2
[N]on-condensing greenhouse gases provide the temperature environment that is necessary for water vapor and cloud feedback effects to operate. …Because carbon dioxide accounts for 80% of the non-condensing GHG forcing in the current climate atmosphere, atmospheric carbon dioxide therefore qualifies as the principal control knob that governs the temperature of the Earth.
Lacis et al., 2010, commenting on Schmidt et al., 2010
2.1Introduction
The nature of global warming and climate change was introduced in Chapter 1. This chapter considers in detail the greenhouse effect and the reactive gases that cause it.
The sun plays the key role in the energy balance of the Earth. The proximity of the sun to the Earth makes life on Earth possible. Shortwave and ultraviolet energy from the sun enters the atmosphere during the day; the visible part is known as sunlight. Not all of it reaches the surface of the Earth. Clouds can reflect incoming radiation back to space. Suspended aerosols and reflection (albedo) from light-colored surfaces such as ice and snow are also effective in reflecting incoming radiation: as much as 50% may be reflected, and even more on cloudy days. Beginning during the day and continuing into the night, some of the incoming radiation absorbed by the Earth returns to space in the form of long wave infrared radiation, but some is trapped down below low clouds and suspended aerosols. Atmospheric gases, such as water vapor, carbon dioxide, methane, nitrous oxide, ozone, and synthetic chloro- and fluorocarbon gases, also absorb and emit infrared radiation. Retention of infrared radiation increases the air and surface temperature of the Earth. This is the natural essential greenhouse effect. Without it, the Earth would be too cold at night to support life (Jacobson, 2002; Karl and Trenberth, 2003; Prentice et al., 2012).
Since the Industrial Revolution, the balance in the Earth’s energy budget between incoming and outgoing radiant energy has been changing slowly, resulting in an increase in the Earth’s temperature which has become known as global warming, or the enhanced or unnatural greenhouse effect.
2.2The Greenhouse Effect: Day and Night Temperatures
Daily minimum temperatures are increasing 40% faster than daily maximum temperatures (Peng et al., 2013). Asymmetric warming is causing night temperatures to increase more rapidly than daytime temperatures. The effects of higher temperatures during the day will provide less relief from higher temperatures at night, with important implications for human health and wellbeing. Davy et al. (2016) have provided a rationale for why night temperatures are increasing more rapidly. The height of the boundary layer – the lowest layer of the atmosphere – above the Earth’s surface is very high during the day (a few kilometers) and quite low at night (a few hundred meters). Using carbon dioxide as the major radiative forcer, it is proposed that only a smaller volume of air at night needs to be warmed by carbon dioxide effects on infrared radiation retention, and hence the nighttime temperatures are higher.
Peng et al. (2013) considered how differences in daytime and nighttime temperatures might affect photosynthesis and respiration by plants and microorganisms. They speculated that warmer nights would not affect photosynthesis, but could increase respiration, and carbon dioxide release, by plants and microorganisms. Effects of this on vegetation and climate warming are largely unknown.
2.3Commonly Used Descriptive Terms
Several terms are used to describe and explain the atmosphere and the implications of changes in a warming atmosphere. These terms are often used interchangeably, especially in common usage, and this results in confusion and possible misinformation. This is especially true for global warming and climate change. NASA has provided some useful definitions and explanations.
2.3.1The Atmosphere
The air in the atmosphere contains a mixture of gases and aerosols (particles) and clouds. The gases include 78% nitrogen, 21% oxygen, and 1% other gases such as argon, carbon dioxide, and variable water vapor. There are five regions or layers in the atmosphere from the Earth’s surface to outer space, based on temperature changes. We live in the troposphere that extends from the surface of the Earth to about 8 to 16 km into space, being greatest at the equator. This is a zone of turbulence, and weather happens here. The next layer is the stratosphere, where a calm layer of ozone protects us from excessive ultraviolet radiation in sunlight (Vallier-Talbot, 1996).
2.3.2Weather and Climate
“Weather” includes atmospheric conditions and events that occur locally or daily or over very short periods of time. Events include wind, temperature, rain, humidity, storms, and clouds, reported and predicted by the media in weather forecasts. Weather events can change quickly and unpredictably. “Climate” includes monthly and long-term regional or global averages of temperature and precipitation over a season, years, decades, or centuries. Humidity, wind speed, and wind direction are also included. Integrating this information over time allows determination of climate for an area (Whitaker, 1996).
