Greenhouse Effect
Definition and meaning of Greenhouse Effect in chemistry.
The greenhouse effect is the warming of a planet's surface and lower atmosphere that occurs when certain atmospheric gases absorb outgoing infrared radiation and re-emit part of it back toward the surface, trapping heat that would otherwise escape to space.
In more detail
Sunlight (mostly visible and UV) passes through the atmosphere largely unabsorbed and warms Earth's surface, which then radiates energy back as longer-wavelength infrared radiation. Greenhouse gas molecules have vibrational modes that change their dipole moment, allowing them to absorb this infrared radiation and re-emit it in all directions, including downward. Symmetric diatomic molecules like N2 and O2 lack a changing dipole moment during vibration, so they are infrared-inactive and do not contribute. This absorption-and-re-emission cycle delays heat loss to space, raising the planet's equilibrium surface temperature.
Key facts
| Field | Physical Chemistry |
|---|---|
| Major greenhouse gases | CO2, CH4, H2O, N2O, O3 |
| Natural warming effect | ~33 °C above the airless equilibrium temperature |
| Requirement for IR activity | vibration must change the molecule's dipole moment |
Carbon dioxide absorbs infrared radiation near 15 micrometers via its bending vibrational mode; together with water vapor and methane, this natural absorption keeps Earth's average surface temperature about 33°C warmer than it would be without an atmosphere.
Frequently asked questions
Why don't nitrogen and oxygen contribute to the greenhouse effect?
N2 and O2 are homonuclear diatomic molecules whose vibrations do not change their (zero) dipole moment, so they cannot absorb or emit infrared radiation and are considered infrared-inactive.
Is the greenhouse effect harmful?
The natural greenhouse effect is essential for life, keeping Earth's average surface temperature near 15°C instead of about -18°C; the concern is the enhanced effect from rising concentrations of anthropogenic greenhouse gases.