3
Most of the Earth’s carbon is stored in rocks. The rest is in the ocean, the atmosphere, plants, soil and fossil fuels. Carbon flows between reservoirs in an exchange called the carbon cycle. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in other reservoirs. Changes that put more carbon gas into the atmosphere result in warmer temperatures for the Earth.
Riebeek, 2011; NASA
Terrestrial carbon-climate feedbacks depend on two large opposing fluxes – soil organic matter decomposition and photosynthesis – that are tightly regulated by nutrients.
Riley et al., 2018
Climate change due to carbon dioxide emissions is irreversible.
Solomon et al., 2009
3.1Introduction
Reradiative or greenhouse gases were reviewed in Chapter 2. Elevated carbon dioxide is considered to be the most persistent cause of global warming and climate change, having greatly exceeded the capacity of natural terrestrial and ocean sinks. Carbon dioxide is the bridge or link between the land and ocean sinks (Box 3.1). Previously these sinks functioned to keep carbon dioxide in the air at levels that did not result in appreciable global warming. Carbon moves in the environment in slow, intermediate, and relatively fast cycles. Understanding the nature of these cycles will enhance understanding of the important role of carbon and carbon dioxide in the environment.
Box 3.1Carbon Storage and Cycling
|
Land |
In biomass and soil organic carbon |
|
Atmosphere |
Carbon dioxide |
|
Oceans |
Primarily dissolved organic carbon |
Both land and ocean carbon stocks exchange carbon as carbon dioxide with the atmosphere.
Mackey et al., 2013
Carbon dioxide is essential for photosynthesis by plants. It has been proposed that future elevated levels of carbon dioxide are likely to increase photosynthesis and plant growth and increase the sink capacity for carbon dioxide for land vegetation. Several factors, however, also affect the efficiency and effectiveness of photosynthesis. (These will be considered in more detail in Chapter 4.) Current levels of periodic atmospheric ozone can cause plant injury and reduce photosynthesis. Ozone can be taken up into plants via leaf stomata at the same time that carbon dioxide enters. Ozone levels have also been projected to increase in the future. This has implications for the effectiveness of photosynthesis and for ozone’s role as a powerful reradiative or greenhouse gas.
More carbon is stored in soils, especially organic forest soils, than in all forms of terrestrial vegetation combined. Carbon dioxide, methane, and nitrous oxide are released from microbial decomposition of organic matter in and on soil surfaces. How warming temperatures affect the rate of emission of carbon dioxide and other gases is of considerable concern regarding present and future global warming.
3.2Carbon
Carbon (atomic number 6) is the fourth most abundant element by mass. Each atom has four available electrons for covalent bond formation. Carbon atoms are the building blocks for formation of a wide variety of large organic molecules, ranging from long chains and carbohydrates to DNA. Carbon is an element found in and essential to all life forms. Most is stored in rocks, ocean sediments, stones, and fossil carbon in soil. The rest cycles between the terrestrial biosphere, the atmosphere, and the oceans. The terrestrial biosphere (mainly forests) and the oceans absorb, store, and release carbon dioxide (Riebeek, 2011; Royal Society of Chemistry). There is a summary for carbon storage and cycling in Box 3.1.
3.2.1The Slow Carbon Cycle: Chemical Weathering and Carbonate
Chemical weathering of rocks in or near streams is a key component of the slow global carbon cycle. It affects the extent of uptake of carbon dioxide from the atmosphere by the oceans and storage as calcium carbonate. The process that occurs very slowly over thousands to millions of years is called the slow carbon cycle. Carbon dioxide in the atmosphere interacts with water in rain to form carbonic acid. Carbonic acid reacts with rocks and releases calcium, magnesium, potassium, and sodium ions that are transported to the oceans by rivers. Calcium ions combine with bicarbonate ions in ocean water to form calcium carbonate, which sinks to the ocean floor. Shellfish, corals and other organisms build shells with calcium carbonate. These also sink to the ocean floor when the organisms die and combine with calcium carbonate to form limestone (Riebeek, 2011). There is concern that the rate of rock weathering and calcium release may not keep up with increasing amounts of carbon dioxide in the air, and this might slow down carbonate and bicarbonate formation (Falkowski et al., 2000).
In prehistoric times, organic matter from dead plants began to accumulate faster than it could completely decay. Plant remains often fell into wetlands. Over time, accumulating layers resulted in increased heat and pressure, and formation of huge deposits of peat, coal, oil, and methane. These have found use as fuel sources and are known as fossil fuels. They are the source of additional carbon dioxide and other gases driving global warming and climate change. The carbon dioxide released from fossil fuel combustion is new carbon dioxide and the amount exceeds normal carbon sink capacities (Prentice et al., 2012).
Volcanoes are also part of the slow carbon cycle. Instead of capturing carbon dioxide from air and storing in an unavailable form, volcanoes emit carbon dioxide when they erupt. Carbon dioxide is released below ground when rock melts and released during volcanic eruptions (Riebeek, 2011). Volcanoes also emit huge quantities of ash, gases, and sulfate and other aerosols that affect air quality and visibility. These may go circumglobal and increase light reflection back to space.
3.2.2The Intermediate Cycle: Oceans
The ocean covers the largest part of the Earth’s surface. As it is divided into different regions by latitude and warmth and other characteristics, the term “oceans” will be used here. Ocean water surfaces are in continuous contact with atmospheric carbon dioxide. Prior to the Industrial Revolution, the oceans exchanged carbon dioxide with the atmosphere in a way that balanced the cations received from rock weathering. A small amount of carbon dioxide was released. This balance has changed in favor of more uptake of carbon dioxide from the atmosphere than carbon dioxide released (Riebeek, 2011). Carbon dioxide is taken up more readily in colder ocean regions and released more readily in warm ocean regions (Prentice et al., 2012). The oceans are the largest of the three carbon reservoirs and contain the most stored carbon. They are estimated to take up 30–40% of the anthropogenic carbon dioxide from fossil fuel combustion, deforestation, urbanization, and land-use change. Model calculations published in 2012 indicate that net carbon dioxide uptake by the oceans has increased during the last 50 years (Ballantyne et al., 2012).
