Greenhouse Effect Lab

Sunlight passes through the atmosphere and warms Earth's surface. The surface then emits infrared (heat) radiation upward. Greenhouse gases - primarily water vapor, CO₂, methane, and nitrous oxide - absorb this outgoing infrared radiation and re-emit it in all directions, including back toward the surface.

This trapping effect raises Earth's surface temperature above what it would be with no atmosphere. Without any greenhouse gases, Earth's average temperature would be around -18°C. The natural greenhouse effect warms it to about 15°C, making life possible.

Human activities, primarily burning fossil fuels and deforestation, have raised CO₂ from pre-industrial levels of 280 ppm to over 420 ppm today, intensifying the greenhouse effect.

At equilibrium, the energy Earth absorbs from the Sun equals the energy Earth radiates back to space. The Stefan-Boltzmann law relates the equilibrium temperature to the energy balance:

Here S is the solar constant (1361 W/m²), alpha is the planetary albedo (0.30), epsilon is the effective emissivity of the atmosphere, and sigma is the Stefan-Boltzmann constant (5.67 x 10⁻⁸ W m⁻² K⁻⁴).

Greenhouse gases reduce the effective emissivity. Lower emissivity means the planet must warm up more to emit enough radiation to balance the incoming energy.

The IPCC uses Representative Concentration Pathways (RCPs) to describe possible future CO₂ trajectories based on different emissions scenarios.

• RCP 2.6 - Aggressive mitigation. CO₂ peaks before 2100 and declines.
• RCP 4.5 - Moderate mitigation. CO₂ stabilizes around 538 ppm by 2100.
• RCP 6.0 - Partial mitigation. CO₂ reaches approximately 670 ppm by 2100.
• RCP 8.5 - Business as usual, no significant climate policy. CO₂ approaches 936 ppm by 2100.

This lab uses a simplified energy-balance model. The actual climate system includes feedbacks (water vapor, ice-albedo, clouds) that amplify warming beyond the direct CO₂ effect.