Greenhouse gas

A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect.[1] The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F),[2] rather than the present average of 15 °C (59 °F).[3][4][5] The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Human activities since the beginning of the Industrial Revolution (around 1750) have produced a 45% increase in the atmospheric concentration of carbon dioxide (CO
2
), from 280 ppm in 1750 to 406 ppm in early 2017. This increase has occurred despite the uptake of more than half of the emissions by various natural "sinks" involved in the carbon cycle.[6][7] The vast majority of anthropogenic carbon dioxide emissions (i.e., emissions produced by human activities) come from combustion of fossil fuels, principally coal, oil, and natural gas, with additional contributions coming from deforestation, changes in land use, soil erosion and agriculture (including livestock).[8][9]

Should greenhouse gas emissions continue at their rate in 2017, global warming could cause Earth's surface temperature to exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and human livelihoods.[10] At current emission rates temperatures could increase by 2 °C, which the United Nations' IPCC designated as the upper limit to avoid "dangerous" levels, by 2036.[11]

Greenhouse-effect-t445
The greenhouse effect of solar radiation on the Earth's surface caused by greenhouse gases

Gases in Earth's atmosphere

The most common gases in Earth's atmosphere are nitrogen (78%), oxygen (21%), and argon (0.9%). The next most common gases are carbon dioxide, nitrous oxides, methane, and ozone. They are trace gases that account for almost one tenth of 1% of Earth's atmosphere.

Atmospheric Transmission
Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest absorption band of carbon dioxide is not far from the maximum in the thermal emission from ground, and it partly closes the window of transparency of water; hence its major effect.

Greenhouse gases

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth.[1] In order, the most abundant greenhouse gases in Earth's atmosphere are:

Atmospheric concentrations 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 or absorption by bodies of water).[12] The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). The annual airborne fraction is the ratio of the atmospheric increase in a given year to that year's total emissions. As of 2006 the annual airborne fraction for CO
2
was about 0.45. The annual airborne fraction increased at a rate of 0.25 ± 0.21% per year over the period 1959–2006.[13]

Non-greenhouse gases

The major atmospheric constituents, nitrogen (N
2
), oxygen (O
2
), and argon (Ar), are not greenhouse gases because molecules containing two atoms of the same element such as N
2
and O
2
have no net change in the distribution of their electrical charges when they vibrate, and monatomic gases such as Ar do not have vibrational modes. Hence they are almost totally unaffected by infrared radiation. Some molecules containing just two atoms of different elements, such as carbon monoxide (CO) and hydrogen chloride (HCl), do absorb infrared radiation, but these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Therefore they do not contribute significantly to the greenhouse effect and often are omitted when discussing greenhouse gases.

Indirect radiative effects

Mopitt first year carbon monoxide
The false colors in this image represent concentrations 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 greenhouse gases 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). 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 Earth's surface is very close to a strong vibrational absorption band of CO
2
(15 microns, or 667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much shorter wavelengths (4.7 microns, or 2145 cm−1), where the emission of radiant energy from Earth's surface is at least a factor of ten lower. Oxidation of methane to CO
2
, which requires reactions with the OH radical, produces an instantaneous reduction in radiative absorption and emission since CO
2
is a weaker greenhouse gas than methane. However, the oxidations of CO and CH
4
are entwined since both consume OH radicals. In any case, the calculation of the total radiative effect includes both 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 (NMVOCs) 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 indirect effects in addition to forming CO
2
. The main chemical that reacts with methane in the atmosphere is the hydroxyl radical (OH), thus more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The oxidation of methane can produce both ozone and water; and is a major source of water vapor in the normally dry stratosphere. CO and NMVOCs 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 NMVOCs 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 stratospheric water vapor.[15]

Contribution of clouds to Earth's greenhouse effect

The major non-gas contributor to Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on greenhouse gas radiative properties. Clouds are water droplets or ice crystals suspended in the atmosphere.[17][18]

Impacts on the overall greenhouse effect

Attribution of individual atmospheric component contributions to the terrestrial greenhouse effect, separated into feedback and forcing categories (NASA)
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 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 determined 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 84 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
 
Concentration in
atmosphere[24] (ppm)
Contribution
(%)
Water vapor and clouds H
2
O
10–50,000(A) 36–72%  
Carbon dioxide CO
2
~400 9–26%
Methane CH
4
~1.8 4–9%  
Ozone O
3
2–8(B) 3–7%  
notes:

(A) Water vapor strongly varies locally[25]
(B) The concentration in stratosphere. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.