Carbon dioxide enters the ocean’s surface by diffusion when the concentration of carbon dioxide above the surface is higher than in the water. Carbon dioxide can remain as a dissolved gas (CO2aq) or react with water to form carbonic acid, H2CO3. Carbonic acid dissociates and reforms releasing bicarbonate ions (HCO3–) and a hydrogen ion (H+). The hydrogen ions can lower the pH of the seawater, which can affect the availability of carbonate for shell-building organisms.
Growing near the surface of the oceans are numerous common photosynthetic plant and plant-like organisms called phytoplankton. They are the basis for the food web, beginning the chain from zooplankton to fish to organisms as high as whales. The end of the chain is death and deposition in the deep oceans. Phytoplankton use carbon dioxide for photosynthesis, and this reduces carbon dioxide in the ocean which results in more carbon dioxide diffusion from the atmosphere for use in photosynthesis. The amount of soluble carbon dioxide in water is greatly influenced by the extent of phytoplankton. Photosynthesis by phytoplankton accounts for most of the removal of carbon dioxide from the atmosphere to the oceans.
Atmospheric carbon dioxide is regulated by surface exchanges with ocean carbon dioxide. The ability of ocean water to dissolve carbon dioxide from the air decreases as temperature increases, so the concentration of carbon dioxide in water is determined by water temperature. Carbon dioxide from the atmosphere is maintained in a thin surface layer of water regulated by wind-driven turbulence. Carbon flux between oceans and the atmosphere depends on surface mixing and wind speed and on the carbon dioxide gradient between water and air (Falkowski et al., 2000).
3.2.3The Faster Carbon Cycle: Atmosphere, Forests, and Soils
Carbon moves faster through the terrestrial ecosystem, both in and out of organisms, mostly trees and forests, and is stored in organic matter, primarily wood and soils. While uptake of carbon dioxide by photosynthesis and carbon storage is less than in the oceans, it is an essential part of atmospheric carbon management.
3.2.3.1Carbon Dioxide in the Atmosphere
Before the Industrial Revolution, there was a balance between carbon dioxide uptake from the atmosphere and release that maintained carbon dioxide at approximately 280 ppm. It was the major contributor to the essential natural greenhouse effect that allowed life on Earth as we know it. Since that time, an ever-expanding human population has used increasing quantities of fossil fuels to generate energy for heating and cooling, transportation, and industrial use. Deforestation and land-use change have reduced carbon sink capacity. Carbon dioxide has increased in the atmosphere from the pre-industrial levels of approximately 280 ppm to 400 ppm and above. This global average concentration is predicted to remain and possibly increase slowly (Mann et al., 2016). Industrial growth (65%), fossil fuels (17%), and reduced sinks (18%) have been identified as reasons for the increased rate of carbon dioxide (Canadell et al., 2007). El Nino events have also contributed to record high carbon dioxide levels (Betts et al., 2016).
Carbon dioxide greater than 280 ppm is anthropogenic in origin. Anthropogenic carbon dioxide exceeds the capacity of the oceans and the terrestrial biosphere to absorb and store all of it. Approximately 40% of anthropogenic carbon dioxide does not find sink storage. This carbon dioxide will remain in the atmosphere for 1000 years or more (Schimel, 2007; Archer et al., 2009; Solomon et al., 2009; Ryan et al., 2010; Riebeek, 2011). It is the major cause of the global warming and climate change that we are experiencing today.
Keeling, at the Mauna Loa observatory in Hawaii, determined that carbon dioxide levels in the Northern Hemisphere were lower in summer than in winter. He attributed the summer decrease in carbon dioxide to uptake by seasonal vegetation. These seasonal variations have been documented since 1960 (Keeling et al., 1996; Graven et al., 2013). Using long-term data from the Mauna Loa observatory (1958–2015), Curran and Curran (2016a, 2016b) documented that carbon dioxide concentrations in the Northern Hemisphere typically decrease (drop) during a four-month period from May to September, coinciding with plant growth and carbon sequestration. Carbon dioxide concentrations begin to increase in November and remain higher than in summer until early spring of the following year. The magnitude of the intra-annual drop cycle was examined and calculated over a 57-year period, using weekly data from Mauna Loa. Curran and Curran determined that the magnitude of the summer carbon dioxide concentration began to drop rapidly in the 1960s to 1970s, reaching a peak in 2006, with small increases since then. They concluded that if the early rates of drop in carbon dioxide had continued to 2013, the summer drop in carbon dioxide would have been 8.9 ppm. The value that they determined for 2013 was 7.5 ppm. If less carbon dioxide is being taken up by vegetation in the summer, then perhaps the terrestrial carbon sink is becoming saturated. This would mean more carbon dioxide in the atmosphere in the summer, which would be a positive forcer for global warming.
3.2.3.2Carbon Movement in Trees and Soils
Carbon dioxide is taken up by trees through photosynthesis and used for growth and carbon storage. Trees and forests are the major long-term sinks for carbon storage. Forests are estimated to absorb as much as 30% of anthropogenic carbon dioxide (Luo et al., 2015). Carbon is allocated to stems, roots, branches, leaves, and seeds. It is stored in stems, branches, and roots. Small branches, leaves, and seeds are transient and become part of litter on the forest floor. Carbon is returned as carbon dioxide through respiration by shoots and roots and by microbial respiration from breakdown of litter and soil organic matter. It has been estimated that the carbon dioxide taken up in photosynthesis in a year is nearly balanced by the amount returned to the atmosphere by respiration (Griffin and Prager, 2017). The small margin between photosynthesis and respiration is stored carbon. Heterotrophic respiration from microbes decomposing litter on the forest floor releases carbon dioxide. Forest fires release stored carbon (Falkowski et al., 2000).
