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Main articles: Greenhouse effect, Global warming and Carbon dioxide in Earth's atmosphere Greenhouse gases refer to caption and adjacent text Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest absorption band of carbon dioxide is in the infrared. Greenhouse gases are those that can absorb and emit infrared radiation,[1] but not radiation in or near the visible spectrum. In order, the most abundant greenhouse gases in Earth's atmosphere are: Water vapor (H 2O) Carbon dioxide (CO 2) Methane (CH 4) Nitrous oxide (N 2O) Ozone (O 3) CFCs Atmospheric concentrations of greenhouse gases are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound).[12] The proportion of an emission remaining in the atmosphere after a specified time is the "Airborne fraction" (AF). More precisely, the annual AF is the ratio of the atmospheric increase in a given year to that year's total emissions. For CO 2 the AF over the last 50 years (1956–2006) has been increasing at 0.25 ± 0.21%/year.[13] Non-greenhouse gases Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N 2), oxygen (O 2), and argon (Ar), are not greenhouse gases. This is because molecules containing two atoms of the same element such as N 2 and O 2 and monatomic molecules such as argon (Ar) have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared radiation. Although molecules containing two atoms of different elements such as carbon monoxide (CO) or hydrogen chloride (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Because they do not contribute significantly to the greenhouse effect, they are usually omitted when discussing greenhouse gases. Indirect radiative effects world map of carbon monoxide concentrations in the lower atmosphere The false colors in this image represent levels of carbon monoxide in the lower atmosphere, ranging from about 390 parts per billion (dark brown pixels), to 220 parts per billion (red pixels), to 50 parts per billion (blue pixels).[14] Some gases have indirect radiative effects (whether or not they are a greenhouse gas themselves). This happens in two main ways. One way is that when they break down in the atmosphere they produce another greenhouse gas. For example methane and carbon monoxide (CO) are oxidized to give carbon dioxide (and methane oxidation also produces water vapor; that will be considered below). Oxidation of CO to CO 2 directly produces an unambiguous increase in radiative forcing although the reason is subtle. The peak of the thermal IR emission from the Earth's surface is very close to a strong vibrational absorption band of CO 2 (667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much higher frequencies (2145 cm−1), where the ~300 K thermal emission of the surface is at least a factor of ten lower. On the other hand, oxidation of methane to CO 2 which requires reactions with the OH radical, produces an instantaneous reduction, since CO 2 is a weaker greenhouse gas than methane; but it has a longer lifetime. As described below this is not the whole story, since the oxidations of CO and CH 4 are intertwined by both consuming OH radicals. In any case, the calculation of the total radiative effect needs to include both the direct and indirect forcing. A second type of indirect effect happens when chemical reactions in the atmosphere involving these gases change the concentrations of greenhouse gases. For example, the destruction of non-methane volatile organic compounds (NMVOC) in the atmosphere can produce ozone. The size of the indirect effect can depend strongly on where and when the gas is emitted.[15] Methane has a number of indirect effects in addition to forming CO 2. Firstly, the main chemical which destroys methane in the atmosphere is the hydroxyl radical (OH). Methane reacts with OH and so more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The second effect is that the oxidation of methane can produce ozone. Thirdly, as well as making CO 2 the oxidation of methane produces water; this is a major source of water vapor in the stratosphere which is otherwise very dry. CO and NMVOC also produce CO 2 when they are oxidized. They remove OH from the atmosphere and this leads to higher concentrations of methane. The surprising effect of this is that the global warming potential of CO is three times that of CO 2.[16] The same process that converts NMVOC to carbon dioxide can also lead to the formation of tropospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. Finally hydrogen can lead to ozone production and CH 4 increases as well as producing water vapor in the stratosphere.[15] Contribution of clouds to Earth's greenhouse effect 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 greenhouse gases. Clouds are water droplets or ice crystals suspended in the atmosphere.[17][18] Impacts on the overall greenhouse effect refer to caption and adjacent text Schmidt et al. (2010)[19] analysed how individual components of the atmosphere contribute to the total greenhouse effect. They estimated that water vapor accounts for about 50% of the Earth's greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and the minor greenhouse gases and aerosols accounting for the remaining 5%. In the study, the reference model atmosphere is for 1980 conditions. Image credit: NASA.