Greenhouse Gas Science
Compare incoming short-wave solar radiation with Earth's outgoing long-wave infrared. Describe how suitable molecular vibrations let greenhouse gases absorb infrared, while O2 and N2 absorb very little. Explain the natural greenhouse effect and its strengthening by human emissions. Interpret gas warming comparisons for equal released masses over a stated time.
What a learner can do afterwards
- Compare short-wave solar radiation with Earth's long-wave infrared emissions
- Identify greenhouse gases and explain why suitable molecular vibrations absorb infrared
- Describe infrared absorption, energy transfer through collisions and infrared emission
- Distinguish the natural greenhouse effect from its strengthening by human emissions
- Interpret a stated equal-mass, 100-year warming comparison and explain why emission amounts and timing also matter
1 · Read
You have seen how sunlight warms land and water. Light carries energy as electromagnetic radiation. Wavelength is the spacing between neighboring wave crests. The electromagnetic spectrum groups radiation by wavelength. Sunlight includes visible light, ultraviolet and near-infrared. Earth's cooler surface emits mostly long-wave infrared. Solar radiation is called short-wave in comparison. Infrared is invisible; absorbing it can warm matter.
Greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and water vapor (H2O). Their bonded atoms vibrate, moving back and forth. They absorb certain infrared wavelengths when a vibration changes their electric dipole. This is the size or direction of the separation between positive and negative charge in the molecule.
Oxygen (O2) and nitrogen (N2) also vibrate. Their atoms can stretch apart and move together, but this does not change their electric dipole. They absorb very little of Earth's outgoing infrared.
CO2 gains vibration energy by absorbing matching infrared. Collisions can pass energy to nearby air molecules. Greenhouse gases also emit infrared in many directions, including toward the surface. The absorbing molecule need not emit immediately.
One possible path for absorbed energy. The boxes show events, not molecule shapes.
Greenhouse gases occur naturally. By absorbing and emitting infrared, they keep Earth's surface warmer than it would be without them. This natural greenhouse effect helps keep Earth warm enough for liquid water and life. It existed before human industry. Removing it would leave Earth's average surface colder.
Human activities add greenhouse gases. Burning coal, oil and gas releases CO2. Fuel leaks and farming release methane. With the same incoming sunlight, added greenhouse gases initially reduce energy escaping to space. Earth warms, and outgoing energy increases toward a new balance. This strengthening of the natural effect is the enhanced greenhouse effect.
As Earth warms, outgoing energy increases toward a new balance.
Global warming potential (GWP) compares the warming effects of equal released masses over a stated time. CO2 has a score of 1. A 100-year GWP adds up the effects over 100 years after release. The score reflects infrared absorption and how long the gas affects the atmosphere. A historical score of 265 for nitrous oxide means 265 divided by 1, or 265 times CO2's effect for equal masses. It is not a per-molecule rating.
People release different amounts of each gas. To judge total warming, also consider how much was released and when. A smaller release of a high-GWP gas need not cause more warming than a much larger release of a lower-GWP gas. An equal-mass score alone cannot tell you which gas makes the larger total contribution.
Greenhouse gases absorb and emit some infrared. Human emissions strengthen the natural effect. Compare warming scores for equal released masses over the same time, not molecule by molecule.
Lesson, questions and flashcards written for this dataset. Historical gas comparisons use IPCC 2013 estimates for equal released masses over 100 years, without climate-carbon feedbacks.
Four readings and six flashcards written for this dataset about electromagnetic radiation and the greenhouse effect.
More to read
From the surface into the air
Follow energy leaving a warm patch of ground. The ground emits infrared radiation. Some infrared reaches space. At other wavelengths, a greenhouse gas can absorb it and gain energy.
After absorption, a CO2 molecule has more vibration energy. A collision can transfer energy to a neighboring air molecule. The energy need not remain in CO2, and the absorbing molecule need not immediately emit it again.
Greenhouse gases also emit infrared. Some travels toward the surface. Absorption, collisions and emission move energy through the atmosphere. They help keep Earth's surface warmer than it would be without greenhouse gases. Adding greenhouse gases changes this energy flow and can cause extra warming.
Why some infrared passes through
A greenhouse gas does not absorb every wavelength. An infrared wavelength must match a suitable molecular motion for the absorption discussed here. Other wavelengths may pass through that gas.
This means the atmosphere is not a solid lid. Some energy escapes directly from the surface, and some is absorbed and later emitted within the atmosphere. The path depends on the radiation's wavelength and the gases it meets.
Keep absorption separate from reflection. Absorption transfers radiation's energy to matter. Reflection changes the direction of radiation at a surface. Explaining greenhouse gases as tiny mirrors misses the energy that their molecules take in and can share through collisions.
Wavelength measures spacing
Wavelength measures the distance between neighboring wave crests. A shorter wavelength means closer crests. It does not mean the wave travels faster through empty space.
The electromagnetic spectrum puts radiation into bands. From longer to shorter wavelengths, these are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Our eyes detect the visible band. Invisible bands carry energy too.
Sunlight includes infrared as well as visible light and ultraviolet. Earth's much cooler surface emits mostly longer infrared wavelengths. The terms short-wave sunlight and long-wave Earth radiation compare their wavelength ranges. They do not mean that the Sun emits only visible light or that Earth emits every kind of long-wavelength radiation equally.
How sunlight reaches a book
Imagine a book resting in a sunbeam. It absorbs some incoming radiation and warms. The energy did not arrive because the book touched the Sun. Air did not have to flow from the Sun to the book. Electromagnetic radiation can cross empty space.
Contact can transfer energy by conduction. Moving air or water can carry energy by convection. These are different paths from radiation crossing space.
The warm book also emits infrared, although it does not glow visibly. Earth's surface emits infrared too. Winds redistribute energy within the atmosphere, but radiation carries energy across the space between Earth and the Sun. Greenhouse gases affect some of the infrared travelling outward from the surface.
2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.