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.[26]

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.[27]

Atmospheric lifetime

Aside from water vapor, which has a residence time of about nine days,[28] major greenhouse gases are well mixed and take many years to leave the atmosphere.[29] 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)[30] defines the lifetime of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically can be defined as the ratio of the mass (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box (), chemical loss of X (), and deposition of X () (all in kg/s): .[30] If output of this gas into the box ceased, then after time , its concentration would decrease by about 63%.

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.[31] The atmospheric lifetime of CO
2
is estimated of the order of 30–95 years.[32] 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.[33] Although 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.[34] [35] [36] 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 121 years.[21]

Radiative forcing

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.[37] Earth's surface temperature depends on this balance between incoming and outgoing energy.[37] If this energy balance is shifted, Earth's surface becomes warmer or cooler, leading to a variety of changes in global climate.[37]

A number of natural and man-made mechanisms can affect the global energy balance and force changes in Earth's climate.[37] Greenhouse gases are one such mechanism.[37] Greenhouse gases absorb and emit some of the outgoing energy radiated from Earth's surface, causing that heat to be retained in the lower atmosphere.[37] As explained above, some greenhouse gases remain in the atmosphere for decades or even centuries, and therefore can affect Earth's energy balance over a long period.[37] Radiative forcing quantifies the effect of factors that influence Earth's energy balance, including changes in the concentrations of greenhouse gases.[37] Positive radiative forcing leads to warming by increasing the net incoming energy, whereas negative radiative forcing leads to cooling.[37]

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 when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 3 years. The 2007 IPCC report lists the GWP as 72 over a time scale of 20 years, 25 over 100 years and 7.6 over 500 years.[38] A 2014 analysis, however, states that although methane's initial impact is about 100 times greater than that of CO
2
, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane's relative role continues to decline.[39] 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:

Atmospheric lifetime and GWP relative to CO
2
at different time horizon for various greenhouse gases
Gas name Chemical
formula
Lifetime
(years)[21]
Global warming potential (GWP) for given time horizon
20-yr[21] 100-yr[21] 500-yr[38]
Carbon dioxide CO
2
30–95 1 1 1
Methane CH
4
12 84 28 7.6
Nitrous oxide N
2
O
121 264 265 153
CFC-12 CCl
2
F
2
100 10 800 10 200 5 200
HCFC-22 CHClF
2
12 5 280 1 760 549
Tetrafluoromethane        CF
4
50 000 4 880 6 630 11 200
Hexafluoroethane C
2
F
6
10 000 8 210 11 100 18 200
Sulfur hexafluoride SF
6
3 200 17 500 23 500 32 600
Nitrogen trifluoride NF
3
500 12 800 16 100 20 700

The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties.[40] The phasing-out of less active HCFC-compounds will be completed in 2030.[41]

M15-162b-EarthAtmosphere-CarbonDioxide-FutureRoleInGlobalWarming-Simulation-20151109
Carbon dioxide in Earth's atmosphere if half of global-warming emissions[42][43] are not absorbed.
(NASA simulation; 9 November 2015)
15-233-Earth-GlobalAirQuality-2014NitrogenDioxideLevels-20151214
Nitrogen dioxide 2014 – global air quality levels
(released 14 December 2015).[44]

Natural and anthropogenic sources

Carbon History and Flux Rev
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.
Diagram showing a simplified representation of the Earth's annual carbon cycle (US DOE)
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).[45]

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, because the large natural sources and sinks roughly balanced. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.[46][47]