2.3.3Global Warming
This is a specific term that refers to the upward temperature trend across the Earth, often considered to have begun in the 1750s when the Industrial Revolution resulted in release of fossil fuel emissions, primarily from the combustion of coal. It became increasingly evident in the 1980s when aerosol air pollution began to decline.
2.3.4Climate Change
“Climate change” is a collective term that refers to a broad range of enhanced changes in environmental and biological systems and events. Sea level rise, loss of ice mass in the Arctic and glaciers in mountain regions, and increased severe weather events are examples of climate change. Warming atmospheric temperatures drive all these changes and thus are included in climate change. Global warming causes climate change; climate change and global warming are not interchangeable terms.
2.4Greenhouse Gases: Relative Importance
Global warming continues because of increasing concentrations of a variety of radiative and reactive gases (Solomon et al., 2010). Water vapor is the most effective in absorbing and radiating infrared radiation. Ozone is formed in the atmosphere. Water vapor and ozone do not occur in persistent concentrations, but carbon dioxide, methane, nitrous oxide, and synthetic chloro- and fluorocarbon gases do exist in persistent concentrations in the atmosphere.
Carbon dioxide is the major persistent contributor to global warming (65%) (Stocker et al., 2013). The warming impacts of other radiative gases can be determined by calculating how much they would contribute to global warming compared with carbon dioxide. The gases differ from each other in how long they persist in the atmosphere and in their radiative efficiency in absorbing infrared thermal energy. Comparisons can be made between them by using “global warming potentials” (GWPs) (American Chemical Society 2012c; US EPA 2017).
2.4.1Global Warming Potentials
Characteristics that determine a gas molecule’s GWP include:
1.Wavelength absorption window: infrared radiation returning from the Earth is in the thermal infrared region.
2.Radiative efficiency: how much energy is absorbed. The more absorbed, the more effective the molecule.
3.Lifetime: how long the molecule persists in the atmosphere. Effectiveness increases with persistence.
GWPs estimate how much energy a known quantity of a radiative gas would add to atmospheric warming compared with the same quantity of carbon dioxide, often on a 100-year timescale (American Chemical Society, 2012c; US EPA, 2017). GWPs for a 100-year scale include the following.
· Carbon dioxide: has been assigned a GWP of 1.00 and is used as the reference gas. It has not been assigned a lifetime. Half of anthropogenic carbon dioxide emissions may be gone in 100 years, but the rest may persist for thousands of years.
· Methane: GWP of 28–36. More effective than carbon dioxide, but lifetime is only about 10–15 years, although concentrations are increasing.
· Nitrous oxide: GWP of 265–298. Lifetime of more than 100 years in the atmosphere.
· Chloro- and fluorocarbons: High-GWP gases. More effective than carbon dioxide. Lifetimes range from 1,400 to 22,800 years.
· Ozone and water vapor: GWPs are not calculated, as they are both variable in concentration and short-lived. As global temperatures increase, so does the amount of water vapor in the air from evaporation and sublimation, and its potential to warm the air. This adds to the effects of other more persistent radiative forcers (American Chemical Society, 2012c). Episodes of ozone at concentrations high enough to act as a radiative forcer occur in warm weather. Lifetimes of these concentrations are often short-lived. Ozone also enhances the effects of other radiative gases.
2.5Water Vapor: A Condensable Gas
Water vapor is a naturally occurring, variable concentration, gaseous form of water (H2O). As a positive radiative forcer, it is the major contributor to the Earth’s energy flow and the natural greenhouse effect that allows life on Earth to occur (American Chemical Society, 2012b; Chung et al., 2014). Water vapor influences transfer of latent heat from evaporation and vapor condensation, and affects incoming and outgoing shortwave and longwave radiation via clouds (Allan, 2012).
Sources include evaporation of water from bodies of water, transpiration from plants, and sublimation from ice and snow. Two of the products from burning fossil fuels are carbon dioxide (CO2) and water vapor, with ratios for CO2 and water vapor for gasoline, fuel oil, and natural gas (methane CH4) of 1:1, 1:1, and 1:2, respectively. Compared with coal, fuel oil, and gasoline, natural gas is considered to be the “clean fuel”, as complete combustion results in release of only carbon dioxide and water vapor. Both, however, are strong radiative forcers. Twice as much water vapor as carbon dioxide is released. Water vapor is much more effective in absorbing outgoing infrared radiation than carbon dioxide. Unlike carbon dioxide, water vapor concentrations are variable and short-lived.
While humans can have some local influence on water vapor, temperature controls water vapor on a global basis. This affects evaporation and precipitation. A warming atmosphere increases water vapor and increases its interactions with other positive radiative forcers (American Chemical Society, 2012a, 2012b, 2012c; Chung et al., 2014).
