The greenhouse effect is a process that traps the incoming thermal radiation from the sun in the atmosphere of the earth. It is needed to keep the earth at a habitable temperature. Some gasses like carbon dioxide affect the intensity of the greenhouse effect. With rising carbon dioxide levels the greenhouse effect becomes stronger leading to global warming. To prevent a snowball effect of global warming, we need to reach net zero emissions of greenhouse gasses as soon as possible.
The greenhouse effect is a natural process that keeps the earth at a habitable temperature. Without the greenhouse effect earth’s average temperature would be at -18 °C (0 °F) instead of the 14°C (59°F) we experience today. We rely on the greenhouse effect, yet the temperature on earth is rapidly increasing. It has always fluctuated, but for some decades there has been a clear upward trend in addition to natural fluctuations, as the graph shows.
Since 1880, the average global temperature has risen by about 1 °C. If global warming reaches a certain limit, a vicious cycle will occur, making it worse and worse. Therefore, the Paris Agreement set the goal to limit global warming to well below 2, preferably to 1.5 degrees Celsius, compared to pre-industrial levels.
To understand the reason for global warming, we first need to take a closer look at how the greenhouse effect works.
The sun radiates short-wave sunlight onto the earth. This is surrounded by the atmosphere, which consists largely of nitrogen and oxygen. About 30% of the incident sunlight (radiant solar energy that hits the earth's surface) is reflected back into space. The rest heats the atmosphere and the earth's surface. The warmed earth surface re-radiates energy with a longer wavelength. This radiation is absorbed by gasses in the atmosphere, causing the temperature on Earth to rise.
Not all of the gasses in the atmosphere cause the temperature to rise. Nitrogen and oxygen, for example, are transparent to infrared light. The gasses that do not have this property (and therefore contribute to global warming) are called greenhouse gasses (GHGs).
Symmetrical molecules with only two atoms are not greenhouse gasses, because their molecular structure doesn’t allow them to absorb thermal radiation. Examples of these gasses are nitrogen (N2) and oxygen (O2). Gasses that do not fit this description are called greenhouse gasses.
Carbon dioxide (CO2)
Nitrous oxide (N2O)
Sulfur hexafluoride (SF6)
Nitrogen trifluoride (NF3)
Water vapor is the most abundant greenhouse gas. Even though it causes up to 85% of the greenhouse effect, it is often not listed as a GHG because its concentration in the atmosphere is not a cause of human activity.
Relative danger vs absolute danger
Some greenhouse gasses are more dangerous than others when comparing equal amounts of different gasses. This is referred to as the Global Warming Potential (GWP). It can be calculated for different time horizons, e.g. 20 years, 100 years or 500 years. The GWP for different time horizons varies because different GHGs have different lifetimes. The table below shows the global warming potential for some greenhouse gasses.
Carbon dioxide shows a 100-years potential of 1. To be able to compare different compounds, the global warming potential is shown in CO2e (carbon dioxide equivalents). The carbon dioxide equivalent is the mass of CO2 that would warm the earth as much as the mass of that gas.
From the table, one could conclude that CO2 is the "least dangerous" of the greenhouse gasses shown. Relatively speaking, this is true, but not in absolute terms. In absolute terms, the amount of the respective gas in the atmosphere must also be taken into account.
If we also look at the atmospheric levels of the gasses, carbon dioxide is the greenhouse gas with the greatest impact. Currently the CO2 level is at 419 ppm (October 2022), but it is steadily rising as the graph below shows.
Aerosols are small solid or liquid particles contained in the atmosphere. They can be of natural origin, for example dust, volcanic ash, vapor or sea salt or they can be human made like agricultural dust, vehicular exhaust, industrial emissions and soot from combustion processes.
Aerosols reflect incoming shortwave radiation so they reduce the heating of the earth. At the same time aerosols affect the weather on the earth. Water vapor condenses on aerosols and forms droplets. More aerosols in the atmosphere lead to more, smaller droplets (vs less, larger droplets with fewer aerosols in the air). Since the droplets are smaller, they might not reach the critical size they need to fall as rain therefore preventing rain. Since the droplets live longer without raining down, the size of the clouds get larger and larger. When it finally rains, it rains much stronger, causing floods.
The Ozone Hole
Besides being a GHG in the lower part of the atmosphere, higher up in the stratosphere, the Ozone layer absorbs solar ultraviolet radiation. The ultraviolet (UV) radiation is known to cause cancer. Therefore Ozone plays an important role in protecting us. In the past the increasing usage of chlorofluorocarbons (CFCs) has damaged the Ozone layer.
In 1987, the Montreal Protocol was adopted to protect the ozone layer. The Montreal Protocol regulated the usage of chemicals that deplete the ozone layer. Today, the Ozone hole still exists but it is slowly healing.
Knowing that the reason for global warming is the increase in greenhouse gasses in the atmosphere, one can also deduce what needs to be done to achieve the Paris Agreement goal (to limit global warming to well below 2 degrees Celsius): Emissions of GHGs must go to "net zero" or, better yet, "net negative." This means that emissions must first be reduced as much as possible. All emissions that cannot currently be reduced must be removed from the atmosphere in some other way, so that the concentration of GHGs does not continue to rise.