Ch4: Greenhouse Effect

Humans observe steadily rising atmospheric \text{CO}_2 and a parallel increase in global mean temperature.
Core questions examined:
- What molecular-level process links \text{CO}_2 to warming?
- Which other gases enhance or counterbalance this effect?
- How does Earth’s atmosphere differ from those of Venus and Mars?
Greenhouse effect (GHG effect) \approx selective absorption of outgoing infrared (IR) radiation by certain atmospheric gases, trapping heat and elevating near-surface temperatures.
Without this mechanism Earth’s average surface temperature would be -18^\circ \text{C} (similar to Mars), not the current \approx 15^\circ \text{C}.

Principal greenhouse gases (GHGs):
- \text{CO}_2 (carbon dioxide)
- \text{H}_2\text{O} vapor (water)
- \text{CH}_4 (methane)
- Halocarbons (e.g., CFCs, HCFCs, HFCs)
- \text{N}_2\text{O} (dinitrogen monoxide / nitrous oxide)
Atmospheric coolants / negative radiative-forcing agents:
- Mineral dust
- Particulate matter (PM).
– Discussed in Ch. 2 as criteria air pollutants yet confer minor cooling by reflecting sunlight.

Comparative models:
- “Natural-only” forcing (volcanism, solar variability) \Rightarrow small temperature variations.
- “Natural + Anthropogenic” forcing \Rightarrow large observed warming trend.
Anthropogenic (human-caused) = “anthropogenic” factors. Clear divergence between the two model curves is strong evidence for human influence.

Venus: very dense \text{CO}_2 atmosphere \rightarrow extremely hot surface (\approx 735\,\text{K}). Demonstrates an extreme greenhouse scenario.
Earth: moderate, balanced atmosphere \rightarrow habitable temperatures (was “ideal”).
Mars: almost no atmosphere, scant \text{CO}_2 \rightarrow cold surface.
Correlation highlighted: higher \text{CO}_2 concentration \rightarrow higher equilibrium temperature.

Incoming solar spectrum: ultraviolet (UV), visible, infrared (IR).
Fate of incident solar energy:
- 46\% reaches Earth’s surface.
- 37\% is re-emitted upward as IR.
- Portion is absorbed in atmosphere (drives photochemistry, warms air).
Greenhouse mechanism steps:
1. Sunlight (mainly visible + near-IR) warms ground/oceans.
2. Surface emits IR (longer wavelength than incoming).
3. GHG molecules absorb this outgoing IR, vibrate, and re-radiate in all directions \Rightarrow net heating of troposphere.

Radiation type	Typical energy outcome	Key examples
UV (A,B,C)	Can break covalent bonds if h\nu \ge bond energy	Ozone photolysis, \text{O}_2 dissociation
Visible	Causes electronic excitation / is reflected \rightarrow vision	Color perception
IR	Induces molecular vibration	Greenhouse warming
Microwave	Causes molecular rotation	Microwave ovens (food plate rotates to distribute energy)

Absorption requires wavelength match between photon energy and a molecule’s vibrational transition.
Incoming solar IR has "wrong" wavelengths for \text{CO}_2, so it passes through.
Surface-emitted IR is longer wavelength \rightarrow matches \text{CO}_2 bending/ asymmetric stretch modes \rightarrow strong absorption.

Requirement for IR activity: the vibration must change the molecule’s dipole moment.
Examples drawn in lecture:
- \text{H}_2\text{O} (bent) \rightarrow IR-active.
- \text{CO}_2 (linear, but asymmetric stretch creates dipole) \rightarrow IR-active.
- \text{CH}_4 (tetrahedral) \rightarrow several IR-active modes.
- \text{N}2, \text{O}2 (homonuclear diatomics) \rightarrow symmetric, no dipole change \Rightarrow IR-inactive.

Non-GHG bulk gases: \text{N}2 (\sim78 %), \text{O}2 (\sim21 %) do not absorb terrestrial IR.
Active GHGs (trace): \text{H}2\text{O}, \text{CO}2, \text{CH}4, \text{N}2\text{O}, halocarbons.
Shape and dipole criteria explain selectivity.

Annual concentration increase rates:
- \Delta [\text{CO}_2] \approx 1.5\,\text{ppm yr}^{-1}
- \Delta [\text{CH}4] and \Delta [\text{N}2\text{O}] smaller (exact numbers from table).
Atmospheric lifetimes:
- \text{CO}_2: 50\text{--}200\,\text{yr} (multiple sinks, carbon cycle dynamics)
- \text{CH}_4: 12\,\text{yr}
- \text{N}_2\text{O}: 114\,\text{yr}
Global Warming Potential (GWP, 100-yr horizon):
- \text{CO}_2: 1 (reference)
- \text{CH}_4: 23
- \text{N}_2\text{O}: 296
Interpretation:
- Although \text{CH}4 & \text{N}2\text{O} are lower in concentration, their per-molecule heat-trapping potency far exceeds that of \text{CO}_2.
- \text{CO}_2 still dominates radiative forcing because of its huge, rapidly rising concentration.

Cooling via particulate matter offers an example of an environmental “double-edge sword”: pollutants that harm health yet modestly offset warming (Ch. 2 - Air Pollutants).
Ozone layer (stratospheric chemistry, Ch. 3) shields Earth from high-energy UVC/UVB, demonstrating another protective atmospheric process.
Carbon cycle sinks (oceans, forests) dictate \text{CO}_2 residence time—ties to biogeochemical cycles discussed earlier.
Climate policy must weigh: longevity (atmospheric lifetime), potency (GWP), emission rate, and co-benefits/harms of each gas.

Define greenhouse effect at molecular level: “IR absorption \rightarrow molecular vibration \rightarrow heat retention.”
List five major GHGs and two primary cooling agents.
Explain why \text{N}2 and \text{O}2 are not GHGs despite abundance.
State the approximate portion of outgoing IR absorbed (\approx 80\%).
Recite lifetimes and GWP values for $$\text{CO}_