*Numbers may not add to 100 due to independent rounding
*Data from the U.S. Environmental Protection Agency, released 2020
*Does not include carbon sinks
The transportation sector is the largest producer of greenhouse gas emissions, at 28% of all emissions in 2018. It is clear what the transportation sector encompasses; all the cars (60% of transportation emissions), trucks (23% of transportation emissions), buses, trains (2% of transportation emissions), airplanes (9%), etc. (2% marine vehicles, 4% other).
Instead of coal, vehicles burn gasoline/petroleum. Burning one gallon of gasoline releases 19.6 pounds of carbon dioxide into the atmosphere (8.9 kilograms), and the average car releases 0.4 kilograms of carbon dioxide per mile, and that figure is significantly more with other modes of transportation. Also, the average American burns approximately 500 gallons (1892 liters) of gasoline in a year. In all, that releases 4.85 U.S tons (4.4 metric tons) of carbon dioxide annually, per person.
The transportation sector is difficult to change because people interact with it daily. This is unlike other sectors, where people are not aware of the behind-the-scenes. As people interact with a sector more, they are more resistant to change. It is very difficult to force the change of electric cars on to people when it is still just a relatively emerging sector.
How Does it Work?
All motor vehicles work with the same primary design. They use a combustion engine that powers the vehicle. The engine is loaded with gasoline, which is ignited by the spark plug in an engine. As a result, heat, light, and carbon dioxide is produced. The heat and light powers the car, whereas the carbon dioxide is released as tailpipe emissions into the air.
The electric sector is responsible for 27% of the United State's greenhouse gas emissions in 2018. About 63% of electricity comes from the combustion of fossil fuels, usually coal or natural gas. Coal releases 385 kilograms of carbon dioxide for every megawatt-hour of electricity; that's a lot. In fact, coal supplied 23.5% of the total electrical power in the United States 2019, which means that it released 352,590 megatonnes (352 trillion, 590 billion kilograms) kilograms of carbon dioxide last year. This amount of carbon dioxide is a serious number and majorly contributes to exacerbating the greenhouse effect.
Natural gas is the other fossil fuel that is primarily used for electrical generation. Natural gas is a significantly cleaner fossil fuel (though still potent). It releases 202 kilograms of carbon dioxide per megawatt-hour, which is still a significant amount. In fact, natural gas supplied 38.4% of the total electricity in the United States in 2019, which means that it released 319,564 megatonnes (319 trillion, 564 billion kilograms) of carbon dioxide last year. It is clear that which natural gas is touted by supporters as a "clean fuel," it's total carbon dioxide output is close to that of coal, showing how it really is not the "clean" solution of the future.
Natural gas is primarily methane, meaning that it releases methane into the atmosphere when burned. Methane is a very potent greenhouse gas. It only lives in the atmosphere for about 100 years, but it is over 84 times more potent than carbon dioxide. This means that methane emissions actually accounts for 25% of all global warming. This is an important factor with natural gas that is not often taken into account.
How Does it Work?
Coal is primarily made of carbon (60% to 80% carbon), which means that during oxygen combustion, the carbon bonds with the oxygen atoms and they create carbon dioxide, which is then released into the atmosphere. The byproduct of this reaction is energy, which is then converted into electricity.
Natural gas, as mentioned above, is primarily methane, the most basic alkane hydrocarbon. During combustion, the bonds are broken and the carbon in methane forms bonds with oxygen atoms, to create carbon dioxide.
Industry, accounting for 22% of U.S. greenhouse gas emissions in 2018, includes all the direct industrial emission: manufacturing, processing, mining, construction, etc. The greenhouse gas emissions come from a variety of means, including on-site combustion for off-grid energy and power, fossil fuel-run machines, other uses of fossil fuels, and chemical processes that incur. However, most emissions arise from material processing; the conversion of natural resources into material stock. Iron, steel, and non-metallic mineral production results in 44% of all global industrial emissions. In addition, most manufacturing emissions arise from chemical reactions and fossil fuel combustion. Industrial emissions are a significant section of global and national emissions, yet they are the hardest to mitigate because we have not developed good methods to not use fossil fuels.