Many factors, such as elevated carbon dioxide, light, temperature, available soil water, soil nutrients, and ozone, affect photosynthesis and respiration. With warming global temperatures, the impacts of elevated carbon dioxide, temperature, and water availability become important. Water vapor from stomata during photosynthesis cools air and affects low clouds. Models predict that elevated carbon dioxide can close stomata and increase radiative forcing owing to reduction in low cloud formation (Doutriaux-Boucher et al., 2009). Increased temperatures increase photosynthesis and respiration globally, while availability may be more important locally (Jung et al., 2017). There are indications of adaptation of respiration rates to higher temperatures (Griffin and Prager, 2017). Higher temperatures may stimulate and accelerate carbon dioxide release from litter by heterotrophic microbial respiration (Falkowski et al., 2000).
3.2.3.3Carbon Dioxide and Tree Growth
It has been extensively demonstrated under experimental conditions that elevated concentrations of carbon dioxide stimulate growth and biomass of trees. Data from satellite surveillance and results from models also indicate that extensive increases in global vegetative gross (GPP) and net (NPP) primary productivity, or “global greening,” occurred during the twentieth century (Mao et al., 2016; Sun et al., 2017; Yakir, 2017). This is also known as “global carbon dioxide fertilization” (Friedlingstein et al., 1995; Pan et al., 2011). This trend is also consistent with other observations (Ballantyne et al., 2012; Smith et al., 2015; Campbell et al., 2017). McMahon et al. (2010) presented empirical evidence that forest growth was increasing. They obtained data on tree biomass increase from 55 temperate forest plots, with known use histories and conditions, over a 22-year period. They concluded that recent biomass increase was greater than what was expected to have occurred by natural recovery from past events in the forest plots. Increased growth was attributed to the effects of utilization of increasing carbon dioxide in photosynthesis. Others have suggested that it may be due to increased water-use efficiency. Incidence of deforestation, drought, insect infestations, and fires did not appear to affect estimations of global increased GPP and NPP.
3.2.3.4Forests as Carbon Sinks
European forests are considered to be strong sinks for carbon dioxide. There are indications, however, that forest growth may be slowing, and that this could reduce carbon sink strength. Nabuurs et al. (2013) reported significant declines in the rates of increase of tree net stem volume from 2005 to 2010 for all European forests. Associated with this, land-use change and deforestation are increasing, resulting in decreased uptake of carbon dioxide. Storms, fires, and other disturbances also contribute to tree losses and reduced uptake of carbon dioxide. Taken together, they clearly indicate that the European forest biomass is in danger of becoming saturated and less effective as a carbon sink. Reichstein et al. (2013) raise concerns that extreme events, such as fire, drought, storms, and high temperatures, reduce the strength of the forest carbon sink. Neumann et al. (2017) conclude that warmer summers and changes in precipitation may be responsible for increased tree mortality in Europe.
3.2.3.5Soil Carbon Sink
Soils are the largest reservoirs of organic carbon in plant and animal remains, and major sources of carbon dioxide (Jackson et al., 2017). The carbon dioxide released from root and microbial respiration from soils is estimated to be 60 billion tonnes per year. It is estimated that this amount exceeds that from fossil fuel combustion and is nine times the total carbon dioxide emitted from all combined human inputs (Wang et al., 2013; Giardina et al., 2014; van Groenigen et al., 2014; Carey et al., 2016). Rising air temperatures and increased carbon dioxide promote shoot/biomass, larger root systems, greater respiration, and increased organic matter for decomposition (van Groenigen et al., 2014). Roots exude organic compounds that accelerate decomposition and respiration. This process of accelerated decomposition has been called “priming” (Sulman et al., 2014; Carey et al., 2016). Decomposition is favored by well-drained and aerated mineral soils and by temperatures above 25 °C (Davidson and Janssens, 2006; Carey et al., 2016). This has been confirmed by long-term study of emissions from warmed and non-warmed soil plots in forests. Warmer temperatures stimulate microbial activity which increases breakdown of organic matter and release of carbon dioxide and methane (Mellilo et al., 2017). Carey et al. (2016) reviewed 27 soil warming studies in temperate areas, finding that soil respiration increased in all areas up to 25 °C and decreased after that. The effect was more pronounced in colder areas. Warmer temperatures in a tropical rainforest in Panama caused increased tree growth and litterfall. Additional litter stimulated microbial activity and release of soil carbon. Sayer et al. (2011) conclude from their findings that much of the carbon taken up by tropical tree growth could be lost because of stimulation of soil microbial activity by increased litterfall. They estimate that the amount of carbon lost from soil could be greater than the increase in tree biomass due to warming and higher levels of carbon dioxide. Deforestation also leads to soil carbon loss. This process is accelerating rapidly in tropical areas where original forests are burned and replaced by plantations of trees grown in response to demand for food and fuels by people in developed and rapidly developing countries. Native trees and shrubs are replaced by plantations of avocado, banana, cacao, citrus, macadamia, oil palm, and rubber trees. Van Straaten et al. (2015) reported that oil palm, rubber, and cacao plantations in Cameroon, Indonesia, and Peru resulted in up to 50% losses of soil carbon through emissions of carbon dioxide and soil leaching.
Permafrost soils in the Arctic contain large amounts of frozen organic matter. Rising temperatures there are accelerating thawing and exposure of the organic matter to decomposition and microbial decomposition, releasing carbon dioxide and methane (Schadel et al., 2016).
