Which of the following best describes the mechanism of the greenhouse effect?
AGreenhouse gases trap solar radiation like a physical glass barrier, preventing heat from escaping
BGreenhouse gases are opaque to incoming solar radiation and reflect it back to warm the surface
CGreenhouse gases absorb outgoing infrared radiation from Earth's surface and re-emit it in all directions, including back toward the surface
DGreenhouse gases increase the amount of solar radiation reaching Earth's surface by thinning the atmosphere
The key is the wavelength selectivity: greenhouse gases are largely transparent to shortwave solar radiation (visible/UV) but absorb longwave infrared emitted by Earth's surface. They then re-emit that energy in all directions — some back toward the surface, raising its temperature. This is fundamentally different from a physical greenhouse, which traps warm air by preventing convection.
Question 2 True / False
Carbon dioxide is the most abundant greenhouse gas in Earth's atmosphere.
TTrue
FFalse
Answer: False
Water vapor (H₂O) is the most abundant and most powerful greenhouse gas by concentration. However, water vapor concentration is controlled by temperature (it evaporates or condenses in response to temperature), making it a feedback rather than a forcing. CO₂ is the most important anthropogenically controlled greenhouse gas, which is why it receives the most policy attention.
Question 3 Short Answer
What does 'radiative forcing' measure, and why is it useful for comparing different greenhouse gases?
Think about your answer, then reveal below.
Model answer: Radiative forcing measures the change in net energy flux at the top of the atmosphere (in W/m²) caused by a change in atmospheric composition. It is useful because it provides a common unit for comparing how much warming different greenhouse gases or other factors contribute, regardless of their chemical mechanisms.
Because different greenhouse gases absorb at different wavelengths and have different atmospheric lifetimes, we need a common currency for comparison. Radiative forcing (W/m²) serves this role. A gas with a higher global warming potential (GWP) produces more radiative forcing per unit mass. CO₂ is set as the reference (GWP = 1), and methane's GWP of ~80 over 20 years means it produces 80× the forcing of the same mass of CO₂ over that period.