How Does it Work?
Industrial emissions, as discussed, come mainly from manufacturing processes and mining. The vast majority of industrial processes rely on fossil fuels to power their machines. They use standard combustion techniques that have been in use since the Industrial Revolution. In addition, mining produces natural gas, methane, as a byproduct during the pressure reduction process of oil, or oil refinement. While some natural gas is captured, a lot is released into the atmosphere, where methane acts as a very potent greenhouse gas. Carbon dioxide is also produced as a result of burning fossil fuels to power machines needed for industrial processes.
The building sector is a combination of commercial and residential emissions and made up 12% of all greenhouse gas emissions in 2018. Residential and commercial buildings use vast quantities of energy to heat/cool, light, etc. Furthermore, as buildings get larger and as energy use increases, then these emissions will also increase; from 1990 to 2015, residential emissions increased by 20.4%, and commercial emissions increased by 7.8%. However, these figures can be misleading because they include indirect emissions, which is the electrical energy needed to power the building sector. Indirect emissions can lead to double counting of emissions, which is something that should be avoided at all costs.
The direct emissions are counted in the 12% figure for 2018, and they include on-site fossil fuel combustion for heating and cooling, such as a boiler or a generator, as well as for hot water and cooking. Many buildings also have refrigerant and air conditioning leakage, which leads to hydrofluorocarbons being released into the atmosphere.
How Does it Work?
The slow turnover of homes in the U.S. leads to outdated infrastructure and heating systems. As a result, many residential buildings still use out-of-date heating oil boilers to heat water and the building itself. Heating oil is a type of crude oil, which is a fossil fuel, meaning that it is primarily made of carbon. When combusted, carbon dioxide is released, just like with any other fossil fuel. In cooking, many people use gas stoves, which use the third fossil fuel, natural gas. Natural gas not only releases carbon dioxide but also methane. Finally, refrigerants and air conditioners leak powerful greenhouse gases, hydrofluorocarbons, into the air.
Agriculture, representing 10% of 2018 greenhouse gas emissions, is the least emitting sector, but still very important. Emissions primarily come from livestock, as flatulence is made up of methane. More than 70 billion animals are raised annually for human consumption. The animal that contributes most to emissions is cattle, with a new study showing that the average American's beef consumption causes the release of 1,984 pounds of carbon dioxide equivalent.
Agricultural practices also contribute heavily to these emissions, though not as much as livestock does. A third of all ice-free land in the world is devoted to livestock, so the subsequent deforesting is another climate change concern. Trees are a carbon sink, which means that they act as a natural barrier to climate change. By taking them away, we are exacerbating the effects of climate change. This is called land-use change and degradation. Fertilizers also release nitrous oxide, another greenhouse gas. Furthermore, tractors typically use far outdated combustion engines that release enormous amounts of carbon dioxide.
How Does it Work?
Livestock flatulence is made up of methane, so the 70 billion animals across the world release a multitude of methane. Agricultural processes are more complicated. Fertilizers help feed plants by putting nitrogen into the soil. In the process of nitrification, ammonia-oxidizing bacteria produce nitrates and nitrous oxide as a byproduct through nitrate leakage. A study by Johnny Soares et al. showed a significant reduction in N2O emissions when nitrification inhibitors were used in the urea (fertilizer). Furthermore, another fertilizer, the polymer and sulfur coated urea (PSCU), showed lower peaks than the traditional urea, yet emission cycles lasted for longer, despite claims that PSCU is more environmentally-friendly.
Denitrification (reducing nitrogen to nitrogen gas) can release nitrous oxide in the process, but it will not be the result. Denitrification converts nitrous oxide into nitrogen gas. Therefore, it is only released as a byproduct. Most of the nitrous oxide comes from the urea fertilizer; 73% of nitrous oxide of all nitrous oxide comes from agriculture. 87% of that (63.5% of the total) comes from fertilizer.
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“Greenhouse Gas Emissions from a Typical Passenger Vehicle.” EPA Office of Transportation and Air Quality, March 2018. EPA-420-F-18-008.
“Sources of Greenhouse Gas Emissions.” EPA. Environmental Protection Agency, April 11, 2020.