Greenhouse Effect

The Earth receives energy primarily from the Sun in the form of short-wavelength radiation (visible, ultraviolet). Some of this incoming solar radiation is:

• Reflected back to space by clouds, aerosols, and Earth’s surface (albedo effect).
• Absorbed by the atmosphere and Earth’s surface, increasing internal energy.

The Earth then emits energy back into space as long-wavelength infrared radiation. At equilibrium, the power absorbed by Earth equals the power emitted. If S₀ is the solar constant (≈ 1,366 W·m⁻²) and A is the planetary albedo (≈ 0.30), then the absorbed solar power per unit area (averaged over the entire Earth) is:

P_in = S₀/4 (1−A).

At equilibrium, the outgoing power per unit area (treated as a blackbody) is given by Stefan–Boltzmann’s law:

P_out = σ T_eff⁴,

where σ = 5.67×10⁻⁸ W·m⁻²·K⁻⁴ and T_eff is Earth’s effective radiating temperature (~255 K). For energy balance:

S₀/4 (1−A) = σ T_eff⁴.

Greenhouse gases (GHGs) in the atmosphere—primarily water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)—are largely transparent to incoming shortwave solar radiation but absorb outgoing longwave infrared radiation emitted by Earth’s surface.

When GHGs absorb infrared photons, their molecular bonds vibrate or rotate, raising the internal energy of the gas molecules. These molecules subsequently re-emit infrared radiation in all directions; some returns downward, warming Earth’s lower atmosphere and surface.

This trapping of heat raises the surface temperature above the effective radiating temperature; without GHGs, Earth’s average surface temperature would be about –18 °C instead of the observed +15 °C.

Radiative forcing is the change in net (downward minus upward) radiative flux at the top of the troposphere caused by a perturbation (e.g., increased CO₂). Positive radiative forcing leads to warming; negative forcing leads to cooling. It is typically measured in W·m⁻².

Since the pre-industrial era (~1750), CO₂ concentration has risen from ~280 ppm to over 410 ppm, producing a positive radiative forcing of approximately +1.82 W·m⁻² (IPCC estimate).

● Carbon Dioxide (CO₂): Emitted by fossil fuel combustion, deforestation, and cement production. CO₂ absorbs infrared strongly at wavelengths near 4.3 µm and 15 µm, making it a potent GHG.

● Water Vapor (H₂O): The most abundant GHG; its concentration is temperature-dependent (warmer air holds more moisture). Acts as a feedback amplifier: as Earth warms, more water evaporates, leading to additional GHG effect.

● Methane (CH₄): Emitted by wetlands, agriculture (rice paddies, ruminant digestion), and fossil fuel extraction. CH₄ has a shorter atmospheric lifetime (~12 years) but a global warming potential ~28–36 times that of CO₂ over a 100-year period.

● Nitrous Oxide (N₂O): Emitted by agricultural soils (fertilizer use), industrial processes, and combustion. Lifetime ~114 years; global warming potential ~265–298 times that of CO₂ (per kg).

● Ozone (O₃): In the lower atmosphere (troposphere), O₃ is a GHG formed by chemical reactions involving pollutants (NOₓ, volatile organic compounds). Stratospheric O₃, by contrast, protects life from ultraviolet radiation.

Increased atmospheric concentrations of GHGs cause an imbalance between absorbed solar radiation and emitted infrared radiation, leading to a net warming of Earth’s surface and lower atmosphere.

Feedback Mechanisms:

• Positive Feedback: Ice-albedo feedback—melting ice reduces surface reflectivity, leading to more absorption of solar radiation and further warming.
• Negative Feedback: Enhanced infrared emission at higher temperatures can partially offset warming.

Consequences of Global Warming:

• Rising global average temperatures, with associated increases in the frequency and intensity of heat waves.
• Melting of glaciers and polar ice, contributing to sea-level rise.
• Altered precipitation patterns, leading to droughts in some regions and increased flooding in others.
• Ocean acidification, as increased CO₂ dissolves in seawater to form carbonic acid, affecting marine life.
• Impacts on biodiversity and ecosystem services (e.g., shifts in species ranges, coral bleaching).