As seen in the figure below.

The larger the temperature difference between surface and cloud, the more positive the radiative forcing is from the cloud.

So as the cloud moves up, it supposedly should move into a colder altitude. But doesn't climate change also increase the temperatures at any given altitude too? If so, why does the cloud move up into a colder altitude due to climate change? (even after accounting for heating at the given altitude?)

enter image description here

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    $\begingroup$ IPCC permits you to post their figures, but only if you give a full citation and include the full caption. Please do so. (I suppose this is to prevent misinterpretation of figures) $\endgroup$ – gerrit May 2 '14 at 13:03

The primary mechanism at work here is the type of clouds that exist at different altitudes.

Low clouds of the cumulus or strato-cumulus variety are optically thick and reflective. These types of clouds will scatter more light than they transmit or absorb, and the energy scattered upward and back into space will not contribute to warming.

High clouds outside of convection tend to be cirrus clouds, which are icy and not optically thick. These clouds let light pass through freely (you can see the sun through these clouds). This lets solar energy through but they will absorb longwave (terrestrial) radiation, and this is what will contribute to warming.

This is where a lot of the uncertainty in cloud feedbacks in a warming climate comes from. If we get more clouds -- low, bright clouds, then this could contribute to cooling, wheras high, transparent (to shortwave) clouds would contribute to warming. Again, it is not so much the high vs low that matters here, it is the properties of typical clouds at these heights.

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    $\begingroup$ I'd say it's certainly both the type and high vs. low. A thin cloud near the surface (liquid or ice) acts very differently radiatively speaking, than a thin cloud in the upper troposphere, even if it has the same optical depth for both solar and terrestrial radiation. $\endgroup$ – gerrit May 2 '14 at 13:05
  • $\begingroup$ Clouds also tend to (for the most part) cool the planet's surface during day time when the sun is high and warm it by trapping heat during night, and there's a relation between clouds and atmospheric water vapor, which is a greenhouse gas. Taken as a whole it's an equation that took years to work out and there's still some measure of uncertainty. $\endgroup$ – userLTK Jun 29 '16 at 15:59

The primary reason behind the occurrence of this positive feedback is in the radiative properties of high and low clouds. According to the Stefan–Boltzmann law, the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. While the change in altitude changes the optical properties of the clouds, meaning, there is an increased transmittance of shortwave radiation coming from the sun toward the Earth's surface, there is also a decrease in the re-emission of the absorbed longwave radiation being emitted from the earth's surface.

To summarize, higher clouds let more sunlight pass through them. They absorb earth's emissions but let out decreasing amounts with decrease in their temperature.


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