Earth enjoys mild temperatures because of its atmosphere which is the thin layer of gases that surround and protect the planet. However, 97 percent of climate scientists agree upon the fact that human activities have changed the Earth's atmosphere drastically over the past two centuries, resulting in global warming. To understand global warming, it is necessary to understand greenhouse gases and their effects upon the planet.
Gases that trap heat energy in the atmosphere are called greenhouse gases. Each of the gas's effect on climate change depends on three main factors:
- Concentration, or abundance, the amount of a particular gas in the air.
- The amount of time these greenhouse gases are present in the earth’s atmosphere.
- Some gases are more effective in making the planet warmer and "thickening the Earth's blanket”. For each greenhouse gas, a Global Warming Potential (GWP) is calculated to reflect its effect on the atmosphere. Gases showing higher GWP are known to absorb more energy, per pound, than gases with a lower GWP.
Some of the greenhouse gases that contribute to the greenhouse effect are discussed below.
- Carbon dioxide (CO2): CO2 enters the earth’s atmosphere through the burning of solid waste, trees, other biological materials and fossil fuels (coal, natural gas, and oil). Carbon dioxide is removed from air (or "sequestered") by plants as part of the biological carbon cycle.
- Methane (CH4): CH4 is released into the atmosphere during the production and transport of coal, natural gas, and oil. Methane gas is also emitted from the decay of organic waste in municipal solid waste landfills.
- Nitrous oxide (N2O): N2O is released into the atmosphere during agricultural and industrial activities, burning of fossil fuels and solid waste, as well as during wastewater treatment.
- Fluorinated gases: Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic and powerful greenhouse gases. These are released into the atmosphere from a variety of industrial processes. These gases are referred to as High Global Warming Potential gases ("High GWP gases") because even in smaller quantities, they contribute immensely to global warming.
Figure 1. Biological mechanisms to the greenhouse effect. Image Credit: clinton5.nara.gov
The Greenhouse Effect
Climate change is a complex study and is even more challenging to monitor. The sun emits solar radiation in the visible and near visible parts of the spectrum, which passes through the Earth’s atmosphere. Solar radiation that is at a frequency similar to visible light is responsible for warming the Earth’s surface. Radiations in the form of Gamma rays, X-rays, and ultraviolet rays (all with a wavelength frequency of less than 200 nm) are absorbed by oxygen and nitrogen, a reaction that generates heat, thus warming up the planetary surface. Ozone (O3), which occupies the stratosphere absorbs solar radiation with a wavelength between 200 to 300 nm. CO2, water vapor, and ozone molecules are collectively responsible for absorbing solar radiation that is 700 nm or greater in wavelength.
Only half of the solar radiation penetrating the Earth’s atmosphere is absorbed. The atmosphere reflects some of the solar radiation back into space; whereas part of the solar radiation is absorbed by greenhouse gases (GHG) that is then reemitted as energy to the lower atmosphere (Figure1). As the Earth cools in the dark, the heat begins to escape; however, due to the abundance of GHGs in the atmosphere, this heat becomes trapped.
Figure 2. (a) Global annual emissions of anthropogenic GHGs from 1970 to 2004. (b) Share of different anthropogenic GHGs in total emissions in 2004 in terms of carbon dioxide equivalents (CO2-eq). (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO2-eq. (Forestry includes deforestation.) (IPCC Fourth Assessment Report, Climate Change 2007 (AR4), Synthesis report [chapter 2: Figure 2-1])
Figure 3. Graphical interpretation of Global anthropogenic GHG emission levels from the period of 1970 to 2004. Global CO2 emissions for 1940 to 2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100 (left-hand panel); and the corresponding relationship between the stabilisation target and the likely equilibrium global average temperature increase above pre-industrial (right hand panel). Approaching equilibrium can take several centuries, especially for scenarios with higher levels of stabilisation. Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The right-hand panel shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). Black dashed lines in the left panel give the emissions range of recent baseline scenarios published since the SRES (2000). Emissions ranges of the stabilisation scenarios comprise CO2-only and multi gas scenarios and correspond to the 10th to 90th percentile of the full scenario distribution. Note: CO2 emissions in most models do not include emissions from decay of above ground biomass that remains after logging and deforestation, and from peat fires and drained peat soils. (IPCC Fourth Assessment Report, Climate Change 2007 (AR4), Synthesis report [chapter 2: Figure 5-1])
Natural efforts whereby plants absorb CO2 from the atmosphere to maintain the carbon cycle in the ecosystem may to some degree help lower the CO2 levels in the atmosphere. The IPCC Fourth Assessment Report, Climate Change 2007, suggests that by stabilizing the GHG emission in the atmosphere, global temperature increase should start to slow down as demonstrated in figure 3.
Infrared Gas Analyser
An infrared gas analyzer (sensor) is used to measure the quantities of various gases. The basic principle of an infrared gas analyzer involves two chambers (one chamber being the reference chamber and the other chamber allowing for the measurement of the type of gas and quantity). Infrared light of a particular frequency is emitted from one end of the chamber through to a series of gas chambers that contain given concentrations of different gases. As the photons from the infrared source pass through the different gas chambers, they excite symmetric and asymmetric vibrations in the gas molecules. The detector, being the end chamber to this sensor, is responsible for converting the amount of infrared radiation absorbed by the gas into a voltage (e.g., the signal from the detector [end chamber] will change in response to varying levels of CO2 in a given sample) (see figure 4).
Figure 4. Infrared greenhouse gas analyzer. Image Credit: esrl.noaa.gov
An infrared gas analyzer measures the amount of different gases (greenhouse and others) present in the atmosphere. Different molecules in the air absorb different frequencies of light. Gases are determined by the amount of a particular frequency of light absorbed when it is passed through them. Presently, GHG sensors focus on satellite technology using light detectors and ranging methods to estimate gas emission levels in the atmosphere and have the advantage of monitoring emission levels over longer periods of time.
Sources and Further Reading
This article was updated on 13th February, 2020.