If atmospheric CO₂ concentrations double, radiative-convective equilibrium theory predicts which of the following temperature changes?
ABoth the troposphere and stratosphere warm, because CO₂ traps outgoing radiation at all atmospheric levels
BThe troposphere warms while the stratosphere cools, because CO₂ raises the effective emission altitude in the troposphere and enhances radiative cooling from the stratosphere
CThe troposphere cools and the stratosphere warms, because increased CO₂ reflects incoming solar radiation back to space
DOnly the surface warms; the free troposphere and stratosphere remain unaffected
Adding CO₂ makes it harder for longwave radiation to escape the lower atmosphere, raising the effective emission altitude to a colder level — which reduces outgoing radiation until the troposphere warms to restore energy balance. In the stratosphere, more CO₂ means more efficient radiative cooling to space (the stratosphere is optically thin and CO₂ there radiates directly to space), so the stratosphere actually cools. This tropospheric warming + stratospheric cooling signature is a diagnostic fingerprint of greenhouse forcing, distinguishing it from solar-driven warming (which would warm both layers).
Question 2 Multiple Choice
What would Earth's lower atmosphere look like if vertical energy transport were purely radiative, with no convection allowed?
AThe lower atmosphere would be stable, with a gentle lapse rate of about 6.5°C/km as observed today
BThe surface would be cooler than observed, because radiative transfer efficiently carries heat away from the surface
CThe lower atmosphere would have an extremely steep lapse rate — far steeper than the moist adiabat — making it statically unstable
DThe tropopause would disappear because there would be no temperature inversion to define the boundary
Pure radiative equilibrium concentrates intense heating near the surface and strong cooling of the mid-troposphere, producing a steep temperature gradient. By the criterion for static stability, a parcel lifted from the surface would find itself warmer and less dense than its environment — it would keep rising spontaneously. This static instability is inherent to the pure radiative profile: it cannot be maintained because convection will spontaneously break out and mix the atmosphere until the lapse rate stabilizes near the moist adiabatic value. The observed 6.5°C/km lapse rate is set by convection, not radiation.
Question 3 True / False
In radiative-convective equilibrium, adding greenhouse gases to the atmosphere warms the troposphere and simultaneously cools the stratosphere.
TTrue
FFalse
Answer: True
This counterintuitive asymmetry is a cornerstone of greenhouse theory. In the troposphere, greenhouse gases trap outgoing longwave radiation, reducing the efficiency of radiative cooling — the troposphere must warm to restore energy balance. In the stratosphere, CO₂ is an efficient radiator to space (nothing above it traps its emission), so more CO₂ actually enhances stratospheric cooling. The observed pattern of tropospheric warming combined with stratospheric cooling is one of the empirical fingerprints confirming the greenhouse mechanism and distinguishing it from natural solar variability.
Question 4 True / False
The observed tropospheric lapse rate of approximately 6.5°C/km is primarily determined by radiative transfer processes, since the troposphere is where most atmospheric absorption and emission of longwave radiation occurs.
TTrue
FFalse
Answer: False
The tropospheric lapse rate is primarily set by convection, not radiation. The moist adiabatic lapse rate (~6.5°C/km for Earth's average humidity) is the temperature gradient at which a rising moist air parcel is just neutrally buoyant — neither buoyant nor negatively buoyant. Convective mixing drives the lapse rate toward this value. If the radiative lapse rate were steeper, it would be convectively unstable; if gentler, convection would cease. Radiation dominates in the stratosphere, but convection sets the lapse rate in the troposphere — this is precisely why the model is called radiative-*convective* equilibrium.
Question 5 Short Answer
Why is the purely radiative equilibrium atmosphere statically unstable, and what process actually establishes the observed tropospheric lapse rate?
Think about your answer, then reveal below.
Model answer: In pure radiative equilibrium, the surface is strongly heated while the middle troposphere is strongly cooled by emission of longwave radiation. This creates a temperature profile much steeper than the moist adiabatic lapse rate — meaning a lifted air parcel would always be warmer and less dense than its surroundings, and would keep rising. This static instability cannot be maintained: convection spontaneously breaks out and mixes heat upward until the lapse rate is reduced to approximately the moist adiabatic value (~6.5°C/km). Convection — not radiation — is therefore the dominant process setting the tropospheric temperature profile, which is why the actual equilibrium state requires both processes.
This is the 'C' in radiative-convective equilibrium: convection is not a minor correction to radiation but the dominant vertical transport mechanism in the lower atmosphere. The equilibrium that actually exists has a division of labor: convection handles vertical heat transport in the troposphere, and radiation handles the stratosphere and defines the energy balance at the top of the atmosphere. This framework is the conceptual backbone of every general circulation model used in climate projections.