Anyway, this one is the most sensible so far.
From memory, with some garnish.
The core hypothesis is that Venus is the norm for terrestrial planets, and that Earth billions of years ago was similar, namely thick atmosphere, very high in CO2, extreme barometric pressure (~90 times Earth), and basically no atmospheric oxygen.
Then the normative freak collision, a huge celestial object (that itself broke up) plowed through the atmosphere and got a big chunk of crust, too. Thus creating a vast halo of particulates which accreted into a moon.
Anyway here is where it gets interesting, with some of Noel's timing being uncertain (maybe because it is impossible).
The moon back then was closer (scientists agree), and if the Earth was also sufficiently smaller than it is now then the barycenter of the moon-Earth together would have been outside the Earth! Meaning as a pair they would technically have been dual planets.
But because Earth had an atmosphere, there would have been (for at least hundreds of millions of years) a shared gravity well, and the Earth's atmosphere would have bled over to the moon's.
Meanwhile the moon, being too small to retain lighter gases, would then have bled gas to outer space.
Since the Earth would be slowly losing atmosphere, the barometric pressure at the surface would be constantly dropping. This would allow the gravity-compressed Earth to begin to expand. Cracks would form in the crust, destined to become oceans (inter alia: why current ocean floors are so young).
Noel does not say this, but maybe something about the special situation allowed edge-of-space micro-rain.
As I read it, this hypothesis holds the lowering surface barometric pressure allowed the creation of anaerobic life similar to coccolithophores. Namely protozoa that fixed CO2 with calcium (on the ground) into calcium carbonate.
This early life - Noel suggests - isolated oxygen. Which did not get a chance to all float into space ...
Meanwhile the moon is still moving away. And the Earth is bigger. So eventually, the barycenter moves inside the Earth, meaning the gravity well disappears, there is no more atmospheric bleed-over, and the planet has a true moon.
Thanks to new atmosphere, oxygen-based life appears, which fixes even more CO2 out of the air.
Now here we are, hypothetically, on a planet that once had maybe as much as 95% CO2 in its atmosphere, but is now a trace gas.
- I guess since CO2 is heavier than N or O, the surface barometric pressure would have continued to drop? Thus, even after the moon became a true moon, there would have been expansion, albeit at a a slower rate
- re: micro-rain: the hydrogen for the water apparently from outer space (too light to be atmospheric), combining with the oxygen (freed up by the protozoa) only the H adding to Earth's mass, thus about 0.01% not 0.02%
- in this Earth Expansion hypothesis (EEH) the expansion has a natural cessation point
- the lithosphere being real, the loosest bit nearest the equator would move away the fastest, centrifugal force, hence the Himalayas -- the first EEH that is satisfying on this point
- the onset (which the planet is still in) of glaciation not just because of the position of Antarctica but because of all the CO2 fixing?
- All the changes guaranteed periodic mass extinctions. Poor Barney.
- the planet has a vast amount of limestone. That from CO2 permanently fixed out of the atmosphere, the #volcanoes adding CO2 during the billions of year is unknown, ASFAIK
In 2013 famous NASA Venusian & Earth climatologist James Hansen clarified his remarks about Earth's potential future. Because his opinion is strikingly like an opposite of Noel's (and because, frankly, he is the type of negativist who sometimes turns out to be an opposite indicator) I will quote a big bit.
“I get questions from the public about the Venus Syndrome: is there a danger of
'runaway' greenhouse warming on Earth leading to Venus-like conditions?”
“Venus today has a surface pressure of about 90 bars, compared with 1 bar on Earth. The Venus atmosphere is mostly CO2. The huge atmospheric depth and CO2 amount are the reason Venus has a surface temperature of nearly 500°C.”
“Venus and Earth probably had similar early atmospheric compositions, but on Earth the carbon is mostly in Earth's crust, not in the atmosphere. As long as Earth has an ocean most of the carbon will continue to be in the crust, because, although volcanoes inject crustal carbon into the atmosphere as CO2, the weathering process removes CO2 from the air and deposits it on the ocean floor as carbonates. Venus once had an ocean, but being closer to the Sun, its atmosphere became hot enough that hydrogen could escape from the upper atmosphere, as confirmed today by the extreme depletion on Venus of normal hydrogen relative to heavy hydrogen (deuterium), the lighter hydrogen being able to escape the gravitational field of Venus more readily.”
“Earth can "achieve" Venus-like conditions, in the sense of ~90 bar surface pressure, only after first getting rid of its ocean via escape of hydrogen to space”
As an amateur I am skeptical.
Note the 'similar early atmospheric compositions' bit. Hansen does not say what those are, precisely. (Noel has more guts). Then he changes tenses very quickly. He seems to want Earth as the more archetypal, with a kind of preCopernican feel.