How the Greenhouse Gas Effect works

In summary, the greenhouse gas effect works because of the absorption of infrared energy by greenhouse gas molecules. This energy is then re-radiated by the molecules in all directions. Together with the additional energy continually being received from the Sun, this warms the surface more than would be the case without the GHG, and so causes increased radiation from the surface.
  • #1
AEBanner
5
0
How the Greenhouse Gas Effect Works

We all know by now that the greenhouse gas effect depends upon absorption by greenhouse gas molecules of infrared energy radiated from the Earth’s surface, and which is then re-radiated by these molecules in all directions. Effectively, half the energy is emitted to outer space, and half is emitted to the Earth’s surface where it is absorbed again. Together with the additional energy continually being received from the Sun, this warms the surface more than would be the case without the GHG, and so causes increased radiation from the surface.

Consider an area of 1 square metre of the surface, and let us take a value of 235 Watts for the power incident upon this area. Assume an emissivity of 1.0 for the atmosphere. Follow the process through second by second as in the table below.

Sun’s radiation to____Surface radiation____Back radiation____Radiation to
Earth’s surface(W)___to atmosphere(W)___to surface(W)_____space(W)

235________________235_______________117.5___________117.5
235________________352.5______________176.25__________176.25
235________________411.25_____________205.625_________205.625
235________________440.625____________220.3125________220.3125
235________________455.3125___________227.65625_______227.65625
235________________462.65625__________231.328125______231.328125
235________________466.328125_________233.1640625_____233.1640625
235________________468.1640625________234.0820313_____234.0820313
235________________469.0820313________234.5410156_____234.5410156
235________________469.5410156________234.7705078_____234.7705078
235________________469.7705078________234.8852539_____234.8852539
235________________469.8852539________234.942627______234.942627
235________________469.942627_________234.9713135_____234.9713135
235________________469.9713135________234.9856567_____234.9856567
235________________469.9856567________234.9928284_____234.9928284
235________________469.9928284________234.9964142_____234.9964142
235________________469.9964142________234.9982071_____234.9982071
235________________469.9982071________234.9991035_____234.9991035
235________________469.9991035________234.9995518_____234.9995518
235________________469.9995518________234.9997759_____234.9997759


We see that the radiation to space converges towards 235 Watts, which in fact is the limit set by the power received from the Sun since, in equilibrium, the power emitted must equal the power received.

Also, the power radiated to the atmosphere converges towards 470 Watts, which is twice the power emitted from the atmosphere to space. Now, the power radiated from a body is proportional to the fourth power of the absolute temperature, by the Stefan-Boltzmann Law.

Let Ts be the absolute temperature of the Earth’s surface.
Let Ta be the absolute temperature of the top of atmosphere.

Therefore, we have
( Ts )^4 = 2 × ( Ta )^4

and Ts = (2)^0.25 × ( Ta)

Finally, Ts = 1.189 × Ta

That is, the surface temperature of the Earth is 18.9% greater than the temperature of the GHG emitting to space. This will be true provided that there is at least sufficient greenhouse gas present to be able to cope with the amount of energy being received from the Sun.

Note that this analysis can be applied to other planets which have an atmosphere containing at least sufficient greenhouse gases, like Venus. It also means that there cannot be a runaway greenhouse gas effect, because the ToA temperature is governed by the emission which must be equal to the energy received from the Sun, and the surface temperature is limited to twice the ToA temperature. This reasoning, of course, assumes that no energy is entering the atmosphere from other sources, such as geothermal energy, and it assumes an emissivity of 1.0 for the atmosphere.

Aubrey E Banner
Sale, Cheshire, UK
 
Earth sciences news on Phys.org
  • #2
You're missing the point entirely, AE. You're correct that if the volume of greenhouse gas is constant, there will be no runaway greenhouse effect. Let me be the first to say: DUH! That's not what a runaway greenhouse effect is!

If the concentration of greenhouse gases is constant, then there is no feedback path, and the Earth will simply reach equilibrium. The actual temperature of that equilibrium will depend on the amount of greenhouse gas present.

However, if there's a positive feedback path involved -- say, higher temperatures lead to more dissociation of water in the upper atmosphere, leading to less dissolved greeenhouse gas, leading to higher temperatures, leading to more dissociation of water, etc. -- then you have a runaway greenhouse effect.

Your naive "analysis" totally neglects the feedback path that makes a runaway greenhouse effect a runaway in the first place.

- Warren
 
  • #3
chroot

Please refer to my post "Greenhouse Gas Effect and Carbon Dioxide".

AEBanner
 
  • #4
I don't need to refer to any other posts to show that your statements here are wrong.

- Warren
 

1. What is the Greenhouse Gas Effect?

The Greenhouse Gas Effect is a natural process where certain gases in the Earth's atmosphere, such as carbon dioxide, methane, and water vapor, trap heat and warm the planet. This helps to regulate the Earth's temperature and make it habitable for living organisms.

2. How does the Greenhouse Gas Effect work?

The Greenhouse Gas Effect works by allowing short-wavelength solar radiation to pass through the Earth's atmosphere and reach the Earth's surface. The Earth then absorbs this radiation and heats up, emitting longer-wavelength infrared radiation. The greenhouse gases in the atmosphere then trap some of this infrared radiation, preventing it from escaping into space and keeping the Earth's surface warm.

3. What causes an increase in the Greenhouse Gas Effect?

An increase in the concentration of greenhouse gases in the atmosphere, mainly from human activities such as burning fossil fuels and deforestation, leads to an increase in the Greenhouse Gas Effect. This is because more greenhouse gases are present to trap and retain heat, resulting in a warmer Earth.

4. What are the effects of an intensified Greenhouse Gas Effect?

The intensified Greenhouse Gas Effect can lead to global warming, which can have various consequences such as rising sea levels, changes in weather patterns, and disruptions in ecosystems. It can also contribute to climate change, which can lead to more frequent and severe natural disasters.

5. Can we reduce the Greenhouse Gas Effect?

Yes, we can reduce the Greenhouse Gas Effect by reducing our carbon footprint and limiting our emissions of greenhouse gases. This can be achieved through actions such as using renewable energy sources, practicing sustainable agriculture, and promoting reforestation. It is essential to take action to reduce the Greenhouse Gas Effect to mitigate the negative impacts of climate change.

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