On Monday, February 19, 2024 at 8:23:13 AM UTC-5, Ernest Major wrote:
According to a press report, a passing star caused the PETM.
"According to a new study published in The Astrophysical Journal
Letters, the relatively close passing of a star 56 million years ago
caused the Earth’s temperature to rise by eight degrees centigrade."
https://www.indy100.com/science-tech/earth-orbit-chance-encounter-star
The actual paper says that the star passed by 2.8 million years ago
(only a factor of 20 difference). What the paper says is that taking
account of perturbations caused by the flyby, the uncertainty in the
eccentricity of the earth's orbit at the time of the PETM is
considerably larger, which makes an orbital forcing more tenable as a
cause of the PETM.
It is true that due to it's chaotic nature, we cannot predict the evolution of orbital parameters infinitely far into the past.
But observations (and such things as conservation of energy) do provide
some constraints.
In rocks laid down well before this event, (e.t. the Triassic) the temporal signature of Milankovitch variations in orbital parameters is clear, and it is the
same signature we see in more recent records. It is not clear to me
that a passing object could induce major changes in orbital variations, which later resume their previous behavior.
The long eccentricity cycles are controlled by interactions between
the Earth, Jupiter, and Saturn, will little impact from other bodies.
If either of these outer planets were seriously perturbed by
a passing object, the tempo of eccentricity changes should vary.
Eccentricity itself has little direct effect on the amount of sunlight reaching
the earth. It's major impact is in combination with the precession of
the equinoxes, and this is due to the gravitational pull of the moon
and sun. It will not be altered by perturbations in the orbits of
other planets, or by a plausibly higher eccentricity in the earth's orbit.
The linear effect of all orbital variations, to a very good approximation,
is to redistribute sunlight among latitudes and seasons, not to
contribute to a net warming or cooling of the planet. It takes
nonlinear feedbacks (e.g. ice albedo) to rectify the signal one
way or another.
William Hyde
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