What Is The Greenhouse Effect?
QUICK ANSWER
The greenhouse effect is the warming caused when atmospheric gases absorb infrared radiation emitted from Earth's surface and re-radiate part of it back down. Without the greenhouse effect, Earth's average temperature would be around -18°C (0°F), too cold for most life. The natural greenhouse effect keeps Earth's average temperature around 15°C (59°F).
The greenhouse effect is one of the most important processes in Earth's climate system, keeping our planet warm enough for liquid water and life. Without it, Earth would be a frozen world. The basic mechanism was discovered in the 1820s and has been refined ever since. Today, the greenhouse effect is central to discussions of climate change, as human activities are increasing certain greenhouse gases and warming the planet.
What is the natural greenhouse effect?
The natural greenhouse effect is the warming Earth gets from greenhouse gases that have always been present in the atmosphere. Water vapor, carbon dioxide, methane, and other gases absorb infrared radiation from Earth's surface and re-emit it in all directions, including back toward the surface. This raises Earth's average temperature from what it would be without an atmosphere (about -18°C or 0°F) to the current average of about 15°C (59°F). Without this 33°C boost, Earth would be a frozen, lifeless world.
How does the greenhouse effect work?
The greenhouse effect works in a specific sequence. Sunlight reaches Earth and warms the surface. The warmed surface emits infrared radiation upward. Greenhouse gas molecules absorb some of this infrared radiation, gaining energy. The energized molecules re-emit infrared in random directions, with about half going back down toward the surface. This downward radiation adds to the heating from direct sunlight. The result is a warmer surface than if all infrared radiation escaped directly to space. The same process happens in Earth's atmosphere day and night.
How is the greenhouse effect different from a real greenhouse?
Despite the name, the greenhouse effect works differently from a real greenhouse. A real greenhouse traps heat primarily by preventing air convection: the glass walls keep warm air from flowing away. Atmospheric greenhouse warming works through radiation absorption and re-emission, not convection blocking. The name 'greenhouse effect' was coined when scientists initially compared the warming mechanisms, but the analogy is imperfect. The atmospheric effect is real and important, but the physics differs significantly from a literal greenhouse, where glass is selectively transparent to different wavelengths.
How is the greenhouse effect changing?
Human activities are increasing greenhouse gas concentrations, enhancing the greenhouse effect beyond natural levels. CO2 has risen about 50% since pre-industrial times. Methane has more than doubled. According to NASA's explanation of the enhanced greenhouse effect, the enhanced effect from these increases is causing Earth's average temperature to rise. The change is gradual on human timescales but rapid in geological terms. Climate scientists project continued warming as long as greenhouse gas concentrations keep rising, with effects on weather patterns, sea levels, and ecosystems worldwide.
The greenhouse effect is the warming caused by atmospheric gases absorbing infrared radiation from Earth's surface and re-radiating it back down. The natural effect keeps Earth habitable at 15°C average instead of -18°C. Human-caused increases in greenhouse gases are enhancing the effect, driving the climate change we're observing today.
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