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For example, after the eruption of Mt. Pinatubo in 1991 according to this paper,

The introduction of large amounts of sulphuric acid aerosol into the stratosphere increases the planetary albedo (essentially the Earth's reflectivity of solar radiation) because these aerosol particles are efficient scatterers but only weak absorbers at solar wavelengths.

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The changes in the Earth's albedo observed by ERBE resulted in a net cooling of approximately 8 W m-2 between 5°S and 5o N, with a net cooling of 4.3 W m-2 between 40° S and 40° N.

What I'm interested in is how exactly this cooling happens. Which is correct:

  1. the atmosphere is experiencing a deficit of input energy, or

  2. the surface is experiencing a deficit of input energy, which is then communicated to the atmosphere by a decreases longwave surface emission

I suspect (2) is mostly correct, since about 75% of input shortwave solar radiation to the earth is absorbed by the surface, while only 25% by the atmosphere itself, according to ref [1] (and perhaps even less so by volcanic aerosols).

If this is correct, then once the surface does communicate that energy deficit to the atmosphere, will it happen locally? That is, will a slab of the atmosphere nearest the surface be the first to experience a cooling rate?

[1] Petty, G.W., 2006. A first course in atmospheric radiation, 2nd ed. ed. Madison, Wis: Sundog Pub.

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  • $\begingroup$ Neither. The aerosols in the atmosphere scatter incomong solar light (which peaks in the visible) but lets the infrared radiation emitted by the Earth out. Less radiation coming in + the same amount as before going out = more net going out = colder. $\endgroup$
    – Spencer
    Jan 1 at 21:12
  • $\begingroup$ This skips what I'm asking though. There is less radiation coming in, yes. This means that some specific transfer process is in deficit. I'm asking if that is occurring between radiation and surface, or radiation and atmosphere directly. $\endgroup$
    – pretzlstyle
    Jan 1 at 21:28
  • $\begingroup$ The comment by @Spencer raised an issue that you completely missed, pretzlstyle. Neither of the options you are considering is correct. $\endgroup$
    – David Hammen
    Jan 2 at 12:28
  • $\begingroup$ @DavidHammen I think I disagree. I understand how TOA energy balance works. I'm essentially asking how the troposphere is actually warmed by solar radiation, and whether or not the surface itself is a necessary mediator in that process. It seems that the answer is yes. $\endgroup$
    – pretzlstyle
    Jan 2 at 19:10

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What I'm interested in is how exactly this cooling happens. Which is correct:

  1. the atmosphere is experiencing a deficit of input energy, or
  2. the surface is experiencing a deficit of input energy, which is then communicated to the atmosphere by a decreases longwave surface emission

Neither is correct. You are forgetting about reflection. Some of the incoming shortwave radiation is reflected back into space by the Earth's surface, and also by clouds and particles in the atmosphere. That reflected shortwave radiation does not cause warming. Increasing the reflectivity of the Earth increases the Earth's albedo and thereby decreases the amount of sunlight absorbed by the atmosphere or the surface.

The aerosols emitted by volcanos increase the Earth's shortwave reflectivity. The emissions from powerful volcanos can reach well into the stratosphere, thereby enabling those aerosols to stay in the atmosphere for a much longer time compared to aerosols that remain within the troposphere. Those stratospheric aerosols also help in the formation of stratospheric clouds, once again increasing the Earth's shortwave reflectivity.

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    $\begingroup$ I think you've misunderstood the question. I am on the same page with everything that you've said. My question is, once the albedo increases, and less radiation is able to penetrate down to the troposphere, is the cooling driven by the fact that (1) there is less radiation delivering heat to the surface, or (2) there is less radiation delivering heat to the troposphere itself $\endgroup$
    – pretzlstyle
    Jan 2 at 19:08
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    $\begingroup$ Not 1 OR 2 , it will be 1 AND 2; less sunlight will heat the surface and less will heat clouds in the troposphere - which will radiate a bit less downwards. Both will result in lower average temperatures at the surface. $\endgroup$
    – Ken Fabian
    Jan 2 at 22:05

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