[Coral-List] coral bleaching

Douglas Fenner douglasfennertassi at gmail.com
Thu Apr 21 20:34:43 UTC 2022

    Earth not only had higher sea levels 100,000 years ago, and much lower
just 20,000 years ago, but this 100,000 year cycle has been going on for
many cycles in a period of time some people call the ice age over the past
several million years.  The sea level changes are due to ice being locked
up on land in giant ice caps on North America and northern Europe, similar
to the ice caps now on Greenland and Antarctica.  Ice locked up on land
from more snow falling than melting, comes from water that evaporates off
the oceans, so it reduces ocean height, and the changes in sea level are
enormous as Gene pointed out.  One small detail is that the end of the last
ice age produced rapid sea level rise starting about 13,000 years ago (when
sea levels were about 120 METERS lower than today) and ending a few
thousand years ago, when sea levels were a little higher than today, maybe
a couple meters.  Then sea level went down from then to the present, and of
course now they are rising rapidly on a geological time scale.  Anyhow,
there is a roughly 100,000 year cycle, with sea level usually decreasing
gradually, then increasing suddenly on a geological time scale (it's more
of a sawtooth curve than a sinusoidal wave curve).  A variety of lines of
evidence show such cycles, such as data from ice cores removed from
Greenland and Antarctica, and oxygen isotopes in fossil foraminifera shells.
       This and much more is explained in much more detail on the Wikipedia
page on "Ice Age" and the section on that page on "Glacials and
https://en.wikipedia.org/wiki/Ice_age#Glacials_and_interglacials  You can
see, for instance, five different major ice ages on the graph at the
beginning of the section on "Major ice ages," with the first set starting
nearly 2500 million years ago.  The next graph shows the gradual increase
in the amplitude of the glacial cycles over the last 5 million years.  This
is the most recent ice age, which we are still in.  Then in the section on
"glacials and interglacials" it has a graph of the last 5 glacial cycles
over the last 450 thousand years, and you can see how closely the different
data sources agree, including an ice volume measure and two temperature
proxy measures (oxygen isotopes).  I'd like to point to the last sentence
in the "Glacials and Interglacials" section:  "Moreover, anthropogenic
forcing from increased greenhouse gases
<https://en.wikipedia.org/wiki/Greenhouse_gas> is estimated to potentially
outweigh the orbital forcing of the Milankovitch cycles for hundreds of
thousands of years.[49] <https://en.wikipedia.org/wiki/Ice_age#cite_note-49>
[5] <https://en.wikipedia.org/wiki/Ice_age#cite_note-PIK2016-5>[4]
<https://en.wikipedia.org/wiki/Ice_age#cite_note-LiveScience2007-4>"   So
the effects of the greenhouse gases we are putting into the atmosphere
could last for as much as hundreds of thousands of years (not just 50
       The best available current explanation of the cause of the glacial
cycles in the past is "Milankovitch cycles."  These are cycles in the
motions of earth, such as the eccentricity of its orbit around the sun and
the tilt of the axis of rotation.  Also, the gravitational attraction of
other planets is involved.  Although Jupiter is far from us on human scales
of distance, it has more mass than all other planets combined.  Anyhow,
some of the cycles influence climate more than others.  One of the ways
that they influence climate is that most of the earth's land masses are in
the northern hemisphere.  So if the northern hemisphere is tilted towards
the sun in the season when earth is closer to the sun and the eccentricity
of the earth's orbit is near maximum, the earth's surface heats more,
because land warms much faster than water, mainly due to the great heat
capacity of water.  (That effect is easily seen in "sea breezes" in which
during the day on temperate coastlines in the summer, the land during sunny
days heats up, the ocean doesn't, so air rises over land and sucks water
off of the ocean, producing a strong, cold wind from the ocean onto land.
At night, the land cools and the sea breeze reverses, blowing from land to
the water.)
     Wikipedia has a page on Milankovitch cycles:
https://en.wikipedia.org/wiki/Milankovitch_cycles  Their first graph shows
the temperature and sea level cycles as measured in ice cores and
foraminifera, at the bottom of the graph, and various Milankovitch cycles
in the upper part of the graph.  It is quite true that glacial cycles
before the present time could not have been produced by greenhouse gases
released by humans.  However, Milankovitch cycles explain a lot of the
variation from one cycle to the next during an ice age period.  Carbon
dioxide concentration in the atmosphere has also varied greatly at
different times.  One likely source of that CO2 was supervolcano eruptions
such as at Yellowstone in the USA, and another possible source is "flood
basalts."  The latter are gigantic lava flows that were so fluid that they
spread out like a flood over wide areas.  The most recent I know of is the
Columbia River Basalt Group in the US Pacific Northwest (Washington, Oregon
and Idaho).  An older and larger set is in India, the "Deccan Traps" and
the largest and even older is the "Siberian Traps."  Typically, they
consist of many layers of horizontal basalt lava flows on top of each other
(these eruptions did not form volcanoes.)  There are many such flood
basalts, the three I refer to are some of the larger or more famous ones.
These enormous flows surely released huge volumes of gases, likely
including CO2 and SO2.  By the way, the dark patches on the moon, called
"Mares" because they look like oceans, are also flood basalts (oceans of
solidified lava not water).  Flood basalts have also been suggested to
possibly be the cause of some of the great extinction events in the
geological record.  The Wikipedia page on them says "Large igneous
provinces have been connected to five mass extinction
<https://en.wikipedia.org/wiki/Mass_extinction> events, and may be
associated with bolide <https://en.wikipedia.org/wiki/Bolide> impacts.[5]
<https://en.wikipedia.org/wiki/Flood_basalt#cite_note-5>"  Though causes of
some of the major extinction events are still debated, I believe.
Wikipedia has a page on flood basalts:
https://en.wikipedia.org/wiki/Flood_basalt   with many more details of
course.  And of course it has a page on extinction events, and their
possible causes, including flood basalts:
     As the Wikpedia pages on ice ages and Milankovich cycles point out,
not every detail has been worked out, there are still puzzles to explain.
That is, of course, typical of science and human knowledge in general.
     Cheers, Doug

