The Greenhouse Effect
How does the Sun heat the Earth?
The most important source of energy for life on Earth is the Sun. Not only is it the ultimate source of energy in various food chains, it is also responsible for the heating of Earth.
The Sun emits energy in the form of mainly UV and visible light (and also some near-infrared). A little over a quarter of this energy is reflected right back into space upon reaching our atmosphere. The rest of the energy is either absorbed by the atmosphere and the surface of the planet. Upon absorbing the energy, they become warmer and then emit the energy back out. However, this energy is emitted not in the same form, but as infrared radiation.
The Role of Greenhouse Gases
Certain gases in the atmosphere, known as greenhouse gases, prevent heat from escaping back into space. They do so by absorbing the emitted IR radiation and then re-emitting some of it back down toward Earth.
This results in Earth absorbing more energy than it emits and causes it to warm.
Without the greenhouse effect, life on Earth would not exist. Earth's average temperature would be significantly lower (at roughly -18°C rather than 14°C), and we would have extremely hot days and extremely cold nights.
Not all greenhouse gases have the same effect.
Global Warming Potential refers to how much a ton of gas can contribute to the warming of the atmosphere over a given period of time, usually 100 years. It is based on two major factors: the "lifetime" or "residence time" (how long the gas stays in the atmosphere) and the infrared absorption (how well the molecule is able to absorb and radiate IR radiation). This measurement is relative to one ton of carbon dioxide.
Carbon Dioxide (CO2) has a GWP of 1, as all others are described relative to it. It is the biggest driver of climate change currently, with atmospheric levels having increased by 50% due to human activities since the Industrial Revolution. It is released primarily through the burning of fossil fuels and deforestation. While other greenhouse gases have greater effects on a molecule-to-molecule basis, there is significantly more CO2 being released than the other GHGs.
Methane (CH4) has a GWP of roughly 27-30 over 100 years. It absorbs much more energy than CO2, making the same amount of gas result in stronger warming. However, while CO2 lasts in the atmosphere for thousands of years, methane lasts only for about ten years. In fact, if you were to look at the GWP of methane over a 20-year period, rather than 100 years, it would have a GWP of roughly 84. It is released from anaerobic decomposition in landfills, leaks from fossil fuel production, and from animal agriculture.
Nitrous Oxide (N2O) has a GWP of 273 for a 100-year period. It will stay in the atmosphere for over 100 years and absorbs significantly more IR than CO2 does. It is released primarily from fertilizer production and use (both commercial and organic fertilizers) as well as from the burning of fossil fuels.
Chlorofluorocarbons (CFCs) trap waaaay more heat than CO2 does, resulting in them having a GWP in the thousands or tens of thousands. CFCs are now being regulated due to the Montreal Protocol, but were heavily used as refrigerants, solvents, and spray-can propellants.
Another principal greenhouse gas is water vapor. It does serve to absorb and radiate heat down toward Earth, however, because it has such a short residence time, it does not cause any meaningful warming in the context of climate change. In fact, rather than more water vapor causing warming, a warming climate will result in more evaporation, which in turn will increase the concentration of water vapor.
