# How much colder would that asteroid make my planet?

I've seen some posts here (for example this one) for the effects of certain sizes of impactors, but I'm not finding any definite answer and all my google and google scholar searches have not turned up any proper answer. I'm looking at an asteroid a bit over 1km in diameter, possibly 1.6km, iron density (7.8kg/m^3) hitting at 16km/s at a 45° angle. Basically think Mistastin impactor. That's the basis for it. (Highest recorded surface temperature and all).

Now I'm wondering how much cooler the weather would get and for how long. Because this is essentially early days for my species and they're basically where we were at around 4 million years ago and I need to know how big the changes to vegetation might be and all that to know how it would affect them.

Follow-up question: how do you think they'd fare if, around the equivalent of their 4,000 BC they got hit by an 8 km impactor, rock density, 17 km/s (think Popigai).

Thanks!! :D

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

• Both effects would have exactly the effects needed by the plot of your story. It's not as if we have some sort of exact formula. You want insolation to decrease sharply and plunge the world in continuous winter for three years, fine. You want only half the world to go in winter for one year, fine. You want two years of deep winter followed by eight more years of mild winter, fine. – AlexP Mar 30 at 12:52
• Possibly relevant, though a little short of hard science: Where would an asteroid have to hit earth to cause an impact winter? – Starfish Prime Mar 31 at 9:04
• If the provided answer was helpful, it would be nice if you could upvote or accept it (or both). If it was not helpful, perhaps you could explain why, and that will either help it to be refined or assist later answerers. – Starfish Prime Apr 12 at 12:27
• @StarfishPrime sorry! I'm not sure how to accept an answer, and I thought maybe someone else would answer it but I haven't checked in a while. This was my first question and I'm still trying to work out how to use the website, apologies! – Nierninwa Apr 12 at 22:32
• @Nierninwa that's no problem; you're under no obligation to accept or upvote, i'm just making sure you haven't forgotten the question entirely ;-) The help pages for the site and for stackexchange contain everything you need, but it isn't always organised well. Have a look at What should I do when someone answers my question? and how does accepting an answer work? – Starfish Prime Apr 12 at 22:38

## 1 Answer

Lazy answer, basically following AlexP: it will be as long and as serious as you wish it to be.

There are so many variables here (exactly where the meterorite hits, the nature of the geology underneath the impact, the kind of vegetation around it, secondary effects like post-impact volcanism or ice-sheet breakup, etc etc) that giving a simple answer backed up with figures is basically impractical.

That said: here's a few thoughts.

I had a quick look at the ImpactEarth calculator, which says a few useful things, but nothing about the most important stuff like dust production, which is what will really affect the climate. The energy released by your Mistastin-equivalent is something like $$5*10^{20}J$$, which is in the 100 gigatonne TNT-equivalent range. That's big, equivalent to a major volcanic eruption (but not a supervolcano eruption, like yellowstone, which was about 10 times more powerful).

Now, I know that meterorite impacts can't be trivially compared to volcanic impacts, but big volcanic eruptions happen more often and their impact on human society has been documented and with a bit of handwaving and squinting their scale and effects may be close enough for your needs.

There is at least one eruption which is relevant to you here: Tambora, 1815, which involved a similar release of energy and was conveniently recent. Lots of stuff has been written about it as a result... one random example: After Tambora. Take home message: everything within a few hundred km of the impact is pretty stuffed, but the environment further away isn't so badly effected. Global average temperature drops by under a degree for about a year (but the averaging masks serious local cooling).

Global effects are pretty bad for societies that rely on agriculture (see Year Without A Summer) due to reduced temperatures and significant drop in rainfall. Longer-term effects are limited, though it may take a little while for plant and animal species to recolonise badly affected areas.

Your 4Mya species might be OK, depending on how widespread they are, how far away they are from the impact, and what sort of things they eat. If they're equivalent to humans they won't even have stone tools or fire yet, so you may as well look at prehistoric animal extinction events for an idea of what might happen to them.

You might also consider looking up the Laki eruptions (recent-ish, wikipedia link) or the effects of the eruption of Thera (bronze age, more detailed and interesting paper).

Your Popigai impact, however, is a rather different scenario. It releases about $$5*10^{22}J$$ (I'm assuming 8km chondrite impactor, rather than a denser rocky material) so there's an order of magnitude more energy released than even the Yellowstone or Toba supervolcanos. As it happens, the Toba eruption has been linked to a human population bottleneck, though they were ancient humans rather than the neolithic/bronze age civilization you're thinking of.

The Popigai impact was linked to the Eocene–Oligocene extinction event. This should give you some idea of the severity of the impact effects and the scale of environmental disruption. (TL;DR: really, really bad).

I wouldn't hold out much hope for your species, but their extinction isn't necessarily guaranteed, especially if they're as widespread as humans are at that time.

For specific cooling figures, I can't help you, but the Chixulub event was suggested to have caused prompt reduiction of average global temperatures of ~13 degrees in the first year, with significant cooling effects lasting several years afterwards. Your popigai-ish event wouldn't be as bad, but I can't give you anything more specific than "more than 1 degree for a year, but less than 10 degrees for 10 years".

• I had been looking at the different impacts and all, and to be fair my calculations with ImpactEarth back in 2016 gave me 2x10^21 joules for Mistastin and 1.3x10^23 joules for Popigai. For the impact as cause for the extinction, from what I understood, the "multiple impact" theory is the lead possibly with Chasepeake as well. I've come to the conclusion I need to move Popigai earlier in a Toba bottleneck equivalent, but I still want Chasepeake to be within written history, so… I think they'll see massive weather changes – Nierninwa Apr 12 at 22:41
• I was looking for more info to flesh out the evolution of my species but oh well. I'd have also liked more data on larger impactors but if it's a bit of a "it's up to you", then cool. Also I might make Chasepeake less of a big deal because water vapour allows to heat things according to this question. I guess I was just hoping for an easy way to calculate the length and severity of an impact winter from the amount of soot/area burned… – Nierninwa Apr 12 at 22:42
• @Nierninwa yeah, there's nothing easy to be found here, alas. Lots of postgrad/postdoc level work on this sort of thing. Burnt biomass is probably a minor contribution to the overall effect when dealing with big impactors and large quantities of ejecta. Water vapour release can indeed cause warming, even after cooling caused by denser materials has passed. – Starfish Prime Apr 12 at 22:46
• Oh that's a very interesting thought! So there could be a short-ish winter/cooling period of a couple years and then a decade or so of heating due to water vapour! A kind of impact winter and impact summer to mirror the nuclear effects ^_^ Thank you for this! – Nierninwa Apr 12 at 22:49