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Difference between Effective Temp and Actual Surface Temp is due to what?

by harrycallahan
Tags: actual, difference, effective, surface, temp
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harrycallahan
#1
Apr29-11, 12:09 PM
P: 5
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"
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/Fea...ance/page6.php

There's a lot of warming in the atmosphere which is not due to surface emitted IR absorption.

Also the following quote
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.
(emph. mine)

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...res/radiation/
http://acmg.seas.harvard.edu/people/...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?
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Studiot
#2
Apr29-11, 12:37 PM
P: 5,462
Welcome to PF Harry.

Be aware that there is an absoute ban on climate change discussion anywhere onPF, since such discussion tends to grow so quickly as to overwhelm the server and the rest of physics.

I hope the mods here allow your question to progress however.

The figures I have are not -19C if there was no atmosphere (then we would be at the same temperature as the moon, ie much colder) but -19C if there was no carbon dioxide or water vapour in the atmosphere
(source O'Neil - Environmental Chemistry)

The +15C seems pretty universal average for the actual average, however.
harrycallahan
#3
Apr29-11, 09:22 PM
P: 5
Quote Quote by Studiot View Post
Welcome to PF Harry.

Be aware that there is an absoute ban on climate change discussion anywhere onPF, since such discussion tends to grow so quickly as to overwhelm the server and the rest of physics.

I hope the mods here allow your question to progress however.

The figures I have are not -19C if there was no atmosphere (then we would be at the same temperature as the moon, ie much colder) but -19C if there was no carbon dioxide or water vapour in the atmosphere
(source O'Neil - Environmental Chemistry)

The +15C seems pretty universal average for the actual average, however.
Ouch!

Thanks for the welcome Studiot, didn't realise the topic was so sensitive to be banned. I did read that peer reviewed papers should be cited, I haven't quite done that but have quoted academic sources. Perhaps we can discuss this topic as it is mostly grounded in basic physics.

Your statement "but -19C if there was no carbon dioxide or water vapour in the atmosphere" is kind of answering my question, though contrary to my current hunch.

Of course the hypothesis of the Earth with no atmosphere is very unrealistic. For example water is a large part of our world and it will always evaporate when warmed, creating an atmosphere, however thin. And the atmosphere does a lot more than just allow IR warming so when the statement "no atmosphere" is made it isn't the same as saying no greenhouse gas warming.

Again back to your statement "no carbon dioxide or water vapour in the atmosphere", well that also rules out evaporative warming which isn't an example of IR greenhouse warming. You can see it becomes hard to interpret these statements where they are not made with a very careful qualification of the circumstances they suppose.

Evo
#4
Apr30-11, 09:21 AM
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Evo's Avatar
P: 26,444
Difference between Effective Temp and Actual Surface Temp is due to what?

Sorry, this will eventually draw people with heated opinions and the wars will start. Even if this thread somehow managed to remain civil, then others would point to it as an exception being allowed to the rules.


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