Recognizing the importance of understanding and estimating microbe-mediated carbon dioxide release from organic matter in soils, a new carbon cycling model (the MEND model) was developed at Oak Ridge National Laboratory. By modeling rates of soil microbial decomposition in relation to temperature, estimates of carbon dioxide emissions were determined.
3.3Lakes and Rivers
There are more than a million lakes in the world, plus additional water reservoirs. Microbial activity in organic sediments releases carbon dioxide. Weyhenmeyer et al. (2015) included carbon dioxide from surrounding land that moves via streams to lakes, especially in Sweden. They estimated that small lakes in Sweden, surrounded by agricultural land, emit more carbon dioxide than a similar lake in a forest area. Carbon dioxide emissions from lakes and reservoirs are estimated to be as much as 25% of the carbon dioxide emitted from fossil fuel combustion.
The rivers of the world accumulate large quantities of organic matter from terrestrial and aquatic vegetation, and, as we have said, microbial action releases large quantities of carbon dioxide. The Amazon river and tributaries form one of the most important river systems in the world. Sawakuchi et al. (2017) have extensively determined carbon dioxide concentrations and release in the lower Amazon river, which drains 13% of the total Amazon basin. This area is influenced by tides which result in wind and wave action and turbulence that increase as the river reaches the ocean. This in turn increases carbon dioxide release from the river. The surface area of the river expands as it widens, resulting in increased emission. Carbon dioxide emissions from this region of the river are considerably higher than those measured in the central basin of the river. Carbon dioxide emissions from the Amazon river system may closely match the amount of carbon dioxide taken up by tropical trees in the region. If the Amazon basin is net carbon-neutral, this has profound implications for expectations that the Amazon is or will be a major sink for anthropogenic carbon dioxide.
3.4Cities and the Global Carbon Cycle
The 2017 United Nations estimate of the world’s human population size is 7.6 billion. The UN further estimates that 183 million new humans are added daily, with most growth occurring in India and China and in developing countries such as Nigeria. Half of the world’s population growth is projected to occur in Africa: 40 African countries could double the size of their populations by 2050 (Friedman, 2016; United Nations, 2017). The population size in Nigeria could increase from number seven to number three in the world, which would make it larger in population size than the USA (Friedman, 2016). Every single human has requirements and activities that have profound effects on the global carbon cycle. As population grows, these effects will increase. The population factor has received little attention in studying global warming and climate change.
Human populations require food, water, energy, and shelter. Governments have concluded that, since cities use only 2% of global land area, they are the most efficient places to provide these requirements. The current world estimate of world population living in cities is 54.4%. This is estimated to reach 60% by 2030 (Churkina, 2016; United Nations, 2017). In Western Europe, 80% of the population lives in cities. Consolidated urban areas of as many as 30 million people exist in China.
Cities require large amounts of energy to make them function. Electricity for lighting, heating, and cooling comes from fossil-fueled power plants adjacent to the cities. Fossil fuels, such as coal, gasoline, methane, and fuel oil, and in some places biofuels such as wood, are burned for transportation and heating. The products of this incomplete combustion include carbon dioxide, oxides of nitrogen, small hydrocarbons, and much more. The irregular structure of cities makes it difficult to accurately determine or estimate urban-scale fluxes and emissions of carbon dioxide. Hutyra et al. (2014) point out that this has discouraged detailed investigations. Velasco and Roth (2010) proposed that the eddy covariance method (EC) can be used to directly measure carbon dioxide fluxes in cities and suburban areas. They summarized results from 30 EC systems, mostly in mid-latitude cities. The largest fluxes of carbon dioxide occurred in city centers, decreasing with distance to the suburbs. They concluded that cities are a net source of carbon dioxide and attributed this to emissions from vehicles and heating buildings. Cities are considered to be the main sources of carbon dioxide from energy source combustion that accumulates in the atmosphere as anthropogenic carbon dioxide (Velasco and Roth, 2010; Lauvaux et al., 2013; Hutyra et al., 2014; Churkina, 2016).
Because of their nature, structure, and low albedo, cities experience heat island effects that increase temperatures, especially at night. As global warming increases, and cities increase in size, the demand for air conditioning in cities for cooling will expand rapidly in tropical and mid-latitude areas. This is particularly true in Asia where middle-class lifestyles are emerging, resulting in increased energy use (Friedman, 2016). Abel et al. (2017) found that increased use of air conditioners during hot weather meant that fossil-fueled power plants have to increase fuel to generate more electricity, releasing more carbon dioxide, oxides of nitrogen, and sulfur dioxide. Using data from 27 states, they found that power plants averaged an increased release of 3.32% per degree C in temperature. They estimated that increased air temperatures correlated with an increase of 140,000 metric tonnes of carbon dioxide.
