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

In summary, the effective temperature is the temperature of a blackbody that would emit the same amount of radiation. The difference between the actual average surface temperature and the effective temperature is due to the greenhouse effect.
  • #1
harrycallahan
5
0
First let me describe what I'm talking about :smile: 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/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
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/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:
 
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  • #2
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.
 
  • #3
Studiot said:
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 realize 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.
 
  • #4
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.
 
  • #5


I can provide a response to this content by explaining the difference between effective temperature and actual surface temperature and how it relates to the greenhouse effect.

The effective temperature is the theoretical temperature of a blackbody (an object that absorbs all radiation that falls on it) that would emit the same amount of radiation as the Earth does. This temperature does not take into account the reflection of incoming solar radiation by the Earth's atmosphere and surface. Therefore, the effective temperature of the Earth is much lower (-19°C) than the actual average surface temperature (15°C).

The actual average surface temperature of the Earth is a result of various processes, including the absorption and reflection of solar radiation, the greenhouse effect, and other atmospheric processes such as evaporation. The greenhouse effect refers to the trapping of heat in the Earth's atmosphere by certain gases, such as carbon dioxide and water vapor. These gases absorb and re-emit infrared radiation, preventing it from escaping into space and thus warming the Earth's surface and atmosphere.

The 30-degree difference between the effective temperature and the actual surface temperature can be largely attributed to the greenhouse effect. However, as mentioned in the content, there are other processes that also contribute to the warming of the atmosphere, such as evaporative warming. This process involves the conversion of liquid water into water vapor, which absorbs heat from the Earth's surface and warms the atmosphere.

Overall, the difference between effective temperature and actual surface temperature is due to a combination of factors, including the greenhouse effect, atmospheric processes, and the reflection of incoming solar radiation. It is important for climate scientists to continue studying and understanding these processes in order to accurately predict and mitigate the effects of climate change.
 

What is the difference between effective temperature and actual surface temperature?

The effective temperature refers to the temperature that an object would have if it were a perfect black body, meaning it absorbs and emits all radiation. The actual surface temperature refers to the temperature measured at the surface of an object, taking into account factors such as reflectivity and atmospheric effects.

What causes the difference between effective temperature and actual surface temperature?

The difference between the two temperatures is mainly due to the object's reflectivity, or albedo, which determines how much incoming radiation is absorbed or reflected. Atmospheric effects, such as greenhouse gases, can also impact the actual surface temperature.

Why is effective temperature used in scientific calculations?

Effective temperature is used in scientific calculations because it provides a standard measure for comparing the temperatures of different objects, regardless of their reflectivity or atmospheric conditions. This allows for more accurate and consistent analysis and predictions.

How does the difference between effective temperature and actual surface temperature vary among different objects?

The difference between effective temperature and actual surface temperature can vary greatly among different objects depending on their characteristics. For example, a planet with a high albedo, such as Venus, will have a larger difference between the two temperatures compared to a planet with a low albedo, like Earth.

How can scientists measure the difference between effective temperature and actual surface temperature?

Scientists use various methods, including remote sensing techniques and ground-based measurements, to determine the effective temperature and actual surface temperature of an object. These measurements are then used to calculate the difference between the two temperatures and analyze their impact on the object's overall temperature.

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