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".[48] 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[49]
Gas Pre-1750
tropospheric
concentration[50]
Recent
tropospheric
concentration[51]
Absolute increase
since 1750
Percentage
increase
since 1750
Increased
radiative forcing
(W/m2)[52]
Carbon dioxide (CO
2
)
280 ppm[53] 395.4 ppm[54] 115.4 ppm 41.2% 1.88
Methane (CH
4
)
700 ppb[55] 1893 ppb /[56][57]
1762 ppb[56]
1193 ppb /
1062 ppb
170.4% /
151.7%
0.49
Nitrous oxide (N
2
O
)
270 ppb[52][58] 326 ppb /[56]
324 ppb[56]
56 ppb /
54 ppb
20.7% /
20.0%
0.17
Tropospheric
ozone (O
3
)
237 ppb[50] 337 ppb[50] 100 ppb 42% 0.4[59]
Relevant to radiative forcing and/or ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial[49]
Gas Recent
tropospheric
concentration
Increased
radiative forcing
(W/m2)
CFC-11
(trichlorofluoromethane)
(CCl
3
F
)
236 ppt /
234 ppt
0.061
CFC-12 (CCl
2
F
2
)
527 ppt /
527 ppt
0.169
CFC-113 (Cl
2
FC-CClF
2
)
74 ppt /
74 ppt
0.022
HCFC-22 (CHClF
2
)
231 ppt /
210 ppt
0.046
HCFC-141b (CH
3
CCl
2
F
)
24 ppt /
21 ppt
0.0036
HCFC-142b (CH
3
CClF
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
2
FCF
3
)
75 ppt /
64 ppt
0.0108
Carbon tetrachloride (CCl
4
)
85 ppt /
83 ppt
0.0143
Sulfur hexafluoride (SF
6
)
7.79 ppt /[60]
7.39 ppt[60]
0.0043
Other halocarbons Varies by
substance
collectively
0.02
Halocarbons in total 0.3574
Vostok Petit data
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.[61] 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.[62][63][64] 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.[65] 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.[66] 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 approximately 0.645 billion tons of CO
2
per year, whereas humans contribute 29 billion tons of CO
2
each year.[67][66][68][69]

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.[70] 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,[71] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO
2
variability.[72][73] 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

CO2 increase rate
Recent year-to-year increase of atmospheric CO
2
.
Major greenhouse gas trends
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 to 400 ppm, or 120 ppm over modern pre-industrial levels. The first 30 ppm increase took place in about 200 years, from the start of the Industrial Revolution to 1958; however the next 90 ppm increase took place within 56 years, from 1958 to 2014.[74][75]

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.[76]

Total cumulative emissions from 1870 to 2017 were 425±20 GtC (1539 GtCO2) from fossil fuels and industry, and 180±60 GtC (660 GtCO2) from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2017, coal 32%, oil 25%, and gas 10%.[77]

Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock. This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions.[78]

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

NOAA Annual Greenhouse Gas Index 2012
This graph shows changes in the annual greenhouse gas index (AGGI) between 1979 and 2011. [79] The AGGI measures the levels of greenhouse gases in the atmosphere based on their ability to cause changes in Earth's climate.[79]
Global greenhouse gas emissions by sector, 1990-2005, in carbon dioxide equivalents (EPA, 2010)
This bar graph shows global greenhouse gas emissions by sector from 1990 to 2005, measured in carbon dioxide equivalents.[80]
Global Carbon Emissions
Modern global CO2 emissions from the burning of fossil fuels.

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.[81] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[82] 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.[83]

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.[84] Future warming is projected to have a range of impacts, including sea level rise,[85] increased frequencies and severities of some extreme weather events,[85] loss of biodiversity,[86] and regional changes in agricultural productivity.[86]

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.[83]
  • livestock enteric fermentation and manure management,[87] paddy rice farming, land use and wetland changes, man-made lakes,[88] 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
    2
    O
    ) concentrations.

The seven sources of CO
2
from fossil fuel combustion are (with percentage contributions for 2000–2004):[89]

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[90]
 4 %

Carbon dioxide, methane, nitrous oxide (N
2
O
) and three groups of fluorinated gases (sulfur hexafluoride (SF
6
), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major anthropogenic greenhouse gases,[91]:147[92] and are regulated under the Kyoto Protocol international treaty, which came into force in 2005.[93] Emissions limitations specified in the Kyoto Protocol expired in 2012.[93] The Cancún agreement, agreed on in 2010, includes voluntary pledges made by 76 countries to control emissions.[94] At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions.[94]

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. On 15 October 2016, negotiators from over 170 nations meeting at the summit of the United Nations Environment Programme reached a legally binding accord to phase out hydrofluorocarbons (HFCs) in an amendment to the Montreal Protocol.[95][96][97]

Sectors

Greenhouse Gas by Sector
This figure shows the relative fraction of anthropogenic greenhouse gases coming from each of eight categories of sources, as estimated by the Emission Database for Global Atmospheric Research version 4.2, fast track 2010 project. These values are intended to provide a snapshot of global annual greenhouse gas emissions in the year 2010. The top panel shows the sum over all anthropogenic greenhouse gases, weighted by their global warming potential over the next 100 years. This consists of 72% carbon dioxide, 20% methane, 5% nitrous oxide and 3% other gases. Lower panels show the comparable information for each of these three primary greenhouse gases, with the same coloring of sectors as used in the top chart. Segments with less than 1% fraction are not labeled.