3.4.1Urban Forestry and Carbon Dioxide
There is increasing interest in preserving woodland fragments and parklands in cities to reduce carbon dioxide release by city centers. Large campaigns to plant street trees are in force in many cities and towns. In addition to enhancing life quality, it is hoped that additional trees will increase uptake of carbon dioxide and help to slow global warming and climate change. Slogans like “Plant a Tree: Save the Planet” resonate well and promote tree planting. New York City and Los Angeles have begun million-tree planting programs. Even Amherst Massachusetts, USA (population 33,000), has begun to plant 2000 new trees. The purpose is to get people involved in global warming and climate change, and to be more aware of their environment. While there are indications of significant carbon dioxide uptake by trees in temperate climate cities, there is no empirical evidence that urban trees effectively reduce large amounts of carbon dioxide or other air pollutants (Unger, 2014). Nowak and Crane (2002) modeled carbon dioxide sequestration by urban trees and concluded that the shading of buildings to reduce the need for electricity generation (and thus carbon dioxide emissions from power plants burning fossil fuels) was the principal benefit. Any benefit from carbon dioxide sequestration by urban trees was offset by the use of power equipment to prune and maintain them. Tang et al. (2016) concluded that total carbon sequestration by Beijing’s street trees was equivalent to approximately 0.2% of the carbon dioxide emitted from generation and use of energy. Velasco et al. (2016) investigated the role of evergreen (sub)tropical trees and soil (the biogenic component) in carbon sequestration in parts of Mexico City and Singapore. Their assessment included tree species and the amount of open soil suitable for respiration. They concluded that the biogenic component in the area assessed in Mexico City acted as a sink for carbon dioxide (+1.4%) and in Singapore as a source of carbon dioxide (−4.4%). By including trees and soil respiration in their analysis, they were able to obtain a more realistic assessment of the role of urban vegetation as a sink or source for carbon dioxide. The value of urban trees to significantly reduce urban carbon dioxide remains in doubt.
3.5Human Influence on Carbon Dioxide: Energy Sources and Emissions
Human influence on the carbon cycle began with combustion, when humans learned how to start and control fires, releasing carbon dioxide back to the atmosphere. As populations grew, release of carbon dioxide from deforestation, developing human sites, and agriculture began to affect the capacity of the carbon sinks. Industrialization and growing urban development required the use of wood and coal as fuels, greatly increasing release of carbon dioxide to the atmosphere. The discovery of oil and natural gas, and their use along with coal as fuels as replacement for wood, began the process whereby the amount of carbon dioxide emitted by combustion of these fossil fuels began to exceed the capacity of the land and ocean sinks to absorb it. Le Quéré et al. (2016) estimate that emissions from fossil fuel combustion became the dominant source of carbon dioxide to the atmosphere around 1920.
Continued anthropogenic influence through deforestation, agriculture, and fossil fuel combustion has resulted in a disruption of the balanced land/ocean air sinks that existed at approximately 280 ppm carbon dioxide at the beginning of the Industrial Revolution to an increase to 400+ ppm carbon dioxide today (NASA, 2019). The 40% of anthropogenic carbon dioxide remaining in the atmosphere will remain there for 1000 years or more (Archer et al., 2009).
Humans require large amounts of energy for transportation, heating/cooling, agriculture, cooking, and industry. With the exception of the growing solar panel and windfarm energy programs, this energy comes from combustion of some kind of fuel containing carbon. Carbon dioxide emissions for different fuels by pounds of carbon dioxide emitted per million British thermal units (Btu) of energy are listed in Box 3.2. Methane is the most efficient fuel in terms of energy content to carbon dioxide released. Emissions from methane are 2:1 water vapor to carbon dioxide, both strong radiative forcers (US Energy Information Administration, 2017a, 2017b).
Box 3.2CO2 Emissions of Fuels in Pounds of CO2 per Million Btus of Energy
All common fuels contain carbon and release CO2.
|
Coal: |
|
|
anthracite |
228.6 |
|
bituminous |
205.7 |
|
lignite |
215.4 |
|
sub-bituminous |
214.3 |
|
Diesel and heating oil |
161.3 |
|
Gasoline (no ethanol) |
157.2 |
|
Propane |
139.0 |
|
Natural gas methane |
117.0 |
Methane has the lowest CO2 to energy content
Source: US Energy Information Administration 2017a
3.5.1Gasoline and Diesel
Estimates for 2016 indicate that in the United States, gasoline-powered vehicles emitted approximately 1,102 million metric tonnes of carbon dioxide, and diesel-powered vehicles emitted 437 million metric tonnes of carbon dioxide, for a total of about 1,540 million metric tonnes of carbon dioxide. This was equal to 30% of total carbon dioxide emissions from energy generation. Addition of 10% ethanol to gasoline and 20% biodiesel to diesel can reduce emissions (US Energy Information Administration, 2017a, 2017b).
3.5.2Wood
Wood is often a readily available non-hydrocarbon inexpensive fuel for home heating. People have been burning wood for heating and cooking for thousands of years. In addition to release of stored carbon dioxide, other elements in the wood, or those that form during the combustion process, are released to the atmosphere in smoke. These species can cause serious human health problems. McDonald et al. (2000) identified more than 350 elements or compounds emitted from wood burning, including fine particulates. Cooper (1980) identified compounds that were pollutants, carcinogens, hydrocarbons, aldehydes, carbon monoxide and carbon dioxide, and many more.
3.5.3Global Waste Burning
The generation and disposal of waste or rubbish from human activities is generally managed effectively in developed countries. In large areas of the world, however, rubbish is openly burned, releasing all the products of incomplete combustion, including carbon dioxide. Wiedinmyer (2014) calculated that carbon dioxide emitted from open burning approximated 5% of global anthropogenic carbon emitted in 2010, and concluded that emissions of carbon dioxide and other gases from rubbish burning are not included in estimates of emission rates by countries.
3.5.4Food and Household Products
Human activity and metabolism cause carbon dioxide release into the atmosphere (Prairie and Duarte, 2007). On any day, people can visit a local food store and observe an enormous variety of fruits and vegetables, meat and meat products, dry and canned goods, and household products. They can also find large sections devoted to pet foods. In 2015, there were 163 million pet dogs and cats, compared to 321 million people, in the United States (Okin, 2017). Everything in the stores is brought there by fossil-fueled trucks and airplanes, many from distances as great as several thousand miles. The amount of anthropogenic carbon dioxide emitted during transport is part of the human carbon budget.
