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Albedo is a measure of the amount of light reflected back by an object that is incident on it.

So, high albedo means that a greater portion of light falling on something is reflected back. This also means a that greater amount of light goes back towards outer space which is then absorbed by the greenhouse gases present in the atmosphere. Thus high albedo contributes to global warming.

Now, if we consider low albedo, then that means a greater portion of light is absorbed thus raising the temperature or otherwise the heat content of the body. Less light is reflected back for greenhouse gases to capture. But then absorbed heat itself contributes towards global warming.

So as I understand, both low and high albedo contribute towards global warming. Is this understanding of mine correct?

When we do climate change engineering, what do we try to do in terms of albedo? Do we try to achieve high albedo or low albedo?

Additionally I want to know about the effect of 'concrete jungles' in urban areas on global warming?

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tl,dr: low albedo = warming (more energy absorbed); high albedo = cooling (more energy reflected).

In principle, and grossly simplifying, energy from the sun warms the ground and less the air. The overlying air is then warmed by the ground, leading to convection, pressure differences, and all that.

So, a high albedo atmosphere would prevent sunlight from reaching the ground, having a cooling effect. Equally, a high albedo ground would reflect more sunlight, less warming the air. That's why glaciers or pole caps of ice have a feedback effect on cooling.

Furthermore, wavelengths reflected by clouds can help cooling an area by day, but also trap warmth radiating from the ground when they move over it. That depends on the situation.

A low albedo atmosphere would let more sunlight pass. In conjunction with a low albedo ground it has a warming effect. Again a potential feedback when ice starts to melt and exposes more ground, leading to more warming, etc.

An open sky will allow warming of the ground by day, but also partly cools back to space by night (continental or desert climate). Greenhouse gases are those that reflect this emitted radiation back instead of letting it escape into space.

Highly populated areas can have a "heat island effect", because of the low albedo of asphalt and concrete and the lack of plants and foliage to absorb sunlight for photosynthesis and evaporate ground water and thus cool.

That (highly controversial and IMO only driven by economic interest) geoengineering thing is about raising the albedo of the stratosphere in order to reflect more sunlight before it reaches the ground. Its adverse effects on ocean acidification and damage to marine life and terrestrial plants and animals ("acid rain") and unresolved side effects could outweigh the potential benefits. But that is a discussion on its own, IMO.

Hope that helps.

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    $\begingroup$ In "and evaporate ground water" are you referring to evapotranspiration in plants? $\endgroup$ – HARVEER RAWAT Dec 25 '19 at 11:07
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This also means a that greater amount of light goes back towards outer space which is then absorbed by the greenhouse gases present in the atmosphere.

No, it depends on the wavelength. Just like our body is transparent to X-Rays but doesn't let visible light through, greenhouse gases let sunlight through but absorb infrared light radiated by Earth.

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  • Sunlight reflected by ice or snow (=high albedo) has almost the same spectrum as sunlight and can go back to space through the atmosphere.
  • Sunlight landing on dark material on Earth (=low albedo) will be absorbed and a portion of it will be emitted back as infrared, according to Stefan–Boltzmann and Planck laws. A portion of this infrared light will be absorbed by greenhouse gases.
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    $\begingroup$ This is the correct answer: OP's confusion comes from not distinguishing between visible and infrared wavelengths. The albedo reflects visible light, leading to cooling. What it does in the IR is mostly irrelevant as the IR light is trapped by greenhouse gases already. $\endgroup$ – AtmosphericPrisonEscape Dec 25 '19 at 17:17
  • $\begingroup$ @AtmosphericPrisonEscape: Thanks. I wanted to keep the answer as simple as possible. Note that even though sunlight is composed of almost 50% of infrared, it is near-infrared, which gets absorbed much less by greenhouse gases than the long-wavelength infrared radiated by the Earth. $\endgroup$ – Eric Duminil Dec 25 '19 at 17:40
  • $\begingroup$ 50% measured how? The integrated energy flux, computed as $ \int_a^b \lambda F_{\lambda}$ is only 30% (with $a=800nm, \, b=\infty$) of the total blackbody-flux (with $a=0,\, b=\infty$). I think that number makes a difference, because it does give the correct impression that most of the energy arrives in the UV+visible window of the atmosphere. $\endgroup$ – AtmosphericPrisonEscape Dec 25 '19 at 18:32
  • $\begingroup$ @AtmosphericPrisonEscape: I guess it depends on the boundary that is chosen between visible and infrared. en.wikipedia.org/wiki/Sunlight#Measurement mentions "52-55%" for infrared, starting at 700nm. $\endgroup$ – Eric Duminil Dec 25 '19 at 19:38
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No, it is low albedo which contributes to global warming because ground with low albedo absorbs more of the sun's rays. High albedo reflects them back into space, and therefore has a cooling effect. When the polar caps increase in size, this can lead to an ice age if the caps reflect enough heat into space. There can be a feedback effect whereby the more the caps reflect the heat away, the more cold it gets and the more the caps increase in size, reflecting even more heat back into space.

To change the earth's albedo artificially by a significant amount is an impossible task, and any attempt to do so would be prohibitively expensive as well as having the environmentalists up in arms. Whatever the solution to global warming, it isn't geo-engineering of the kind you suggest.

The concrete canyons that you mention would be much the same as natural canyons except for one thing: they consume an enormous amount of energy, much of it derived from fossil fuels, so make a contribution to global warming.

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    $\begingroup$ Isn't the outgoing radiation captured by the greenhouse gases up in atmosphere? $\endgroup$ – HARVEER RAWAT Dec 25 '19 at 8:09
  • $\begingroup$ Some is, but not enough to compensate for the cooling effect of high albedo. $\endgroup$ – Michael Walsby Dec 25 '19 at 8:19
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    $\begingroup$ Does not then a body having low albedo also lose absorbed heat by emitting radiations and thus produce cooling effect? $\endgroup$ – HARVEER RAWAT Dec 25 '19 at 8:30
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    $\begingroup$ I am talking about Stefan-Boltzmann Radiation Law which states that radiation emitted by a body is proportional to the 4th power of absolute temperature. As the body which has absorbed radiation incident on it will be at higher temperature and thus emitting greater radiation. $\endgroup$ – HARVEER RAWAT Dec 25 '19 at 9:19
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    $\begingroup$ Citing sources would make @MichaelWalsby's answer better and more exact. Under a greeanhouse scenario, that emitted radiation is trapped by the gases in the atmopshere. It cannot escape to space any more. $\endgroup$ – user18411 Dec 25 '19 at 10:11

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