Grades 9–12 · Earth & Space
How does the greenhouse effect fit an energy balance?
Earth keeps giving off thermal radiation even after sunset. Its surface absorbs sunlight and returns energy as longer-wavelength radiation, while the atmosphere absorbs and emits some of that radiation too. Greenhouse gases change the routes by which energy leaves the surface and eventually reaches space. They do not create a new energy supply.
In the diagramToy energy flux account • W/m²100 solar in = 30 reflected + 20 air + 50 groundAir layerAbsorbs 40% of surface IRSolar absorbed: 20Ground: solar absorbed 50↑ 75↓ 25↑ 25To space: direct IR 45 + air IR 25Net gain = 100 − 30 − 70 = 0
The fictional budget begins with 100 watts per square meter of incoming sunlight: 30 is reflected, 20 is absorbed in the air layer and 50 at the ground. The whole system absorbs 70. In the baseline, 45 of the surface's thermal emission escapes directly to space and the air emits 25 upward, returning 70 in total. Incoming and outgoing energy balance at this outer boundary.
Inside that boundary, the same energy can be exchanged more than once. Greenhouse gases absorb selected thermal wavelengths and emit in several directions, including toward the ground. The baseline surface receives 50 from sunlight plus 25 from the atmosphere and emits 75 upward. This downward radiation belongs to the internal energy exchange; it is not an extra 25 of sunlight entering the entire planet–atmosphere system.
The supplied perturbation changes the air layer's infrared absorptivity and emissivity while initially holding temperatures fixed. Direct surface escape falls to 15 and upward atmospheric emission becomes 50. Only 65 now leaves against 70 absorbed, so the system gains 5 watts per square meter. Energy accumulating in a system changes its state rather than disappearing; in this simplified example the adjustment includes warming. A warmer surface with fixed emissivity emits more thermal radiation, and the supplied readjusted state restores a total outgoing loss of 70.
This is why energy balance and a warmer surface are compatible. Equal incoming and outgoing totals describe a balanced state, but greenhouse exchange helps determine the temperatures at which that balance occurs. The fictional numbers make the accounting visible; they are not a full climate model, a temperature forecast, or a prediction of how quickly the adjustment takes place.
About this explanation
Starting knowledge: Use energy conservation, signed flux accounts and the distinction between solar and thermal radiation.
Sources
- NASA Science — The Earth's Radiation Budget
- National Oceanic and Atmospheric Administration — The Earth-Atmosphere Energy Balance
- NOAA Global Monitoring Laboratory — Basics of the Carbon Cycle
- OpenStax — Mechanisms of Heat Transfer: Radiation