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Assume Earth of today was altered overnight.

Some percentage of the more common elements of Earth's bulk (silicon, iron, and so on) are replaced with something much heavier. This changes the over mass of Earth and its gravity increases according.

How much of a change to gravity would it take to cause an extinction level event for the higher lifeforms? In other words, no need to worry about microorganisms as they may not notice.

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    $\begingroup$ Note that increased content of elements heavier than iron would also increase radioactive fission, and you might get extinction from heat or radiation before you will get one from gravity. $\endgroup$ – Mołot Oct 18 '17 at 7:21
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    $\begingroup$ Which organisms are you targeting? Killing an elephant with gravity is not the same as killing an Escherichia Coli. $\endgroup$ – L.Dutch - Reinstate Monica Oct 18 '17 at 7:22
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    $\begingroup$ Not really. I just think you should be aware of this fact when you will be writing. But maybe specify that radiation is not the concern for you in this question. Similarly, you might want to specify that geological activity changes should be ignored, or taken into account, whichever is your intention. $\endgroup$ – Mołot Oct 18 '17 at 7:27
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    $\begingroup$ Agreed with @RonJohn: ditch the science and reality-check. This is magic... pure magic. So trying to suss if you need 5%, 10% or 50% magic to make it happen is kind of pointless. $\endgroup$ – MichaelK Oct 18 '17 at 7:49
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    $\begingroup$ The results of changes in complex systems are not predictable. Anything might happen. So it should happen faster if the change is higher, but you might get ecosystem breakdown with very very little changes. On the other side: "Earth of today" undergoes already an extinction event for the higher lifeforms with "today" meaning the millenium before and after now. $\endgroup$ – Henning M. Oct 18 '17 at 8:40
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The main cause of extinction will not be direct effect on biology but geophysics. A higher gravity changes many aspects of the geosphere, oceans and atmosphere.

The big one is suddenly increased pressures in the geosphere. Even if the change in gravity is small it would likely both trigger earthquake faults and more importantly, volcanism. Large magma chambers would suddenly be under significantly more pressure and the strength of the rock above would not have increased: expect at least numerous synchronous volcanic eruptions. It is not implausible that this might include supervolcanism, in which case we easily get a basic mass extinction. This volcanism not just a quick transient since the change would also make mantle plumes much more vigorous, leading to a long-term increase in plate tectonics and eruptions.

The fundamental reason for this is that buoyancy forces are due to a density difference times gravity: $F=g(\rho_{heavy}-\rho_{light})$. Boost gravity and they become proportionally stronger.

This also affects the atmosphere. Surface pressure goes up a bit. Convection will become more vigorous since the density changes due to temperature gets multiplied by a stronger gravity. Hence the weather and climate will change in complex ways. Exactly how is a bit hard to tell without simulation. One obvious effect is that the scale height will be reduced: pressure will decrease faster at high altitudes (it will be $7400 (g_{now}/g_{changed})$ meters), changing the stratosphere and likely the size of the Hadley convection cells (several factors working against each other there; I think they will become proportionally broader, meaning that the current 30 degree size might shift to a 45 degree size with a very different climate distribution).

A lot of mountains and continental slopes will become unstable since they will have a flatter angle of repose, so there will be lots more landslides, seaslides and erosion. That likely also includes a lot of glaciers and ice shelves.

The moon would get a tighter orbit, likely more elliptical if the change was sudden. That would make tides slightly stronger.

In short, I think a few percent extra gravity might be enough to cause a mass extinction even though most animals would not even notice the extra gravity. The disaster effect would just be a combination of simultaneous (super)volcanism, a reorganisation of the climate zones, and instability of a lot of material on continental shelves.

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  • $\begingroup$ Wow! When I first started thinking about this, my mind went for biology and engineering first. So I didn't get to plate tectonics. This is beautifully simple! Thank you :) $\endgroup$ – Leezard Oct 19 '17 at 19:45
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I never played with animal in a centrifuge to see how they react to increased gravity, nor I am aware of any studies doing this (probably because putting an elephant for days or months into a centrifuge would certainly be awarded an IgNobel), but I can still try to draw some parallel with what we know about effects on human body.