On Thu, Apr 21, 2022 at 3:06 AM Dennis Hubbard via Coral-List <
coral-list at coral.aoml.noaa.gov> wrote:

> Hi Gene:
> While the specifics have faded in my memory, "natural" sea level is the
> result of many factors including the obliquity of Earth's orbit, Earth's
> tilt and its proximity to the sun (probably others that I've omitted due to
> failing memory since I taught these things). However, as I remember, both
> the magnitude and slope of temperature changes were much smaller than what
> we have seen since the "industrial revolution". So, we are, at the very
> least" "among" the leading causes. Absolute temperatures have been higher
> in the past than what is proposed, but I believe that these intervals
> represent significant declines or absence of reefs as wew define them.
> Denny
> On Wed, Apr 20, 2022 at 8:08 AM Eugene Shinn via Coral-List <
> coral-list at coral.aoml.noaa.gov> wrote:
> > Regarding our recent discussions concerning coral bleaching and
> > temperature here is something I thought would be relevant for readers of
> > the list. It is well known that the Florida Keys were once a coral reef
> > as were many elevated Pleistocene age coral reefs around the planet.
> > They had to be formed when sea level was several meters above present.
> > They did not grow in air. They grew during what is known as isotope
> > stage 5e. The newest information suggests that was 129-116 thousand
> > years ago. Most scientists agree the higher sea level at that time was
> > due to melting polar ice and therefore Earth was likely warmer that
> > today's temperature. If bleaching today is due to warm temperatures,
> > then coral growth that produced those elevated stage 5e coral reefs
> > should have bleached. We have no way of knowing for sure, but it is
> > evident that those 5e reefs did produce what is now land in many places
> > including the Florida Keys. If there was bleaching back then it is clear
> > those corals did not go extinct until sea level dropped and left them
> > high and dry. As readers know, sea level dropped as much as 120 meters
> > when those reefs were left high and dry. Sea level then began rising as
> > it is doing now. Was the higher sea level during stage 5e caused by
> > higher CO2 levels? If so, what caused the high CO2 levels? There were no
> > automobiles or fossil fuel power plants back then. The CO2 could have
> > been expelled by a warming sea but what caused the sea to warm? Could
> > whatever cause the sea to warm be what is causing today's warming? I
> > thought these questions would be relevant subjects for discussion by
> > members of the coral-list. Gene
> > _______________________________________________
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> > Coral-List at coral.aoml.noaa.gov
> > https://coral.aoml.noaa.gov/mailman/listinfo/coral-list
> >
> --
> Dennis Hubbard - Emeritus Professor: Dept of Geology-Oberlin College
> Oberlin OH 44074
> (440) 935-4014
> * "When you get on the wrong train.... every stop is the wrong stop"*
>  Benjamin Stein: "*Ludes, A Ballad of the Drug and the Dream*"
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