# Could a comet cool down global tempatures? [duplicate]

In warlords I wanted Terrans to have a fad of big (synthetic) fur jackets, but I didn't want too have everyone running around in winter clothing for no good reason so the solution my mind instantly came up with was for a comet to burn up in the atmosphere causing temperatures to drop (for at least a few years and nothing too drastic). Now is this possible and if so what size of comet would we be talking?

Note: The comet was being mined for organic compounds and/or other valuable substances when a engine malfunction on the galleon sent the comet straight towards Earth. Long story short it burned up in the atmosphere.

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• Futurama addressed a rather similar question. – user535733 Nov 6 '18 at 8:42
• When a comet cools global temperatures that simply moves winter clothing "belts" towards the equator. This might increase their area and population, but unless the planet cools enough that the tropics have constant "winter" it won't be global and it will be seasonal, so no global fad from that. And obviously constant winter would cause some issues to agriculture, after which people would probably live their lives in climate controlled indoors wearing T-shirts and shorts. – Ville Niemi Nov 6 '18 at 9:58

I can picture comet-caused cooling but it's going to be pretty hard to do without a big splat.

As L.Dutch says, burning a comet simply adds heat. What you need is dust. Your comet could be old, most of the volatiles gone, it's mostly dirt by now. However, any object that can deliver that much dust is too massive to burn before it hits. It's going to plunge through the atmosphere and make a big crater, complete with major splash effects.

Ok, we need more. The engine malfunction sends the comet Earthward, the world panics. The scientists know what to do--a series of standoff nuclear detonations to nudge it aside. (An Orion drive doesn't actually care where the bombs come from, only where they detonate.) Some megalomaniac doesn't listen and a missile goes out with a big impact-fused bomb, the comet is blown to bits. The comet turns out to be very fragile, this is far more successful than expected but it's not enough--many of the bits are still a threat and deflection is now hopeless. The deflection missiles are retargeted against the most threatening bits, impact detonation.

While they have no hope of stopping everything most of it is reduced to stuff small enough to burn and since the chunks have been targeted based on the threat they pose the bigger stuff that gets through falls in remote areas. Much of the dirt that comprised the comet has been dumped into the upper atmosphere--you've got your winter.

A comet burning up in the atmosphere will add up energy to it, increasing its temperature.

If you want to cool down the planet, you have to rely on a side effect: something shielding the solar radiation and altering the energetic balance of the planet.

You can achieve this in two ways:

1. the comet itself turns to dust/aerosol during burning
2. as a result of the comet impacting the surface, a lot of dust is lifted above the troposphere

Considering that in 1. the comet trail in the sky is not even a 1% of the total sky (and thus the effect on the energetic balance is minuscule), I would opt for 2.

• Option 2 has side effects, such as a continent-wide shower of debris. – Renan Nov 6 '18 at 4:37
• The % of the sky the comet tail occupies is irrelevant for figuring what happens if it burns. – Loren Pechtel Nov 6 '18 at 9:23
• @LorenPechtel, it gives a rough indication of how much radiation it shields. It is relevant. – L.Dutch Nov 6 '18 at 9:35
• @L.Dutch But the tail has nothing to do with the amount of dust in the head and it's that dust in the head that's going to be released. – Loren Pechtel Nov 6 '18 at 9:48
• @LorenPechtel, my answer says trail, not tail. If the comet burns up in the atmosphere it doesn't release enough dust to effectively shield solar light. – L.Dutch Nov 6 '18 at 10:04

## Yes - if you let it hit

Drops in global mean surface temperatures, known as impact winters, are a common result of asteroidal and cometary collisions with planetary bodies. The mechanism is changes in solar radiation caused by increased absorption or refraction of light in Earth's upper atmosphere lasting a period of years, and three principal means would be:

• particulate and aerosol ejection from explosive force of impact
• triggering chains of volcanic eruptions that spew sulfur dioxide
• energy of impact causing widespread forest fires and smoke blanketing the Earth

If your comet absolutely has to burn up in atmosphere, it will generate a large amount of energy as it explodes - what is known as a superbolide (think the Tunguska event or more recent one in Chelyabinsk). In this case, your best bet for global cooling is likely scenario 3 (forest fires) but I'm afraid I can't help crunch the numbers.

## However...

• Initial cooling caused by aerosols, sulfur dioxide etc would usually give way to warming driven by CO2 and other processes over the long term

• The extent of initial cooling would depend on the composition, size, density, velocity and trajectory of the object and the characteristics of impact site - especially whether the comet impacted in water or on land, and the water depth and extent of vegetative cover respectively. This online impact calculator allows you to simulate impacts under different conditions.

• Impact winters tend to be drastic: sudden drops in temperature of 14 degrees celcius, photosynthesising life perishes under the thick blanket, food chains collapse, any single or double figure % of human population could perish. But there is some research that points to less drastic chains of events in history, such as an impact winter that might have helped bring about Europe's 'Dark Ages'

• the fact you are talking about a comet, rather than an asteroid, increases the chances of an impact with drastic effects because: (1) comets coast on more random circumsolar orbits compared to the Earth-like orbits of asteroids, meaning they are more likely to hit Earth head-on, and (2) comets can move up to three times faster than asteroids, and so unleash nine times more destructive power (the energy increases as a square of speed)