Impact of GHG on Global warming. Global C, N, S and H cycles, greenhouse effect and causes of climate change. Greenhouse gases - CO₂, CH₄, NO₃, CFCs - Change in Conc. greenhouse gases in atmosphere and global warming potential

The Greenhouse effect is a leading factor in keeping the Earth warm because it keeps some of the planet's heat that would otherwise escape from the atmosphere out to space. Without the greenhouse effect the Earth's average global temperature would be much colder and life on Earth as we know it would be impossible. Greenhouse gases include water vapor, CO₂, methane, nitrous oxide (N₂O) and other gases. Carbon dioxide (CO­₂) and other greenhouse gases turn like a blanket, gripping Infra-Red radiation and preventing it from escaping into outer space. The clear effect of the greenhouse gases is the stable heating of Earth's atmosphere and surface, thus leading to global warming. The ability of certain gases, greenhouse gases, to be transparent to inbound visible light from the sun, yet opaque to the energy radiated from the earth is one of the best still events in the atmospheric sciences. The existence of greenhouse effect is what makes the earth a comfortable place for life.

The factor that earth has an average surface temperature pleasurably between the boiling point and freezing point of water, therefore suitable for our kind of life, cannot be clarified by merely proposing that planet Earth orbits at just the precise space from the sun to absorb just the right amount of solar radiation. The moderate temperatures are also the outcome of having just the precise kind of atmosphere.

The atmosphere in planet Venus would produce hellish, Venus like conditions on planet Earth; the Mars troposphere would leave earth shivering in a Martian-type deep freeze. Additionally, parts of the earth’s atmosphere act as shielding blanket of just the right thickness, receiving appropriate solar energy to keep the global average temperature in an amusing range. The Martian blanket is too thin, and the Venusian blanket is way too thick. The 'blanket' as stated here, is termed as a collection of atmospheric gases called greenhouse gases based on the knowledge that the gases also capture heat similar to the glass walls of a greenhouse. These gases, mostly water vapor, carbon dioxide, methane, and nitrous oxide, all perform as effective global insulators. The conversation of inbound and outward-bound radiation that warms the Earth is often referred to as the greenhouse effect because a greenhouse works in much the same way.

Inbound Ultra Violet (UV) radiation easily passes through the glass walls of a greenhouse and is absorbed by the plants and hard surfaces inside. Weaker Infrared (IR) radiation, however, has difficulty passing through the glass walls and is trapped inside, that is, warming the greenhouse. This outcome lets tropical plants flourish inside a greenhouse, even during a cold winter. The greenhouse influence upsurges the temperature of the Earth by trapping heat in our atmosphere. This retains the temperature of the Earth higher than it would be if direct heating by the Sun was the only source of warming. When sunlight reaches the surface of the Earth, some of it is absorbed which warms the ground and some jumps back to space as heat. Most Greenhouse gases that are in the atmosphere fascinate and then transmit some of this heat back towards the Earth. The greenhouse effect is a foremost factor in keeping the Earth heartfelt because it keeps some of the planet's heat that would otherwise escape from the atmosphere out to space. In fact, without the greenhouse effect the Earth's average global temperature would be much colder and life on Earth as we recognize it would not be possible. The difference between the Earth's actual average temperature 14°C (57.2°F) and the expected effective temperature just with the Sun's radiation -19°C (-2.2°F) gives us the strength of the greenhouse effect, which is 33°C

 The greenhouse effect is a natural process that is millions of years old. It plays a critical role in a variable the overall temperature of the Earth. The greenhouse effect was first discovered by Joseph Fourier in 1827, experimentally verified by John Tyndall in 1861, and quantified by Svante Arrhenius in 1896.

The prime forcing gases of the greenhouse effect are: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases.

Carbon dioxide is one of the greenhouse gases. It involves one carbon atom with an oxygen atom bonded to each side. As soon as its atoms are bonded tightly together, the carbon dioxide molecule can absorb infrared radiation and the molecule starts to vibrate. Eventually, the vibrating molecule will emit the radiation again, and it will likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface from the cold of space. Carbon dioxide, water vapor (H₂O), methane (CH₄), nitrous oxide (N₂O), and some limited other gases are greenhouse gases. They all are molecules made up of more than two constituents atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The foremost mechanisms of the atmosphere (N₂ and O₂) are two-atom molecules too closely bound together to vibrate and consequently, they do not absorb heat and subsidize to the greenhouse effect

Carbon dioxide, methane, nitrous oxide and the fluorinated gases are all well-mixed gases in the atmosphere that do not react to changes in temperature and air pressure, so the levels of these gases are not affected by condensation effect. Water vapor also is a highly active component of the climate system that retorts briskly to fluctuations in conditions by either dwindling into rain or snow or evaporating to return to the atmosphere. Consequently, the imprint of the greenhouse effect is principally circulated through water vapor, and it turns as a fast reaction effect. Carbon dioxide and the other non-condensing greenhouse gases are the vital gases within the Earth's atmosphere that tolerate the greenhouse effect and rheostat its strength.

Water vapor is a fast-acting feedback but its atmospheric concentration is controlled by the radiative forcing supplied by the non-condensing greenhouse gases.

The Solar Radiation

The sun radiates gigantic quantities of energy into space, crosswise a wide spectrum of wavelengths. Utmost of the radiant energy from the sun is concentrated in the visible and near-visible portions of the spectrum. The narrow band of visible light, between 400 and 700 nm, signifies 43% of the total radiant energy emitted. Wavelengths shorter than the visible account for 7 to 8% of the total, but are extremely important because of their high energy per photon. The shorter the wavelength of light, the more energy it contains. Accordingly, ultraviolet light is very energetic (accomplished by breaking apart stable biological molecules and instigating sunburn and skin cancers). The residual 49 - 50% of the radiant energy is spread over the wavelengths longer than those of visible light. These lie in the near infrared range from 700 to 1000 nm; the thermal infrared, between 5 and 20 microns; and the far infrared regions. Various components of earth's atmosphere absorb ultraviolet and infrared solar radiation before it penetrates to the surface, but the atmosphere is quite transparent to visible light

Absorbed by land, oceans, and vegetation at the surface, the visible light is transformed into heat and re-radiates in the form of invisible infrared radiation. During the day, earth heats up, but at night, all the accumulated energy would radiate back into space and the planet's surface temperature would fall far below zero very rapidly. The reason why this doesn't happen is that earth's atmosphere contains molecules that absorb the heat and re-radiate the heat in all directions. This reduces the heat radiated out to space called greenhouse gases because they serve to hold heat in like the glass walls of a greenhouse, these molecules are responsible for the fact that the earth enjoys temperatures suitable for our active and complex biosphere.

Sources of Greenhouse

Gas Emissions In recent times, one of the major sources of greenhouse gas (GHG) emission is from water resource recovery facilities (wastewater treatment plants (WWTPs). Wastewater treatment plants (WWTPs) are recognized as one of the larger minor sources of GHG emissions. The WWTPs emit gases such as nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). Increasing emission of GHG from this source harm to our climate.  Biological mechanisms such as emissions of CO₂ due to microbial respiration, emission of N₂O by nitrification and denitrification, and emission of CH₄ from anaerobic digestion processes are direct emissions from WWTPs. Sources that not regulated directly within the WWTP are indirect internal emission sources; consumption of thermal energy and indirect external emission sources; third-party biosolids hauling, chemical productions and their transportation to the plant, etc.

