Aha! Chemistry with Prof Bob
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  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
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    • Playful dolphins
    • The University of Western Australia
    • Kings Park
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    • 999 Thermodynamics

Module 2705



The “greenhouse effect”

What is the “greenhouse effect”? 

How does carbon dioxide in the atmosphere disrupt Earth’s energy balance?

Carbon dioxide molecules gobbling up photons and spitting out other photons
​

In which direction do photons emitted from molecules go?

What is the “greenhouse effect”?
​
The greenhouse effect is the phenomenon discussed in Module 2704 Does carbon dioxide affect Earth’s energy balance?

In a nutshell, by “capturing” some of the radiation emitted from Earth, carbon dioxide (and other greenhouse gases) affect Earth's energy balance (See Module 2701 Is Earth in energy balance?) so that the environment becomes warmer than it would be if there were no carbon dioxide.


It is called the “greenhouse effect” because the greenhouses (or glasshouses) used in agriculture also maintain a higher temperature.
​
Picture
A greenhouse used in agriculture to maintain a warm environment. A greenhouse works by preventing warm air from escaping. This is different from the mechanism of operation of “greenhouse gases” in the atmosphere.
Before we go on to discuss how carbon dioxide causes a warmer environment …. 
​
​

The nature of radiation
​

We can think about radiation as waves, or as particles called photons. This is often referred to as the “dual nature of light” (See Module 2202 Light: wave-particle “duality”).

I emphasise that surely there is no duality of identity of radiations: they are what they are. The duality resides in us, in our sensemaking. The nature of radiations are outside of our normal experiences, so for some purposes we resort to explaining phenomena by considering radiations as waves, and for other purposes we consider radiations as a stream of photons.

In relation to the mode of operation of greenhouse gases, it suits us to think of the radiation emitted from Earth’s surface as photons.
​


At last …. So how does carbon dioxide bring about a warmer environment?
​

In other words, what is the mechanism by which carbon dioxide acts as a greenhouse gas?
 
At every moment, many photons leave the surface of Earth, and move at the speed of light toward space.

When a photon collides with a molecule of carbon dioxide, the molecule absorbs/traps the energy of the photon. The energy of the carbon dioxide molecule increases. It is said to be “excited”.

 
Where does the energy go? The molecule vibrates faster. See Module 2706 Why do carbon dioxide molecules absorb radiations emitted from Earth?
 
But then ....

after a few milliseconds, the excited molecule “relaxes” - it returns to its previous low-energy condition. It does so by emitting a photon with the same energy as the one that it absorbed. [And the molecules of carbon dioxide are again available to capture other photons of outgoing radiation.]

The key to the greenhouse effect is that the photons from the “relaxing” carbon dioxide molecules are emitted in random directions - some of them back toward Earth.
​
Picture
A simplified portrayal of the greenhouse effect. Blue lines represent the paths of photons emitted from Earth and captured by carbon atoms. Green lines portray the random-direction paths of the photons emitted when the “excited” carbon dioxide molecules “relax”. Sometimes these photons collide with, and “excite”, another carbon dioxide molecule. Then yet another photon will be emitted (red line).
So, some of the energy that was on its way to space returns to Earth. Less energy is emitted to space than if there were no carbon dioxide in the atmosphere.

Earth’s incoming and outgoing energy flows are out of balance. Which is larger: the rate of incdoming energy, or the rate of outgoing energy?

Earth is warmer than it would otherwise be. This is the “greenhouse effect”.
​
​

A little less scientifically ....

Carbon dioxide molecules gobble up photons that are on a straight-line trajectory toward space, and then spit out other photons every which-way. Simple.

​
Greenhouse gases
​
Gases that bring about the greenhouse effect are called “greenhouse gases”.
​
The greenhouse gas in the public eye is carbon dioxide. Other important ones are methane (CH4, See Module 2714 Methane: How does it compare with carbon dioxide as a greenhouse gas?), nitrous oxide (N2O), fluorocarbons (a range of substances whose molecules contain fluorine atoms) and sulfur hexafluoride (SF6).
 
The most important greenhouse gas is water vapour (See Module 2710 Does water vapour dominate all other greenhouse gases?)
 
Picture
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  • HOME
  • NAVIGATION
    • Table of contents
    • Index
    • TALK WITH PROF BOB?
  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
    • Family
    • Travel
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
    • Perth
    • At work
    • 999 Thermodynamics