252 million years ago, the worst event in the history of life on Earth occurred. 70% of all terrestrial species and 96% of all marine species became extinct through, according to geological records, a combination of events--flood basalt eruptions in Siberia, runaway greenhouse effect caused by the melting of methane ice, even the formation of the supercontinent Pangaea itself.

In an alternate Earth, a reverse Great Dying occurred, one in which 96% of all terrestrial species and only 70% of all marine species became extinct. What combination of factors would create such specified damage?

Just a reminder, this question does not ask how long life would recover, who would fill in the vacant niches, etc. The question is what would cause an extinction event as described in the paragraph above. It doesn't matter when it happened.

  • 1
    $\begingroup$ considering we are not sure what caused it it is basically impossible to say. what dies is not guaranteed you can just say that is the ratio and no one will be able to call you on it. $\endgroup$
    – John
    Commented Aug 6, 2018 at 1:02
  • $\begingroup$ The cause IS the question. $\endgroup$ Commented Aug 6, 2018 at 1:43
  • $\begingroup$ @JohnWDailey and that's what I am saying, we know there was a massive impact event and its trigger volcanism, but that is the cause of most mass extinctions, it is not something we understand down to percentages. $\endgroup$
    – John
    Commented Aug 6, 2018 at 5:08
  • $\begingroup$ Your use of the term inverse and reverse (which one is it?) is a bit confusing. The reverse of dying is....? $\endgroup$
    – user3106
    Commented Aug 6, 2018 at 13:22

5 Answers 5


Gamma ray shower from a close-by supernova.


The explosion of a supernova close (few tens of light year) to the Sun would bathe the Solar System in a shower of highly energetic gamma rays, which are lethal for life. Moreover, stripping over the ozone layer, it will allow also radiation from the Sun (UV, X-ray, gamma ray) to easily reach the surface.

Being shielded by water, the organisms living immediately after the surface layer would get a better protection from such shower, thus explaining why there is an higher mortality on land than in water.

  • $\begingroup$ How much would it affect the atmosphere? It would cause a lot of ionization, but that would end fairly fast. It wouldn't necessarily blow a significant amount away, and it wouldn't change the isotopic distribution. $\endgroup$ Commented Aug 7, 2018 at 21:30
  • $\begingroup$ @DavidThornley In other words...? $\endgroup$ Commented Aug 9, 2018 at 3:42
  • $\begingroup$ @L.Dutch How long would the atmosphere re-establish itself? $\endgroup$ Commented Aug 13, 2018 at 2:35
  • $\begingroup$ @JohnWDailey, it depends on many factors. Years to decades, I guess. $\endgroup$
    – L.Dutch
    Commented Aug 13, 2018 at 2:41
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    $\begingroup$ Gas generation rates (volcanic activity on all), strength of the magnetic field that holds ionized gases, entity of the damage... $\endgroup$
    – L.Dutch
    Commented Aug 13, 2018 at 3:02

Snowball Earth

snowball earth https://252mya.com/products/snowball-earth-wallpaper

The Snowball Earth hypothesis proposes that for a period during the Precambrian, the Earth froze over entirely or almost entirely.


The Snowball Earth hypothesis proposes that Earth surface's became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record... A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.

For this answer, the Snowball Earth event occurs instead of the Permian extinction, as stated on the question. There would be a thin band of open water at the equator (as in above image). This open water would act as a refuge for the 30% of aquatic species surviving the event. On the ice covered land only 4% of species survive: land based insect scavengers of dead fish washing up, and specialists eating snow algae.

How would the Snowball Earth come to be, you may ask? Just as with the Precambrian one - the earth cools because of volcanic winter or some similar event, new ice increases the earths albedo and reflects away heat, and runaway cooling ensues.


Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a supervolcano, a reduction in the atmospheric concentration of greenhouse gases such as methane and/or carbon dioxide, changes in solar energy output, or perturbations of Earth's orbit. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen equator as cold as modern Antarctica.

But how, you may ask, will the Snowball Earth ever thaw? In my proposed scenario, giant burps of volcanic CO2 and microbially-produced methane from the ocean floor increase the greenhouse effect and the earth melts.

Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium.

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    $\begingroup$ People are leaning more towards Slushball than Snowball. $\endgroup$ Commented Aug 6, 2018 at 1:42
  • $\begingroup$ That makes sense for the real world, but I think snow would be more likely to give the requested 96% extinction rate for land creatures. So for this answer it is snow. $\endgroup$
    – Willk
    Commented Aug 6, 2018 at 2:06

A much less lucky meteor strike.

Here's the thing with the K-T extinction event, which is most famous for having wiped out most the non-avian dinosaurs (and actually wiped out most of the avian ones, too: today's birds are descended from no more than five species that survived the strike).

It could have been much, much worse for land animals.

Here's a map I made of where the meteor hit, with images from this website:


enter image description here

It doesn't seem very big, does it? And in relation to the continent, it isn't. It also happened to hit a relatively benign spot, geologically speaking - that shallow area, which would become part of the Yucatan Peninsula, was mostly a centuries-old tropical reef and the porous limestone it had formed.

Now just imagine a slightly larger meteor hitting harder continental rock, like the granites of a mountain chain. The K-T event killed off about 70% of marine species because species living at lower depths were the least affected by its worst effects - turning the atmosphere into a huge oven, killing off the food chain by blocking out the sun, etc. With a larger meteor hitting harder rock, that advantage doesn't change.

However, that slight change in luck makes it that much harder for land-based species. A larger meteor hitting continental rock is going to throw even more mass into the atmosphere, blocking the sun out for even longer, and continuing to bake the surface for longer as it rains back down.

It's very likely, and it's all it takes to wipe out life in the ratios you're looking for.