The increasing rate of GHG emissions is due to the changes in the economic output, extended energy consumption, increasing emission from landfills, livestock, rice farming, septic processes, and fertilizers as well as other factors. Increase of industrialization, use of fertilizers, burning of fossil fuels and other human and natural activities result in a rise above normal average atmospheric temperature; thus posing threat to our environment. Research identifies methane and carbon dioxide as the main greenhouse gases. Therefore, the reduction of methane concentration in the atmosphere, both from natural and anthropogenic sources, is indispensable to tackle the negative outcomes of global warming.

Greenhouse Effect

Atmospheric scientists first used the word 'greenhouse effect' in the later 1800s. At that time, it was used to designate the naturally happening functions of trace gases in the atmosphere and did not have any negative implications. It was not up until the mid-1950s that the term greenhouse effect was attached to concern over climate alteration. And in contemporary decades, we often hear about the greenhouse effect in somewhat negative terms. The negative concerns are related to the possible impacts of an improved greenhouse effect. It is important to remember that without the greenhouse effect, lifecycle on earth as we know it would not be possible.

While the earth's temperature is reliant on upon the greenhouse-like action of the atmosphere, the extent of heating and cooling are toughly influenced by several factors just as greenhouses are pretentious by various factors. In the atmospheric greenhouse effect, the type of surface that sunlight first happenstances are the most important factor. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate radiation differently. Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark desert soil is strongly absorbed, on the other hand, and contributes to significant heating of the surface and lower atmosphere.

Cloud cover also affects greenhouse warming by both reducing the amount of solar radiation reaching the earth's surface and by reducing the amount of radiation energy emitted into space. Scientists outline the percentage of solar energy reflected back by a surface. Understanding local, regional, and global effects are life-threatening to foretelling global climate change.

Greenhouse Gases and Global Warming

Greenhouse gases (GHGs) such as carbon dioxide, methane, nitrous oxide, and halogenated compounds emissions are caused by human activities and some do occur naturally. The GHGs absorb infrared radiation and trap heat in the atmosphere, thereby enhancing the natural greenhouse effect defined as global warming. This natural occurrence warms the atmosphere and make life on earth possible, without which the low temperature will make life impossible to live on earth. "Gas molecules that captivate thermal infrared radiation, and are in a substantial amount, can force the climate system. These type of gas molecules are called greenhouse gases,"

Carbon dioxide (CO₂) and other greenhouse gases turn like a blanket, gripping Infrared (IR) radiation and preventing it from evading into outer space. The net effect is the steady heating of Earth's atmosphere and surface, and this process is called global warming. These greenhouse gases include water vapor, CO₂, methane, nitrous oxide (N₂O) and other gases. Since the dawn of the Industrial Revolution in the early 1800s, the scorching of fossil fuels like coal, oil, and gasoline have greatly increased the concentration of greenhouse gases in the atmosphere, specifically CO₂, National Oceanic and Atmospheric Administration (NOAA). "Deforestation is the second largest anthropogenic basis of carbon dioxide to the atmosphere ranging between 6% and 17%,".  Some human activities like the production and consumption of fossil fuels, use of various chemicals agriculture, burning bush, waste from incineration processes and other industrial activities have increased the concentration of greenhouse gases (GHG), particularly CO₂, CH₄, and N₂O in the atmosphere making them harmful. This increase in atmospheric GHG concentration has led to climate change and global warming effect, which is motivating international efforts such as the Kyoto Protocol, signing of Paris Agreement on climate change and other initiatives to control negative outcomes of the greenhouse effect.

The contribution of a greenhouse gas to global warming is commonly expressed by its global warming potential (GWP) which enables the comparison of global warming impact of the gas and that of a reference gas, typically carbon dioxide. Atmospheric CO₂ intensities have increased by more than 40% since the beginning of the Industrial Revolution, from about 280 parts per million (ppm) in the 1800s to 400 ppm today. The last time Earth's atmospheric levels of CO₂ reached 400 ppm was during the Pliocene Epoch, between 5 million and 3 million years ago, according to the University of California, San Diego’s Scripps Institutions of Oceanography. The greenhouse effect, collective with growing levels of greenhouse gases and the resultant global warming, is expected to have profound consequences, according to the near-universal consensus of scientists. If global warming undergoes unimpeded, it will cause noteworthy climate change, a rise in sea levels, increasing ocean acidification, life threatening weather events and other severe natural and societal impacts, according to NASA, the Environmental Protection Agency(EPA) and other scientific and governmental bodies.

The majority of the Earth's atmosphere is composed of a mixture of only a few gases-nitrogen, oxygen, and argon; combined these three gases comprise more than 99.5% of all the gas molecules in the atmosphere. These gases which are most abundant within the atmosphere exhibit almost no effect on warming the earth and its atmosphere since they do not absorb visible or infrared radiation. However, there are minor gases which comprise only a small portion of the atmosphere (about 0.43% of all air molecules, most of which are water vapor at 0.39%) that do absorb infrared radiation. These "trace" gases contribute substantially to warming of the Earth's surface and atmosphere due to their abilities to contain the infrared radiation emitted by the Earth. Since these trace gases influence the Earth in a manner somewhat similar to a greenhouse, they are referred to as GreenHouse Gases, or GHGs.

Composition of Earth's Dry Atmosphere (as of 2009)

Nitrogen	78.1%
Oxygen	20.9%
Argon	0.9%
Carbon Dioxide	0.039%
Methane	.00018%
Nitrous Oxide	.000032%
Sulfur Hexafluoride	.00000000067%

Water vapor is the most important GHG, since globally it is the most abundant of these gases, although it varies from 0-3% in a given location. NOAA's Carbon Cycle Greenhouse Gases (CCGG) group is concerned with the abundances of many of the other GHGs, since humans have a dominant role in the growing atmospheric concentrations of these gases. The gases measured by the CCGG include carbon dioxide (the second most important GHG), methane, nitrous oxide, sulfur hexafluoride, ozone, and a few others. While these gases constitute only a tiny fraction of Earth's very large atmosphere, their amounts are sufficient to absorb a major fraction of the infrared light in the atmosphere.

Influential Greenhouse Gases

Carbon Dioxide (CO₂) is a colorless, odorless gas consisting of molecules made up of two oxygen atoms and one carbon atom. Carbon dioxide is produced when an organic carbon compound (such as wood) or fossilized organic matter, (such as coal, oil, or natural gas) is burned in the presence of oxygen. Carbon dioxide is removed from the atmosphere by carbon dioxide "sinks", such as absorption by seawater and photosynthesis by ocean-dwelling plankton and land plants, including forests and grasslands. However, seawater is also a source, of CO₂ to the atmosphere, along with land plants, animals, and soils, when CO₂ is released during respiration.

Methane (CH₄) is a colorless, odorless non-toxic gas consisting of molecules made up of four hydrogen atoms and one carbon atom. Methane is combustible, and it is the main constituent of natural gas-a fossil fuel. Methane is released when organic matter decomposes in low oxygen environments. Natural sources include wetlands, swamps and marshes, termites, and oceans. Human sources include the mining of fossil fuels and transportation of natural gas, digestive processes in ruminant animals such as cattle, rice paddies and the buried waste in landfills. Most methane is broken down in the atmosphere by reacting with small very reactive molecules called hydroxyl (OH) radicals.