2.5.1Clouds
Water occurs in the atmosphere in several forms or states. Water vapor is invisible, but when combined with temperature, we can feel it as relative humidity. Water is visible as ice, snow, rain, and clouds. All forms of water are involved in climate change. Clouds are particularly important as they can be either positive or negative radiative forcers. Warm air containing water vapor rises, and the vapor condenses to water droplets or ice particles, making clouds visible. As much as 70% of the Earth’s surface may be covered with clouds at any point in time (US National Science Foundation).
A warmer atmosphere could result in more clouds. By their nature, some of them could be positive radiative forcers, increasing the amount of returning infrared radiation trapped beneath them, or they could be negative radiative forcers, reflecting incoming shortwave radiation from the sun. Elevated concentrations of carbon dioxide can cause plant stomata to close, reducing transpiration and photosynthesis (and thus carbon dioxide uptake). The reduction in transpiration output can cause a reduction in low cloud, resulting in warming (Field et al., 1995; Doutriaux-Boucher et al., 2009). Furthermore, biogenic volatile organic compounds (BVOCs), emitted by trees, play a key role in cloud formation. Extensive coverage of BVOCs and their function is found in Chapters 4 and 7.
The three most important types of clouds are stratus, cirrus, and cumulus. How effectively they function depends on their thickness and heights (US National Science Foundation).
2.5.1.1Stratus Clouds
These are low clouds that are widespread and consisting of ice crystals. As they are low-hanging and gray in color, they can reflect incoming shortwave radiation from the sun. This can cause a cooling effect below, so they are negative radiative forcers.
2.5.1.2Cirrus Clouds
These are high-elevation, white, detached clouds consisting of ice crystals that allow considerable passage of sunlight to the Earth. Effective in trapping outgoing infrared radiation, they may function as positive radiative forcers enhancing atmospheric warming.
2.5.1.3Cumulus Clouds
These are high-elevation, large, dense clouds shaped like irregular mounds or towers that are white when exposed to sunlight, but with darker horizontal bases. These clouds can block sunlight, serving as negative radiative forcers, or trap outgoing infrared radiation and function as positive radiative forcers. Size and thickness or density affect whether the effects are positive or negative (US National Science Foundation; National Weather Service).
2.6Non-condensable Gases
Unlike water vapor, radiative gases such as carbon dioxide, methane, nitrous oxide, ozone, and human-synthesized chloro- and fluorocarbons do not condense at atmospheric pressure and temperatures. This allows them to exceed any sink capacity and accumulate in the atmosphere (American Chemical Society, 2012a).
2.6.1Carbon Dioxide
Carbon dioxide is a normal, important part of the atmosphere. Until the Industrial Revolution, atmospheric concentrations were relatively stable, with sources regulated by natural sinks such as oceans and forests. More than 50 years ago, however, Charles D. Keeling began a continuous air monitoring program for carbon dioxide at Mauna Loa, Hawaii. Two things soon became evident. The concentration of carbon dioxide in the atmosphere had increased considerably from the estimated average concentration of 280 ppm and was likely to continue to rise. Keeling also identified a regular seasonal cycle of lowered carbon dioxide concentrations in the summer in the Northern Hemisphere, implying that forests and other vegetation through photosynthesis were acting as a sink for some of the elevated carbon dioxide. Both were key findings that helped to begin the science of global warming and climate change (Keeling, 2008).
2.6.1.1Anthropogenic Emissions of Carbon Dioxide
Elevated concentrations of carbon dioxide are considered to be the most important persistent cause of global warming and climate change. Carbon dioxide is the best-known radiative forcer and has become the “poster child” for global warming. Most of the research and attention has focused on the sources and effects of anthropogenic emissions of carbon dioxide. A more detailed consideration of carbon dioxide in relation to the carbon cycle will be found in Chapter 3.
2.6.1.2Carbon Dioxide Sources and Sinks
Natural sources of carbon dioxide include emissions from volcanoes, evaporation from the surface of oceans, and decomposition of dead plants and animals. The impact of carbon dioxide emissions through respiration by billions of humans and animals is not clear. Natural sinks include the oceans, phytoplankton and other aquatic plants, and land-mass vegetation, mostly trees and forests (American Chemical Society, 2012b).
