The greenhouse effect
The greenhouse effect occurs when “greenhouse gases” (GHGs) trap solar radiation within the Earth’s atmosphere, particularly infrared radiation, which is re-emitted from the Earth’s surface after it has been warmed by the Sun.
Luckily, this infrared radiation or “heat radiation”—at the core of the greenhouse effect—has kept Earth’s surface temperatures warm at night for millions of years.
Unluckily, a slight imbalance of greenhouse gases linked to human activity is increasing the trapping of infrared radiation and causing average temperatures to rise. This phenomenon is called global warming or climate change and is slowly overheating the planet since the ignition of the Industrial Revolution.
The solar radiation heats up the Earth in different ways:
• Incoming (visible) sunlight travels through the transparent atmosphere to heat up black objects absorbing sunlight, and also to heat up colored objects or colored liquids (e.g., seawater) absorbing sunlight while reflecting some light seen as colors by the human eye.
• Incoming infrared radiation is mostly reflected (i.e., absorbed and re‑emitted later) back to space by the atmosphere, which is “not-so-transparent” for infrared radiation. In other words, infrared radiation is partially absorbed to heat up the not-transparent gases in the atmosphere, while some infrared radiation manages to reach the Earth’s surface and to heat it up. These non-transparent atmospheric gases are the so-called greenhouse gases, such as—in order of importance—water vapor, carbon dioxide, methane, nitrous oxide, while nitrogen and oxygen are transparent for infrared radiation.
• Incoming ultraviolet radiation plays a very small part to heat up Earth’s surface as most of it is absorbed by the ozone layer in the upper atmosphere.
• Outgoing infrared radiation is re-emitted by the Earth’s surface and the atmosphere warmed by Sun radiation. This outgoing infrared radiation bounces (i.e., absorbed and re-emitted later) as “heat radiation” in all directions, back to space, trapped in the atmosphere, and to some extent, back to Earth’s surface.