In the most comprehensive collection of greenhouse gas measurements to date, a new report confirms that the most important greenhouse gases are rising drastically. The report states that atmospheric greenhouse gas concentrations are at record levels for the last 800,000 years. But what are the sources of greenhouse gases and what are their environmental impacts?
Greenhouse gases remain in the atmosphere for different periods of time due to differences in rates of decomposition and removal by sinks. Therefore, the radiative forcing of the gas or its capacity to affect that energy balance, thereby contributing to climate change, to varying degrees, with time.
Greenhouse gases enter the atmosphere through natural processes and human activities. The portfolio of natural processes includes above ground sources like animal and plant respiration and underground sources such as volcanic activities and sedimentary basins.
Human sources of greenhouse gas emissions consist of (percent of 2014 greenhouse gas emissions from anthropogenic activities):
• Electricity production (30 percent)
• Transportation (26 percent)
• Industry (21 percent)
• Commercial and Residential (12 percent)
• Agriculture (9 percent)
• Land Use and Forestry (offset of 11 percent) – sinks, the opposite of emissions sources and absorb CO2 from the atmosphere.
Carbon dioxide is also naturally present in the atmosphere as part of the Earth's carbon cycle - the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals. Human activities can alter the carbon cycle—both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like oceans and forests, to remove CO2 from the environment. While debates concerning the impact of elevated CO2 levels on climate change are slowly converging on agreement of CO2 induced global warming, public opinion seems to diverge on whether high levels of atmospheric CO2 are beneficial or harmful to plant life.
It is a fact that plant life depends on atmospheric carbon dioxide, light energy from the sun, water and nutrients to produce oxygen and sugars that builds roots, stems and leaves during photosynthesis. Without carbon dioxide, plants cannot get carbon and therefore, cannot live. Common logic then suggests that, as the level of atmospheric CO2 increases, so would plant growth. But is this true?
In Can Plants Overdose on CO2?,a survey of scientific studies investigating the impact of elevated levels of CO2 on plant growth, the author found agreement between investigators “that higher levels of atmospheric CO2 do increase plant growth when viewed only from the standpoint of CO2.” However, when other factors were considered, several investigations supported the opposing view that excessive amounts of ambient CO2 can adversely influence plant growth. These “secondary” factors – ambient temperature, local precipitation, soil condition, nutrient availability, and microorganism plant interactions – are the observable changes in weather patterns from accelerated atmospheric CO2 concentrations.”
Then where did the assumption originate that anthropogenic activities contribute to the rise in these GHG? Scientific America points out that that the human race “has subsisted for at least 200,000 years on a planet that has oscillated between 170 and 280 ppm of atmospheric CO2, according to records preserved in air bubbles trapped in ice.” Over the last 160 years, CO2 levels have risen above 400 parts per million (ppm). Thus, implying a direct relationship between the Industrial revolution, which began around 1760 to 1840, and increasing levels of atmospheric CO2. This finding is also substantiated by other indirect measures like “tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in Earth’s orbit around the sun,” per the EPA.
The rise of Industrial Revolution coincided with the railroad industry’s reliance on steam locomotives powered by a relatively inexhaustible supply of cheap coal rather than firewood and charcoal which were less efficient and in short supply. Around the same time, coal-fired steam engines made inroads in the maritime industry, as steamboats began to replace barges and flatboats in the transport of goods around the United States. By mid-19th century, petroleum started to make inroads as domestic oil refining provided abundant supplies of kerosene for lighting and heating homes and industrial facilities. Later petroleum made a huge impact on the transportation industry as oil drillers in America learned how to supply an endless stream of low-cost petroleum-based fuels - diesel and gasoline - that fueled the rapid growth of the automotive, shipping and aviation industries.
In late 2010, the U.S. EPA issued a report concluding that fugitive emissions from the natural gas system from wellhead to burner may be far greater than previously thought. These fugitive emissions are therefore a particular concern since it is the major component of natural gas. As such, small leakages are important. Fugitive leakage becomes even more important as natural gas serves a greater role in America’s energy mix. Currently, the EPA exempts the Oil and Gas industry from direct controls of natural gas discharges.
Global Warming Potential Of Greenhouse Gases
To keep a level playing field between greenhouse gases, which have different absorbance and atmospheric lifetimes, the International Panel on Climate Change developed a comparative measurement system called Global Warming Potential (GWP). GWP compares the ability of each greenhouse gas to trap-heat in the atmosphere relative to carbon dioxide, which accounts for about 82 percent of all U.S. greenhouse gas discharges.
