limited by temperature amplitude itself.
What limits the diversity of modern vegetation of Lena and Mackenzie valleys - vegetation that still is reasonably diverse and forms a blanket of forest?
While absolute thermal minima in winter are a factor, a winter with -30 January mean would kill a few plants that are not killed at -20 January mean, I have an impression it is not the major factor.
The bigger factor limiting vegetation seems to be length of vegetation season. The time of year when it is warm enough for plants to complete growing their shoots, fruits etc.
Inland Antarctica, with hotter summers for equally cold winters, would have a longer vegetation season. And thus be suitable for more diverse vegetation.

 

[snorkack]

So far I have addressed exclusively temperature.
However, the modelling also covers precipitation. Yet, again, in inconveniently inconsistent way.
Note that some data are in Supplement only.

 

[snorkack]

Precipitation data and the relevant temperature data:

• Article Figure 2: 4xPIC summer and winter temperature maps
• Article figure 3: 4xPIC and 2xPIC monthly temperature and precipitation averaged over regions
• Article figure 5: 2xPIC summer, winter and annual temperature and precipitation profile along a line
• Article figure 8: Warmest month temperature map, comparison against 3 other authors
• Supplement figure S8: Summer temperature map, several model variants
• Supplement figure S9: Winter temperature map, several model variants
• Supplement figure S10: Annual precipitation map, several model variants
• Supplement figure S11: Summer precipitation map, several model variants
• Supplement figure S12: Winter precipitation map, several model variants