So, Ice Ball or Venus.
Seems like climate is more sensitive than we think.
Man made impact or not, the whole system seems on a knifes edge of tipping towards the extremes. That we are living right now is rare in-between point.
Seems like climate is more sensitive than we think.
Man made impact or not, the whole system seems on a knifes edge of tipping towards the extremes. That we are living right now is rare in-between point.
Another part of the system that could be like that is the oxygen content of the atmosphere. The time constant for variation in O2 level is just a few million years (due to consumption of O2 by release of reducing gases and weathering of rocks to expose reduced materials like ferrous iron.) If it had ever dropped too much during the Phanerozoic higher life would have been wiped out. I don't believe any plausible feedback control mechanism has been discovered. In particular, fire involving plants (which couldn't have occurred before the evolution of large land plants) can be a positive feedback, as charcoal is very resistant to further oxidation and, when buried, represents a net increase in atmospheric oxygen.
Maybe, if mankind explores the galaxy, we'll find all these formerly life bearing planets where stability failed.
> Since 2009, some researchers have argued that during the Cryogenian Period, potentially the oldest known fossils of sponges, and therefore animals, were formed. However, it is unclear whether these fossils actually belong to sponges, though the authors do not rule out the possibility of such fossils to represent proto-sponges or complex microbial precursors to sponge-grade organisms. The issue of whether or not biology was impacted by this event has not been settled, for example Porter (2000) suggests that new groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.
> The end of the period also saw the origin of heterotrophic plankton, which would feed on unicellular algae and prokaryotes, ending the bacterial dominance of the oceans.
Even during catastrophic events, life persists in certain niches: deep sea life usually flourishes during massive extinctions because the marine snow becomes a blizzard; ducks survived the dinosaur extinction because rivers obtain bioavailable energy from gravity and erosion, not photosynthesis. So I suspect something like an "Mars vs Venus" cycle is necessary for the evolution of complex life. A highly predictable habitable planet might have cyanobacteria but not algae, since there wouldn't be any selective crises for mutated bacteria to adapt to. It would be awfully hard for a spectrometer to tell the difference between a planet full of bacteria and a pre-civilization planet full pf complex life.
But the really interesting thing is that that derived from this [2], which is Earth's temperature over the past 65 million years. Those previous extremes just blur, because now you're talking about a temperature difference of 20 degrees celcius, no ice on the planet, and giant beasts roaming in lush greenery. Greenery which was basically everywhere, even Antarctica! [3]
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[1] - https://en.wikipedia.org/wiki/Geologic_temperature_record#/m...
[2] - https://en.wikipedia.org/wiki/Geologic_temperature_record#/m...
[3] - https://en.wikipedia.org/wiki/Eocene#/media/File:Ypresian_Ea...
Climate and Biosphere is delicate with lots of tipping points and cascades to uninhabitable.
A few billion years ago, the inner planets of the Sol system were capable of supporting abundant surface life - now only Terra.