Sunlight is the fundamental source of energy for Earth’s systems, powering everything from weather patterns to biological life. After traveling through space at nearly 186,000 miles per second, sunlight is either reflected or absorbed upon striking the Earth.
Absorption and Heat
When sunlight is absorbed by a surface of an object, it increases molecular vibration of that surface, thereby raising its temperature. All objects that contain heat energy gradually lose that energy by radiating heat waves.
This energy is then re-radiated by the surface as longwave infrared radiation, or heat. The more sunlight a surface absorbs, the warmer it becomes, and the more heat it releases. This released heat is then absorbed and re-radiated by greenhouse gases and clouds, warming the atmosphere through the greenhouse effect.
Reflection
If light is not absorbed by a surface, it is mostly reflected. Reflection occurs when sunlight strikes a surface in the air, on land or in water, and rebounds back without getting absorbed, and later getting converted into heat.
Note that the yellow arrows in the diagram below represent just the light energy that is absorbed by the Earth. There will be some light that is reflected back into space by the Earth, but we ignore that, since none of that energy will stay on the Earth. It all goes through the greenhouse gases without being affected.
The red arrows represent energy in the form of heat radiated from the Earth as a result of the light energy that has been absorbed. The heat is radiated from the Earth at night and in the daytime, even though the light energy is incident on the Earth only in the daytime.
The arrows represent the daily average energy values of each energy type.
The Role of Greenhouse Gases
The image on the left illustrates an Earth without greenhouse gases. In the absence of greenhouse gases, all the heat energy generated from the absorbed light energy is radiated into the atmosphere. In that case, since there is no net energy received by the Earth, its average temperature would remain constant (and much colder).
The image on the right depicts the mechanisms of solar absorption and radiation, highlighting their critical role in the global climate crisis.
Currently, about 71% of incident solar radiation is absorbed by the Earth’s atmosphere and surface. The remaining 29% is reflected back into space. Unfortunately Earth’s reflectivity (albedo) has been trending downward over the past few decades, further intensifying global warming.
Human Impact on Climate Crisis
The current climate crisis is primarily driven by an unprecedented surge in greenhouse gas emissions (indicated by the dashed circle in the image at right) resulting from fossil fuel combustion since the Industrial Revolution began in the 1800s. Furthermore, human activities such as deforestation, agriculture, urbanization, pollutants and waste also play a major role in the release of greenhouse gases.
The Physics of Trapping Heat
Greenhouse gases allow sunlight to pass through the atmosphere and warm the Earth. But some of the heat that is then radiated from the Earth is trapped by the greenhouse gases. Because heat energy has a much longer wavelength than light, and it interacts with greenhouse gas molecules via resonance, it cannot all get through the greenhouse gases, like the light energy can.
Greenhouse gases absorb specific wavelengths of infrared radiation like those of the heat energy. These long heat waves have a lower energy than light waves, so they are more easily absorbed by the greenhouse gas molecules than the light waves. Once they absorb this heat, they re-emit it in all directions. Some is released into space, but a significant portion of it radiates back toward the Earth’s surface.
In the above image on the right, the small red arrow pointing toward the Earth represents this heat energy re-radiated by the greenhouse gases.
More energy arrives on the Earth than can escape, so global temperature rises.
This has been going on for a century and a half, and the world is now seeing significant shifts in Earth’s average temperature and weather patterns.