What is global warming?

Figure 1: Atmospheric gas composition

Global warming due to increasing concentrations of atmospheric CO2 has been a hot topic discussed by both scientific communities and governments all over the world in the past couple of decades. To understand global warming, one must know the natural mechanism that keeps the planet earth’s temperature at the levels habitable for humans. Our earth’s atmosphere gas composition is below.

Figure 2: Heat Trapping Mechanism (Greenhouse effect)

Majority of the atmosphere is filled with two major gases oxygen and nitrogen (99%). Other gases make up only 1%. Carbon dioxide is only 0.04% yet play a huge role in keeping our planet warm. When sunlight reaches earth some of the energy is absorbed by earth and reflected as infrared radiation to the atmosphere. In the absence of any resistance, these infrared waves will escape back into the space. Oxygen and Nitrogen gases do not interact with infrared waves. They have special affinity to shorter wavelengths (200nm) than infrared rays that travel at longer wave lengths of 700 to 1,000,000 nm. Carbon dioxide tends to interact with wavelengths 2000 – 15,000 nm which overlaps with that of infrared spectrum. When CO2 absorbs infrared energy, it vibrates and emits the energy in all directions. Half of the energy emitted into the space and remaining half returned to earth as heat energy. There are few gases that have same kind of tendency. They are Methane (CH4), Nitrous oxide (N2O), hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs), Sulfur hexafluoride (SF6), Nitrogen trifluoride (NF3), Ozone (O3) and water vapor. These gases are collectively known as greenhouse gases.

Figure 1 indicates that these greenhouse gases are present in an insignificant quantity in the atmosphere, yet they are powerful in trapping heat. Heat trapping mechanism is a normal phenomenon that has been happening for millions of years and has been acting as a mechanism to provide thermal balance to our planet. Without greenhouse gases, Earth would be at -18 degree Celsius (0-degree Fahrenheit). Everyday earth’s natural activities emit CO2 and methane into the atmosphere. The largest sink for methane (CH4) is the troposphere. Hydroxyl radicals react with methane (CH4) and Nitrous oxides and destroy them. We will not be focusing our discussion on methane in this blogpost. Reader is encouraged to find the resources that talk about impact of methane (CH4) elsewhere. There has been plenty of parallel activities to curb methane emissions as well.

Figure 3 – CO2 Concentration Trends in Atmosphere

Most of the CO2 emitted is absorbed by natural carbon sinks such as photosynthesis of plants, creation of soil and peat, direct absorption into the ocean, etc. It is called carbon cycle. Carbon Cycle is nature's way of reusing carbon atoms, which travel from the atmosphere into organisms in the Earth and then back into the atmosphere over and over again [What is the carbon cycle? (noaa.gov)]. Carbon cycle has kept the atmospheric CO2 levels at or below 300 ppm for millions of years. i.e., It may not be an issue if we leave the natural balance as it is. However, due to industrial development, human activities started emitting huge volumes of these gases into the atmosphere, particularly CO2. Due to imbalance of supply and removal process, the amount of carbon dioxide in the atmosphere started increasing every year. The graph (Figure 3) taken from NOAA Climate.gov website shows how much CO2 concentration has increased over the years. The curve is very steep. According to NOAA, the annual rate of increase is 100 times faster than previous natural events such as ice age. In the 1960s, the rate of atmospheric CO2 increase was 0.8 ppm per year. In the 1980s, it became double and remained steady at 1.5 ppm per year in the 1990s. In the 2000’s it increased to 2.0 ppm per year. In the past decade the rate of increase accelerated to 2.4 ppm per year. At the time of writing this article, the CO2 concentration in atmosphere is measured to be ~421 ppm. The high concentration of greenhouse gases trap more heat and cause global warming.