[20] The contribution of each gas to the greenhouse effect is affected by the characteristics of that gas, its abundance, and any indirect effects it may cause. For example, the direct radiative effect of a mass of methane is about 72 times stronger than the same mass of carbon dioxide over a 20-year time frame[21] but it is present in much smaller concentrations so that its total direct radiative effect is smaller, in part due to its shorter atmospheric lifetime. On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation. Shindell et al. (2005)[22] argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.[23] When ranked by their direct contribution to the greenhouse effect, the most important are:[17] Compound Formula Contribution (%) Water vapor and clouds H 2O 36 – 72% Carbon dioxide CO 2 9 – 26% Methane CH 4 4–9% Ozone O 3 3–7% In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in very small quantities.[24] Proportion of direct effects at a given moment It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect. This is because some of the gases absorb and emit radiation at the same frequencies as others, so that the total greenhouse effect is not simply the sum of the influence of each gas. The higher ends of the ranges quoted are for each gas alone; the lower ends account for overlaps with the other gases.[17][18] In addition, some gases such as methane are known to have large indirect effects that are still being quantified.[25] Atmospheric lifetime Aside from water vapor, which has a residence time of about nine days,[26] major greenhouse gases are well-mixed, and take many years to leave the atmosphere.[27] Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999)[28] defines the lifetime \tau of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically \tau can be defined as the ratio of the mass m (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box (F_{out}), chemical loss of X (L), and deposition of X (D) (all in kg/s): \tau = \frac{m}{F_{out}+L+D}.[28] If one stopped pouring any of this gas into the box, then after a time \tau, its concentration would be about halved. The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the mean lifetime. Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely.[29] The atmospheric lifetime of CO 2 is estimated of the order of 30–95 years.[30] This figure accounts for CO 2 molecules being removed from the atmosphere by mixing into the ocean, photosynthesis, and other processes. However, this excludes the balancing fluxes of CO 2 into the atmosphere from the geological reservoirs, which have slower characteristic rates.[31] While more than half of the CO 2 emitted is removed from the atmosphere within a century, some fraction (about 20%) of emitted CO 2 remains in the atmosphere for many thousands of years.[32][33][34] Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes than CO 2. E.g., N2O has a mean atmospheric lifetime of 114 years.[21] Radiative forcing The Earth absorbs some of the radiant energy received from the sun, reflects some of it as light and reflects or radiates the rest back to space as heat.[35] The Earth's surface temperature depends on this balance between incoming and outgoing energy.[35] If this energy balance is shifted, the Earth's surface could become warmer or cooler, leading to a variety of changes in global climate.[35] A number of natural and man-made mechanisms can affect the global energy balance and force changes in the Earth's climate.[35] Greenhouse gases are one such mechanism.[35] Greenhouse gases in the atmosphere absorb and re-emit some of the outgoing energy radiated from the Earth's surface, causing that heat to be retained in the lower atmosphere.[35] As explained above, some greenhouse gases remain in the atmosphere for decades or even centuries, and therefore can affect the Earth's energy balance over a long time period.[35] Factors that influence Earth's energy balance can be quantified in terms of "radiative climate forcing."[35] Positive radiative forcing indicates warming (for example, by increasing incoming energy or decreasing the amount of energy that escapes to space), while negative forcing is associated with cooling.[35] Global warming potential The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO 2 and evaluated for a specific timescale. Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO 2 its GWP will increase with the timescale considered. Carbon dioxide is defined to have a GWP of 1 over all time periods. Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO 2 through chemical reactions in the atmosphere. Examples of the atmospheric lifetime and GWP relative to CO 2 for several greenhouse gases are given in the following table:[21] Atmospheric lifetime and GWP relative to CO 2 at different time horizon for various greenhouse gases. Gas name Chemical formula Lifetime (years) Global warming potential (GWP) for given time horizon 20-yr 100-yr 500-yr Carbon dioxide CO 2 See above 1 1 1 Methane CH 4 12 72 25 7.6 Nitrous oxide N 2O 114 289 298 153 CFC-12 CCl 2F 2 100 11 000 10 900 5 200 HCFC-22 CHClF 2 12 5 160 1 810 549 Tetrafluoromethane CF 4 50 000 5 210 7 390 11 200 Hexafluoroethane C 2F 6 10 000 8 630 12 200 18 200 Sulfur hexafluoride SF 6 3 200 16 300 22 800 32 600 Nitrogen trifluoride NF 3 740 12 300 17 200 20 700 The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties.[36] The phasing-out of less active HCFC-compounds will be completed in 2030.[37] Natural and anthropogenic sources refer to caption and article text Top: Increasing atmospheric carbon dioxide levels as measured in the atmosphere and reflected in ice cores. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning fossil fuel. refer to caption and image description This diagram shows a simplified representation of the contemporary global carbon cycle. Changes are measured in gigatons of carbon per year (GtC/y). Canadell et al. (2007) estimated the growth rate of global average atmospheric CO 2 for 2000–2006 as 1.93 parts-per-million per year (4.1 petagrams of carbon per year).