Nitrous oxide (N₂O) is a colorless, non-flammable gas with a sweetish odor, commonly known as "laughing gas", and sometimes used as an anesthetic. Nitrous oxide is naturally produced in the oceans and in rainforests. Man-made sources of nitrous oxide include the use of fertilizers in agriculture, nylon and nitric acid production, cars with catalytic converters and the burning of organic matter. Nitrous oxide is broken down in the atmosphere by chemical reactions driven by sunlight.

Sulfur hexafluoride (SF₆) is an extremely potent greenhouse gas. SF₆ is very persistent, with an atmospheric lifetime of more than a thousand years. Thus, a relatively small amount of SF₆ can have a significant long-term impact on global climate change. SF₆ is human-made, and the primary user of SF₆ is the electric power industry. Because of its inertness and dielectric properties, it is the industry's preferred gas for electrical insulation, current interruption, and arc quenching (to prevent fires) in the transmission and distribution of electricity. SF₆ is used extensively in high voltage circuit breakers and switchgear, and in the magnesium metal casting industry.

The Greenhouse Effect

Many of the atmospheric trace gases, despite their relatively minor abundances, have a significant influence on Earth's climate, due to a phenomenon called the "Greenhouse Effect".

The Sun ultimately drives Earth's climate by emitting energy in the form of sunlight. Sunlight is solar radiation mostly in the form of visible and a smaller portion as ultraviolet (UV) energy. This is also called shortwave radiation. Clouds and the Earth's surface reflect some of this incoming solar radiation back out to space (approximately 30%), some (mostly UV) is absorbed by the atmosphere (about 20%), and the remaining half is absorbed at the Earth's surface. Sunlight absorbed by Earth's surface acts to warm the surface.

The solar energy that has been absorbed by Earth's surface is then emitted in a different form. Since Earth is much cooler than the Sun, it emits weaker radiation with longer wavelengths, in the infrared range. Some of this infrared radiation passes through the atmosphere unimpeded, but the majority is absorbed by GHGs and then reemitted in all directions-towards space, to other GHG molecules, and back to Earth's surface. In this way, GHGs block most of the infrared radiation within the atmosphere that would otherwise escape directly into space.

This process is naturally occurring and beneficial, as it maintains favorable living conditions for Earth's microbial, animal and plant inhabitants. The global average temperature is 14°C (57°F), which is approximately 33°C (59°F) warmer than temperatures would be without an atmosphere and GHGs. Due to their ability to absorb infrared radiation, GHG molecules have a significant impact on Earth's climate by acting as a barrier for escaping "heat".

For over a century, scientists have realized that concentrations of atmospheric gases may significantly affect Earth's climate through this process. Scientists have been measuring GHGs in the atmosphere for more than 50 years. Charles Keeling began continuous measurements of CO₂ concentrations in 1958 and others, including NOAA scientists, followed shortly thereafter.

Today, there is unequivocal scientific evidence that the abundance of these gases is increasing in the atmosphere. Evidence includes decades of carefully calibrated, global measurements of these trace gases, combined with measurements of "old" air preserved in bubbles embedded in ice cores and measurements of carbon isotopes, in tree rings (from which past atmospheric CO₂ can be reconstructed). This increase in atmospheric GHGs has a significant impact on Earth's climate because Earth's incoming and outgoing radiation is out of balance --which forces the climate to change.

As the concentrations of GHGs increase within the atmosphere, more infrared radiation is absorbed and less escapes directly to space, resulting in amplified warming. This is called the Enhanced Greenhouse Effect.

Note: This atmospheric process is referred to as the Greenhouse Effect, since both the atmosphere and a greenhouse act in a manner which retains energy as heat. However, this is an imperfect analogy. A greenhouse works primarily by preventing warm air (warmed by incoming solar radiation) close to the ground from rising due to convection, whereas the atmospheric Greenhouse Effect works by preventing infrared radiation loss to space. Despite this subtle difference, we refer to this atmospheric process as the Greenhouse Effect and these gases as Greenhouse Gases because of their role in warming the Earth.

The Carbon Cycle

Of the GHGs, CO₂ is of greatest concern because it contributes the most to the Enhanced Greenhouse Effect and climate change. For this reason, scientists have been studying this molecule carefully and attempting to quantify its abundance in the atmosphere and track how and why it changes. The CO₂ molecule is involved in a complex series of processes called the carbon cycle, where the carbon atom within the molecule moves between many different natural reservoirs. As carbon is transferred between reservoirs, processes which release CO₂ into the atmosphere are called sources, and processes which remove CO₂ from the atmosphere are called sinks.

Carbon is continuously exchanged and recycled among the reservoirs through natural processes. These processes occur at various rates ranging from short-term fluctuations which occur daily and seasonally to very long-term cycles which occur over hundreds of millions of years. For example, there is a clear seasonal cycle in atmospheric CO₂ as plants photosynthesize during the growing season, removing large amounts of CO₂. Respiration (from both plants and animals) and decomposition of leaves, roots, and organic compounds release CO₂ back into the atmosphere. On a scale spanning decades to centuries, CO₂ levels fluctuate gradually between the ocean and atmospheric reservoirs as ocean mixing occurs (between surface and deep waters) and the surface waters exchange CO₂ with the atmosphere. Much longer cycles also occur, on the scale of geologic time, due to the deposition and weathering of carbonate and silicate rock.

Carbonate rocks like limestone are formed from the shells of marine organisms buried on the ocean floor, and they are chemically eroded by reaction with CO₂ (remember that CO₂ mixed with water is an acid) in the air and in soils.  Silicate rock reacts with carbonate rock deep underground, producing CO₂ gas coming out of volcanoes. Fossil fuels form a relatively small part of these natural geologic cycles.

Carbon Reservoirs and Exchange

At time scales of most interest to humans (years to decades to centuries) the atmosphere exchanges carbon with three main reservoirs: the terrestrial biosphere, the oceans, and fossil fuels.

Source: NOAA

Terrestrial Biosphere

The terrestrial biosphere defines the part of the earth system that supports organisms living on land, and includes plants, animals, soil microbes, and decomposing organic material. Since carbon is a main component of organic molecules that are the building blocks for all life, a large amount of organic material is stored in the terrestrial biosphere-it is one of the main reservoirs for carbon.

In addition, there is a large amount of carbon exchanged seasonally between the terrestrial biosphere and the atmosphere. Surface exchanges (or "fluxes" ) result from organisms living within the terrestrial biosphere, and they naturally include both sources and sinks. Some of the terrestrial biosphere's major sources of atmospheric CO₂ include respiration by land biota(plants, animals, microorganisms, humans, etc) and the burning and decomposition of organic material. The removal of atmospheric CO₂ by the terrestrial biosphere occurs through photosynthesis. Plants use CO₂ from the atmosphere to build food in the form of organic matter--which in turn becomes food for microbes, fungi, insects, and higher organisms. Human activities have a considerable impact on the terrestrial biosphere's ability to remove or emit carbon dioxide through practices such as deforestation and other forms of land management.

Oceans

The oceans continuously exchange CO₂ with the atmosphere. Due to the large surface area of the oceans and the high solubility of carbon dioxide in water (which creates carbonic acid ), the oceans store very large amounts of carbon - about 50 times more than is in the atmosphere or terrestrial biosphere. Each year, some of that carbon is released to the atmosphere, and a similar amount is taken back up into the oceans (although the two processes might occur in different parts of the world's oceans). In addition, organisms within the marine biosphere photosynthesize and respire CO₂.