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.[98]

Trucking and haulage

The trucking and haulage industry plays a part in production of CO
2
, contributing around 20% of the UK's total carbon emissions a year, with only the energy industry having a larger impact at around 39%.[99] Average carbon emissions within the haulage industry are falling—in the thirty-year period from 1977 to 2007, the carbon emissions associated with a 200-mile journey fell by 21 percent; NOx emissions are also down 87 percent, whereas journey times have fallen by around a third.[100] Due to their size, HGVs often receive criticism regarding their CO2 emissions; however, rapid development in engine technology and fuel management is having a largely positive effect.

Plastic

Plastic is produced mainly from Fossil fuels. Plastic manufacturing is estimated to use 8 percent of yearly global oil production. The EPA estimates as many as five ounces of carbon dioxide are emitted for each ounce of polyethylene terephthalate (PET) produced—the type of plastic most commonly used for beverage bottles,[101] the transportation produce greenhouse gases also.[102] Plastic waste emits carbon dioxide when it degrades. In 2018 research claimed that some of the most common plastics in the environment release the greenhouse gases Methane and Ethylene when exposed to sunlight in an amount that can affect the earth climate.[103][104]

From the other side, if it is placed in a landfill, it becomes a carbon sink[105] although biodegradable plastics have caused methane emissions. [106] Due to the lightness of plastic versus glass or metal, plastic may reduce energy consumption. For example, packaging beverages in PET plastic rather than glass or metal is estimated to save 52% in transportation energy, if the glass or metal package is single use, of course.

In 2019 a new report "Plastic and Climate" was published. According to the report plastic wiil contribute Greenhouse gases in the equivalent of 850 million tons of Carbon dioxide (CO2) to the atmosphere in 2019. In current trend, annual emissions will grow to 1.34 billion tons by 2030. By 2050 plastic could emit 56 billion tons of Greenhouse gas emissions, as much as 14 percent of the earth’s remaining carbon budget[107]. The report says that only solutions which involve a reduction in consumption can solve the problem, while others like biodegradable plastic, ocean cleanup, using renewable energy in plastic industry can do little, and in some cases may even worsen it[108].

Role of water vapor

BAMS climate assess boulder water vapor 2002
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 directly affect water vapor concentrations except at local scales, such as near irrigated fields. Indirectly, human activity that increases global temperatures will increase water vapor concentrations, a process known as water vapor feedback.[109] 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 in saturated air at about 32 °C.[110] (See Relative humidity#other important facts.)

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
.[111] 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.[112]

Direct greenhouse gas emissions

Between the period 1970 to 2004, greenhouse gas emissions (measured in CO
2
-equivalent
)[113] 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.[114][115] Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO
2
-equivalent.[116]:15 These emissions include CO
2
from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other greenhouse gases 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.[117]

Regional and national attribution of emissions

According to the Environmental Protection Agency (EPA), GHG emissions in the United States can be traced from different sectors.

There are several different ways of measuring greenhouse gas emissions, for example, see World Bank (2010)[118]:362 for tables of national emissions data. Some variables that have been reported[119] include:

  • Definition of measurement boundaries: Emissions can be attributed geographically, to the area where they were emitted (the territory principle) or by the activity principle to the territory produced the emissions. These two principles result in different totals when measuring, for example, electricity importation from one country to another, or emissions at an international airport.
  • Time horizon of different gases: Contribution of a given greenhouse gas is reported as a CO
    2
    equivalent. The calculation to determine this takes into account how long that gas remains in the atmosphere. This is not always known accurately and calculations must be regularly updated to reflect new information.
  • What sectors are included in the calculation (e.g., energy industries, industrial processes, agriculture etc.): There is often a conflict between transparency and availability of data.
  • The measurement protocol itself: This may be via direct measurement or estimation. The four main methods are the emission factor-based method, mass balance method, predictive emissions monitoring systems, and continuous emissions monitoring systems. These methods differ in accuracy, cost, and usability.