[38] Image credit: U.S. Department of Energy Genomic Science program[39] Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused sources. During the pre-industrial Holocene, concentrations of existing gases were roughly constant. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.[40][41] The 2007 Fourth Assessment Report compiled by the IPCC (AR4) noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century".[42] In AR4, "most of" is defined as more than 50%. Abbreviations used in the two tables below: ppm = parts-per-million; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 = watts per square metre Current greenhouse gas concentrations[5] Gas Pre-1750 tropospheric concentration[43] Recent tropospheric concentration[44] Absolute increase since 1750 Percentage increase since 1750 Increased radiative forcing (W/m2)[45] Carbon dioxide (CO 2) 280 ppm[46] 395.4 ppm[47] 115.4 ppm 41.2% 1.88 Methane (CH 4) 700 ppb[48] 1893 ppb /[49] 1762 ppb[49] 1193 ppb / 1062 ppb 170.4% / 151.7% 0.49 Nitrous oxide (N 2O) 270 ppb[45][50] 326 ppb /[49] 324 ppb[49] 56 ppb / 54 ppb 20.7% / 20.0% 0.17 Tropospheric ozone (O 3) 237 ppb[43] 337 ppb[43] 100 ppb 42% 0.4[51] Relevant to radiative forcing and/or ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial[5] Gas Recent tropospheric concentration Increased radiative forcing (W/m2) CFC-11 (trichlorofluoromethane) (CCl 3F) 236 ppt / 234 ppt 0.061 CFC-12 (CCl 2F 2) 527 ppt / 527 ppt 0.169 CFC-113 (Cl 2FC-CClF 2) 74 ppt / 74 ppt 0.022 HCFC-22 (CHClF 2) 231 ppt / 210 ppt 0.046 HCFC-141b (CH 3CCl 2F) 24 ppt / 21 ppt 0.0036 HCFC-142b (CH 3CClF 2) 23 ppt / 21 ppt 0.0042 Halon 1211 (CBrClF 2) 4.1 ppt / 4.0 ppt 0.0012 Halon 1301 (CBrClF 3) 3.3 ppt / 3.3 ppt 0.001 HFC-134a (CH 2FCF 3) 75 ppt / 64 ppt 0.0108 Carbon tetrachloride (CCl 4) 85 ppt / 83 ppt 0.0143 Sulfur hexafluoride (SF 6) 7.79 ppt /[52] 7.39 ppt[52] 0.0043 Other halocarbons Varies by substance collectively 0.02 Halocarbons in total 0.3574 refer to caption and article text 400,000 years of ice core data. Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO 2 and CH 4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates CO 2 mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago CO 2 levels were likely 10 times higher than now.[53] Indeed higher CO 2 concentrations are thought to have prevailed throughout most of the Phanerozoic eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Ma.[54][55][56] The spread of land plants is thought to have reduced CO 2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO 2 have since been important in providing stabilising feedbacks.[57] Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the CO 2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day.[58] This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.[58][59][60] Ice cores Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO 2 mole fractions were about 280 parts per million (ppm), and stayed between 260 and 280 during the preceding ten thousand years.[61] Carbon dioxide mole fractions in the atmosphere have gone up by approximately 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago,[62] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO 2 variability.[63][64] Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels. Changes since the Industrial Revolution Refer to caption Recent year-to-year increase of atmospheric CO 2. Refer to caption Major greenhouse gas trends. Since the beginning of the Industrial Revolution, the concentrations of most of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm by about 36% to 380 ppm, or 100 ppm over modern pre-industrial levels. The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973.[citation needed]; however the next 50 ppm increase took place in about 33 years, from 1973 to 2006.[65] Recent data also shows that the concentration is increasing at a higher rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007.[66] Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock.[clarification needed] This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions.[67] The other greenhouse gases produced from human activity show similar increases in both amount and rate of increase. Many observations are available online in a variety of Atmospheric Chemistry Observational Databases. Anthropogenic greenhouse gases This graph shows changes in the annual greenhouse gas index (AGGI) between 1979 and 2011.[68] The AGGI measures the levels of greenhouse gases in the atmosphere based on their ability to cause changes in the Earth's climate.[68] This bar graph shows global greenhouse gas emissions by sector from 1990 to 2005, measured in carbon dioxide equivalents.[69] Modern global anthropogenic carbon emissions. Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmospheric concentrations of carbon dioxide are currently 100 ppm higher than pre-industrial levels.[70] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[71] but over periods longer than a few years natural sources are closely balanced by natural sinks, mainly photosynthesis of carbon compounds by plants and marine plankton. As a result of this balance, the atmospheric mole fraction of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the end of the last glacial maximum and the start of the industrial era.[72] It is likely that anthropogenic (i.e., human-induced) warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems.[73] Future warming is projected to have a range of impacts, including sea level rise,[74] increased frequencies and severities of some extreme weather events,[74] loss of biodiversity,[75] and regional changes in agricultural productivity.[75] The main sources of greenhouse gases due to human activity are: burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations in the air. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic CO 2 emissions.[72] livestock enteric fermentation and manure management,[76] paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane. use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes. agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide (N 2O) concentrations. The seven sources of CO 2 from fossil fuel combustion are (with percentage contributions for 2000–2004):[77] Seven main fossil fuel combustion sources Contribution (%) Liquid fuels (e.g., gasoline, fuel oil) 36% Solid fuels (e.g., coal) 35% Gaseous fuels (e.g., natural gas) 20% Cement production 3 % Flaring gas industrially and at wells < 1% Non-fuel hydrocarbons < 1% "International bunker fuels" of transport not included in national inventories[78] 4 % Carbon dioxide, methane, nitrous oxide (N 2O) and three groups of fluorinated gases (sulfur hexafluoride (SF 6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major anthropogenic greenhouse gases,[79]:147[80] and are regulated under the Kyoto Protocol international treaty, which came into force in 2005.[81] Emissions limitations specified in the Kyoto Protocol expire in 2012.[81] The Cancún agreement, agreed in 2010, includes voluntary pledges made by 76 countries to control emissions.[82] At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions.[82] Although CFCs are greenhouse gases, they are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the media. Sectors [icon] This section requires expansion with: Information on emissions from other sectors. (July 2013) Tourism According to UNEP global tourism is closely linked to climate change. Tourism is a significant contributor to the increasing concentrations of greenhouse gases in the atmosphere. Tourism accounts for about 50% of traffic movements. Rapidly expanding air traffic contributes about 2.5% of the production of CO 2. The number of international travelers is expected to increase from 594 million in 1996 to 1.6 billion by 2020, adding greatly to the problem unless steps are taken to reduce emissions.[83] Role of water vapor Increasing water vapor in the stratosphere at Boulder, Colorado. Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for clear sky conditions and between 66% and 85% when including clouds.[18] Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass at in saturated air at about 32 °C.(see Relative humidity#other important facts) [84] The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as CH 4 and CO 2.[85] Thus, water vapor responds to and amplifies effects of the other greenhouse gases. The Clausius–Clapeyron relation establishes that more water vapor will be present per unit volume at elevated temperatures. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor (assuming that the relative humidity remains approximately constant; modeling and observational studies find that this is indeed so). Because water vapor is a greenhouse gas, this results in further warming and so is a "positive feedback" that amplifies the original warming. Eventually other earth processes offset these positive feedbacks, stabilizing the global temperature at a new equilibrium and preventing the loss of Earth's water through a Venus-like runaway greenhouse effect.[86] Direct greenhouse gas emissions Between the period 1970 to 2004, GHG emissions (measured in CO 2-equivalent)[87] increased at an average rate of 1.6% per year, with CO 2 emissions from the use of fossil fuels growing at a rate of 1.9% per year.[88][89] Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO 2-equivalent.[90]:15 These emissions include CO 2 from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other GHGs covered by the Kyoto Protocol. At present, the primary source of CO 2 emissions is the burning of coal, natural gas, and petroleum for electricity and heat.[91] A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.[1] The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would average about 33 °C colder, which is about 59 °F below the present average of 14 °C (57 °F).[2][3][4] Since the beginning of the Industrial Revolution (taken as the year 1750), the burning of fossil fuels and extensive clearing of native forests has contributed to a 40% increase in the atmospheric concentration of carbon dioxide, from 280 to 392.6 parts per million (ppm) in 2012.[5][6] and has now reached 400 ppm in the northern hemisphere. This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle.[7][8] Anthropogenic carbon dioxide (CO 2) emissions (i.e., emissions produced by human activities) come from combustion of carbon-based fuels, principally wood, coal, oil, and natural gas.[9] Under ongoing greenhouse gas emissions, available Earth System Models project that the Earth's surface temperature could exceed historical analogs as early as 2047 affecting most ecosystems on Earth and the livelihoods of over 3 billion people worldwide.[10] Greenhouse gases also trigger[clarification needed] ocean bio-geochemical changes with broad ramifications in marine systems.[11] In the Solar System, the atmospheres of Venus, Mars, and Titan also contain gases that cause a greenhouse effect, though Titan's atmosphere has an anti-greenhouse effect which reduces the warming. |
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