Greenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. In our solar system, the atmospheres of Venus, Mars and Titan also contain gases that cause greenhouse effects.
Greenhouse gases, mainly water vapor, are essential to helping determine the temperature of the Earth; without them this planet would likely be so cold as to be uninhabitable. Although many factors such as the sun and the water cycle are responsible for the Earth’s weather and energy balance, if all else was held equal and stable the planet’s average temperature should be considerably lower without greenhouse gases.
Human activities have an impact upon the levels of greenhouse gases in the atmosphere, which has other effects upon the system, with their own possible repercussions. The most recent assessment report compiled by the IPCC observed that “changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system”, and concluded that “increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century”.
Types of Major Greenhouse Gases in Earth’s Atmosphere
In order, Earth’s most abundant greenhouse gases are:
- Carbon Dioxide
- Carbon Monoxide
- Nitrous Oxide
- Sulfur Hexafluoride
When these gases are ranked by their contribution to the greenhouse effect, the most important are:
Effects of Greenhouse Gases
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere. As a result, the average surface temperature is higher than it would be if direct heating by solar radiation were the only warming mechanism.
Solar radiation at the high frequencies of visible light passes through the atmosphere to warm the planetary surface, which then emits this energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which in turn re-radiate much of the energy to the surface and lower atmosphere. The mechanism is named after the effect of solar radiation passing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by reducing airflow, isolating the warm air inside the structure so that heat is not lost by convection.
Causes of Greenhouse Gases
The primary greenhouse gases include:
- Carbon dioxide
- Nitrogen Oxides, and
- Fluorinated Gases
Sources of Greenhouse Gases
Many scientists feel one of the main reasons for global warming is the addition of carbon dioxide (CO2) to the atmosphere. Even though CO2 is actually only a small percentage of the overall composition of the atmosphere (about 0.035%), its physical properties make it a strong absorber of infrared energy. The concentration of CO2 has been increasing primarily due to the burning of fossil fuels and deforestation. Current computer models predict that doubling the amount of CO2 in the atmosphere will cause Earth’s average surface temperature to rise between 1 to 3.5 degrees Celsius (2 to 6 degrees Fahrenheit) over the next 100 years. This might not sound like a great increase, but Earth’s climate is very sensitive to even small changes in surface temperature (see Glaciation Cycles).
Solutions of Greenhouse Gases
A global effort, they suggest, to reduce air pollution, especially tropospheric ozone and black carbon (soot), would not only slow global warming but also help unite the interests of developed and developing countries.