The greenhouse effect is a phenomenom which you can perceive when you enter a greenhouse. Energy in the form of sunlight flows into the greenhouse unhindered, but the flow out of the greenhouse is affected by the glass. The balance of energy is disrupted, and the temperature inside the greenhouse rises. This principle also applies in the very same way, but on a very much larger scale – all around our planet.
There certainly are a lot of discussions going on about this subject, ranging anywhere from the famous Kyoto Protocol to everyday political debates. What exactly is it that governs this so called greenhouse effect – and perhaps more importantly, how exactly do we contribute to this greenhouse effect? To answer these questions, we first have to get some much needed insight into what the greenhouse effect is and how it works. As previously stated, the same general principle that applies for a regular greenhouse also applies to the entirety of our planet, but how does this work? Surely there are no giant invisible glass panels around our planet, right?
While there certainly are no giant glass panels around our precious planet, there are instead the so called greenhouse gases, which act very much in the same way as glass panels do in a greenhouse. The temperature of the Earth is decided by the amount of energy that is radiated from the sun and onto our planet, as well as how much of that energy that is reflected back into space. If the amount of energy that is radiated onto the Earth is greater than the amount of energy that is radiated out from the Earth, the temperature will continously rise.
This, however, is not the case. Apart from the day and night cycle and the seasons, the average temperature is generally speaking the same at all times. The Earth itself reflects as much energy as it absorbs. We can use this fact, which is known as the Earth’s radiation balance, in order to make different scenarios and then calculate on what the temperature would be within those given scenarios. The sun shines at the Earth with the intensity of 1367 Watts per square meter. This value only applies outside our atmosphere though. The atmosphere itself absorbs around 30 percent of the intensity, which results in a netflow of 950 or so watts per square meter after penetrating the atmosphere. What function does our atmosphere serve? Let us go into detail in a moment, but before we do that, we will first take a glance at what our planet would look like without any kind of protecting atmosphere.
As you can see, the temperature on Earth, without any kind of protecting atmosphere, would indeed be very cold. Luckily for us, we have a naturally occuring greenhouse effect – without which we would not be able to survive.
With that said, you may be left wondering what the atmosphere actually does. While the incoming solar radiation has most of its energy in the wavelengths of visible light (luckily for us), there still are other wavelengths that, unlike the visible light, do not pass through the atmosphere unhindered. The greenhouse gases in the atmosphere have their electrons excited by the photon energy of certain wavelengths of sunlight, which results in some of the total energy being absorbed from the radiation to the atmospheric layer of greenhouse gases, this is where about 30 percent of the total energy directed from the sun is absorbed, and ultimately sent out as heat radiation in all possible directions, which serves to increase the temperature on our planet. The multiple-atom greenhouse gases such as ozone, carbon dioxide, methane, ammonia and various types of CFCs can absorb infrared radiation as a result of being composed of several types of atoms, which results in them having an unsymmetric charge distribution. The molecule receives kinetic energy from the electromagnetic radiation that previously put the atoms in a vibrating and rotating state.
If you were to assume that 100 units of solar radiation is approaching the Earth, you could then take note that 30 of these units are absorbed by the atmospheric greenhouse gases, which in turn convert the energy of the photons to heat, which is then radiated in all kinds of directions. This brings a net flow of 50 percent of the heat towards the Earth and the remaining 50 percent towards space. As for the remaining 70 units, they will penetrate through the atmospheric layer of gases, and continue down towards the earth. If you were to lie on the beach during a hot day, you would come in contact with UV-radiation, which is what gives you a tan after prolonged exposure. An extreme exposure of UV-radiation can result in skin cancer. Anyway, these 70 units will then be reflected back towards the atmosphere, where 35 units will be reflected towards the Earth, and the remaining 35 units will be sent off into space. This process repeats; when the 35 remaining units are reflected back towards the atmosphere, about half of it will be sent back down, and half will be sent out towards space. We should take note that the radiation from the sun and the radiation being redirected towards space are unaffected by the greenhouse effect. The greenhouse effect only affects what happens between the atmosphere and the Earth. The Earth receives just as much re-radiation from the atmosphere as direct radiation from the sun.
If we were to apply this to the equation seen above, all we have to do is to multiply the numerator by two because of the solar radiation emission being double the value when accounting for the greenhouse effect compared to not adding it into account.
This equation, as you can see, is the same one we used a while back, but this time we take the atmosphere into effect. Without an atmosphere to speak of, the temperature would be well below the freezing point of water, landing at -18 degrees celsius or -0.4 degrees fahrenheit.
As you can see, the average temperature with an atmosphere consisting of greenhouse gases results in a drastic change in the average temperature. It should however be noted that this average global temperature is this high due to us counting with the maximum possible values for the greenhouse effect as well as total absorbation of the heat radiation. Due to only small amounts of carbon dioxide, water and so on present in the atmosphere, the absorbed amounts for most wavelengths is well below 100 percent. It should be noted that higher amounts of these greenhouse gases will result in an absorbation value closer and closer to 100 percent – which in turn serve to increase the temperature on our planet. The higher the absorbation, the higher the re-redation, which in turn results in a higher temperature for us down here.
We can, based on this information, come to the conclussion that our emissions of the greenhouse gases impact the equilibrium temperature.