When you increase gravity, the first effects will be:

  • increased effort to pump blood against the gravity field (a.k.a. up)
  • increased load on limbs to sustain the body
  • increased effort on moving against the gravity field

All these (especially the first two) are likely to generate long term issues (cardiovascular stress, increased wear on joints, etc.) which will shorten lifetime. But this alone won't lead to extinction, as long as the birth rate is still decent.

Then it comes the question on how gravity affects embryo's growth. Again, as far as I know nobody ever put any (big enough for the scope of your question) animals in a centrifuge for long enough to have mating and successful pregnancy carried over. I suspect that increased gravity will surely affect the development of the embryo, resulting in increased miscarriage.

As you can guess, it is likely that there is no sharp threshold for gravity lethality. It is more a fuzzy change from the "business as usual" at 1 g to the "flat day, isn't it?" at black hole level g.

To make a parallel, consider obesity: being overweight is known to increase the chances of pathologic conditions, but being 10% or 100% above ideal weight are two different games.

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I think this is a multifaceted question, that needs an answer for each animal or plant type. So I'll consider humans as the main case then breify a few other examples.

Humans
If the increase was substantial, say greater than 2g, I doubt that any humans would survive for long. A few hardly individuals might last a few weeks or even a few months but the casualty rate would be terrible. Older people often break bones when they fall, in part because they can’t react quickly enough and in part because their bones are weaker than those of younger people. In a 2g environment even the young would suffer these problems because their reaction times would have to be so much faster to prevent damage (by extending arms etc) and the increased speed of impact would make their bones more likely to break.

At 1.5g it’s a tougher call, but I suspect not in part because of the reasons given above for 2g conditions, but also because of the break down in society caused by multiple dislocations to civilization. Huge numbers of people in hospital due to fractures and many other complaints exacerbated by the increased gravity eg asthma. Any mechanism that relied on gravity in any way might change its behaviour or stop functioning. Such things might be hard to predict but would range from the inconvenient to the seriously damaging for example doors becoming jammed or hard to open, most, if not all, planes grounded and many plants like modern wheat ending up bent over and hard to harvest.

So at 1.5g I would say it’s borderline but non-survivable due to general collapse. Much less than that I think it’s survivable by at least some for years.

At 1.2g I suspect a lot would survive for long enough to ensure the continuity of the species. The young would have the advantage (if that’s the right word) of growing their bodies in the new gravity so would almost certainly be better adapted than their parents even without any genetic adaption. Beyond that natural selection would ensure that those best capable of adapting would survive in the greatest numbers and the human genome would be whittled down a fair bit (less tall people more short people etc) leaving just the fittest.

Over the longer term evolutionary adaption by mutation would become an important factor but might take hundreds of thousands of years.

Birds
Birds are much more vulnerable to gravitational issues and would be seriously affected by any change more than a few percent with the heaviest birds suffering most. It is very hard to say exactly but I would estimate somewhere between 1.05 and 1.1g would see off all flying bird species.

Fish
Fish should be less affected due to buoyancy, but might still struggle with swim bladder issues. 1.5-2g?

Plants
Large species or those with fine structures would be seriously affected and would be first to go the more sturdy squat varieties would last longer and ground hugging species would survive longest.

Lower forms
Things like algae, lichen, plankton and bacteria would all be affected but should be capable of withstanding much higher g forces. It is hard to predict but I suspect 5-10g would kill many even of these species. The ultimate survival limit is almost impossible to say as the world would be a very different place.

Edit

Evidence and references
There is some information of interest in these references

https://space.stackexchange.com/questions/6154/maximum-survivable-long-term-g-forces

https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-2

https://www.amazon.com/Biology-Human-Survival-Extreme-Environments/dp/0195165012

But I doubt that there is any really hard evidence available because the situation is inaccessible to any form or simple experiment. So this is a matter of opinion to some degree. Although the g forces quoted in the second reference might appear to suggest that 4g is doable, bear in mind this is for astronauts in acceleration couches. Not for joe public trying to do his shopping and climb stairs etc.This reference tends to support my view about injury (page 53). I would suggest this is especially true when not lying on an acceleration couch.