Humans and their pets consume food containing carbon (agricultural carbon) directly fixed recently by plants and indirectly by animals. Food is metabolized and carbon dioxide is released when oxygen is inspired. Excreta also are decomposed by microorganisms, releasing more carbon dioxide and methane. It is generally concluded that the amount of carbon taken in by humans and their pets is roughly balanced by the amount released, resulting in carbon-neutrality (West et al., 2009).
Humans have very strong dependencies on animals for food. Prairie and Duarte (2007) estimated that in 2007 the world’s population of domestic animals was three times the size of the human population (possibly 15 billion domestic animals). This has grown considerably since then and will continue to grow as population increases. Most of the food for domestic animals is plant-based, and the carbon fixed from the air in growing the plants to feed the animals is returned when the animals respire. Considerable quantities of methane are released directly by cattle and their excreta. It should also be noted that more land will have to be cleared (deforested) to provide agricultural land to feed ever-growing populations of domestic animals and humans. This will require ever-larger quantities of chemical fertilizers. Electricity from power plants is required in the formulation of fertilizers. Land clearing and chemical fertilizer use could change the carbon budget for humans from carbon-neutral to carbon source. With increased land clearance, electricity use for fertilizer production, and carbon dioxide emissions during food and materials transport, humans could be considered as carbon sources rather than being carbon-neutral.
3.5.5Wood Biofuels
There is considerable interest in moving away from oil and coal to biofuels such as crop residues and wood. Wood has been used since the beginning of humanity as a fuel. Thinning forests, using logging wastes, or using plantations of trees to produce biofuels has been proposed to be carbon-neutral as forest trees will grow back and replace any that have been removed. If forest trees are used, however, the time for replacement with new comparable large trees will be very long. Hudiburg et al. (2011) investigated the possible effects of tree removals for biofuels from 80 forest types in Oregon, Washington, and California and found that projected tree removal and use as biofuel would release as much as 14% more carbon dioxide than that released by current forest management practices.
Wibe (2012) examined wood fuels obtained from wood residues from logging in Sweden. He concluded that burning logging residues released carbon dioxide and decreased nutrient levels in the forest to support forest regeneration. He proposed that time was a key element in determining carbon-neutrality for wood biofuels. It was possible in the very long term only. He concluded that burning wood from logging residues would result in emission of approximately 60% of the carbon dioxide emitted if oil had been burned to produce the same amount of energy.
3.5.6Wood Pellets
Small milled wood pellets can be fashioned from waste wood from logging. These wood pellets were initially thought to be very useful clean or green fuel as an alternative to use of fossil fuels like coal, oil, and gas. Homeowners with wood stoves appreciated the portability, ease of use, and low cost of the pellets.
Wood pellet use expanded greatly, and the amount of waste wood available from logging was no longer sufficient for expanded production. Wood source expansion focused on using whole softwood and hardwood trees and waste wood from logging on a very large scale. Drouin (2015) has provided a review of the nature and implications of the expansion of the use of wood pellets. Enormous wood-pellet mills have developed in the Southeastern United States. Wood pellets became the new green energy fuel. In addition to using wood pellets for heating, they also have been used on a very wide scale for electricity generation, primarily in the UK and other parts of Europe. Millions of tonnes of wood pellets, from numerous enormous American pellet mills, are shipped each year to the UK and other parts of Europe, and the amounts are expected to keep growing. The European Union considers wood pellets as a renewable carbon-neutral fuel (Cornwall, 2017). To be carbon-neutral, the carbon dioxide released from burning wood pellets into the atmosphere must be recaptured by regenerated forests. Drouin (2015) poses the question: “Wood pellets: green energy or new sources of CO2 emissions?” Wood pellets are an energy source, but it is not clear how “green” they are. Wood pellets are a new source of carbon dioxide. How much carbon dioxide is released from the wood is unknown and the relationship between its recapture by different types of regenerating forests is also unknown. Given the phenomenal growth of the wood pellet industry, these questions will probably not be answered.
3.5.7Forest Fragments and Edge Effects
Deforestation and land-use change for plant and animal agriculture are increasing in the world’s tropical areas. Some areas are clear cut, while others are selectively harvested, resulting in numerous forest fragments or islands. Using satellite data, Brinck et al. (2017) estimate that there are 50 million fragments in the tropics, and that the length of the edges of these fragments is 50 million kilometers. Trees in the edges of these forest fragments experience more decline and mortality. Together with the effects of land-use change, this results in significant release of carbon dioxide into the atmosphere.
3.6Deforestation and Land-Use Change
Tree harvesting so that land can be used for agriculture and other purposes removes not only tree-stored carbon, but also the utility of the harvested trees to take up carbon dioxide via photosynthesis, and results in increased soil carbon release. The long-term loss of forest carbon sinks due to deforestation is estimated to account for 10–15% of the increase in carbon dioxide in the atmosphere (Mahowald et al., 2016). Baccini et al. (2017) acknowledge that tropical forests can be either sinks or moderate sources of carbon. Land-use and land-cover changes release carbon, while intact forests are concluded to be effective carbon sinks with carbon release and sequestration approximately equal in magnitude. General agreement is lacking about whether tropical forests are net sinks or sources of carbon dioxide. To address this, Baccini et al. (2017) analyzed MODIS (Moderate Resolution Imaging Spectroradiometer) pantropical satellite data from 2003 to 2014 across tropical forests in America, Africa, and Asia. Their purpose was to obtain direct measurement of net annual changes in above-ground density of live trees in tropical forests. They reported that the world’s tropical forests are a net carbon source of 425.2 ± 92.0 teragrams of carbon per year (Tg C yr−1). Carbon gains from forest growth were 436.5 ± 31.0 Tg C yr−1. Carbon losses from deforestation, land-use change, degradation and disturbances in forests were 861.7 ± 80.2 Tg C yr−1. Land-use changes and deforestation are weakening the carbon sink strength of tropical forests, changing them overall from net sinks to net sources. They concluded that tropical forest carbon gains were lower than carbon losses on all three continents.
