Quote:
Originally Posted by engineerdude
Quote:
Originally Posted by belovelife
...yes at which point it accelerates the methane molecule that is currently in a gel like state, o the point of releasing a gas, the amount of this gas added to the atmoshere would hold more of the suns energy in the atmoshere...
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This isn't how our world works. There is no energy trapped or "held" in our atmosphere. The greenhouse effect is like a blanket around our planet - it does not trap energy, it just slows the movement of heat, both coming in and going out. Again, there is no significant energy trapped any place in the atmosphere - changes in air composition just make heat move faster or slower. |
I believe Belovelife's characterization of "holding more of the sun's energy in the atmosphere" is appropriate.
The sun's radiation peaks around the center of the visible spectrum (0.5 μm). As the sun's temperature is much greater than earth's, it radiates at a higher frequency and shorter wavelength than earth’s radiation—as Planck's law says it should.
It’s not difficult to calculate the temperature we would expect earth’s surface to be if there were no contribution from greenhouse gas. If we assume that the heat absorbed by planet earth from the sun is equal to the heat radiated from the earth then we can find temperature using Stefan–Boltzmann’s law.
The heat absorbed by earth is equal to its albedo times solar irradiance per unit area times the cross-sectional area of absorption.
where a is earth’s albedo, r is its average polar and equatorial radius, and
is the radiation flux at our distance from the sun (1360 w/m^2).
The heat radiated by earth at a given temperature is given by
Stefan–Boltzmann’s law 
where
is earth’s emitted irradiance and
is the Stefan–Boltzmann constant (5.67E-8 W/m^2/K^4). So then,
To solve for temperature we say heat absorbed = heat radiated,
and solve algebraically for T,
![T = \left[(1-a) I_S / 4 \sigma \right]^{1/4}](http://hypography.com/forums/latex/img/e5a8a1df0b5385cd4bc9104247397953-1.gif "T = \left[(1-a) I_S / 4 \sigma \right]^{1/4}")
or,
![T = \left[\frac{(1-0.367) (1360 \ W/m^2)}{4 \times (5.67 \times 10^{-8} W/m^2/K^4)}\right]^{1/4}](http://hypography.com/forums/latex/img/debad249f6dd5d3496637a377a66efde-1.gif "T = \left[\frac{(1-0.367) (1360 \ W/m^2)}{4 \times (5.67 \times 10^{-8} W/m^2/K^4)}\right]^{1/4}")
Without any greenhouse effect, earth’s average temperature would be well below freezing around -25° C. This is far below the surface’s actual average temp of approximately 15° C due to greenhouse gasses. This is supported by
Historical Overview of Climate Change Science which says (on page 97):
Quote:
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To emit 240 W m–2, a surface would have to have a temperature of around –19°C. This is much colder than the conditions that actually exist at the Earth’s surface (the global mean surface temperature is about 14°C)... The reason the Earth’s surface is this warm is the presence of greenhouse gases, which act as a partial blanket for the longwave radiation coming from the surface. This blanketing is known as the natural greenhouse effect.
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They give a slightly higher result of -19° C which I can’t reconcile because they don’t show their work, but the conclusion remains the same.
The earth does “hold more of the sun's energy in the atmosphere” due to the greenhouse effect. The average kinetic energy of the surface and lower atmosphere is greater than it would otherwise be exactly because it holds on to the sun's energy.
~modest
