Answer by SJuan76 for Dropping ice in the ocean to stop global warming

Comets are tiny

• Halley's mass is 2.2×10¹⁵ kg.

• A cubic kilometer of water has a mass^※1 of 1 × 10¹² kg

• Oceans hold 1.35 × 10 ⁹ cubic kilometers, which gives us 1.35 ²¹ kg of water.

• So, the mass of the Halley is half a millionth of the ocean. If it is 200 C colder than Earth oceans (say −180° C for the comet, 20° C for the ocean) when dropped, the calculus of how much would it cool the ocean is approximately as follows:

  SpecificHeat_ice-180to0 = 180 K × 0.513^※2 cal/K = 92.34 cal

  Enthalpy_ice = 79.72 cal

  SpecificHeat_water0to20= 20 K × 1 cal/K = 20 cal

  TotalSpecificHeat = 92.34 cal + 79.72 cal + 20 cal = 192.16 cal

  All of the above values are per gram. This gives the approx. value of cooling as^※3

  EnergyChange = 192.16 cal/g * 2.2*10¹⁸ g = 4.22 × 10²⁰ cal.

  Which, with a specific heat of 1 cal per gram of water gives a temperature change of

  TemperatureChange = 4.22 × 10²⁰ cal × 1 K·g/cal / 1.35 × 10²⁴ g = 3,125 × 10⁻⁴ K = 3,125 × 10⁻⁴ C = 0.0003125 C of variation.

Comets are not (only) water

Comets usually have ice of it, but not only water ice. If we see the Halley's composition, it shows things like CO and CO2 ice. So, while you may get some to reduce the heat by dropping it, you also get to release greenhouse gases to the atmosphera, somewhat reducing the effect.

Comets have energy

Comets are moving at astronomical speeds, and slowing their mass will release energy as a lot of heat.

Even if we somehow "stop" the Halley comet to allow for it to slowly fail into the ocean, it will still have gravitatory energy. Maybe it would be not enough to cook a steak, but it would still heat the falling water and the air around it considerably, reducing (if not negating completely) any possible gain due to the comet temperature.

Comet will be in our orbit.

In order to drop it, the comet will have to get into our orbit. That means that the Sun will be warming it from some time, reducing the temperature differential.

Despite the popular image of an icy rock, when a comet approachs the sun it begins to melt and evaporate (in fact, that is what the whole "tail" of the comet is, gas and particles projected away from the comet).

Obviously, the "hotter" a comet is, the less effect we will get by dropping it on the sea.

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※1 Water density varies slightly with pressure and temperature, but for comparing orders of magnitude the standard value is good enough.

※2 Specific heat of ice seems to be 2,108 J/kg·K, or 0,513 calories

※3 Since the mass of the comet is nine orders of magnitude lower than the mass of the oceans, I would not change that value.