Due to the slow rate of mixing between surface and deep ocean waters, only the surface waters are responsible for short-term changes of atmospheric CO₂. As the atmospheric CO₂concentration increases, the ocean sink also increases slightly. The oceans will eventually absorb the majority of the CO₂ released from human activities, but this will take thousands of years. CO₂ in the form of carbonic acid is a weak acid, and there are profound implications on marine ecosystems due to the increasing acidity of the oceans.

Fossil Fuels

Over the course of millions of years, as biomass from dead plants and microorganisms accumulated in sediments and was subjected to high temperature and pressure deep below Earth's surface, organic remains from the biosphere (both terrestrial and marine) have been converted to fossil fuels (coal, oil, and natural gas). However, since the beginning of the Industrial Revolution in the 1800s, humans have been burning these fossil fuels, releasing the carbon from them back into the atmosphere as CO₂.

Processes that took millions of years to remove carbon from the biosphere have been reversed so that the same carbon is being released at unprecedented rates as a result of human activities. Atmospheric CO₂ levels have increased 38% [as of 2009] since Preindustrial times and are higher than at any time in the past 800,000 years.

Currently, atmospheric CO₂ levels continue to rise at an accelerating rate as humans burn fossil fuels at increasing rates. In human terms, the CO₂ emitted by the combustion of fossil fuels (along with cement manufacturing and other human activities) remains "forever" due to the stability and longevity of CO₂ within the atmosphere and oceans. This will have significant implications on the Earth System, as the resulting radiation imbalance from the Enhanced Greenhouse Effect will noticeably alter the global climate for centuries to millennia.

Global warming potential

Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide and is expressed as a factor of carbon dioxide (whose GWP is standardized to 1).

A GWP is calculated over a specific time horizon, commonly 20, 100, or 500 years. User related choices such as the time horizon can greatly affect the numerical values obtained for carbon dioxide equivalents. In the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, methane has a lifetime of 12.4 years and with climate-carbon feedbacks a global warming potential of 86 over 20 years and 34 over 100 years in response to emissions. For a change in time horizon from 20 to 100 years, the GWP for methane therefore decreases by a factor of approximately 2.5.

The GWP depends on the following factors:

·         the absorption of infrared radiation by a given species

·         the spectral location of its absorbing wavelengths

·         the atmospheric lifetime of the species

Thus, a high GWP correlates with a large infrared absorption and a long atmospheric lifetime. The dependence of GWP on the wavelength of absorption is more complicated. Even if a gas absorbs radiation efficiently at a certain wavelength, this may not affect its GWP much if the atmosphere already absorbs most radiation at that wavelength. A gas has the most effect if it absorbs in a "window" of wavelengths where the atmosphere is fairly transparent. The dependence of GWP as a function of wavelength has been found empirically and published as a graph.

Because the GWP of a greenhouse gas depends directly on its infrared spectrum, the use of infrared spectroscopy to study greenhouse gases is centrally important in the effort to understand the impact of human activities on global climate change.

The substances subject to restrictions under the Kyoto protocol are either rapidly increasing their concentrations in Earth's atmosphere or have a large GWP.

Calculating the global warming potential

Just as radiative forcing provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, global warming potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense. GWP is based on a number of factors, including the radiative efficiency (infrared-absorbing ability) of each gas relative to that of carbon dioxide, as well as the decay rate of each gas (the amount removed from the atmosphere over a given number of years) relative to that of carbon dioxide.

The radiative forcing capacity (RF) is the amount of energy per unit area, per unit time, absorbed by the greenhouse gas, that would otherwise be lost to space.

The Intergovernmental Panel on Climate Change (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report. The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas:

{\displaystyle GWP\left(x\right)={\frac {\int _{0}^{TH}a_{x}\cdot \left[x(t)\right]dt}{\int _{0}^{TH}a_{r}\cdot \left[r(t)\right]dt}}}Use in Kyoto Protocol

Under the Kyoto Protocol, the Conference of the Parties decided (decision 2/CP.3) that the values of GWP calculated for the IPCC Second Assessment Report are to be used for converting the various greenhouse gas emissions into comparable CO₂ equivalents when computing overall sources and sinks.

Global Temperature change Potential (GTP)

The Global Temperature change Potential is another way to quantify the ratio change from a substance relative to that of CO₂, in global mean surface temperature, used for a specific time span.

Importance of time horizon

A substance's GWP depends on the time span over which the potential is calculated. A gas which is quickly removed from the atmosphere may initially have a large effect, but for longer time periods, as it has been removed, it becomes less important. Thus methane has a potential of 34 over 100 years but 86 over 20 years; conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoting a GWP it is important to give a reference to the calculation.

The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases. Commonly, a time horizon of 100 years is used by regulators (e.g., the California Air Resources Board).

Values

Carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared).

GWP values and lifetimes from 2013 IPCC AR5 (with climate-carbon feedbacks)	Lifetime (years)	GWP
		20 years	100 years
Methane	12.4	86	34
HFC-134a (hydrofluorocarbon)	13.4	3790	1550
CFC-11 (chlorofluorocarbon)	45.0	7020	5350
Nitrous oxide (N2O)	121.0	268	298
Carbon tetrafluoride (CF4)	50000	4950	7350

 

 

GWP values and lifetimes from 2007 IPCC AR4   (2001 IPCC TAR in parentheses)	Lifetime in years	GWP
		20 years	100 years	500 years
Methane	12 (12)	72 (62)	25 (23)	7.6   (7)
Nitrous oxide	114 (114)	289 (275)	298 (296)	153 (156)
HFC-23 (hydrofluorocarbon)	270 (260)	12,000 (9400)	14,800 (12,000)	12,200 (10,000)
HFC-134a (hydrofluorocarbon)	14 (13.8)	3,830 (3,300)	1,430  (1,300)	435  (400)
Sulfur hexafluoride	3200 (3,200)	16,300 (15,100)	22,800 (22,200)	32,600 (32,400)

The values given in the table assume the same mass of compound is released; different ratios will result from the conversion of one substance to another. For instance, burning methane to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of carbon dioxide released (ratio 1:2.74). If you started with 1 tonne of methane which has a GWP of 25, after combustion you would have 2.74 tonnes of CO₂, each tonne of which has a GWP of 1. This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74 (approximately 9 times.

The global warming potential of perfluorotributylamine(PFTBA) over a 100-year time horizon has been estimated to be approximately 7100. It has been used by the electrical industry since the mid-20th century for electronic testing and as a heat transfer agent. PFTBA has the highest radiative efficiency (relative effectiveness of greenhouse gases to restrict long-wave radiation from escaping back into space) of any molecule detected in the atmosphere to date. The researchers found an average of 0.18 parts per trillion of PFTBA in Toronto air samples, whereas carbon dioxide exists around 400 parts per million.

