Now, getting back to the topic, I think that your quote deserves further attention Doc:
Doc wrote: ↑Wed Jun 15, 2022 8:54 pm
Reconstruction of the past 5 million years of climate history, based on oxygen isotope fractionation in deep sea sediment cores (serving as a proxy for the total global mass of glacial ice sheets), fitted to a model of orbital forcing (Lisiecki and Raymo 2005)[2] and to the temperature scale derived from Vostok ice cores following Petit et al. (1999).[3]
https://en.wikipedia.org/wiki/Geologic_ ... ure_record
Description of the temperature record
The last 3 million years have been characterized by cycles of glacials and interglacials within a gradually deepening ice age. Currently, the Earth is in an interglacial period, beginning about 20,000 years ago (20 kya).
The cycles of glaciation involve the growth and retreat of continental ice sheets in the Northern Hemisphere and involve fluctuations on a number of time scales, notably on the 21 ky, 41 ky and 100 ky scales. Such cycles are usually interpreted as being driven by predictable changes in the Earth orbit known as Milankovitch cycles. At the beginning of the Middle Pleistocene (0.8 million years ago, close to the Brunhes–Matuyama geomagnetic reversal) there has been a largely unexplained switch in the dominant periodicity of glaciations from the 41 ky to the 100 ky cycle.
The gradual intensification of this ice age over the last 3 million years has been associated with declining concentrations of the greenhouse gas carbon dioxide, though it remains unclear if this change is sufficiently large to have caused the changes in temperatures. Decreased temperatures can cause a decrease in carbon dioxide as, by Henry's Law, carbon dioxide is more soluble in colder waters, which may account for 30ppmv of the 100ppmv decrease in carbon dioxide concentration during the last glacial maximum. [1]
Similarly, the initiation of this deepening phase also corresponds roughly to the closure of the Isthmus of Panama by the action of plate tectonics. This prevented direct ocean flow between the Pacific and Atlantic, which would have had significant effects on ocean circulation and the distribution of heat. However, modeling studies have been ambiguous as to whether this could be the direct cause of the intensification of the present ice age.
This recent period of cycling climate is part of the more extended ice age that began about 40 million years ago with the glaciation of Antarctica.
Since warm water seas flanked both sides of Panama I doubt very much whether the closure of the Darien Gap would have had much of an effect either on ocean circulation or to the earths zones of climate. What would have had more of an impact on both would have been the emergence from below the sea of the broad Beringia land bridge that unified Eurasia and North America into a single unitary continental landmass for as long as the most recent Ice Age lasted.
Why so? Because (firstly) it would have stopped the flow of icy water from the Arctic Sea through Bering Strait into the northern Pacific Ocean. That would have meant a permanent bank-up of icy water within that Sea extending south into the northern Atlantic Ocean as far south as the vicinity of Iceland, The British Isles and Newfoundland - adjacent to where the great northern ice sheets formed and no doubt helping to keep these along with the underlying land masses chilled. And so the Ice Age perpetuated itself.
It also would have meant that on the Pacific side the warm ocean currents that originated in the vicinity of the central Pacific and/or Indonesia would have been forced into a permanent clockwise rotating circulation thanks to Beringia: hence the warmer Ice Age regional climate of eastern Siberia.
But now, what about the south? I have a direct interest in what happened in the Earth's southern half simply because I live there.
Doc's quoted info duly notes that the glaciation of Antarctica began only around 40 million years ago. Prior to 60 million years ago it was part of a larger continental mass known as
Gondwana (sometimes as
Gondwanaland). Along with Antarctica and fused into it were the landmasses of Australia; New Zealand; India and Madagascar. Another 100 million years or so earlier South America and the southern part of Africa were part of a supercontinent the approximate size of present day Eurasia. Then the mass broke up with tectonic plates carrying different parts of it in a generally northerly direction.
The glaciated island-continent of Antarctica has retained its ice cover even during the relatively warm inter-glacial periods because it is surrounded by the eternally circulating, chill waters of the Southern Ocean. Pushed along from west to east by the eternal westerly winds flowing around our high temperate latitudes the broad drift acts like a barrier that prevents any warm water current from further north to ever penetrate closer than a couple of thousand sea kilometers closer to the Antarctic coast - so there is nothing to warm Antarctica up.
There is no continental land mass closer to interrupt the flow of either air or water until it reaches the Patagonia peninsula of South America; which seems to act like a gigantic snag-hook that diverts the more slow moving northern waters of the drift into the high-speed Humboldt Current along the coast of Chile and Peru. The more southerly waters flow on unimpeded through the strait between Tierra del Fuego and the Antarctic Peninsula.
Australia was apparently the last piece of continent to break away from former Gondwana and drift north - which it has done at high speed ever since. The water filled gap that opened between southern Australia and the adjacent Antarctic coast as it grew wider permitted that circular flow of cold sea water to get underway and sustain itself since.
While Australia was still a large "hump" protruding north from Antarctica (
considering that presently Australia, including Tasmania, currently extends from 11 degrees south to 43 degrees south latitude that would have put Australia's most northern extreme in those times at about 34 degrees south of Equator: which is the present day latitude of Sydney) when without a doubt the western coast of that hump would have deflected cold water north towards the Equator while making room for a flow of warm water from more northerly latitudes probably flowing down along its eastern coastline. This would have led to a much more benign, most likely low temperate regional climate for the combined island continent.