These different measures are sometimes used by different countries to assert various policy/ethical positions on climate change (Banuri et al., 1996, p. 94).[120] The use of different measures leads to a lack of comparability, which is problematic when monitoring progress towards targets. There are arguments for the adoption of a common measurement tool, or at least the development of communication between different tools.[119]

Emissions may be measured over long time periods. This measurement type is called historical or cumulative emissions. Cumulative emissions give some indication of who is responsible for the build-up in the atmospheric concentration of greenhouse gases (IEA, 2007, p. 199).[121]

The national accounts balance would be positively related to carbon emissions. The national accounts balance shows the difference between exports and imports. For many richer nations, such as the United States, the accounts balance is negative because more goods are imported than they are exported. This is mostly due to the fact that it is cheaper to produce goods outside of developed countries, leading the economies of developed countries to become increasingly dependent on services and not goods. We believed that a positive accounts balance would means that more production was occurring in a country, so more factories working would increase carbon emission levels.[122]

Emissions may also be measured across shorter time periods. Emissions changes may, for example, be measured against a base year of 1990. 1990 was used in the United Nations Framework Convention on Climate Change (UNFCCC) as the base year for emissions, and is also used in the Kyoto Protocol (some gases are also measured from the year 1995).[91]:146, 149 A country's emissions may also be reported as a proportion of global emissions for a particular year.

Another measurement is of per capita emissions. This divides a country's total annual emissions by its mid-year population.[118]:370 Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–07).[120]

While cities are sometimes considered to be disproportionate contributors to emissions, per-capita emissions tend to be lower for cities than the averages in their countries.[123]

From land-use change

Greenhouse gas emissions from agriculture, forestry and other land use, 1970-2010
Greenhouse gas emissions from agriculture, forestry and other land use, 1970–2010.

Land-use change, e.g., the clearing of forests for agricultural use, can affect the concentration of greenhouse gases in the atmosphere by altering how much carbon flows out of the atmosphere into carbon sinks.[124] Accounting for land-use change can be understood as an attempt to measure "net" emissions, i.e., gross emissions from all sources minus the removal of emissions from the atmosphere by carbon sinks (Banuri et al., 1996, pp. 92–93).[120]

There are substantial uncertainties in the measurement of net carbon emissions.[125] Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri et al., 1996, p. 93).[120] For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

Greenhouse gas intensity

GHG intensity 2000
Greenhouse gas intensity in the year 2000, including land-use change.
Carbon intensity of GDP (using PPP) for different regions, 1982-2011
Carbon intensity of GDP (using PPP) for different regions, 1982–2011.
Carbon intensity of GDP (using MER) for different regions, 1982-2011 (corrected)
Carbon intensity of GDP (using MER) for different regions, 1982–2011.

Greenhouse gas intensity is a ratio between greenhouse gas emissions and another metric, e.g., gross domestic product (GDP) or energy use. The terms "carbon intensity" and "emissions intensity" are also sometimes used.[126] Emission intensities may be calculated using market exchange rates (MER) or purchasing power parity (PPP) (Banuri et al., 1996, p. 96).[120] Calculations based on MER show large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

Cumulative and historical emissions

Cumulative energy-related carbon dioxide emissions between 1850-2005 for low-income, middle-income, high-income, the EU-15, and OECD countries
Cumulative energy-related CO
2
emissions between the years 1850–2005 grouped into low-income, middle-income, high-income, the EU-15, and the OECD countries.
Cumulative energy-related carbon dioxide emissions between 1850-2005 for different countries
Cumulative energy-related CO
2
emissions between the years 1850–2005 for individual countries.
CO2 responsibility 1950-2000
Map of cumulative per capita anthropogenic atmospheric CO
2
emissions by country. Cumulative emissions include land use change, and are measured between the years 1950 and 2000.
Yearly trends in annual regional carbon dioxide emissions from fuel combustion between 1971 and 2009
Regional trends in annual CO
2
emissions from fuel combustion between 1971 and 2009.
Regional trends in annual per capita carbon dioxide emissions from fuel combustion between 1971 and 2009
Regional trends in annual per capita CO
2
emissions from fuel combustion between 1971 and 2009.

