3. How exactly does the greenhouse effect work in detail?

3. How exactly does the greenhouse effect work in detail?

The greenhouse effect can be explained in several steps. It all begins with the sun emitting electromagnetic waves in form of ultraviolet, visible and near-infrared radiation towards the earth, with a common wavelength of around 500 nm. This wavelength maximum of the spectrum is only scarcely absorbed by the atmosphere, since greenhouse gases like water, carbon dioxide, methane and ozone are permeable for the short-wave radiation. The atmosphere and clouds reflect around 26% of the solar energy back into space and absorb 19%. After passing the atmosphere, the remaining solar energy hits the Earth’s surface, where a small part of the energy is reflected back into space, while the rest is absorbed. The photons induce their energy into the surface and cause a heating effect. The heated surface then emits infrared radiation with a wavelength of 10.000 nm.

Figure 2: Diagram showing light energy (white arrows) emitted by the sun, warming the earth’s surface which then emits the energy heat (orange arrows), which warms the atmosphere and is then emitted as heat by three of the greenhouse gas molecules (water, carbon dioxide and methane). Figure adopted from wikipedia.org.

The long-wave infrared radiation emitted by the surface is more likely to be absorbed by greenhouse gases in the atmosphere. Thus, only a small part of this radiation escapes the atmosphere back into space. The infrared radiation can be absorbed and emitted by greenhouse gases due to their molecular structure with two different atoms (carbon monoxide) and all gases with three or more atoms. The energy absorbed by the greenhouse gases causes the loosely bound molecules to vibrate and, at some point, release the radiation again. This energy is then emitted evenly into space or back to the surface. Hitting the surface, this energy is absorbed again, and an additional heating effect occurs due to the energy being trapped in the lower atmosphere (figure 2). Increasing the concentration of greenhouse gases increases the amount of absorption and reradiation and thereby further warms the atmosphere and the surface below. Around 99% of the dry atmosphere is infrared transparent because the main constituents are nitrogen, oxygen and argon. These gases are composed of either one atom or two identical atoms, thus these gases are not able to directly absorb or emit infrared radiation. Intermolecular collisions, however, cause the energy absorbed and emitted by the greenhouse gases to be shared with the non-infrared active gases. Even though the atmosphere is composed largely of non-reactive gas molecules, the small amount of greenhouse gases has a huge impact on global warming and by increasing their concentration, the greenhouse effects develops a positive feedback loop, which results in temperatures increasing more quickly.

Effects of the albedo

As already mentioned above, one important factor that contributes to the greenhouse effect is the ability of the Earth’s surface to absorb or reflect the solar radiation. These parameters are described by the albedo, which determines the measure of the diffuse reflection of solar radiation out of the total solar radiation received by the Earth. This effect can be explained with a simple example. Snow reflects a lot of sunlight, resulting in lower heat gain for the surface and thus, less warming. If the area of the snow cover decreases, the proportion of reflected sunlight decreases, resulting in higher heat gain and therefore warming.

The albedo is a dimensionless parameter, measured on a scale from 0, which describes a black body that absorbs all radiation, to 1, corresponding to a body that reflects all incident radiation. The average albedo of Earth from the upper atmosphere is around 0.3 – 0.35 because of the cloud cover, but widely varies locally across the surface due to different geological and environmental features. The average albedo of 0.3 – 0.35 means that 30 – 35% of the incoming solar radiation is reflected by the surface. The most important surface albedos are described by the oceans (0.06), forests (0.08 to 0.18), the continental surface (0.1 to 0.4), ocean ice (0.5 to 0.7) and fresh snow (0.8). The albedo varies with latitude, being highest near the poles and lowest in the subtropics, with a local maximum in the tropics. Albedo affects climate by determining how much radiation is absorbed by the planet. Uneven heating from albedo variations between land, ice or ocean surfaces can drive weather.

Climate change and albedo

The intensity of albedo temperature effects depend on the amount of albedo and the level of local insolation (solar irradiance). The Arctic and Antarctic regions are cold due to low solar irradiance and high albedo, whereas also higher albedo areas like the Sahara Desert are hotter due to higher insolation. Arctic regions notably release more heat back into space than what they absorb, effectively cooling the planet, but snow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to 0.4 for melting snow and 0.2 for dirty snow. With the current climate change, arctic ice and snow are melting at higher rates due to increasing temperatures. With the warming of snow-covered areas, the snow tends to melt, lowering the albedo and hence leading to more snowmelt because more radiation is being absorbed. As a consequence of the melting the underlying surface is exposed (water or ground with lower albedo), which then results in even less solar radiation being reflected back into space. This process creates a positive feedback loop, which results in a reduced albedo effect. In summary, the lower the reflection of the solar radiation, the more infrared radiation is produced. The higher the amount of infrared radiation, the stronger is the effect of greenhouse gases resulting in global temperatures to increase.




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