Impact winter

If an asteroid were to strike land or a shallow body of water, it would eject an enormous amount of dust, ash, and other material into the atmosphere, blocking the radiation from the Sun.

[5] Each year, the Earth is hit by 5 m (16 ft) diameter meteoroids that deliver an explosion 50 km (31 mi) above the surface with the power equivalent of one kiloton of TNT.

[2] These metallic objects are the most likely to impact the surface because they stand up better to the stresses of ram pressure induced flattening and fragmentation during deceleration in the atmosphere.

Those that do make it to the surface need a minimum energy of about 10 Mt (4×1016 J) or about 50 m (160 ft) diameter to breach the lower atmosphere (this is for a stony object hitting at 20 kilometres per second (40,000 mph)).

The porous comet-like objects are made up of low-density silicates, organics, ice, volatile and often burn up in the upper atmosphere because of their low bulk density (≤1 g/cm3 (60 lb/cu ft)).

In this scenario massive amounts of debris injected into the atmosphere would block some of the Sun's radiation for an extended period of time and lower the mean global temperature by as much as 20 °C after a year.

In a study conducted by Curt Covey et al., it was found that an asteroid about 10 km (6.2 mi) in diameter with the explosive force of about 108 MT could send upward of about 2.5x1015 kg of 1 μm sized aerosol particles into the atmosphere.

[3] These particles would then be spread throughout the atmosphere and absorb or refract the sunlight before it is able to reach the surface, cooling the planet in a similar fashion as the sulfurous aerosol rising from a megavolcano, producing deep global dimming.

These pulverized rock particles would remain in the atmosphere until dry deposition and due to their size, they would also act as cloud condensation nuclei and would be washed out by wet deposition/precipitation, but even then, about 15% of the sun's radiation might not reach the surface.[why?]

[3] However, this effect could be largely mitigated, even reversed, by a release of enormous quantities of water vapor and carbon dioxide caused by the initial global heat pulse after the impact.

[citation needed] If the impact event is sufficiently energetic it might cause mantle plume (volcanism) at the antipodal point (the opposite side of the world).

These wood fires might release enough amounts of water vapor, ash, soot, tar and carbon dioxide into the atmosphere to perturb the climate on their own and cause the pulverized rock dust cloud blocking the sun to last longer.

Alternatively it could cause it to last for a much shorter time, as there would be more water vapor for the rocky aerosol particles to form cloud condensation nuclei.

Gypsum, a sulfate-containing rock that is usually present in the shallow seabed of the region, had been almost entirely removed and must therefore have been almost entirely vaporized and entered the atmosphere, and that the event was immediately followed by a huge megatsunami (a massive movement of sea waters) sufficient to lay down the largest known layer of sand separated by grain size directly above the peak ring.

This global dispersion of dust and sulfates would have led to a sudden and catastrophic effect on the climate worldwide by causing large temperature drops, devastating the food chain.

[6] Depending on location and size of the initial impact, the cost of clean-up efforts could be so high as to cause an economic crisis for the survivors.

Greenhouses in underground complexes with fossil or nuclear energy power stations could conceivably keep artificial sunlight growing lamps on until the atmosphere began to clear.

This death of plants might lead to a long period of famine if enough people survived the initial blast wave and would result in increased food costs in undeveloped countries only a few months after the first crop failures.