Anthropogenic emissions of carbon dioxide result primarily from combustion of fossil fuels, wood, and other carbon-containing materials. Fossil fuel combustion and global cement production account for 75% of emitted anthropogenic carbon dioxide (American Chemical Society, 2012b; Betts et al., 2016). Deforestation and land-use changes for agriculture and permanent development make up the rest (Prentice et al., 2012).
Increased levels of carbon dioxide have caused approximately an 81% increase in radiative forcing during the past decade and approximately an 82% increase during the past 5 years (World Meteorological Organization, 2016).
Coal, oil, and natural gas are natural carbon sources derived from prehistoric vegetation millions of years old. The carbon dioxide emitted from them is somewhat different than the carbon dioxide in the air before the Industrial Revolution. Different isotopic forms of carbon dioxide occur in ratios, and fossil fuels have a lower ratio of 13C to 12C than the carbon dioxide in the atmosphere (because of the preference of the ancient plants for the lighter isotope). The atmospheric ratio of 13C to 12C has been declining, beginning in the twentieth century. This has been proposed as convincing evidence that increasing carbon dioxide concentrations in the atmosphere are caused by anthropogenic carbon dioxide emissions from fossil fuel combustion (American Chemical Society, 2012b).
Carbon dioxide concentrations in the atmosphere have increased on average by approximately 2.1 ppm/year for the past 10 years. The annual growth rate is variable, depending on how climate affects carbon dioxide sources and sinks. This became evident in 2016 when a record high increase of carbon dioxide was noted at Mauna Loa. Correlation was found between increased carbon dioxide and the effects of the 2015–2016 El Nino/Southern Oscillation (ENSO). Carbon dioxide concentrations tend to increase during the peak of the cyclical El Nino events in the Southern Pacific Ocean. In this case, less carbon was taken up by vegetation in tropical areas, and carbon dioxide was released from trees killed by drought and fires. Betts et al. (2016) investigated the 2015–2016 El Nino effects on carbon dioxide. Based on their model results, incorporating carbon dioxide emission data and Southern Pacific sea surface temperatures, they forecast that carbon dioxide at Mauna Loa would for the first time remain above 400 ppm all year.
The sinks for anthropogenic carbon dioxide are the same as for carbon dioxide from natural sources. Before the Industrial Revolution, the concentration of carbon dioxide in the air was maintained by the balance between the atmosphere and the ocean and land-mass sinks. By 2015, carbon dioxide levels had increased to 144% of pre-industrial levels (World Meteorological Organization, 2016). The oceans and the land-mass sinks are no longer capable of absorbing all of the elevated concentrations of carbon dioxide. It is estimated that as much as 40% remains in the air and persists on a timescale of 100 years to several thousand years. This carbon dioxide is the most important contributor (representing 65% of the radiative forcing by long-lived greenhouse gases) to global warming and climate change (Archer et al., 2009; Prentice et al., 2012; Arneth et al., 2017). There are no indications from the results from models or in the scientific literature that the concentrations and effects of anthropogenic carbon dioxide can be mitigated or substantially reduced in the foreseeable future (Archer et al., 2009).
2.6.1.3Carbon Dioxide Controls the Temperature of the Earth
Water vapor is a condensing gas in the atmosphere, with varying concentrations in time, and, while not self-sustaining, yet plays a major role in the greenhouse effect. Non-condensing greenhouse gases, such as carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons, determine the air temperatures that determine how long water vapor and clouds can be sustained in the atmosphere. The effects of the non-condensing gases determine the relative contribution of water vapor and clouds to radiative and reradiative effects (Lacis et al., 2010). Schmidt et al. (2010) estimated that water vapor accounted for 50% of the greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and other greenhouse gases and aerosols 5%. As water vapor and clouds are not self-sustaining, carbon dioxide (20%) and the other greenhouse gases (5%) control the extent of the greenhouse effect. Carbon dioxide is 80% of the non-condensing greenhouse gases that control the extent of the greenhouse effect. Lacis et al. (2010) conclude that carbon dioxide therefore can be considered as the thermostat or “control knob” that governs the temperature of the Earth.
2.7Methane
Methane (CH4) gas is a normal part of the atmosphere. It is a natural, powerful radiative climate forcer that absorbs some longwave infrared radiation as it moves from Earth back to space. This results in some beneficial warming of air in the atmosphere. Together with other gases, it helps to regulate the Earth’s temperature budget to allow life on Earth as we know it. Methane concentrations, however, slowly began to rise with the start of the Industrial Revolution. Methane is now increasing more rapidly with the continuing growth in extensive agriculture needed to feed an ever-growing population (Montzka et al., 2011; Saunois et al., 2016; Rigby et al., 2017). This has resulted in an increase of the radiative climate forcing capacity of methane and other gases to cause increased global warming and resulting climate change. Methane is the second most important greenhouse gas after carbon dioxide. It has increased to 256% of its value in 1750 (World Meteorological Organization, 2016). It is estimated that methane contributes approximately 17% to radiative climate forcing (Montzka et al., 2011; Stocker et al., 2013).