Water vapour

Water vapour is one of the primary greenhouse gases, but some issues prevent its GWP to be calculated directly. It has a profound infrared absorption spectrum with more and broader absorption bands than CO₂, and also absorbs non-zero amounts of radiation in its low absorbing spectral regions. Next, its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16 °C, for example, creates an average humidity of ~18,000ppm at sea level (CO₂ is ~400ppm and so concentrations of [H₂O]/[CO₂] ~ 45x). Unlike other GHG, water vapor does not decay in the environment, so an average over some time horizon or some other measure consistent with "time dependent decay," q.v., above, must be used in lieu of the time dependent decay of artificial or excess CO₂molecules. Other issues complicating its calculation are the Earth's temperature distribution, and the differing land masses in the Northern and Southern hemispheres.

GHG Concentrations

GHG	Pre-industrial (1750)	Recent	Increased Radiative Forcing (W/m2)
CO2	280 ppm	395 ppm	1.88
CH4	722 ppb	1893 ppb	0.49
N2O	270 ppb	326 ppb	0.17
O3	237 ppb	337 ppb	0.40
Source: IPCC (2013)

 

Carbon cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration to and release from carbon sinks.

Main components

 

Pool	Quantity (gigatons)
Atmosphere	720
Ocean (total)	38,400
Total inorganic	37,400
Total organic	1,000
Surface layer	670
Deep layer	36,730
Lithosphere
Sedimentary carbonates	> 60,000,000
Kerogens	15,000,000
Terrestrial biosphere (total)	2,000
Living biomass	600 - 1,000
Dead biomass	1,200
Aquatic biosphere	1 - 2
Fossil fuels (total)	4,130
Coal	3,510
Oil	230
Gas	140
Other (peat)	250

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:

 

·         The atmosphere

·         The terrestrial biosphere

·         The ocean, including dissolved inorganic carbon and living and non-living marine biota

·         The sediments, including fossil fuels, freshwater systems, and non-living organic material.

·         The Earth's interior (mantleand crust). These carbon stores interact with the other components through geological processes.

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth. The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments are fairly balanced so that carbon levels would be roughly stable without human influence.

Carbon in the Earth's atmosphere exists in two main forms: carbon dioxide and methane. Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect. Methane produces a larger greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short-lived than carbon dioxide, making carbon dioxide the more important greenhouse gas of the two.

Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (ocean, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity. It can then be absorbed by rocks through weathering. It also can acidify other surfaces it touches or be washed into the ocean.

Human activities over the past two centuries have significantly increased the amount of carbon in the atmosphere, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete.

In the extremely far future, the carbon cycle will likely speed up the rate of carbon dioxide is absorbed into the soil from carbonate–silicate cycle. This is mainly caused by the increased luminosity of the Sun, which speeds up the rate of surface weathering. This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth's crust as carbonate. Though volcanoes will continue to pump carbon dioxide into the atmosphere in the short term, it will not be enough to keep the carbon dioxide level stable in the long term. Once the carbon dioxide level falls below 50 particles per million, C₃photosynthesis will no longer be possible. This is expected to occur about 600 million years from now.

Once the oceans on the Earth evaporate in about 1.1 billion years from now, plate tectonics will very likely stop due to the lack of water to lubricate them. The lack of volcanoes pumping out carbon dioxide will cause the carbon cycle to end between 1 billion and 2 billion years into the future.

The terrestrial biosphere includes the organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils. About 500 gigatons of carbon are stored above ground in plants and other living organisms, while soil holds approximately 1,500 gigatons of carbon. Most carbon in the terrestrial biosphere is organic carbon, while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate. Organic carbon is a major component of all organisms living on earth.

Autotrophs extract it from the air in the form of carbon dioxide, converting it into organic carbon, while heterotrophs receive carbon by consuming other organisms.

Because carbon uptake in the terrestrial biosphere is dependent on biotic factors, it follows a diurnal and seasonal cycle. In CO₂ measurements, this feature is apparent in the Keeling curve. It is strongest in the northern hemisphere because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon.

Carbon leaves the terrestrial biosphere in several ways and on different time scales. The combustion or respiration of organic carbon releases it rapidly into the atmosphere. It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon. Carbon stored in soil can remain there for up to thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration. Between 1989 and 2008 soil respiration increased by about 0.1% per year. In 2008, the global total of CO₂ released by soil respiration was roughly 98 billion tonnes, about 10 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel (this does not represent a net transfer of carbon from soil to atmosphere, as the respiration is largely offset by inputs to soil carbon).

 There are a few plausible explanations for this trend, but the most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO₂. The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change.

The ocean can be conceptually divided into a surface layer within which water makes frequent (daily to annual) contact with the atmosphere, and a deep layer below the typically mixed layerdepth of a few hundred meters or less, within which the time between consecutive contacts may be centuries. The dissolved inorganic carbon (DIC) in the surface layer is exchanged rapidly with the atmosphere, maintaining equilibrium. Partly because its concentration of DIC is about 15% higher but mainly due to its larger volume, the deep ocean contains far more carbon—it's the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere—but the timescale to reach equilibrium with the atmosphere is hundreds of years: the exchange of carbon between the two layers, driven by thermohaline circulation, is slow.

Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small fraction of which is converted into carbonate. It can also enter the ocean through rivers as dissolved organic carbon. It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the ocean's deeper, more carbon-rich layers as dead soft tissue or in shells as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation. Oceans are basic (~pH 8.2), hence CO₂ acidification shifts the pH of the ocean towards neutral.

Oceanic absorption of CO₂ is one of the most important forms of carbon sequestering limiting the human-caused rise of carbon dioxide in the atmosphere. However, this process is limited by a number of factors.

CO₂ absorption makes water more acidic, which affects ocean biosystems. The projected rate of increasing oceanic acidity could slow the biological precipitation of calcium carbonates, thus decreasing the ocean's capacity to absorb carbon dioxide.

The geologic component of the carbon cycle operates slowly in comparison to the other parts of the global carbon cycle. It is one of the most important determinants of the amount of carbon in the atmosphere, and thus of global temperatures.

Most of the earth's carbon is stored inertly in the earth's lithosphere. Much of the carbon stored in the earth's mantle was stored there when the earth formed. Some of it was deposited in the form of organic carbon from the biosphere. Of the carbon stored in the geosphere, about 80% is limestone and its derivatives, which form from the sedimentation of calcium carbonate stored in the shells of marine organisms. The remaining 20% is stored as kerogens formed through the sedimentation and burial of terrestrial organisms under high heat and pressure. Organic carbon stored in the geosphere can remain there for millions of years.

Carbon can leave the geosphere in several ways. Carbon dioxide is released during the metamorphosis of carbonate rocks when they are subducted into the earth's mantle. This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots. It can also be removed by humans through the direct extraction of kerogens in the form of fossil fuels. After extraction, fossil fuels are burned to release energy, thus emitting the carbon they store into the atmosphere

Although deep carbon cycling is not as well-understood as carbon movement through the atmosphere, terrestrial biosphere, ocean, and geosphere, it is nonetheless an incredibly important process. The deep carbon cycle is intimately connected to the movement of carbon in the Earth's surface and atmosphere. If the process did not exist, carbon would remain in the atmosphere, where it would accumulate to extremely high levels over long periods of time. Therefore, by allowing carbon to return to the Earth, the deep carbon cycle plays a critical role in maintaining the terrestrial conditions necessary for life to exist.

Furthermore, the process is also significant simply due to the massive quantities of carbon it transports through the planet. In fact, studying the composition of basaltic magma and measuring carbon dioxide flux out of volcanoes reveals that the amount of carbon in the mantle is actually greater than that on the Earth's surface by a factor of one thousand. Drilling down and physically observing deep-Earth carbon processes is evidently extremely difficult, as the lower mantle and core extend from 660 to 2,891 km and 2,891 to 6,371  km deep into the Earth respectively. Accordingly, not much is conclusively known regarding the role of carbon in the deep Earth. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of said layer. Furthermore, techniques like seismology have led to a greater understanding of the potential presence of carbon in the Earth's core.

Carbon in the Lower Mantle

Carbon principally enters the mantle in the form of carbonate-rich sediments on tectonic platesof ocean crust, which pull the carbon into the mantle upon undergoing subduction. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within said region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. The study analysed rare, super-deep diamonds at a site in Juina, Brazil, determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and crytallisation under lower mantle temperatures and pressures. Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle silicates, eventually forming super-deep diamonds like the one found.

However, carbonates descending to the lower mantle encounter other fates in addition to forming diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800 km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite. Other experiments—as well as petrologic observations—support this claim, indicating that magnesite is actually the most stable carbonate phase in most part of the mantle. This is largely a result of its higher melting temperature. Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacityenvironments. Magnesium, iron, and other metallic compounds act as buffers throughout the process. The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle.

Carbon in the Core

Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys. Because the core's composition is believed to be an alloy of crystalline iron and a small amount of nickel, this seismic anomaly indicates the presence of light elements, including carbon, in the core. In fact, studies using diamond anvil cells to replicate the conditions in the Earth's core indicate that iron carbide (Fe₇C₃) matches the inner core's wave speed and density. Therefore, the iron carbide model could serve as an evidence that the core holds as much as 67% of the Earth's carbon. Furthermore, another study found that in the pressure and temperature condition of the Earth's inner core, carbon dissolved in iron and formed a stable phase with the same Fe₇C₃composition—albeit with a different structure from the one previously mentioned. In summary, although the amount of carbon potentially stored in the Earth's core is not known, recent studies indicate that the presence of iron carbides can explain some of the geophysical observations.

Since the industrial revolution, human activity has modified the carbon cycle by changing its components' functions and directly adding carbon to the atmosphere.

The largest human impact on the carbon cycle is through direct emissions from burning fossil fuels, which transfers carbon from the geosphere into the atmosphere. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.

Another direct human impact on the carbon cycle is the chemical process of calcination of limestonefor clinker production, which releases CO₂. Clinker is an industrial precursor of cement.

Humans also influence the carbon cycle indirectly by changing the terrestrial and oceanic biosphere. Over the past several centuries, direct and indirect human-caused land useand land cover change (LUCC) has led to the loss of biodiversity, which lowers ecosystems' resilience to environmental stresses and decreases their ability to remove carbon from the atmosphere. More directly, it often leads to the release of carbon from terrestrial ecosystems into the atmosphere. Deforestation for agricultural purposes removes forests, which hold large amounts of carbon, and replaces them, generally with agricultural or urban areas. Both of these replacement land cover types store comparatively small amounts of carbon so that the net product of the process is that more carbon stays in the atmosphere.

Other human-caused changes to the environment change ecosystems' productivity and their ability to remove carbon from the atmosphere. Air pollution, for example, damages plants and soils, while many agricultural and land use practices lead to higher erosion rates, washing carbon out of soils and decreasing plant productivity.

Humans also affect the oceanic carbon cycle. Current trends in climate change lead to higher ocean temperatures, thus modifying ecosystems. Also, acid rain and polluted runoff from agriculture and industry change the ocean's chemical composition. Such changes can have dramatic effects on highly sensitive ecosystems such as coral reefs, thus limiting the ocean's ability to absorb carbon from the atmosphere on a regional scale and reducing oceanic biodiversity globally.

Arctic methane emissions indirectly caused by anthropogenic global warming also affect the carbon cycle and contribute to further warming in what is known as climate change feedback.

On 12 November 2015, NASA scientists reported that human-made carbon dioxide (CO₂) continues to increase, reaching levels not seen in hundreds of thousands of years: currently, the rate carbon dioxide released by the burning of fossil fuels is about double the net uptake by vegetation and the ocean

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmosphere nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. Human modification of global nitrogen cycle can negatively affect the natural environment system and also human health.

Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, ammonium (NH⁺₄), nitrite (NO⁻₂), nitrate (NO⁻₃), nitrous oxide(N₂O), nitric oxide (NO) or inorganic nitrogen gas (N₂). Organic nitrogen may be in the form of a living organism, humus or in the intermediate products of organic matter decomposition. The processes in the nitrogen cycle is to transform nitrogen from one form to another. Many of those processes are carried out by microbes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. For example, the nitrogenous wastes in animal urine are broken down by nitrifying bacteria in the soil to be used by plants. The diagram alongside shows how these processes fit together to form the nitrogen cycle.

Nitrogen fixation

The conversion of nitrogen gas (N₂) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. Between 5 and 10 billion kg per year are fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds.

Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two-component enzymethat has multiple metal-containing prosthetic groups An example of free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such symbioses. Today, about 30% of the total fixed nitrogen is produced industrially using the Haber-Boschprocess, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.

Assimilation

Plants can absorb nitrate or ammonium from the soil by their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants that have a symbiotic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. It is now known that there is a more complex cycling of amino acids between Rhizobiabacteroids and plants. The plant provides amino acids to the bacteroids so ammonia assimilation is not required and the bacteroids pass amino acids (with the newly fixed nitrogen) back to the plant, thus forming an interdependent relationship. While many animals, fungi, and other heterotrophic organisms obtain nitrogen by ingestion of amino acids, nucleotides, and other small organic molecules, other heterotrophs (including many bacteria) are able to utilize inorganic compounds, such as ammonium as sole N sources. Utilization of various N sources is carefully regulated in all organisms.

Ammonification

When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH⁺₄), a process called ammonification or mineralization.

As a result of extensive cultivation of legumes (particularly soy, alfalfa, and clover), growing use of the Haber–Bosch process in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into biologically available forms. In addition, humans have significantly contributed to the transfer of nitrogen trace gases from Earth to the atmosphere and from the land to aquatic systems. Human alterations to the global nitrogen cycle are most intense in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest.

Generation of Nr, reactive nitrogen, has increased over 10 fold in the past century due to global industrialisation. This form of nitrogen follows a cascade through the biosphere via a variety of mechanisms, and is accumulating as the rate of its generation is greater than the rate of denitrification.

Nitrous oxide (N₂O) has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources. N₂O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. Nitrous oxide is also a greenhouse gas and is currently the third largest contributor to global warming, after carbon dioxide and methane. While not as abundant in the atmosphere as carbon dioxide, it is, for an equivalent mass, nearly 300 times more potent in its ability to warm the planet.

Ammonia (NH₃) in the atmosphere has tripled as the result of human activities. It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging to water droplets, eventually resulting in nitric acid(HNO₃) that produces acid rain. Atmospheric ammonia and nitric acid also damage respiratory systems.

The very high temperature of lightning naturally produces small amounts of NO_x, NH₃, and HNO₃, but high-temperature combustion has contributed to a 6- or 7-fold increase in the flux of NO_x to the atmosphere. Its production is a function of combustion temperature - the higher the temperature, the more NO_x is produced. Fossil fuel combustion is a primary contributor, but so are biofuels and even the burning of hydrogen. However, the rate that hydrogen is directly injected into the combustion chambers of internal combustion engines can be controlled to prevent the higher combustion temperatures that produce NO_x.

Ammonia and nitrous oxides actively alter atmospheric chemistry. They are precursors of tropospheric (lower atmosphere) ozone production, which contributes to smog and acid rain, damages plants and increases nitrogen inputs to ecosystems. Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can damage the health of plants, animals, fish, and humans.

Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species-diverse heathlands.

Impacts on Human Health: Air Quality

Human activities have also dramatically altered the global nitrogen cycle via production of nitrogenous gases, associated with the global atmospheric nitrogen pollution. There are multiple sources of atmospheric Nr fluxes. Agricultural sources of Nr can produce atmospheric emission of ammonia (NH₃₎, nitrogen oxides (NO_x) and nitrous oxide (N₂O). Combustion processes in energy production, transportation and industry can also result in the formation of new Nr via the emission of NO_x, an unintentional waste product.

When those Nr are released to the lower atmosphere, they can induce the formation of smog, particular matter (PM) and aerosols, all of which are major contributors to adverse health effects on human health from air pollution. In the atmosphere, NO₂ can be oxidized to nitric acid (HNO₃), and it can further react with NH₃ to form ammonium nitrate, which facilitates the formation of particular nitrate. Moreover, NH₃ can react with other acid gases (sulfuric and hydrochloric acids) to form ammonium-containing particles, which are the precursors for the secondary organic aerosol particles in photochemical smog.

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole earth system and then specificaly focused on the cycle in terrestrial and aquatic systems.

Humans have caused major changes to the global phosphorus cycle through shipping of phosphorus minerals, and use of phosphorus fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent.

Phosphorus in the environment

Phosphorus is an essential nutrient for plants and animals. Phosphorus is a limiting nutrient for aquatic organisms. Phosphorus forms parts of important life-sustaining molecules that are very common in the biosphere. Phosphorus does enter the atmosphere in very small amounts when dust is dissolved in rainwater and seaspray, but remains mostly on land and in rock and soil minerals. Eighty percent of the mined phosphorus is used to make fertilizers. Phosphates from fertilizers, sewage and detergents can cause pollution in lakes and streams. Overenrichment of phosphate in both fresh and inshore marine waters can lead to massive algae blooms which, when they die and decay, leads to eutrophication of fresh waters only. An example of this is the Canadian Experimental Lakes Area. These freshwater algal blooms should not be confused with those in saltwater environments. Recent research suggests that the predominant pollutant responsible for algal blooms in salt water estuaries and coastal marine habitats is Nitrogen.

Phosphorus occurs most abundantly in nature as part of the orthophosphate ion (PO₄)³⁻, consisting of a P atom and 4 oxygen atoms. On land most phosphorus is found in rocks and minerals. Phosphorus rich deposits have generally formed in the ocean or from guano, and over time, geologic processes bring ocean sediments to land. 

Process of the cycle

Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.

         

Phosphorus and Eutrophication

Eutrophication is an enrichment of water by nutrient that lead to structural changes to the aquatic ecosystem such as algae bloom, deoxygenation, reduction of fish species. The primary source that contributes to the eutrophication is considered as nitrogen and phosphorus. When these two elements exceed the capacity of the water body, eutrophication occurs. Phosphorus that enters lakes will accumulate in the sediments and the biosphere, it also can be recycled from the sediments and the water system. Drainage water from agricultural land also carries phosphorus and nitrogen. Since a large amount of phosphorus is in the soil contents, so the overuse of fertilizers and over-enrichment with nutrients will lead to increasing the amount of phosphorus concentration in agricultural runoff. When eroded soil enters the lake, both phosphorus and the nitrogen in the soil contribute to eutrophication, and erosion caused by deforestation which also results from uncontrolled planning and urbanization.

Nutrients are important to the growth and survival of living organisms, and hence, are essential for development and maintenance of healthy ecosystems. Humans have greatly influenced the phosphorus cycle by mining phosphorus, converting it to fertilizer, and by shipping fertilizer and products around the globe. Transporting phosphorus in food from farms to cities has made a major change in the global Phosphorus cycle. However, excessive amounts of nutrients, particularly phosphorus and nitrogen, are detrimental to aquatic ecosystems. Waters are enriched in phosphorus from farms' run-off, and from effluent that is inadequately treated before it is discharged to waters. The input of P in agricultural runoff can accelerate the eutrophication of P-sensitive surface waters. Natural eutrophication is a process by which lakes gradually age and become more productive and may take thousands of years to progress. Cultural or anthropogenic eutrophication, however, is water pollution caused by excessive plant nutrients; this results in excessive growth in the algal population; when this algae dies its putrefaction depletes the water of oxygen. Such eutrophication may also give rise to toxic algal bloom. Both these effects cause animal and plant death rates to increase as the plants take in poisonous water while the animals drink the poisoned water. Surface and subsurface runoff and erosion from high-phosphorus soils may be major contributing factors to this fresh water eutrophication. The processes controlling soil Phosphorus release to surface runoff and to subsurface flow are a complex interaction between the type of phosphorus input, soil type and management, and transport processes depending on hydrological conditions.

Repeated application of liquid hog manure in excess to crop needs can have detrimental effects on soil phosphorus status. Also, application of biosolids may increase available phosphorus in soil. In poorly drained soils or in areas where snowmelt can cause periodic waterlogging, dereducing conditions can be attained in 7–10 days. This causes a sharp increase in phosphorus concentration in solution and phosphorus can be leached. In addition, reduction of the soil causes a shift in phosphorus from resilient to more labile forms. This could eventually increase the potential for phosphorus loss. This is of particular concern for the environmentally sound management of such areas, where disposal of agricultural wastes has already become a problem. It is suggested that the water regime of soils that are to be used for organic wastes disposal is taken into account in the preparation of waste management regulations.

Human interference in the phosphorus cycle occurs by overuse or careless use of phosphorus fertilizers. This results in increased amounts of phosphorus as pollutants in bodies of water resulting in eutrophication. Eutrophication devastates water ecosystems by inducing anoxic conditions

The sulfur cycle is the collection of processes by which sulfur moves between rocks, waterways and living systems. Such biogeochemical cycles are important in geology because they affect many minerals. Biochemical cycles are also important for life because sulfur is an essential element, being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes.         

Sulfur is found in oxidation states ranging from +6 in SO₄²⁻ to -2 in sulfides. Thus, elemental sulfur can either give or receive electrons depending on its environment. On the anoxic early Earth, most sulfur was present in minerals such as pyrite(FeS₂). Over Earth history, the amount of mobile sulfur increased through volcanic activity as well as weathering of the crust in an oxygenated atmosphere. Earth's main sulfur sink is the oceans SO₄²⁻, where it is the major oxidizing agent.

When SO₄²⁻ is assimilated by organisms, it is reduced and converted to organic sulfur, which is an essential component of proteins. However, the biosphere does not act as a major sink for sulfur, instead the majority of sulfur is found in sea water or sedimentary rocks including: pyrite rich shales, evaporite rocks (anhydrite and baryte), and calcium and magnesium carbonates (i.e. carbonate-associated sulfate).

Dimethylsulfide [(CH₃)₂S or DMS] is produced by the decomposition of dimethylsulfoniopropionate(DMSP) from dying phytoplankton cells in the ocean's photic zone, and is the major biogenic gas emitted from the sea, where it is responsible for the distinctive “smell of the sea” along coastlines. DMS is the largest natural source of sulfur gas, but still only has a residence time of about one day in the atmosphere and a majority of it is redeposited in the oceans rather than making it to land. However, it is a significant factor in the climate system, as it is involved in the formation of clouds.

Global climate change, prior to the 20th century, appears to have been initiated primarily by major changes in volcanic activity. Sulfur dioxide (SO²) is the most voluminous chemically active gas emitted by volcanoes and is readily oxidized to sulfuric acid normally within weeks. But trace amounts of SO₂ exert significant influence on climate. All major historic volcanic eruptions have formed sulfuric acid aerosols in the lower stratosphere that cooled the earth's surface ~ 0.5 °C for typically three years. While such events are currently happening once every 80 years, there are times in geologic history when they occurred every few to a dozen years. These were times when the earth was cooled incrementally into major ice ages. There have also been two dozen times during the past 46,000 years when major volcanic eruptions occurred every year or two or even several times per year for decades. Each of these times was contemporaneous with very rapid global warming. Large volumes of SO₂erupted frequently appear to overdrive the oxidizing capacity of the atmosphere resulting in very rapid warming. Such warming and associated acid rain becomes extreme when millions of cubic kilometers of basalt are erupted in much less than one million years. These are the times of the greatest mass extinctions. When major volcanic eruptions do not occur for decades to hundreds of years, the atmosphere can oxidize all pollutants, leading to a very thin atmosphere, global cooling and decadal drought. Prior to the 20th century, increases in atmospheric carbon dioxide (CO₂) followed increases in temperature initiated by changes in SO₂.

By 1962, man burning fossil fuels was adding SO₂ to the atmosphere at a rate equivalent to one “large” volcanic eruption each 1.7 years. Global temperatures increased slowly from 1890 to 1950 as anthropogenic sulfur increased slowly. Global temperatures increased more rapidly after 1950 as the rate of anthropogenic sulfur emissions increased. By 1980 anthropogenic sulfur emissions peaked and began to decrease because of major efforts especially in Japan, Europe, and the United States to reduce acid rain.

Atmospheric concentrations of methane began decreasing in 1990 and have remained nearly constant since 2000, demonstrating an increase in oxidizing capacity. Global temperatures became roughly constant around 2000 and even decreased beginning in late 2007. Meanwhile atmospheric concentrations of carbon dioxide have continued to increase at the same rate that they have increased since 1970. Thus SO₂ is playing a far more active role in initiating and controlling global warming than recognized by the Intergovernmental Panel on Climate Change. Massive reduction of SO₂ should be a top priority in order to reduce both global warming and acid rain. But man is also adding two to three orders of magnitude more CO₂ per year to the climate than one “large” volcanic eruption added in the past. Thus CO₂, a greenhouse gas, is contributing to global warming and should be reduced. We have already significantly reduced SO₂ emissions in order to reduce acid rain. We know how to do it both technically and politically.

In the past, sudden climate change was typically triggered by sudden increases in volcanic activity. Slow increases in greenhouse gases, therefore, do not appear as likely as currently thought to trigger tipping points where the climate suddenly changes. However we do need to start planning an appropriate human response to future major increases in volcanic activity.

Hydrogen cycle

The hydrogen cycle consists of hydrogen exchanges between biotic (living) and abiotic (non-living) sources and sinks of hydrogen-containing compounds.

Hydrogen (H) is the most abundant element in the universe. On Earth, common H-containing inorganic molecules include water (H₂O), hydrogen gas (H₂), methane (CH₄), hydrogen sulfide (H₂S), and ammonia (NH₃). Many organic compounds also contain H atoms, such as hydrocarbons and organic matter. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen, H₂.

Hydrogen gas can be produced naturally through rock-water interactions or as a byproduct of microbial metabolisms. Free H₂ can then be consumed by other microbes, oxidized photochemically in the atmosphere, or lost to space. Hydrogen is also thought to be an important reactant in pre-biotic chemistry and the early evolution of life on Earth, and potentially elsewhere in our solar system

 

Abiotic cycles

Sources

Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic serpentinization reactions between water and olivine minerals produce H₂ in the marine or terrestrial subsurface. In the ocean, hydrothermal vents erupt magma and altered seawater fluids including abundant H₂, depending on the temperature regime and host rock composition. Molecular hydrogen can also be produced through photooxidation (via solar UV radiation) of some mineral species such as sideritein anoxic aqueous environments. This may have been an important process in the upper regions of early Earth's Archaean oceans.

Sinks

Because H₂ is the lightest element, atmospheric H₂ can readily be lost to space via Jeans escape, an irreversible process that drives Earth's net mass loss. Photolysis of heavier compounds not prone to escape, such as CH₄ or H₂O, can also liberate H₂ from the upper atmosphere and contribute to this process. Another major sink of free atmospheric H₂ is photochemical oxidation by hydroxyl radicals (•OH), which forms water.

Anthropogenic sinks of H₂ include synthetic fuel production through the Fischer-Tropsch reaction and artificial nitrogen fixation through the Haber-Bosch process to produce nitrogen fertilizers.

Biotic cycles

Many microbial metabolisms produce or consume H₂.

Production

Hydrogen is produced by hydrogenases and nitrogenases enzymes in many microorganisms, some of which are being studied for their potential for biofuel production. These H₂-metabolizing enzymes are found in all three domains of life, and out of known genomes over 30% of microbial taxa contain hydrogenase genes. Fermentation produces H₂ from organic matter as part of the anaerobic microbial food chain via light-dependent or light-independent pathways.

Consumption

Biological soil uptake is the dominant sink of atmospheric H₂. Both aerobic and anaerobic microbial metabolisms consume H₂ by oxidizing it in order to reduce other compounds during respiration. Aerobic H₂ oxidation is known as the Knallgas reaction.

Anaerobic H₂ oxidation often occurs during interspecies hydrogen transfer in which H₂produced during fermentation is transferred to another organism, which uses the H₂ to reduce CO₂ to CH₄ or acetate, SO₄^2- to H₂S, or Fe³⁺ to Fe²⁺. Interspecies hydrogen transfer keeps H₂concentrations very low in most environments because fermentation becomes less thermodynamically favorable as the partial pressure of H₂ increases.

Relevance for the global climate

H₂ can interfere with the removal of methane from the atmosphere, a greenhouse gas. Typically, atmospheric CH₄ is oxidized by hydroxyl radicals (•OH), but H₂ can also react with •OH to reduce it to H₂O.

1.   CH₄ + OH àCH₃ + H₂O

2.   H₂ + OH  à H + H₂O

{\displaystyle \mathrm {CH_{4}+OH\longrightarrow CH_{3}+H_{2}O} }{\displaystyle \mathrm {H_{2}+OH\longrightarrow H+H_{2}O} }Implications for Astrobiology

Hydrothermal H₂ may have played a major role in pre-biotic chemistry. Production of H₂ by serpentinization supported formation of the reactants proposed in the iron-sulfur world origin of life hypothesis. The subsequent evolution of hydrogen notrophic methano genesis is hypothesized as one of the earliest metabolisms on Earth.

Serpentinization can occur on any planetary body with chondritic composition. The discovery of H₂on other ocean worlds, such as Enceladus, suggests that similar processes are ongoing elsewhere in our solar system, and potentially in other solar systems as well.