The Greenhouse Effect: Trapping IR Energy through Absorption and Re-emission

In summary, there is no simple answer to the question of how greenhouse gases trap energy. It is not a matter of either reflecting back 50% of the radiated IR energy or the greenhouse gas molecule becoming hotter. The truth is more complex and often oversimplified explanations are not entirely accurate. Eli Rabett's explanation of "How Greenhouse Gases Heat the Surface" provides a more detailed and accurate understanding of the process, where the energy is absorbed and re-emitted, not reflected, and thermalization occurs quickly. There is always a fraction of CO2 molecules in excited levels that can emit energy in the form of IR radiation.
  • #1
John Plant
7
0
Is the mechanism of greenhouse gases trapping energy emitted as Infra Red radiation by the cooling ground either
reflecting back a 50% of the radiated IR energy back to Earth
or/and
the greenhouse gas molecule becoming excited with the extra energy and essentially becoming hotter?
Is it just the first or both?
 
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  • #2
I would say neither. Many explanations are oversimplified.
I like Eli Rabett's "The Simplest Explanation"

I see that just yesterday he is working on an improved version:
"How Greenhouse Gases Heat the Surface"
where he says:
Energy does not stay in the molecule that absorbs the IR photon, to be re-radiated later. This is not so, it is quickly degenerated to thermal motion (translation, zipping about) via collisions. Thermalization requires about a 10 μs at atmospheric pressure. So where does the emission come from the bunnies ask?

Well, there is a considerable thermal energy at room temperature, and even much lower. True this average energy is low compared to even the lowest vibrational excitation of CO2 (which would be ~1000 K), but it is enough that a small, but significant fraction of CO2 molecules are always found in excited levels which can emit in the IR (about 6% at room temperature).
 
  • #3
John Plant said:
Is the mechanism of greenhouse gases trapping energy emitted as Infra Red radiation by the cooling ground either reflecting back a 50% of the radiated IR energy back to Earth or/and the greenhouse gas molecule becoming excited with the extra energy and essentially becoming hotter?

The energy goes back to the ground by absorption and re-emission (not reflection). There is a nice Sankey diagram in the German Wikipedia entry for the greenhouse effect:

Sun_climate_system_alternative_(German)_2008.jpg

[Edit: I just see that FactChecker posted an English version of the diagram in a parallel thread.]
 
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What is CO2 and how does it trap IR energy?

CO2, or carbon dioxide, is a colorless gas that is naturally present in the Earth's atmosphere. It is also a greenhouse gas, meaning it absorbs and traps infrared (IR) energy from the sun. This trapped energy contributes to the Earth's overall temperature.

How does CO2 contribute to climate change?

As CO2 levels increase in the atmosphere, more IR energy is trapped, leading to an increase in the Earth's temperature. This phenomenon, known as the greenhouse effect, is a major contributor to climate change and global warming.

What is the relationship between CO2 levels and temperature?

Scientists have found a strong correlation between CO2 levels and global temperature. As CO2 levels increase, so does the Earth's temperature. This is due to the greenhouse effect and the trapping of IR energy by CO2 in the atmosphere.

How do human activities contribute to the increase in CO2 levels?

The burning of fossil fuels, deforestation, and other human activities release large amounts of CO2 into the atmosphere. This increases the overall concentration of CO2, leading to a stronger greenhouse effect and contributing to climate change.

What are the potential consequences of high levels of CO2 in the atmosphere?

High levels of CO2 can lead to a variety of consequences, including rising sea levels, more frequent and severe natural disasters, changes in weather patterns, and negative impacts on ecosystems and biodiversity. These consequences can have significant impacts on human societies and the natural world.

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