Since the heat-trapping ability of a gas is compared to that of CO2, a measure called carbon dioxide equivalents is used to express its GWP. Carbon dioxide equivalents represents an amount of a greenhouse gas whose atmospheric impact has been standardized to that of one-unit mass of CO2, based on the global warming potential of the gas.
For example, if 1 pound of methane is emitted, this can be expressed as 21 pounds of CO2 equivalents by multiplying; 1kg of CH4 by 21, its global warming potential at 100 years, see Figure 10. In other words, one pound of methane has the equivalent heat-trapping ability as 21 pounds of CO2, after 100 years in the atmosphere. Note, there is international disagreement between the EPA and IPCC on the actual GWP of methane; ranging from a low of 21 by the EPA and a high of 34 by the IPCC. Recently, EPA revised the value to 25.
Figure 10: Relationship between the GWP of methane (CH4) and CO2
(Click to enlarge)
Source: US EPA Website
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Therefore, while methane is valuable as a fuel, it is also a greenhouse gas at least 21 times and possibly as much as 32 times more potent than carbon dioxide over a 100-year period, with even greater relative impacts over shorter periods. The meaning of “21 times more potent” relates to methane’s Global Warming Potential (GWP).
Greenhouse gases remain in the atmosphere for different periods of time due to differences in rates of decomposition and removal by different sinks. Figure 11 illustrates the primary greenhouse gases emitted through human activities, their chemical formula, lifetimes and GWP at 20 and 100-year time horizons - a fixed time in the future.
As shown in red, at a time horizon of 20-years, methane has a global warming potential of 72, meaning that over this time period, the emission of 1 kg of methane will have the same climatic impact as the emission of 72 kg of carbon dioxide and about 3 times that at 100-years. In other words, methane is a far more powerful greenhouse gas than carbon dioxide, though it doesn’t last nearly as long in the atmosphere.
Figure 11: GHG Heat-Trapping Potentials Compared to CO2
(Click to enlarge)
1. Carbon lifetime is specified as “variable.” No single lifetime can be defined for CO2 because of the variety of sinks that remove carbon dioxide from the atmosphere at different rates. For instance, between 65 percent and 80 percent of CO2 released into the air dissolves into the ocean over a period of 20–200 years. The rest is removed by slower processes that take up to several hundreds of thousands of years, including chemical weathering and rock formation. This means that once in the atmosphere, carbon dioxide can continue to affect climate for thousands of years.
2. Water vapor is the most powerful greenhouse gas and the largest contributor to the Earth’s greenhouse effect. On average, it probably accounts for about 60 percent of the warming effect. Water vapor’s role in the Earth’s climate system is defined by the very short time it remains in the atmosphere and actively traps heat. While additional CO2 from factories or airplanes can remain in the atmosphere for centuries, extra water vapor will only remain a few days before raining down as water. The concentration of water vapor in the atmosphere is in equilibrium. The atmosphere can only hold more water vapor if overall temperatures increase. So, a small warming effect caused by human CO2 emissions will increase the amount of water vapor in the atmosphere. Any added water vapor leads to even more warming, thus amplifying the CO2 warming effect. Water vapor follows temperature changes, it doesn’t cause or, as climatologists say, ‘force’ them.
Greenhouse Gas Levels 1990-2014
The one area most agree upon in the Greenhouse Gas / Climate Change debate is the concentration of heat-trapping gases in the atmosphere. It’s irrelevant whether one believes climate change is anthropogenic induced or the result of changes in natural rhythm of the earth, the segue behind the Office of Energy Efficiency and Renewable Energy is the need to reduce these emissions, especially CO2, which is at levels not seen for at least 200,000 years. That is of course unless you are a plant. Yes, plant life absorbs CO2 from the environment to produce gaseous oxygen. Plant life is a very important sink for atmospheric CO2.
Regardless of the relationship between human activities and climate change, the question remains – has the DOE been effective in mitigating greenhouse gas emissions? Figure 12 provides a looking glass into the state of U.S. GHG emissions from 1990 to 2014. It illustrates the net emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases for the stated time period in million metric tons of carbon dioxide equivalents. The data represents the most recent greenhouse gas emissions data from the EPA; coming from their 2016 Annual Report, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2014.
Figure 12. - U.S. Greenhouse Gas Emissions by Gas, 1990–2014
(Click to enlarge)
By Barry Stevens for Oilprice.com
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