2.7.1Natural Sources of Methane
It has been estimated that 40% of methane in the atmosphere is from natural sources. The principal natural source is anaerobic microbial activity in wetlands, lakes, and soils. Slow decomposition of organic matter under anaerobic conditions by methanogenic bacteria results in release of methane. Warm tropical wetlands release the most methane (Montzka et al., 2011; Friedlingstein et al., 2012). Wetlands and lakes occupy approximately 7% of the Earth’s surface but wetlands are disappearing all across the globe.
2.7.1.1Termites
Termites are found on approximately 66% of the Earth’s terrestrial surface. Common in tropical grasslands and forests, often in huge colonies in mounds, termites utilize plant materials for nutrition. Depending on the species, anaerobic digestion involves action by symbiotic bacteria or protozoans. Methane is released in the process. Clearing forests for use in agriculture increases termite infestations (Zimmerman et al., 1982). Methane emission from termite colonies is considered less important than emissions from wetlands, but the potential levels of release have not been intensively investigated.
2.7.1.2Wetland Forests
Trees growing in wetlands can be considered as conduits for methane release from soil to the atmosphere via roots, stems, and leaves (Rice et al., 2010; Pangala et al., 2015). Methane release from trees occurs more extensively and for longer periods of time in warmer tropical and subtropical wetlands. Pangala et al. (2017) concluded that tropical tree stems in the Amazon floodplain are a major source of methane. The Amazon floodplain may contribute as much as one-third of global wetland methane. The contribution of methane release from trees growing in other wetlands to atmospheric levels of methane has not been extensively investigated (Carmichael et al., 2014).
2.7.1.3Upland Temperate Forests
Concern about rising levels of atmospheric methane has stimulated research on the possible contribution of forest trees growing in well-drained upland soils to levels of atmospheric methane. That trees in upland forests release methane to the atmosphere is accepted. The source of the methane and how this happens, however, is more controversial. Two methods have been proposed: the first is formation in and release from leaves, the second release from internal wood decay and coarse debris.
It has been proposed that methane is formed under aerobic or oxic conditions in leaves and then released to the atmosphere. This is based on results from experiments with incubating tree and grass leaves under controlled conditions and then measuring methane. How and why methane was produced and then released was not determined (Keppler et al., 2006). Definitive verification of this proposed process has not been found by others (Covey et al., 2012).
In the alternative proposal of methane release from internal wood decay, it has long been known that as forest trees age, the probability increases that the internal dead heartwood will be invaded by methanogenic bacteria, which cause an anaerobic rot called wet wood, or by brown or white rot fungi (Covey et al., 2012; Warner et al., 2017). Methane is released from tree trunks and from coarse debris on moist soil surfaces. Internal decay can be detected in living trees by sonic and electrical tomography (Brazee et al., 2011). It is likely that internal wood decay is more prevalent than previously thought, and this is not being considered in estimations of atmospheric methane concentrations. (Covey et al., 2012; Carmichael et al., 2014). Internal decay is also not being considered in forest management where stem diameter (known as dbh, for diameter at breast height) is used to assess tree growth and assess carbon storage in wood. Parfitt et al. (2010) detected wood decay fungal DNA in the sapwood of 11 forest tree species (angiosperm species). They suggest that wood decay fungi are latent in sapwood of live angiosperm trees and that environment may control the development of internal wood decay. Part of their title is appropriate for the ecology and productivity of forest trees: “Do all trees carry the seeds of their own destruction?”
2.7.1.4Permafrost
Permafrost is a subsurface layer of continuously frozen soil that contains organic matter. It is found chiefly in polar areas, with more in the Northern Hemisphere than in the Southern Hemisphere. Approximately 24% of the land area in the Northern Hemisphere is in permafrost (National Snow and Ice Data Center, 2017). Surface thawing during short summer periods may allow microbial degradation of some organic matter and release of carbon dioxide and methane (Anthony et al., 2016). Given the huge amounts of organic matter in global permafrost, there is concern that continued global warming will accelerate permafrost thawing, microbial degradation, and release of considerable amounts of carbon dioxide and methane, both of which could increase positive climate feedback that would further accelerate permafrost thawing and release of even more carbon dioxide and methane. Permafrost degradation has been occurring for as long as 40 years. It is not, however, well understood how this has contributed to positive climate feedback (Anthony et al., 2016). A recent analysis in Alaska did not find significant increases in long-term methane emissions with increasing air temperature (Sweeney et al., 2016). They pose two important questions to address about the future of emissions of methane from permafrost:
1.What fraction of mobilized soil organic carbon will be released as methane?
2.What is the sensitivity of methane emissions to temperature change?
The potential for extensive methane release from permafrost is there, but it is not clear if this will happen soon.
2.7.2Human-induced Sources of Methane
Human activities increase natural methane emissions by 60%. An increasing population, with rising expectations of a better, more affluent lifestyle, especially in Asia, fuel the demand for food and energy sources, resulting in increases in methane emissions (Montzka, et al., 2011; World Meteorological Organization, 2016).
2.7.2.1Rice Culture
Rice is grown primarily in Asia in flooded paddy fields that generate and release methane. Rice production and methane emissions are increasing. Emissions are influenced by factors such as temperature, fertilizer applications, and water levels (Augenbraun et al., 1997).
2.7.2.2Animals Raised for Food
Increasing population and rising affluence increase the demand for food from domestic animals. Dairy cows and cattle release the most methane during microbial-aided rumen digestion. Pigs, sheep, and goats also contribute (Augenbraun et al., 1997). To meet demand, there has been an enormous increase in domestic ruminants (Stocker et al., 2013). Increasing animal populations also means increasing quantities of methane from manure. Feedlots release methane and ammonia.
2.7.2.3Methane from Fossil Fuels
Methane is released during coal mining and natural gas extraction from the ground. It is also vented or burned off as waste product during oil refining (Augenbraun et al., 1997). Methane is leaking at increasing rates from ageing underground distribution pipes to homes, institutions, and industry. It is promoted as the “clean fuel.” Complete combustion of methane, however, results in release of carbon dioxide and water vapor (2:1 ratio), the two most important positive radiative forcers.
2.7.3Increasing Levels of Atmospheric Methane
Until 2007, atmospheric methane levels were relatively stable and were increasing slowly. Since 2007 they have been increasing at a rate more rapid than ever before (Saunois et al., 2016; Rigby et al., 2017). As methane has an atmospheric residence time of only approximately 10 years, the explanation for why methane is increasing is not clear. Large increases in agriculture and fossil fuel use, particularly in Asia, are possibilities (Saunois et al., 2016; Rigby et al., 2017). Large increases of methane from warmer tropical wetlands is another (World Meteorological Organization, 2016). Methane is removed from the atmosphere by oxidation with the hydroxyl radical (OH ) (Montzka et al., 2011; Fiore, 2014). Reduction in levels of OH in the atmosphere is another possible explanation. Together with increased emissions of methane, this could result in longer residence time for more methane in the atmosphere (Montzka et al., 2011). Rigby et al. (2017) related a decline in OH to the increase in methane since 2007. Whatever the cause, rising methane is a major concern.
2.8Nitrous Oxide
Nitrous oxide (N2O) is considered to be the third most important positive radiative forcer. It has now increased to 121% of its value in 1750 and is estimated to contribute 6% to the radiative forcer menu (World Meteorological Organization, 2016). It has a residence time in the atmosphere of 125 years. It also migrates slowly to the stratosphere where it can cause breakdown of the ozone layer. Removal occurs very slowly in the stratosphere (Montzka et al., 2011). In the twentieth century, a relationship between terrestrial variations in temperature and rates of emission of nitrous oxide was established (Friedlingstein et al., 2012).
Nitrous oxide is formed in soils and water during microbial nitrification and denitrification during decomposition of organic matter. Water absorption by plant residues increases emissions of nitrous oxide ( Kravchenko et al., 2017). The largest source of natural nitrous oxides is wet tropical areas. Nitrous oxide is also released during fuel combustion. Use of inorganic nitrogen crop fertilizers, growing crops that fix nitrogen, and deposition of nitrogen from combustion to soils have led to increased emissions of nitrous oxide. Deposition increases with temperature (Montzka et al., 2011; Friedlingstein et al., 2012; World Meteorological Organization, 2016). Wen et al. (2017) have reported that alder (Alnus), beech (Fagus), and spruce (Picea) emit nitrous oxides from their stems in seasonal patterns, with alder stems emitting the least nitrous oxide. They concluded that soil-based measurements of nitrous oxide in temperate forests were likely underestimations.
People in emerging economies in Asia and Africa seek to improve their lives through increased consumption of material goods, greater living space, and better-quality food. Arable land must be found to grow more food, using chemical fertilizers, especially in Africa where Nigeria is poised to become the world’s third most populous country. Together with warming air temperatures, increased levels of nitrous oxide should be expected.
2.9Other Radiative Forcers Created by Humans
Humans have synthesized many gaseous molecules that have no natural sources. They exist in the atmosphere when they escape from industrial processes. They are used as coolants and propellants and as insulators in the electric power industry. As there are no known breakdown processes, their persistence time in the atmosphere may be forever. Most are fluorinated gases: they include sulfur hexafluoride, hydrofluorocarbons, chlorofluorocarbons, perfluorocarbons, and nitrogen trifluoride. All are very strong radiative forcers, and some move slowly to the stratosphere where they break down ozone. Concentrations are measured in parts per trillion (ppt). Chlorofluorocarbons are decreasing, but some hydrofluorocarbons are increasing (Montzka et al., 2011; World Meteorological Organization, 2016).
2.10Role of Other Products from Incomplete Combustion
Complete combustion of organic materials would result in release of CO2 + H2O vapor in a 1:2 ratio. This can be true for clean natural gas (methane), but it is not likely for most combustion processes. Incomplete combustion results in release of a wide variety of reactive gases that can indirectly strongly influence global warming and climate change. These include nitric oxide and nitrogen dioxide, carbon monoxide, volatile organic compounds, and sulfur dioxide. Nitric oxide, nitrogen dioxide, and volatile organic compounds are involved in atmospheric chemistry that results in the formation of ozone, an important non-persistent radiative gas. Sulfur dioxide is the main precursor for sulfur aerosol formation, which reflects sunlight and cools the area below (Global Atmosphere Watch, 2017).
Black carbon is principally elemental carbon, primarily from fossil fuel combustion, found in what is commonly called soot. It is the strongest atmospheric absorber of incoming solar radiation. Deposition is either direct or by rain or snow, with a life span of about a week. Deposition to ice or snow may cause melting (Khan et al., 2017). Black carbon may also combine with aerosols to form large brown clouds that can result in global dimming (Wild et al., 2007; Ramanathan and Carmichael, 2008).
2.11Ozone
Ozone (O3) is a natural component of the atmosphere. When present, it is considered by many to be the third most important radiative forcer. It is a more effective radiative forcer than carbon dioxide (Unger, 2012). It is very effective in the upper troposphere (NASA, 2015). Unlike carbon dioxide or methane, ozone is a secondary gas that is formed and can be reformed through complex atmospheric chemistry that requires precursor molecules and sunlight and warm temperatures. Low levels of ozone are considered as normal background. Short-lived higher levels can act as radiative forcers and also cause injury to plants and cause human health problems (Krupa and Manning, 1988; Ashmore, 2005; Sitch et al., 2007). Ozone can be a radiative forcer and an air pollutant at the same time (Unger, 2012).
2.11.1Sources and Sinks of Ozone
Rainstorms clean the air of particulates and ozone. If lightning occurs in a storm, the air afterward seems clean with a pleasant, slightly acrid smell. The smell is due to ozone from static discharge from lightning. Ozone can also be released during the use of older photocopiers and older trolleycar wires. Short-term high concentrations can result from temporary intrusions or dip-downs of ozone from the stratosphere (NASA, 2015).
Ozone 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 and act as a radiative forcer and an air pollutant. The photochemical oxidant cycle that forms ozone is detailed in Chapter 3.
In addition to being a direct radiative forcer, it is becoming 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). It has been proposed that ozone effects on plants could result in more global warming through reduction in uptake of carbon dioxide than direct radiative forcing caused by ozone alone. With current and predicted rising ozone levels, this is of particular concern (Sitch et al., 2007).
2.11.2Carbon Dioxide and Ozone
Global warming and climate change are affected by a variety of human-influenced gases in the atmosphere. Water vapor, methane, and nitrous oxide are powerful radiative forcers, but are relatively short-lived. Carbon dioxide is the largest and most important irreversible persistent radiative forcer, with effects estimated to persist for more than 1000 years (Solomon et al., 2009, 2010). Concentrations continue to increase in the atmosphere (Canadell et al., 2007) and this has resulted in extensive increases in vegetative growth, particularly in the Northern Hemisphere (Mao et al., 2016; Yakir, 2017). This is seen as a possible partial mitigation mechanism for elevated carbon dioxide in the atmosphere (Mao et al., 2016; Yakir, 2017). While the effect of elevated carbon dioxide on vegetation is positive, as much as 40% of the anthropogenic carbon dioxide remains in the atmosphere and is the major cause of global warming and climate change.
Ozone is a much more effective radiative forcer than carbon dioxide, but is short-lived and variable in concentration and occurrence (Unger, 2012). Like carbon dioxide, ozone enters leaves via stomata during gas exchange in photosynthesis. It can cause cellular and tissue damage which reduces growth and gross plant productivity. Depending on the concentration, ozone can also cause stomatal closure and limit uptake of carbon dioxide. With foliar damage or stomatal closure, or both, the uptake of carbon dioxide and resulting carbon sequestration can be reduced by ozone, and more carbon dioxide remains in the atmosphere. Elevated levels of carbon dioxide, however, can also cause stomatal closure, which could limit entry by ozone and thus provide protection from ozone damage and plant growth reduction. This is the potential protective effect of carbon dioxide in ameliorating ozone damage in plants (Paoletti and Grulke, 2005).
As Sitch et al. (2007) point out, there is a complex and not well-understood or documented interaction between ozone and carbon dioxide effects on vegetation. It is known that an increase in carbon dioxide can cause stomatal closure and inhibit ozone uptake, and conversely that an increase in ozone can also cause stomatal closure and exclude carbon dioxide. A modified global land carbon cycle model was used to assess whether stomatal closure caused by carbon dioxide would protect plants from damage due to increases in ozone concentrations, or whether ozone would reduce the effects of carbon dioxide to ameliorate reductions in photosynthesis caused by ozone. Using data from empirical experiments, it was found that as ozone concentrations increased there was a significant decrease in plant production that affected the land sink for carbon (Sitch et al., 2007).
Carbon dioxide is considered to have positive effects on vegetation. Average concentrations of ozone above 40 parts per billion (ppb) are considered to have negative effects on vegetation (Ashmore, 2005). Both carbon dioxide and ozone are present during growing seasons, and both can affect trees in forests. It has been suggested that future effects of ozone on plants and land carbon storage will be determined by the interaction of ozone, carbon dioxide, and climate change factors (Sitch et al., 2007). As trees and forests are the major contributors to terrestrial carbon storage, it is essential to determine how they are affected by carbon dioxide and ozone, and how this might affect their role in cooling or warming the atmosphere.
2.12Summary
Global warming was introduced in Chapter 1. In this chapter, the nature and role of the radiative and reactive gases that cause the greenhouse effect were introduced and discussed. During the day, visible light and shortwave radiation from the sun (sunlight) travels from space through the Earth’s atmosphere. Some is reflected back, the rest absorbed to warm the Earth. As long wave infrared radiation (heat) begins to return to space, some is intercepted by clouds and water vapor, or absorbed by the greenhouse gases and radiated back toward the surface, retaining some warmth, and making life on Earth possible. This is the greenhouse effect.
The greenhouse effect and temperatures are increasing. Night temperatures are increasing faster than daytime temperatures. The atmosphere is generally warming.
Water vapor, a condensable, variable-concentration gas, is the major radiative gas involved in global atmospheric warming. Non-condensable radiative gases include carbon dioxide, ozone, methane, nitrous oxide, and chlorofluorocarbons, all of which are persistent and increasing in concentration, except ozone which is seasonal and variable in incidence and concentration. Together, they provide enough elevated temperature to allow water vapor to be the major greenhouse gas.
Carbon dioxide is 80% of the non-condensable radiative gases. As such, it is considered to regulate the temperature of the Earth’s atmosphere. From this perspective, carbon dioxide is the most important greenhouse gas. Carbon dioxide has been increasing steadily by as much as 2.1 ppm per year for the past decade. It has passed the 400 ppm mark and is expected to remain there indefinitely. It is estimated that 40% of the current level of carbon dioxide is from fossil fuel combustion and cement production. The oceans and the terrestrial plant and soil land mass sink do not provide a sufficient sink for the 40% anthropogenic carbon dioxide. Its persistence in the atmosphere may be several thousand years.
Carbon dioxide is used by plants in photosynthesis to fix carbon used in growth. The influence of elevated carbon dioxide should have a positive effect on trees. Tropospheric levels of ozone can be high enough during the summer growing season to injure leaves and reduce plant growth stimulated by elevated carbon dioxide. This is why both carbon dioxide and ozone are important factors in the role of trees in cooling and warming the atmosphere.
The next chapter will focus on the carbon cycle and the photochemical oxidant cycle.
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