Greenhouse Gases and Global Warming
A greenhouse gas (GHG) is a gas that both absorbs and emits radiation in the infrared range. When present in the atmosphere, these gases trap radiation in the form of heat, causing a warming process called the greenhouse effect. The presence of four major greenhouse gases, viz., H2O, CO2, methane CH4 and nitrous oxide N2O in the Earth's atmosphere maintain its temperature at ~15ºC. Without the presence of these greenhouse gases, the average temperature at the earth’s surface would be ~10ºC lower than it is.
H2O in the Atmosphere
Water Vapor is the most abundant greenhouse gas in the atmosphere. As a greenhouse gas, the higher concentration of water vapor is then able to absorb more thermal IR energy radiated from the Earth, thus further warming the atmosphere. As the temperature of the atmosphere rises, more water is evaporated from ground storage (rivers, oceans, reservoirs, soil). Changes in its concentration are also considered to be a result of climate feedbacks related to the warming of the atmosphere rather than a direct result of industrialization. The feedback loop in which water is involved is critically important to projecting future climate change, but as yet is still poorly measured and understood. Because the air is warmer, the absolute humidity can be higher (in essence, the air is able to 'hold' more water when it's warmer), leading to more water vapor in the atmosphere. The warmer atmosphere can then hold more water vapor and so on and so on. This is referred to as a 'positive feedback loop'.
However, huge scientific uncertainty exists in defining the extent and importance of this feedback loop. As water vapor increases in the atmosphere, more of it will eventually also condense into clouds, which are more able to reflect incoming solar radiation (thus allowing less energy to reach the Earth's surface). The future monitoring of atmospheric processes involving water vapor will be critical to fully understand the feedbacks in the climate system leading to global climate change. As yet, though the basics of the hydrological cycle are fairly well understood, we have very little comprehension of the complexity of the feedback loops. While we have good atmospheric measurements of other key greenhouse gases such as carbon dioxide and methane, we have poor measurements of global water vapor.
It is thus uncertain as to how much atmospheric concentration of water vapour has increased in recent decades or centuries. Satellite measurements, combined with balloon data and some in-situ ground measurements indicate generally positive trends in global water vapor content of the atmosphere.
CO2 in the Atmosphere
Carbon dioxide is a greenhouse gas which acts like a blanket in the atmosphere to trap heat. The atmosphere can handle about 700 billion tons of carbon. Atmospheric carbon dioxide levels have been increasing since the industrial revolution. Today the atmosphere contains about 800 billion tons of carbon, and it continues to rise. Much of the carbon dioxide has been added to the atmosphere on account of coal burning since the industrial revolution. How do we know that the burning of carbon-based fossil fuels like coal, oil and natural gas contribute to the atmospheric carbon overload?
Carbon has a unique fingerprint which allows scientists to determine whether the burning of fossil fuel contributes to the atmospheric carbon overload. Carbon is composed of three isotopes which are C12, C13 and C14. C14 is radioactive and constitutes only a small amount. (Half life 5730 yrs). In the upper atmosphere cosmic rays from the Sun react with nitrogen to create C14. C14 is unstable, and over time is converted back to nitrogen. After 60,000 years there is virtually no C14 remaining in the original sample because it has been completely converted to nitrogen. Fossil fuels comprise coal, oil or natural gas and over time these reservoirs are buried deep in the ocean floor or underground.
The carbon atoms found in both the atmosphere and initially in fossil fuel contain all three carbon isotopes. After 60,000 years fossil fuels contain only C12 (all C14 has been converted to nitrogen) but the atmosphere still maintains a healthy mixture of the three isotopes. Since it takes millions of years to create fossil fuel, the carbon dioxide that is released into the atmosphere from the burning of fossil fuel would have no C14. If the burning of carbon-based fossil releases carbon dioxide into the atmosphere, the amount of C14 isotope found in atmospheric carbon dioxide should decrease over time. Measurements of the isotopic composition of atmospheric carbon dioxide do indeed demonstrate a steady decline of C14. Furthermore, fossil fuels also contain a much lower amount of C13 than does the atmosphere. Over time the amount of C13 found in atmospheric carbon dioxide has decreased. Clearly, the atmosphere’s carbon isotopic composition is changing and this change matches the isotope fingerprint of coal, oil and natural gas. This demonstrates that the burning of fossil fuels is partly responsible for the current atmospheric carbon overload.
Methane is the primary component of natural gas – a common fuel source. Methane in the Earth's atmosphere is a strong greenhouse gas with a global warming potential (GWP) 104 times greater than CO2 in a 20-year time frame. Methane is not as persistent a gas as CO2 and tails off to about GWP of 28 for a 100-year time frame. This means that a methane emission will have 28 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect but for a relatively brief period, having an estimated lifetime of 8.9±0.6 years in the atmosphere, whereas carbon dioxide has a small effect for a long period, having an estimated lifetime of over 100 years.
If methane is allowed to leak into the air before being used—from a leaky pipe, for instance – it absorbs the sun’s heat, warming the atmosphere. For this reason, it’s considered a greenhouse gas, like carbon dioxide. The concentration of methane in Earth's atmosphere has increased by about 150% since 1750. It now accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. According to NOAA, in 2013, the atmospheric methane concentration was measured at 1890 ppb in Ireland (Northern Hemisphere) and 1760 ppb (Tasmania, Southern Hemisphere). This implies that arctic methane release from melting methane clathrates has played a role. Other recent emissions include peat fires in Southeast Asia. In the 420,000 year time-period before the industrial era, the atmospheric methane concentration is known to have been less than half the current level, including the period since the Last Glacial Maximum.
While methane doesn’t linger as long in the atmosphere as carbon dioxide, it is initially far more devastating to the climate because of how effectively it absorbs heat.
In the first decade after its release, methane is 84 times more potent than carbon dioxide. Both these must be addressed to effectively reduce the impact of climate change. It is estimated that about 25% of the manmade global warming today is caused by methane emissions.
Methane in the atmosphere can come from many sources, both natural and manmade. But the largest source of industrial emissions is the oil and gas industry. Sources of Methane in the Atmosphere include:
Nitrous Oxide N2O
Nitrous oxide is emitted due to use of nitrogenous fertilizers and industrial activities, as well as during combustion of fossil fuels and solid waste. Important natural sources include soils under natural vegetation and the oceans. Natural sources create 62% of total emissions. Important human sources come from agriculture, fossil fuel combustion and industrial processes. Human-related sources are responsible for 38% of total emissions. The storage and handling of livestock manure is another direct source of agricultural nitrous oxide emissions. Emissions of nitrous oxide from rivers, estuaries and coastal waters are generally considered as human-caused because most of the reactive nitrogen entering these ecosystems is associated with agricultural activities.
Fossil fuel combustion and industrial processes are an important source of nitrous oxide emissions. These two combined are responsible for 10% of human emissions which equals 700,000 tonnes of nitrous oxide per year. A substantial amount of nitrous oxide is caused by biomass burning, which accounts for 10% of human-caused emissions.
As with animal waste, human waste is a significant source of nitrous oxide emissions. Bacteria break down the nitrogen-based organic materials found in human waste (urea, ammonia, and proteins). This creates 200,000 tonnes of nitrous oxide per year.
Although relatively small amounts are released, it has a high GWP (310 times that of carbon dioxide).
Nitrous oxide also damages the ozone layer, thus reducing the protection offered from harmful UV sun rays. At normal environmental concentrations, nitrous oxide is not harmful to humans. Nitrogen is removed from the atmosphere by plants and converted into forms such as ammonia, which can then be used by the plants. Since the Industrial Revolution, the level of nitrous oxide in the atmosphere has increased by 16%. The lifetime of nitrous oxide in the atmosphere is 114 years, and a GWP of 289 over a 20 year period.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)