- #1
harrycallahan
- 5
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First let me describe what I'm talking about With climate science so popular a certain figure is often quoted which is said to represent the temperature difference the greehouse effect makes. This is (often) said to be the difference between the effective temperature and the actual surface temperature.
The effective temperature is the blackbody temperature where the reflected radiation is not considered. For the Earth about 30 % is reflected and the resulting effective temperature is about -19 deg.
The actual average surface temperature is about 15 degrees and it is said the difference, 30ish degrees, can be accounted for by the greenhouse effect.
Quote wikipedia "Greenhouse Effect"
I'm not so sure it's that simple though.
Refer to the first diagram of this article which shows the primary energy flows
http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php
There's a lot of warming in the atmosphere which is not due to surface emitted IR absorption.
Also the following quote
So now I'm getting confused. Is that 30 degrees extra on top of the effective temperature due to greenhouse IR absorption or is it everything that happens in the atmosphere?
For example we have evaporative warming. Water heats up and evaporates and with it the temperature of the atmosphere is raised. This is significant, at 25 % contributes 5 times more than IR - is this contributing to the warming above Effective Temperature? I think yes.
If the warm ground emits IR the ground cools and the IR that is 'captured' makes the atmosphere warmer; well how is this different from evaporated water molecules rising into the atmosphere - the water source cools and the atmosphere gains heat.
Other sources linked to in wikipedia are
http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/
http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap7.html
but I wouldn't say they've helped to resolve the issue. The Columbia source seems to reiterate the Wikipedia version i.e.
It then goes on to explain the absorbing components as only the greenhouse gases.
So any climate scientists out there can help make it clearer? :uhh:
The effective temperature is the blackbody temperature where the reflected radiation is not considered. For the Earth about 30 % is reflected and the resulting effective temperature is about -19 deg.
The actual average surface temperature is about 15 degrees and it is said the difference, 30ish degrees, can be accounted for by the greenhouse effect.
Quote wikipedia "Greenhouse Effect"
If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth is, it would have a temperature of about 5.3 °C. However, since the Earth reflects about 30%[4] (or 28%[5]) of the incoming sunlight, the planet's effective temperature (the temperature of a blackbody that would emit the same amount of radiation) is about −18 or −19 °C,[6][7] about 33°C below the actual surface temperature of about 14 °C or 15 °C.[8] The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect.
I'm not so sure it's that simple though.
Refer to the first diagram of this article which shows the primary energy flows
http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php
There's a lot of warming in the atmosphere which is not due to surface emitted IR absorption.
Also the following quote
(emph. mine)The natural greenhouse effect raises the Earth’s surface temperature to about 15 degrees Celsius on average—more than 30 degrees warmer than it would be if it didn’t have an atmosphere.
So now I'm getting confused. Is that 30 degrees extra on top of the effective temperature due to greenhouse IR absorption or is it everything that happens in the atmosphere?
For example we have evaporative warming. Water heats up and evaporates and with it the temperature of the atmosphere is raised. This is significant, at 25 % contributes 5 times more than IR - is this contributing to the warming above Effective Temperature? I think yes.
If the warm ground emits IR the ground cools and the IR that is 'captured' makes the atmosphere warmer; well how is this different from evaporated water molecules rising into the atmosphere - the water source cools and the atmosphere gains heat.
Other sources linked to in wikipedia are
http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/
http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap7.html
but I wouldn't say they've helped to resolve the issue. The Columbia source seems to reiterate the Wikipedia version i.e.
The effective temperature of Earth is much lower than what we experience. Averaged over all seasons and the entire Earth, the surface temperature of our planet is about 288 K (or 15°C). This difference is in the effect of the heat absorbing components of our atmosphere. This effect is known as the greenhouse effect.
It then goes on to explain the absorbing components as only the greenhouse gases.
So any climate scientists out there can help make it clearer? :uhh: