Contents“Effects of Combustion on our Environment”.......................................................................................................................1

1. Combustion:..............................................................................................................................................................1

a) Complete combustion:..............................................................................................................................................1

b) Incomplete combustion:...........................................................................................................................................1

c) Smoldering:...............................................................................................................................................................2

d) Rapid combustion:.....................................................................................................................................................2

e) Turbulent combustion:..............................................................................................................................................2

2. Environment Effects of Combustion:.........................................................................................................................2

3. Pollutant Emission:....................................................................................................................................................3

4. Exhaust Emission and Pollution:................................................................................................................................4

a) CO2 :...........................................................................................................................................................................5

b) CO:.............................................................................................................................................................................5

c) NOx:...........................................................................................................................................................................5

d) Volatile Organic Compounds:....................................................................................................................................6

e) Particulate matter:....................................................................................................................................................7

f) SO2:............................................................................................................................................................................7

a) Combustion process:.................................................................................................................................................8

Ecological systems:..................................................................................................................................................11

Natural systems:.......................................................................................................................................................11

Greenhouse gases:..................................................................................................................................................12

Role in climate change:............................................................................................................................................12

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“Effects of Combustion on our Environment”

1. Combustion:Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid phase.

a) Complete combustion: In a complete combustion reaction, a compound reacts with an oxidizing element, such as oxygen or fluorine, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH4 + 2 O2 → CO2 + 2 H2O + energy

CH2S + 6 F2 → CF4 + 2 HF + SF6

A simple example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines:

2 H2 + O2 → 2 H2O(g) + heat

The result is water vapor. Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present such as carbon monoxide and pure carbon (soot or ash). Additionally, any combustion in atmospheric air, which is 78% nitrogen, will also create several forms of oxides. In complete combustion, the reactant burns in oxygen, producing a limited number of products.

b) Incomplete combustion: Incomplete combustion will only occur when there isn't enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink such as a solid surface or flame trap.

For most fuels, such as diesel oil, coal or wood, pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.

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c) Smoldering: Smoldering is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction.

d) Rapid combustion: Rapid combustion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons. Sometimes, a large volume of gas is liberated in combustion besides the production of heat and light. The sudden evolution of large quantities of gas creates excessive pressure that produces a loud noise. Such a combustion is known as an explosion.

e) Turbulent combustion: Combustion resulting in a turbulent flame is the most used for industrial application (e.g. gas turbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

2. Environment Effects of Combustion: Combustion is a hazard, and, besides the many services it provides to humankind, it may cause nuisance (e.g. noise, smoke), damage to property (deformations, loss of strength, burnings and explosions), and damage to people (injury and loss of life). Damage may be ranked in top-down severity order as: a) loss of life (mortality), b) loss of health (morbidity), c) loss of property and d) loss of activity.

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Besides damage caused by the combustion process itself, there is also damage associated to the management of fuels (and oxidisers, if special). Coal handling produces respiratory hazards, and crude-oil derivatives are carcinogenic. Liquefied petroleum gases, LPG, and particularly cryogenic fluids like LNG, may cause severe frostbite and structural damage (carbon- and low-alloy steels show a marked ductile to brittle transition at freezing temperatures).

Damage may be caused to individuals or to the environment in general. Combustion is a physico-chemical hazard, and, to minimise its impact, one has to be aware of it, rely on safer fuels (e.g. diesel instead of gasoline, natural gas instead of butane), reduce unnecessary fuel stores, avoid fuel leakages and provide fuel detectors, reduce uncontrolled ignition-sources (sparks and hot spots), decrease the impact of the controlled combustion processes (emissions), and plan for the best rescue actions in case of uncontrolled combustion (fire detection and fighting). As usual, pyrotechnics hazards (from amusement firecrackers to weaponry) are not considered under the combustion heading.

A quick-summary of hazard types associated to combustion may be:

Physical hazards: mechanical (explosion), thermal (excessive heat, out-range temperatures), radiation (blinding flare).

Chemical hazards: oxygen depletion, gas poison, aerosols, liquid poisons, solid poisons.

The effects of combustion on the environmental may be grouped in two categories: Sudden uncontrolled events (combustion accidents). This impact is fought with proper prevention

(minimising risk and educating people) and proper fire fighting (smoke detectors, automatic fire-sprinklers, portable fire-extinguishers, fire-fighting brigades).

Continuous degradation, due to pollutant emissions during normal combustion. This impact is fight by developing better fuels (sulfur-free diesel, unleaded gasoline), better combustion processes (fluidised bed, porous media) and post-combustion treatment (catalytic conversion).

Only a descriptive view of the subject is here presented (some theoretical insight can be found on Combustion Kinetics). Thermal effects on materials in general, which may be originated from a combustion process, are presented aside. We have tried to follow a top-down approach in the analysis of the environmental effects of combustion, i.e. from deadly explosions to inconvenient electromagnetic interferences.

3. Pollutant Emission:Life is a polluting process, because life must live at the expense of the environment. The problem is that the amount and concentration of pollutants emitted by human activities has gone too far and seriously menace life, both locally and globally.

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The first and traditional approach to fight pollution is to go away or throw it away, e.g. to make a chimney for venting the fireplace, to have out-side-air sealed combustion boilers, to build power stations aside, etc.

The effects on the environment are usually identified with pollutant emission through the tail-pipe of combustors, but handling of fuels, fuel losses at the inlet, and product losses from the combustor shell, are other sources (up to 20% of the hydrocarbon emissions in a car do not go out along the tail-pipe). Mass losses and energy losses may be a danger to humans, animals, plants and goods: explosion danger (in confined places), open flame danger, toxicity from CO (and other toxic gases in chemical fires), suffocation or anoxia from CO2, hyperthermia by heat, respiratory and visual irritation by smoke and noxious gases, etc. Mechanical pollution (noise) and electromagnetic pollution (interferences, EMI), are dealt apart.

Main contaminants, besides the unavoidable CO2 in carbon-containing fuels that contribute to the greenhouse warming (the legacy left by these emissions would be felt mainly by our offspring), are: VOCs (Volatile Organic Compounds), CO, NOx, SOx, and particulate matter (PM, or PM10 to explicitly restrict to sizes <10 m). Soot is formed in non-premixed flames and on premixed flames for equivalence ratios >1.5. Diesel engines produce more pollutants in the stated order (more PM and less VOC), whereas Otto engines do just in the reverse order. Some 40% of man-produced VOCs come from transport (not only through the tail-pipe but from reservoirs and at the stations). Sometimes, instead of VOC, the term HC (hydrocarbons) is used, even splitting between methane and NMHC (non-methane hydrocarbons).

4. Exhaust Emission and Pollution:

The complete combustion of CuHvOwNx fuels would only yield CO2 and H2O as new compounds. Water is thought to be processed globally by the hydrological cycle, but CO2 is already considered a pollutant due to the overall green-house-effect contribution if non-renewable fuels are used. We intend here to analyse pollutants going out the exhaust pipe of mobile and stationary combustors that, although in concentration much lower than CO2 and H2O (typically less than 1%), have a great impact on the environment. Combustion processes where non-commercial fuels are burn (e.g. wild fires, waste incineration, rocket propulsion) give off many more hazardous contaminants not considered here: halogenated (dioxines, furanes, hydrogen chloride), organometallic and others.

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In premixed combustion, emissions depend upon the air-fuel ratio, but unfortunately when the concentration of CO and HC decreases the concentration of NOx increases, and vice versa. Non-premixed combustion may be much more pollutant: the flame sits near the stoichiometric diffusion-rates and thus its temperature is very high, causing the formation of NOx, and the pure fuel approaching it gets pyrolysed with large production of soot (that give the characteristic yellow colour to non-premixed flames) and other volatiles.

a) CO2 : Carbon dioxide (CO2) is an unavoidable emission in the combustion of carbon-containing fuels. There is little concern with local contamination by CO2 emissions, being an inert gas like nitrogen. Like nitrogen, it may cause local suffocation; a 10%CO2 is fatal to a person after a few minutes, by anoxia, and a 5% already produces troubles after 1 hour. The main concern of CO2 emissions is at global scale, with the positive correlation found between anthropogenic CO2 generation and the increase of CO2 fraction in air (e.g. from 310 ppm molar in 1950, to 380 ppm molar in 2007), and the foreseeable consequences on climate change due to the associated increase in the global greenhouse effect, which is feared to be highly non-linear, with a small global warming but larger regional changes (desertification, floods, hurricanes, and so on.

b) CO: Carbon monoxide (CO) is found in exhaust emissions due to a poor combustion process (i.e. too rich a mixture, unburnt fuel pyrolises at crevices, or not enough residence time for equilibrium), particularly in the Otto engine at cold starts, idle, and full power conditions. Unburnt pyrolysed fuel would be in negligible amounts if sufficient time for equilibrium at the low exit temperatures were allowed, as in large combustion chambers and large marine engines, where the residence time is near one second (combustion is similar to eating; it takes time for a good digestion, starting by proper food preparation, chewing and so on).

CO is a deadly poison that reduces the ability of the blood to absorb oxygen and, as a result, lowers the blood oxygen content by producing carboxyhemoglobin. Even as low a proportion as 0.5 percent by volume of CO in the air can prove fatal within 1 hour (>50% carboxyhemoglobin in blood), 0.05% produces headache after 10 hours (10% carboxyhemoglobin in blood). In uncontrolled fires, like in a hotel room, typical concentrations of up to 5% are achieved after the fire runs away of control. Measured in the blood. For clean air standard EU sets a limit of 10 mgCO/m3 in a 8 hours average.

c) NOx: NOx stands for all nitrogen oxides, mainly NO and NO2, but also N2O, N2O2, N2O3, N2O4 and N2O5, and all appear from atmospheric nitrogen during combustion with air (coals and heavy fuel-oils have some intrinsic nitrogen also, up to a few percent by weight). All nitrogen oxides are unstable at ambient conditions when pure, i.e. they dissociate, but their decomposition may be very slow. They are formed at very high temperatures in the presence of air (there is a high peak in the range =1..1.1), and two approaches are followed to avoid their emission: avoidance of high temperature formation (by using very lean mixtures, exhaust gas recirculation, porous burners, catalytic burners, water

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injection), and catalytic reduction at the exhaust. Notice that the peak in NOx production practically coincides with the range of maximum combustion efficiency (minimum entropy production), so that it might be said the NOx emission is a sign of good combustion, contrary to unburnt emissions.

The most polluting of NOx-components is nitrogen dioxide, NO2, a brown gas at normal conditions (but readily condensable, Tb=11 ºC). When heating an ampoule containing NO2 from above, some dinitrogen tetroxide N2O4 (2NO2=N2O4(g)+57 kJ/mol) is exothermically formed; N2O4 is a colourless heavier gas that appears at the bottom (because of buoyancy), although the reaction would be more displaced to the left in equilibrium at high temperature (but kinetics dominates). A third gas appears when heating at 600 ºC, nitrogen oxide, NO (N2O4=2NO+O2, also transparent. Nitrogen oxide, NO, is a colourless gas that in the presence of atmospheric oxygen, rapidly converts to yellow NO2; NO concentration can be measured by chemiluminescence with ozone: NO+O3=NO2+O2+h. NO2 smells pungently and causes pronounced irritation of the respiratory system if > 10 ppm, and is fatal if >100 ppm after minutes; due to the fact that it destroys the lung tissue, for clean air standard EU sets a limit of 40 gNO2/m3. When NO2 or N2O4 are cooled, a blue liquid condensate first develops (a strong mixture of N2O4 in NO2), and after further cooling, a blue solid appears, mainly consisting of N2O3. N2O is a powerful greenhouse gas. Atmospheric ozone, O3, is another pollutant (contrary to stratospheric ozone), and, although not directly emitted in combustors, it is formed by reaction with air of NOx emissions. Nitrogen oxides also combine with water vapour to form acid mists (pH<5.6 at 288 K) that give way to acid rain, damaging forest, lakes and rivers ecosystems, one of the key reactions being NO(g) + (3/4)O2(g) +(1/2)H2O = H+(aq) + NO3

-(aq).

d) Volatile Organic Compounds:Volatile organic compounds, coming from unburnt fuel and pyrolysed fuel, are a group of chemicals with Tb<250 ºC that includes important air pollutants like benzene, 1,3 butadiene, and acrylic aldehyde (CH2CHCHO, deadly if >10 ppm), that is the cause of the bad smell from tail-pipes. From uncontrolled fires, with typical solid substances as wood, wool, plastics and flesh, very toxic substances are released, as hydrogen cyanide (HCN, deadly if >0.3%, and found up to 0.1% in typical home fires), ammonia (NH3, deadly if >0.3% in half an hour), hydrogen sulphide in rubber and flesh burning (H2S, deadly if >0.1%), and phosgene from PVC burning (COCl2, deadly if >0.1 ppm). For clean air standard EU sets a limit of 5 gC6H6/m3.

VOC is a cul de sac, comprising all chemical emissions except the singled-out H2O, CO2, CO and NOx; and the expected trend is to go on with the singularisation of emitted substances, since there have widely different effects on the environment. In that move, separate analyses have been already applied to natural gas combustion, classifying its VOC (or gaseous HC) as methane and non-methane (MHC and NMHC). Other approaches split further the bunch of substances identifying e.g. polycyclic

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aromatic hydrocarbons (PAH), BTEX (benzene, toluene, ethylbenzene and xylene), and others, as the more dangerous to health.

VOC (and CO) emissions should be very low for premixed combustion with excess air (even with stoichiometric air), but Otto engines are the major source of them because of the small residence time (some milliseconds for combustion, against near one second in premixed industrial burners), and the associated small size of the combustion chamber (limited to say half a litre per cylinder for this fact). The most pollutant are the small two-stroke Otto engines used in motorcycles and gardening, because the fresh mixture is directly thrown to the exhaust to sweep the burnt gases in the cylinder. By the way, for reciprocating engines, VOC are not only due to the fuel but to the lubrication oil that seep through the segments and gets burnt (in the small two-stroke engines oil is add directly to the fuel).

e) Particulate matter:Particulate matter (PM, or PM10 to explicitly restrict to sizes <10 m) is harmful to the respiratory system for sizes smaller than say 10 m (larger particles do not follow the air stream and get stuck at the nose and trachea), but the worst are sizes <2 m. Particulate matter consists of soot from all kind of hydrocarbon combustion (mainly in non-premixed flames), and fly ash from coal and waste combustion and incineration. Premixed combustion starts producing soot for air-to-fuel relative ratios <0.5 or 0.6, depending on the fuel.

Particulate matter was also characteristic of diesel engines at low loads (e.g. during acceleration), due to very inefficient burning of fuel drops on cold surfaces, with a dense dark smoke at the exhaust. Remedies have been the elevation in fuel-injection pressure (up to 200 MPa), preheating of air, and exhaust filtering. Nowadays, a non-visible-smoke exhaust is mandatory in practically all types of engines (even for ships). Tobacco smoke, incense burning, charcoal grills and the like, are well-known sources of particulate matter associated to combustion (as well as some related processes as deep frying), but emphasis here is on engineering applications.

Finer particles, of sizes smaller than 2.5 m (named PM2.5), are even worse than PM10; they are issued from combustor exhaust, but also form by atmospheric reactions of NOx and SO2 emissions (forming nitrates and sulphates).

f) SO2:Sulfur dioxide (SO2) emissions mainly depends on type of fuel and not on combustion details, and the trend has been to get off the market sulfur-containing fuels (by desulfurising those that need it), or

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implementing desulfurising agents in fluidised-bed combustion (e.g. adding lime for CaO+SO2+2H2=CaSO3·2H2O, or limestone), or in the exhaust (deSOx dry or wet scrubbers). Nowadays only very large marine engines and large power stations still burn sulfur-containing fuels (residual fuel oil and coal, respectively), causing severe local and global pollution (acid rain); even in 2005 the IMO-MARPOL limit on marine fuel is 4.5% in sulfur (down to 1.5% in special areas like in EU seas).

5. Engines and Pollution:An engine is the main source that is responsible for the large amount of hazardous emissions produced by a vehicle. For this reason, the usage of any vehicle, as simple as a small car, is considered a contaminating everyday activity. You might be asking yourself: How can one simple car be responsible for the damaging of our environment? One additional simple car on the road is a contributor. The summation of all the emissions of the vehicles on the road is the cause.

a) Combustion process: The combustion process is the process in which all the burning of gasoline or diesel fuel takes place. This procedure actually powers vehicles causing it to move. Also, during this process, pollution takes into effect. It causes a vehicle to exhaust by-products and evaporate fuel. Diesel fuel and gasoline are composed of mixtures of compounds of carbon and hydrogen known as hydrocarbons. The oxygen in the air combined with the hydrogen and carbon in the fuel, in a "perfect" engine, will form compounds of water and carbon dioxide. The remaining gas in the air, nitrogen, will not suffer any change. These explanations can be easily expressed in the following chemical equations:

Combustion process of a typical engine:AIR (oxygen and nitrogen) + FUEL (hydrocarbons)--->Water + Carbon Dioxide + Carbon Monoxide + Nitrogen Oxides + Unburned Hydrocarbons

Combustion process of a "perfect" engine:AIR (oxygen and nitrogen) + FUEL (hydrocarbons)--->Water + Carbon Dioxide + Natural Nitrogen

As you can see, the previous equations also portray how a "perfect" engine, in comparison with the typical engine, is less hazardous to the environment.

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6) Evaporative Emissions: All the emissions of pollutants are not only due to the exhausting of a vehicle. Some percentage of this perilous process is owing to the evaporation of fuel. The evaporation of fuel causes hydrocarbon toxins escape into the air. These kind of emissions are reprehensible for the majority of the amount of hydrocarbons that are being exposed to open air and for the increase in the ozone level. Evaporative expulsions of a vehicle can take place in various ways. During the day, when the temperature rises, the amount of the evaporation of fuel also increases. The vehicle's fuel tank begins to receive heat from the sun and gasoline vapors emerge. This kind of emission is called diurnal. Another kind of evaporative emissions are running losses. This occurs when a car is running and its engine, which possesses much heat, continues to vaporize gasoline. Even when a car is in cessation if it had been running for a long period of time it gasoline will continue to vaporize. This is known as hot soak. Another evaporative expulsion possibility is refueling. At some point a car is going to need refueling. Since there will always be gasoline vapors present in the fuel tank, they will escape when it is refueled. The liquid fuel will replace the gas in the tank.

7) Effects of coal mining:

The environmental impact of coal mining and burning is diverse. The following are its harms:

Release of methane, a greenhouse gas causing climate change. Waste products including uranium, thorium, and other radioactive and heavy

metal contaminants Acid mine drainage (AMD) Interference with groundwater and water

table levels Impact of water use on flows of rivers and

consequential impact on other land-uses Dust nuisance Tunnels, sometimes damaging

infrastructure Rendering land unfit for the other uses.

Coal mining causes a number of harmful effects. When coal surfaces are exposed, pyrite (iron sulfide), also known as "fool's gold", comes in contact with water and air and forms sulfuric acid. As water drains from the mine, the acid moves into the waterways, and as long as rain falls on the mine tailings the sulfuric acid production continues, whether the mine is still operating or not. This process is known as acid rock drainage (ARD) or acid mine drainage (AMD). If the coal is strip mined, the entire exposed seam leaches sulfuric acid, leaving the subsoil infertile on the surface and begins to pollute streams by acidifying and killing fish, plants, and aquatic animals which are sensitive to drasticpH shifts.

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Coal mining produces methane, a potent greenhouse gas. Methane is the naturally occurring product of the decay of organic matter as coal deposits are formed with increasing depths of burial, rising temperatures, and rising pressures over geological time. A portion of the methane produced is absorbed by the coal and later released from the coal seam and surrounding disturbed strata during the mining process. Methane accounts for 10.5% of greenhouse gas emissions created through human activity.

8) Effects of Burning of Coal:Coal is a chemically complex fuel. Whenever it is burned, gases are given off and particles of ash, called "fly ash," are released. The sulfur in coal combines with oxygen to form sulfur dioxide, which can be a major source of air pollution if emitted in large enough quantities. Today, many of the effects of coal burning have been reduced significantly or eliminated. Three basic methods are used to reduce the quantity of pollutants resulting from coal combustion.

The first, a pre-combustion method for removing contaminates from coal, is coal cleaning or "coal benefication." In coal cleaning the coal is crushed and screened from impurities. Further processing utilizes the different gravities of coal and impurities to separate them in a liquid medium. Coal cleaning can remove the pyritic sulfur, which can reduce sulfur content by as much as 30 percent.

The second, a post-combustion method, uses flue gas desulfurization systems, commonly called scrubbers. According to the Electric Power Research Institute, scrubbers can remove more than 90 percent of the sulfur dioxide emissions from coal combustion. The flue gas is sprayed with a slurry made up of water and an alkaline agent-- usually lime or limestone. The sulfur dioxide reacts chemically, forming calcium sulfate or calcium sulfite. This is removed and disposed of as a wet sludge. There are currently 134 scrubbers operated by the electric utility industry in the United States.

The final method for reducing or eliminating pollution from coal combustion is the use of electrostatic precipitators or baghouses which are used to remove fly ash. In electrostatic precipitators, the particulate matter is given an electrical charge. The charge attracts it to a collector plate, where the particles are collected, preventing their discharge into the atmosphere. In a baghouse, the particulate matter is filtered out as it passes through a series of filters, similar to a household vacuum cleaner.

The two major environmental concerns today dealing with the use of coal are: increase in atmospheric carbon dioxide levels and acid rain. Much remains to be learned about the relationship between fossil fuels (coal, oil, natural gas) and the environment. It is believed that combustion has partially contributed to the increase in atmospheric carbon dioxide levels. Increased atmospheric carbon dioxide levels may result in warmer climates due to the "greenhouse effect." The increase in atmospheric carbon dioxide prevents heat from escaping from the earth, thus warming the atmosphere.The combustion of coal also appears to contribute to acid rain, although precise measures of the scope and seriousness of acid rain are not clear or well understood. What is clear is that further study of the phenomenon is necessary.

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There is an interesting riddle to the acid rain phenomenon, and that is that acid rain damage has occurred during periods when sulfur dioxide discharges have declined or remained stable (sulfur dioxide is considered to be the principal cause of acid rain).

9) Global Warming: Global warming is the increase in the average temperature of Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Most of the observed temperature increase since the middle of the 20th century has been caused by increasing concentrations of greenhouse gases, which result from human activities such as the burning of fossil fuel and deforestation. An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts. Other likely effects of the warming include more frequent occurrence ofextreme weather events including heat waves, droughts and heavy rainfall events, species extinctions due to shifting temperature regimes, and changes in agricultural yields.

Ecological systems: In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.

Natural systems: Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming. Most of the increase in global average temperature since the mid-20th century is, with high probability, attributable to human-induced changes in greenhouse gas concentrations. Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades. Over the course of the 21st century, increases in emissions at or above their

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current rate would very likely induce changes in the climate system larger than those observed in the 20th century.

10) Greenhouse effect: The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere. As a result, the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism.

Greenhouse gases: By their percentage contribution to the greenhouse effect on Earth the four major gases are:

water vapor , 36–70% carbon dioxide , 9–26% methane , 4–9% ozone , 3–7%

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the atmosphere.

Role in climate change: Strengthening of the greenhouse effect through human activities is known as the enhanced (or anthropogenic) greenhouse effect. This increase in radiative forcing from human activity is attributable mainly to increased atmospheric carbon dioxide levels. CO2 is produced by fossil fuel burning and other activities such as cement production and tropical deforestation. Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm in 1960 to about 389 ppm in 2010. Because it is a greenhouse gas, elevated CO2 levels contribute to additional absorption andemission of thermal infrared in the atmosphere, which produce net warming.