The Greenhouse Effect and Greenhouse Gases

Even without a climate system, Earth would balance energy: energy absorbed = energy emitted.
The climate system moderates Earth’s temperature by trapping, storing, and distributing solar energy.
This keeps air temperature relatively constant.

The atmosphere allows high-energy radiation from the Sun to pass through.
Earth’s surface absorbs this radiation, becoming thermal energy and warming up.
The warm surface emits lower-energy infrared (IR) radiation.
Gases in the atmosphere trap much of the IR radiation and radiate it in all directions, with about half sent back to Earth.
This trapped energy keeps Earth warmer than it would otherwise be.
This energy-trapping process is the greenhouse effect: a natural process where gases and clouds absorb infrared radiation emitted from Earth’s surface and radiate it.
Without the greenhouse effect, Earth’s average global temperature would be about $-18$ °C; with it, the average is around $15$ °C.
The greenhouse effect is a natural process that has been happening for millions of years.

Scientists once thought greenhouses warmed up the same way as the atmosphere.
However, greenhouses warm up because colder air currents are prevented from interacting with the air inside.
Therefore, the term 'greenhouse effect' is misnamed.

Most of the air is nitrogen and oxygen, which do not absorb radiation from Earth’s surface.
The greenhouse effect is caused by gases in low concentrations: greenhouse gases.
Important greenhouse gases are water vapour ( $H<em>2O$ ) and carbon dioxide ( $CO</em>2$ ).
Other greenhouse gases include methane ( $CH<em>4$ ), tropospheric ozone ( $O</em>3$ ), and nitrous oxide ( $N_2O$ ).
Their contribution depends on concentration and how much thermal energy each molecule can absorb.

Earth's atmosphere contains only 385 ppm (parts per million) carbon dioxide, or $0.0385$ . This causes up to a quarter of the greenhouse effect.
Before the industrial age, $CO_2$ concentration was 280 ppm.
Natural sources of $CO_2$ include volcanic eruptions, burning organic matter, and cellular respiration.
The carbon cycle is the movement of carbon through living things, the lithosphere, the atmosphere, and the hydrosphere.
Living things and oceans are important carbon sinks, removing carbon dioxide and storing the carbon atoms.
Trees capture $CO_2$ during photosynthesis. When trees decompose or burn, the carbon is released back into the atmosphere as carbon dioxide.
In the ocean, carbon dioxide dissolves and some forms solid calcium carbonate, which sinks to the bottom of the ocean.

Ozone exists naturally in the stratosphere, protecting Earth from UV radiation.
In the troposphere, ozone acts as a greenhouse gas.
The average concentration of tropospheric ozone is not clearly defined, but it contributes to the greenhouse effect.

About two-thirds of Earth’s natural greenhouse effect is caused by water vapour.
The quantity of atmospheric water vapour depends on the temperature of the atmosphere, varying from trace amounts to about $4$ . Warmer air can hold more water vapour.
As Earth’s temperature increases, more liquid water becomes water vapour.
Because water vapour traps energy, the more water vapour there is, the warmer Earth becomes.
This relationship is a feedback loop.

Feedback loop: the cause creates an effect that affects the original cause.
Positive feedback loop: the effect enhances the original cause (e.g., higher temperatures lead to more water vapour, leading to higher temperatures).
Negative feedback loop: the effect decreases the original cause.

There is less methane than carbon dioxide in the atmosphere.
However, a molecule of methane can absorb much more thermal energy than a molecule of carbon dioxide.
A molecule of methane is about 23 times more powerful as a greenhouse gas than a molecule of carbon dioxide.
Methane comes from natural (plant decomposition in swamps, animal digestion) and human sources.
Before the industrial age, the concentration of methane in the atmosphere was 0.700 ppm (700 ppb). It has now risen to 1.785 ppm (1785 ppb).

A molecule of nitrous oxide ( $N_2O$ ) is almost 300 times more effective than a molecule of carbon dioxide as a greenhouse gas.
There is a much smaller concentration of nitrous oxide in the atmosphere.
Before the industrial age, the concentration of nitrous oxide in the atmosphere was 270 ppb (0.270 ppm). It has since risen to 321 ppb (0.321 ppm).
Nitrous oxide is produced from both natural (reactions of bacteria in soil and water) and human sources.

Carbon dioxide, methane, nitrous oxide, and other greenhouse gases are present in the atmosphere in minute quantities.

Nitrogen and oxygen gas each consist of two identical atoms.
The two atoms in these molecules can only vibrate one way: back and forth. This limits the type of energy the molecules can absorb. When infrared radiation reaches these molecules, they cannot absorb it.
Water, carbon dioxide, and methane consist of three or more atoms, and have diff erent types of atoms.
The atoms in these molecules can vibrate and wiggle in many ways, and can absorb diff erent types of energy. Thus, when infrared radiation reaches water vapour, carbon dioxide, or methane, these molecules trap the infrared energy and re-radiate it back out in every direction.