3.6.1Global Carbon Budget 2016
Le Quéré et al. (2016) have published a Global Carbon Budget for 2016. Seeking input from 68 contributors, their purpose was to determine input of carbon dioxide to the atmosphere by emissions caused by humans and land-use changes, to determine the rate of growth of carbon dioxide in the atmosphere and how this might affect land and ocean sinks. Over time, a better understanding of trends in the carbon cycle is necessary to determine sink strength response to continuing increases of carbon dioxide in the atmosphere.
3.7The Photochemical Oxidant Cycle
3.7.1Ozone
There is considerable confusion in popular media about the nature, formation, and effects of ozone. Ozone is a colorless unstable normal constituent gas (relative molecular mass 47.9) formed secondarily in photochemical cycles involving sunlight or sunlight plus nitrogen oxides and small volatile organic compounds (VOCs) in the troposphere (8–12 km above the surface of the Earth) and the stratosphere (the zone above the troposphere). Because ozone episodes in the troposphere are often accompanied by visible haze and humidity, they are often termed “smog” events. The term smog, however, was coined in England and elsewhere to mean a highly visible toxic combination of smoke from coal fires with fog (Krupa and Manning, 1988). The term is not relevant for ozone episodes and is misleading. Also confusing is the role of stratospheric ozone holes in tropospheric air pollution. The size of ozone holes controls the amount of ultraviolet radiation in the polar regions.
3.7.2Ozone Formation
3.7.2.1Stratospheric Ozone
Ozone in the stratosphere is basically inert. It absorbs ultraviolet radiation and prevents much of it from entering the troposphere, making life on Earth possible (NASA). It does become a factor in the size of polar ozone holes and with occasional stratospheric dip downs. The formation cycle for stratospheric ozone formation is shown in Box 3.3.
Box 3.3Stratospheric Ozone Formation
Incoming ultraviolet radiation in sunlight begins the cycle:
O2 + sunlight → O + O
O2 + O → O3
Oxone is unstable and breaks down and reforms quickly. This results in a continuous protective layer for ultraviolet radiation, preventing much of it from entering the troposphere.
3.7.2.2Tropospheric Ozone
Ozone formation is different in the troposphere. The process begins with sunlight and warm air temperatures. Sunlight reacts with nitrogen dioxide (NO2) rather than oxygen to release an oxygen atom used in O3 formation.
Ozone in the troposphere is formed in the photochemical oxidant cycle. The chemistry is complex and involves several precursor molecules and reactions. Ozone can be formed and then broken down in a catalytic cycle that does not allow for accumulation in the atmosphere beyond normal background. To be an effective radiative forcer, ozone must accumulate beyond normal background levels. Small hydrocarbons from combustion and trees (principally isoprene) interrupt the catalytic cycle and allow ozone to accumulate as a radiative forcer and an air pollutant. A simplified version (Cleveland and Graedel, 1979; Brimblecombe, 1986; Krupa and Manning, 1988; Jacobson, 2002) of background ozone formation in a catalytic cycle and a non-catalytic cycle is shown in Boxes 3.4 and 3.5, respectively.
Box 3.4Tropospheric Ozone Formation
Catalytic Cycle
Incoming ultraviolet radiation in sunlight warms air and starts the cycle that begins with photolysis of nitrogen dioxide NO2:
NO2 + UV → NO + O
O2 + O → O3
NO + O3 → NO2 + O2
Ozone does not accumulate beyond background levels of nitrogen dioxide. This is normal background ozone.
Box 3.5Ozone Accumulation Cycle
Non-catalytic Cycle
Ozone forms in the same way as in the catalytic cycle, except here ozone increases beyond background and may be an air pollutant.
Combustion increases NO2 and peroxyradicals (RO2) form from small hydrocarbons. Isoprene is emitted from tree leaves. Both can react with some NO, allowing some O3 to avoid back-reacting with NO and breaking down. The ozone can then accumulate beyond background O3.
NO2 + RO2 → NO2 + RO
3.7.2.3Background Ozone
Since the Industrial Revolution, the concentration of ozone that occurs in the troposphere during warm weather has increased from an estimated 10 ppb by volume to measured average concentrations ranging from 20 to 40 ppb, sometimes as high as 50 ppb depending on location. These average concentrations are determined from data obtained from remote locations to avoid interference by anthropogenic influence as much as possible. There is some indication, however, that some of these concentrations are influenced by ozone and its precursors in long-range wind transport from Asia, principally China (Fiore et al., 2002; Wittig et al., 2008). Ozone concentrations in the range 20–40 ppb are normal background levels and there is general agreement that ozone is not an air pollutant at these concentrations.
3.7.2.4Ozone as an Air Pollutant
Rapid urbanization and industrial development in the world after World War II resulted in large increases of nitrogen oxides and volatile organic compounds in the troposphere, primarily from fossil fuel combustion from home heating/cooling, industries, and vehicles. Ozone levels began to increase beyond background concentrations. Ozone episodes, with peak concentrations as high as 180–300 ppb, were recorded for recurring periods, ranging from a few days to a few weeks, depending on local climate conditions. These exposures caused breathing and lung injury in humans and visible (acute) injury in plant leaves, with growth and yield reductions. Ozone was considered to be an air pollutant when it caused injury to humans and plants at concentrations above background levels.