“During dynamic flight phases, there is potential for impact and flail injury, which includes crewmember extremities impacting vehicular surfaces or objects, hyperextending, hyperflexing, hyper-rotating, fracturing, or dislocating if proper restraints and supports are not used. Features such as harnesses, form-fitting seats, and tethers may help maintain the proper position of the crewmember's body and limbs to reduce movement or contact with vehicle surfaces that would produce injury. In addition, the design of spacesuits may contribute to reducing injury to the crew. Preventing the inadvertent contact of extremities with vehicular structure or interior components significantly reduces the likelihood of limb fracture or soft tissue injury during a dynamic flight event. Extremity guards, tethers, garters, and hand holds have been used to reduce injury in other spacecraft, aircraft, and automotive vehicles”

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  • $\begingroup$ Some sources of your claims would be nice. it is not hard-science so maybe not research papers, but Wikipedia articles or articles on popular science websites? $\endgroup$ – Mołot Oct 18 '17 at 10:29
  • $\begingroup$ will see wht I can do... $\endgroup$ – Slarty Oct 18 '17 at 10:37
  • $\begingroup$ I would mention insects, because of scale I'm not certain the bird math applies to them. Flying pollinators are held to be pretty important to many ecosystems. $\endgroup$ – user25818 Oct 18 '17 at 15:40
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You've said "over night" I'm going to take that as meaning a more or less immediate effect. You haven't set a time limit on extinction though so there are two scenarios I see A. you want everything bigger than a dog dead instantly or B. you want an actual mass extinction event which will play out over a short but geological timespan.

To get option A. you need something over a 100 fold increase, this will give you a 100g "shock load" across the board, that's long been considered the instant death threshold in crash testing for humans. I'm assuming there are animals that can handle more than we can though so I'd make that the lower limit for scenario A. This absolutely will pancake every building on Earth and most of the vegetation too. This is a level of gravity is likely to eventually kill everything on Earth as the resultant pressure will break down protein synthesis pathways as well. I don't think this can be done the way you're thinking though, it's just too big a jump, in mass and more importantly density the words "degenerate matter" come to mind.

Scenario B. is a lot harder to quantify, there are flying insects that are vital to the ecosystems that virtually all animals rely on that could probably be grounded and whipped out by the atmospheric effects of a tiny change, we wouldn't notice the difference except experimentally, just all the flies and bees etc... disappearing and the crops failing, even corn and other wind pollinated species would suffer from a small shift in air density caused by increasing or decreasing gravity. I can't even guess at this one.

The above is focussed on land animals only, sea life is a different story, many shallow water fish (shallow by habit not depth of the underlying ocean) would have trouble as the air/water density ratio will be shifted by a change in gravity and air density at sea level; this will effect buoyancy and dissolved gas levels. Deep water fish will have far less of a problem and fish without swim bladders, like sharks and rays will only have issues with the gas dissolution. What it would do to the Cetacea is anyone's guess.

Do note that any elemental substitution effecting the Crust of the Earth is going to have immediate and drastic consequences for all life due to poisoning/imbalance effects. As I pointed out in the comments if you switch out any noticeable percentage of the Iron in steel framed buildings for something bigger, and heavier, like Lead you are inviting catastrophic failure. This is true of any alloy or composite material, including concrete both the cement and the aggregate phases. If you the replace Silicon in the Ogallala Aquifer's sand with heavy metals you could poison millions of acres of pump irrigated farmland.

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    $\begingroup$ Scenario B will be easier than you think. Even a small change in gravity everywhere will change the energy need of every animal and plant which needs to push something up more than some centimeters (like feet, blood, water). This will disturb every ecosystem. This will lead to a lot of ecosystem changes with a lot of extinction following. On the other side: no one may see the difference to the actual man made extinction event going on for at least the last 500 years. A lot of ecosystem changes need hundreds of years to show effects like species going extinct or new pests. $\endgroup$ – Henning M. Oct 18 '17 at 21:34

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