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    $\begingroup$ Actually the gypsum and limestone deposits on the seabed at Chicxulub made the K-T event far worse than if it had hit somewhere with less sulfur and carbon, like on a subaerial continent. The carbon dioxide and sulfur aerosols messed with the climate far more, for far longer than the dust and smoke ever could have done alone. $\endgroup$
    – Ash
    Commented Aug 13, 2018 at 18:50
  • $\begingroup$ @Ash That's seriously cool! Where can I read more about it? $\endgroup$ Commented Aug 13, 2018 at 22:57
  • $\begingroup$ Don't know about read, it's something I learned in the last couple of years when I caught Episode 4 of the BBCs Catastrophe series; they had a Planetary Geologist, Professor Peter Schultz, who was talking about the effects of the sulfur and carbon dioxide mobilised from the Yucatan Peninsula. The sulfur caused massive cooling, 5°C, for a few years, then acid rain and then the carbon dioxide caused global warming on the order of 20°C above baseline that they think lasted for centuries. $\endgroup$
    – Ash
    Commented Aug 14, 2018 at 11:10
  • $\begingroup$ @Ash So if the impactor were to hit Vredefort instead of Chicxulub, would the immediate outcome be any different? $\endgroup$ Commented Aug 17, 2018 at 3:20
  • $\begingroup$ @JohnWDailey The immediate effect would be largely similar but instead of several hundred years of climate upheaval the total effect period would be a decade or less; the time it takes for the dust to rain back out of the atmosphere and plant growth to return to normal levels, no sulfur aerosols keeping the temperature low, no acid rain killing plant life just as seeds start to germinate, no long term warming either which is probably a two edged sword. $\endgroup$
    – Ash
    Commented Aug 19, 2018 at 11:05

You need to cool the atmosphere without poisoning the oceans. Oceanic life can survive simply cooling better than terrestrials, water is an excellent insulator. What nearly killed the oceans during the Great Dying was sulfur depleting the oxygen from the water. So to cut light without pumping sulfur into the atmosphere you need smoke, lots and lots of sooty smoke over an extended period of time, hitting a large forest with a meteor would be a start but I'd suggest that you actually want to burn a lot of relatively clean coal with very little oxygen. Coal seam fires can potentially pump out smoke continuously for centuries, probably longer. If you have a reasonably large meteoric impact that burns large areas of forest that's growing on shallowly buried peat and coal deposits then the fire could take hold in those buried carbon deposits. Partial combustion of the deposits, due to poor oxygenation, will pump sun blocking smoke into the atmosphere for years afterwards cooling the world and killing land-based lifeforms rapidly and the oceans slowly, the effect will be enhanced on a super-continent as smoke and fire blankets the continent while leaving the oceans relatively clear.


The great oxygenation, undone - Chloroplast virus.

2050: The ancient enemy of man, genus Plasmodium is driven to extinction. The apicomplexan vestigial chloroplast proves to be a fine therapeutic target.. Even the hypermutable plasmodium cannot out-evolve the synthetic virus targeting its chloroplast. Malaria is eliminated as a human disease.

2055 Crop failures are noted in central Africa, followed by similar failures in central and South America. It is realized that not just the crops, but all plants are dying. The synthetic virus has mutated, and all chloroplasts are now potential hosts.

2085 Green plants are gone and with them all terrestrial vertebrates and most insects. Green algae are not far behind.

  1. With green algae and plants gone, oxygen levels begin to drop. Free swimming organisms die when the oxygen levels of the ocean fall below 15% and many of the rest die at 10%.

  2. The lands of earth are left to the fungi, which spread in riotous profusion, subsisting on the remains of the land. Certain robust insects and mollusks survive by feeding on the fungi. As oxygen levels dwindle, so too the number of these survivors.

  3. Coral reefs explode with vigor. Partly it is the lack of predators. More importantly, the dinoflagellate chloroplast is genetically so distinct from that of the plants (and apicomplexans) that the virus cannot target it. These photosynthetic organisms thrive as do the corals, sponges and gorgonians that host them, their symbionts now providing both food and oxygen to their tissues.

Outside the growing reefs, the oceans are now ceded to cyanobacteria, which also colonize the empty land. Within a few decades, oxygen levels begin to rise once again.

The few terrestrial survivors are the fungi which made it through the lean years and soil / detritus dwellers like earthworms and sowbugs. Ocean life does better thanks to the coral reefs, although survivors are heavily skewed towards sessile organisms with photosynthetic dinoflagellate symbionts and (as on land) acorn worms and similar hypoxia-tolerant dwellers in the subsurface.

  • $\begingroup$ Why are you thinking future? $\endgroup$ Commented Aug 14, 2018 at 1:40
  • $\begingroup$ The question did not specify when the great dying is to take place. Specifically "It doesn't matter when it happened." Known modes for extinction events are pretty well covered in other answers, I thought. So: a fictional future one. $\endgroup$
    – Willk
    Commented Aug 14, 2018 at 1:42
  • $\begingroup$ Emphasis on "happenED". $\endgroup$ Commented Aug 14, 2018 at 2:09
  • $\begingroup$ I thought future made a good story. But you can put this whenever you like - a virus that targets chloroplasts. $\endgroup$
    – Willk
    Commented Aug 14, 2018 at 2:54
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    $\begingroup$ Dropping from a 32% atmospheric oxygen level to 22% is what happened at the end of the Carboniferous period. "Vast tropical rainforests collapsed suddenly as the climate changed from hot and humid to cool and arid. This was likely caused by intense glaciation and a drop in sea levels." That's when all those gigantic insects died; two-foot long dragonflies probably need that air. Terrestrial life had set itself up for failure; I can only assume that aquatic life fared a little better. $\endgroup$
    – Mazura
    Commented Aug 14, 2018 at 4:04

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