Governments in developed countries slowly realized that ozone levels were too high and were affecting human health and plant growth and productivity. Air quality standards were developed, and air quality monitoring networks were established to determine if air quality standards for ozone were being accomplished and if not, how they should be revised. In order to set the standards, quantitative data on human and plant responses in response to ozone exposure were needed. The US EPA has conducted inhalation studies with humans to determine ozone effects on breathing. Most of the data for human response to ozone exposure come from huge multi-year epidemiological studies, mostly in cities. Plants are considered to be more sensitive than humans to lower ozone exposures. Hundreds of papers on ozone effects on plants, mostly under experimental conditions, have been published (Ashmore, 2005). There is a more extensive empirical database for ozone injury to plants than there is for humans.
Reducing pollution levels of ozone will affect the incidence of adverse effects on humans and plants. Reducing ozone pollution effects on plants will also reduce the positive forcing effect of ozone on global warming and climate change. To this end, governments have established air quality standards. In the United States, standards are reviewed every five years. The Clean Air Scientific Advisory Committee (CASAC) of the US Environmental Protection Agency (EPA), consisting of medical researchers and ecological and plant scientists, evaluates new data since the last evaluation to advise the EPA Administrator about whether the standards are effective in protecting humans and plants or need revision. Based on their findings and advice, EPA promulgated new revised levels of both the primary ozone standard (human health effects) and the secondary standard (ecosystem and plant effects) in 2015. They set both standards at 0.070 parts per million (70 ppb). Violations of the standard will be determined the fourth-highest daily maximum, averaged across three consecutive years and averaging times of 8 hours (US Environmental Protection Agency, 2015). Establishing a new protective air quality standard for ozone would seem to be a straightforward and appropriate activity. Like most air quality standards, the new standard was based on consensus and compromise. Reaction to the new standard, however, was highly controversial. Legal challenges from manufacturers, the trucking industry, power plant operators, and others ensued. These were designed to delay, change, or reject the standard. Some politicians and economists saw adverse effects on jobs and the economy. Some environmentalists wanted a lower standard. The 2015 ozone standard is in place until 2020, when it will be reviewed again. It would seem, then, that scientific evidence can be used as the basis to determine when ozone is an air pollutant, but this is compromised and may be changed when science meets politics and economics.
3.8An Ozone Exposure Index for European Forests
The European Union has adopted an accumulated exposure index that considers ozone exposure above a threshold concentration of 40 ppb ozone (AOT40 exposure index). The basis for the index is that forest trees will be exposed to ozone only during daylight hours during a growing season from April to September, with the assumption that stomata will be open for ozone uptake then. At the end of the growing season, the product of all measured concentrations time above the threshold of 40 ppb are summed and expressed as ppb or ppm cumulative hours. This index has been widely adopted by other countries. In areas where ozone levels are low and are near or just above ambient background levels (as high or higher than 40 ppb), this index is not useful (United Kingdom Forestry Commission).
3.9Ozone as an Indirect Radiative Forcer
In addition to being a direct radiative forcer, it is becoming increasingly evident that ozone may be more important as an indirect radiative forcer through negative effects on photosynthesis and plant growth that reduce carbon dioxide removal from the atmosphere (Ashmore, 2005). As mentioned in Chapter 2, ozone effects on plants could result in more global warming through reduction in uptake of carbon dioxide than through direct radiative forcing caused by ozone alone (Collins et al., 2010; Unger, 2012). Increasing ozone is affecting photosynthesis and carbon assimilation in boreal and temperate forests, and this is reducing release of water vapor to the atmosphere, which could increase warming (Wittig, et al., 2008; Lombardozzi et al., 2015). With current and predicted rising ozone levels, this is of particular concern (Sitch et al., 2007). Jacob and Winner (2009) predict that surface ozone in polluted areas will increase by 1–10 ppb in the near future owing to the effects of climate change, with the largest increases occurring in urban areas.
3.10Summary
Consideration of all aspects of carbon movement in the environment allows determination of sources, sinks, and consequences when sink capacities are exceeded by anthropogenic carbon dioxide. Sources not traditionally considered include rivers, lakes, cities, food production, and urban forests. Soils contain the largest reservoirs, and release of carbon dioxide by microbial action increases atmospheric carbon dioxide significantly. Carbon dioxide is removed from the air, used in photosynthesis, and some is sequestered in wood by trees. Temperature affects the balance between carbon gain in photosynthesis and loss in respiration.
Fossil fuel combustion has been the major influence on air quality since 1920. Fuel types vary considerably in energy output in relation to emission of carbon dioxide and other air pollutants. Natural gas (methane) is often considered to be the “clean fuel”, as the products of methane combustion are CO2 and H2O in a 1:2 ratio. Both are reradiative greenhouse gases. There has been an enormous growth of the use of milled wood pellets for heating in the United States and for electricity generation in Europe. This has resulted in extensive deforestation and a large increase in emissions of carbon dioxide. Wood pellets are considered by some to be a carbon-neutral fuel, but this is controversial, and the controversy needs resolution soon, as does the amount of carbon dioxide emitted by burning wood pellets.
Carbon dioxide, oxides of nitrogen, small hydrocarbons (VOCs), and other gases and particulates are emitted by combustion of fossil fuels. Nitrogen dioxide (NO2) in the presence of sunlight begins the photochemical oxidant cycle that produces ozone (O3). VOCs can prevent enough of the breakdown of ozone to nitrogen dioxide and oxygen that ozone can then accumulate to pollutant levels. Ozone at pollutant levels can enter plant leaves via stomata with carbon dioxide during gas exchange. Ozone can then damage internal plant tissues and may also reduce plant growth and carbon storage. Ozone levels are predicted to increase in the future.
Chapter 4 will consider the role of biogeochemical factors such as photosynthesis and biogenic hydrocarbons on the function of forests. The role of biogeophysical factors, such as albedo, evapotranspiration and hydrology, and of ozone will also be considered. The interactive roles of biogeochemical and biogeophysical factors determine the function of trees and forests and how they affect cooling and warming the atmosphere.
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