B2-3. Greenhouse Gases, Mechanism, and Energy Balance

1. Origins of Greenhouse Gases

The main greenhouse gases have both natural and human-generated (anthropogenic) origins. In order of decreasing overall contribution to the greenhouse effect, they are:

  • Water Vapour (H₂O): Naturally sourced from ocean and plant evaporation. Its concentration increases significantly as the surrounding air gets warmer.
  • Carbon Dioxide (CO₂): Naturally sourced from respiration, volcanic eruptions, and wildfires. Anthropogenic sources include the burning of fossil fuels and mass deforestation.
  • Methane (CH₄): Produced naturally by soil/ocean decomposition and termites. Anthropogenic sources include livestock farming, landfill decay, and agriculture.
  • Nitrous Oxide (N₂O): Originates from soils, oceans, and artificial fertilisers.

Note on Ozone (O₃): Ozone strongly absorbs incoming UV rays, protecting life on Earth, but it is not found in high enough concentrations in the lower atmosphere to be a primary contributor to the greenhouse effect.

2. Interaction with Infrared Radiation

The atmosphere interacts completely differently with incoming solar radiation compared to outgoing Earth radiation.

  • Incoming Radiation (Short-wave): The atmosphere is mostly transparent to incoming visible solar radiation. Only about $25\%$ of the Sun's incoming radiation is absorbed (mostly high-energy UV blocked by ozone).
  • Outgoing Radiation (Long-wave): The Earth's surface absorbs the visible light, warms up, and re-radiates it primarily as long-wave Infrared (IR) radiation.
  • The Trapping Effect: Greenhouse gases are incredibly efficient at absorbing this specific outgoing infrared radiation. Roughly $80\%$ of the infrared re-emitted by Earth is absorbed by greenhouse gases on its way back out to space.

The relative significance of any greenhouse gas depends on two factors: its overall concentration in the atmosphere, and its ability to absorb specific infrared wavelengths.

3. The Molecular Energy Level Model

Why do greenhouse gases absorb infrared radiation but ignore visible light? The answer lies in their microscopic structure.

  • High-frequency UV light carries enough energy to completely break atomic bonds within atmospheric molecules.
  • However, Infrared light carries just enough energy to cause the atoms within molecules to vibrate.
  • Greenhouse gases (like CO₂ and H₂O) have complex molecular structures with natural frequencies that fall exactly within the infrared region.
  • When outgoing infrared light hits these molecules, they begin to resonate. They absorb the infrared photon, vibrate violently (heating up), and then subsequently re-emit an infrared photon in a random direction—often straight back down towards Earth's surface.

4. The Natural vs. Enhanced Greenhouse Effect

The greenhouse effect itself is a highly beneficial natural mechanism. Without it, the heat absorbed by Earth would immediately radiate out to space, leaving the planet freezing and uninhabitable.

[Image of the greenhouse effect mechanism]

The Enhanced Greenhouse Effect:

Human activity has rapidly increased the number of greenhouse gas molecules in the atmosphere (CO₂ levels have increased by over $100$ ppm to $>420$ ppm in the last century). With a higher concentration of greenhouse gases, more infrared radiation is forced into resonance and trapped within the Earth's surface-atmosphere system.

Because less long-wave radiation can escape into space, heat accumulates, causing the average global temperatures to steadily increase.

Conceptual Question

Question: Which of the following is the result of the enhanced greenhouse effect?

  1. Increasing global average temperature due to natural causes
  2. Decreasing global average temperature due to human activity
  3. Increasing global average temperature due to human activity
  4. Decreasing global average temperature due to natural causes

Answer: C. While the base greenhouse effect is natural, the "enhanced" greenhouse effect specifically refers to the temperature increases driven by human (anthropogenic) activities.

5. Modelling Earth's Climate (Energy Balance)

To predict climate fluctuations, scientists build models based on Earth's energy balance. If the incoming energy from the Sun perfectly equals the outgoing energy to space, the Earth's temperature will remain constant. At its simplest, we evaluate a one-layer atmosphere located directly above the Earth's surface.

Example 1: A One-Layer Climate Model

Problem: In a simple energy balance model, the Earth's surface receives both incoming solar radiation and downward infrared radiation emitted from the atmosphere. At current greenhouse gas levels, the temperature of Earth's atmosphere is expected to increase by $6$ K.

Data for this model:

  • Current mean temperature of atmosphere ($T_{\text{atm}}$) = $242$ K
  • Current mean temperature of surface ($T_{\text{surf}}$) = $288$ K
  • Solar intensity above the atmosphere ($I_{\text{solar}}$) = $344$ W m⁻²
  • Emissivity of the atmosphere ($e_{\text{atm}}$) = $0.720$
  • Albedo of the atmosphere ($a_{\text{atm}}$) = $0.280$

Use this data to estimate the corresponding increase in the temperature of the Earth's surface.

Solution:

  • Step 1: Outline the knowns.
    The new temperature of the atmosphere will be $T_{\text{new\_atm}} = 242 + 6 = 248$ K.
    We will assume the Earth's surface acts as a perfect black body, so $e_{\text{surf}} = 1$.
  • Step 2: Calculate the solar intensity that successfully reaches and is absorbed by the surface.
    If the atmosphere's albedo is $0.280$, it reflects $28\%$. Therefore, $72\%$ ($0.720$) is transmitted to the surface. Note that in this simplified model, transmission equals emissivity.
    $I_{\text{absorbed}} = e_{\text{atm}} \times I_{\text{solar}} = 0.720 \times 344 = \mathbf{247.68}$ W m⁻² (approx $248$).
  • Step 3: Calculate the new infrared intensity radiated downward by the newly warmed atmosphere.
    Using the intensity form of Stefan-Boltzmann ($I = e \sigma T^4$):
    $I_{\text{atm\_radiated}} = 0.720 \times (5.67 \times 10^{-8}) \times (248)^4$
    $I_{\text{atm\_radiated}} = \mathbf{154.43}$ W m⁻² (approx $154$).
  • Step 4: Calculate the total new intensity absorbed by the Earth's surface.
    The surface absorbs both the incoming sunlight AND the downward atmospheric radiation.
    $I_{\text{total}} = 248 + 154 = \mathbf{402}$ W m⁻².
  • Step 5: Calculate the new temperature of the Earth's surface.
    Since the surface is a black body ($e=1$), all absorbed energy must equal the emitted energy ($I_{\text{total}} = \sigma T^4$) to maintain equilibrium.
    $402 = (5.67 \times 10^{-8}) \times T_{\text{new\_surf}}^4$
    $T_{\text{new\_surf}}^4 = \dfrac{402}{5.67 \times 10^{-8}} = 7.09 \times 10^{9}$
    $T_{\text{new\_surf}} \approx \mathbf{290}$ K.
  • Step 6: Determine the increase in surface temperature.
    $\Delta T_{\text{surf}} = 290 - 288 = \mathbf{2}$ K.