What is the Largest Asteroid Impact that is Survivable?

[Lord MJ]

Was wondering what is the largest Asteroid that impacts the Earth that the human race can survive the impact.

Guess we can break this up into three categories.

1. Humans and most species on Earth survive.
2. Human Civilization survives but many species become extinct (is this even possible?)
3. Humanity survives but Civilization is gone.

[Simon_Jester]

It might depend on your definition of civilization. Civilization directly under the impact is screwed, because it gets crushed by a meteor. Civilization near the impact is likely to be more screwed than civilization far from the impact. If, say, a gigaton-range impact hit the east coast of the US it would effectively destroy that region and cause devastating tidal waves in Europe, but it would have far less direct and immediate effect on points that are neither directly under the impact nor in line-of-sight across an ocean (so to speak). In Beijing the direct effects would be hardly noticeable. Climatic effects would still matter, of course, but those are a bit easier to cope with than the overt problems of being crushed by a meteor or having a giant tidal wave flatten your province.

So world civilization as a cooperative and intercommunicating entity might not survive even though there are still substantial areas of the world that are settled and still engaged in agriculture and still have cities (towns, anyway) and government.

It also depends on whether you consider, say, Iron Age city-states to be civilization; at this point that would be a likely stable fallback point for a collapsing civilization. Regression to the hunter-gatherer state is conceivable, but it's unlikely that we would forget about farming entirely, or that we would be unable to return to it in parts of the world where farmers are more accustomed to making do with pre-industrial techniques.

[Spectre_nz]

Half hour throw togeather during lunch:

It'll depend on what/where the impactor hits; an impact that vaporizes a large amount of carbonate rock has a secondary means to affect the climate, that is, mass release of CO2, aside from just reducing the amount of sunlight that reaches the ground with atmospheric dust.
My understanding of impact physics is mostly derived from interest reading of things on the internet, so other than simple calculations of energy released at impact, I don't have any hard and fast numbers to back this up. It's really just speculation based on 'hypothetical impact on carbonate rock will behave differently from the same hypothetical impact on deep ocean or granite.

For something more meaningful, you could use historic impacts as a benchmark; look at how big the presumed event was, and what died. (unfortunately, there aren't many to look at) If you accept that the K-T event had the singular cause of the Chicxulub impact, then you're looking at a 10-14 km diameter impactor.
(this is a little problematic, as I understand it, there are competing ideas as to how much influence the impact had on the K-T extinction, weather it was a king hit that killed everything itself, or the straw that broke the camel’s back and nudged a lot of species over the brink in what was already a tough environment thanks to volcanically induced climate change and/or multiple impacts)

Short summary;
The K-T impact killed large land herbivores and the food chains that depended on them (dinosaurs) worldwide (this is iffy, as climate change is also implicated) , but didn't do nearly as much damage to the terrestrial aquatic food chains that could subsist on run-off nutrients, but had a more significant impact on the oceans, possibly through ocean acidification in concert with pre-existing climate change altering ocean currents. Almost all north American land plants from New Mexico to Alberta were wiped out (presumably burnt off or covered by debris), evidenced by a sudden abundance of fern pollen about the KT boundary in those regions, while plant life in Japan was much less affected and plant life in the southern hemisphere was practically untouched.

I'd wager from a 10km asteroid, land masses within 4000km can kiss their plant based food webs goodbye and in the longer term (years, but I don't know for how long), most of the plants in the hemisphere of impact will decline to the point you couldn't support large land animals; I'm counting a cow as large here; three quarters of a ton of cow has approximately the same metabolic food requirements as three tons of dinosaur. But then, they didn't have grass like we do now-a-days and grass is amazingly good at recovering from disaster and growing in tough environments.
I'd also cross out rice if the impact released enough CO2 to alter rainfall patterns; China and India I'm looking at you.
Cunning human being could probably subsist on small rodents, algae and seaweed on neighbouring continents in the same hemisphere, and if you're in the other hemisphere, you could probably add wheat, sheep, goats and soy to that list.

Extrapolating; 5km iron nickel asteroid would end a country (or five) and really mess with global economies, 10km would severely alter our way of life but civilization could carry on in another hemisphere in what we'd still think of as a modern state. I don't have enough info to scale to what would be required to drop us back to the stone age or wipe us out completely.

[Spectre_nz]

Post lunch thoughts:

Unsuccessfully trying to find a figure on the volume of ejecta estimated from the K-T event, to compare with the Toba catastrophe at around 3000 cubic kilometres; an event that is suspected of bottlenecking the human race down to around 10000-1000 females around 75,000 years ago.
Toba apparently triggered a six year global winter, dropping the average global temperature 15 degrees C. The environment apparently recovered within a few decades. The next question is how would modern civilization fare with 10-20 years of semi-perpetual winter.

If you can scale impact to ejecta, you'd get an estimate of how big compared to Toba something would be. From a quick brows it looks like impacts are a lot more energetic than volcanic events but throw up a lot less material.

[Anguirus]

three quarters of a ton of cow has approximately the same metabolic food requirements as three tons of dinosaur.

Where are you getting that from? As I understand it, the limited studies that can be/have been made of dinosaur metabolism indicate that many were probably endothermic and highly active. Moreover, many of the bigger, slower ones affected by the impact that might have had lower metabolisms were FUCKING ENORMOUS (i.e. Anatotitan, titanosaurs) so that three tons of dinosaur is still less than one dinosaur.

[Spectre_nz]

I had in my head a rough rule of thumb 1 ton mammal = 4 tons dinosaur, which when I go back and hunt down my source, only looks to be valid for comparisons on predators;
http://www.spiegel.de/international/0,1 ... 81,00.html

That sorta worked backwards based on upper limits for the size of a predator comparing metabolic requirements based on size vs the energy required to move that mass to hunt and concluded that large therapod predators got to around 4 times the size of large mammalian predators, yet the mammalian predators were pretty much at the upper size limit of what they could reach, so concluded that therapods did a little metabolic trickery to lower their energy requirements.

Given that this worked on the principle of the animal having to hunt, no, its not going to work for herbavores.

which means I don't have a quick and dirty way of estimating what size of land animal would be bent over backwards on a global scale by a K-T type impact. Other than the aproximation that dinosaurs died and birds didn't so chickens looks like a good modern candidate for survival...
Obviously your mileage may vary depending on your local food web and weather you're a generalist or a specilist feeder. But if I had to pick food animals on which humans subsist that could survive, chickens sheep and goats win, cow's don't. Obviously I'd like to be able to tie this a little closer to what died out around K-T, but, leap of logic fail if that size conversion isn't apropriate.

[dragon]

Here's a good page that calculate effects of an impact.

link