Cumulative anthropogenic (i.e., human-emitted) emissions of CO
2
from fossil fuel use are a major cause of global warming,[127] and give some indication of which countries have contributed most to human-induced climate change.[128]:15

Top-5 historic CO
2
contributors by region over the years 1800 to 1988 (in %)
Region Industrial
CO
2
Total
CO
2
OECD North America 33.2 29.7
OECD Europe 26.1 16.6
Former USSR 14.1 12.5
China   5.5   6.0
Eastern Europe   5.5   4.8

The table above to the left is based on Banuri et al. (1996, p. 94).[120] Overall, developed countries accounted for 83.8% of industrial CO
2
emissions over this time period, and 67.8% of total CO
2
emissions. Developing countries accounted for industrial CO
2
emissions of 16.2% over this time period, and 32.2% of total CO
2
emissions. The estimate of total CO
2
emissions includes biotic carbon emissions, mainly from deforestation. Banuri et al. (1996, p. 94)[120] calculated per capita cumulative emissions based on then-current population. The ratio in per capita emissions between industrialized countries and developing countries was estimated at more than 10 to 1.

Including biotic emissions brings about the same controversy mentioned earlier regarding carbon sinks and land-use change (Banuri et al., 1996, pp. 93–94).[120] The actual calculation of net emissions is very complex, and is affected by how carbon sinks are allocated between regions and the dynamics of the climate system.

Non-OECD countries accounted for 42% of cumulative energy-related CO
2
emissions between 1890 and 2007.[129]:179–80 Over this time period, the US accounted for 28% of emissions; the EU, 23%; Russia, 11%; China, 9%; other OECD countries, 5%; Japan, 4%; India, 3%; and the rest of the world, 18%.[129]:179–80

Changes since a particular base year

Between 1970 and 2004, global growth in annual CO
2
emissions was driven by North America, Asia, and the Middle East.[130] The sharp acceleration in CO
2
emissions since 2000 to more than a 3% increase per year (more than 2 ppm per year) from 1.1% per year during the 1990s is attributable to the lapse of formerly declining trends in carbon intensity of both developing and developed nations. China was responsible for most of global growth in emissions during this period. Localised plummeting emissions associated with the collapse of the Soviet Union have been followed by slow emissions growth in this region due to more efficient energy use, made necessary by the increasing proportion of it that is exported.[89] In comparison, methane has not increased appreciably, and N
2
O
by 0.25% y−1.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming.[128]:17–18[131] This can be calculated by dividing a country's highest contribution to global warming starting from a particular base year, by that country's minimum contribution to global warming starting from a particular base year. Choosing between different base years of 1750, 1900, 1950, and 1990 has a significant effect for most countries.[128]:17–18 Within the G8 group of countries, it is most significant for the UK, France and Germany. These countries have a long history of CO
2
emissions (see the section on Cumulative and historical emissions).

Annual emissions

GHG per capita 2000
Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.

Annual per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.[91]:144 Due to China's fast economic development, its annual per capita emissions are quickly approaching the levels of those in the Annex I group of the Kyoto Protocol (i.e., the developed countries excluding the US).[132] Other countries with fast growing emissions are South Korea, Iran, and Australia (which apart from the oil rich Persian Gulf states, now has the highest percapita emission rate in the world). On the other hand, annual per capita emissions of the EU-15 and the US are gradually decreasing over time.[132] Emissions in Russia and Ukraine have decreased fastest since 1990 due to economic restructuring in these countries.[133]

Energy statistics for fast growing economies are less accurate than those for the industrialized countries. For China's annual emissions in 2008, the Netherlands Environmental Assessment Agency estimated an uncertainty range of about 10%.[132]

The greenhouse gas footprint refers to the emissions resulting from the creation of products or services. It is more comprehensive than the commonly used carbon footprint, which measures only carbon dioxide, one of many greenhouse gases.

2015 was the first year to see both total global economic growth and a reduction of carbon emissions.[134]

Top emitter countries

CO2 emission pie chart
Global carbon dioxide emissions by country.
Ghg-co2-2012
The top 40 countries emitting all greenhouse gases, showing both that derived from all sources including land clearance and forestry and also the CO2 component excluding those sources. Per capita figures are included. "World Resources Institute data".. Note that Indonesia and Brazil show very much higher than on graphs simply showing fossil fuel use.

Annual

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related CO
2
emissions.[135]

Top-10 annual energy-related CO
2
emitters for the year 2009
Country % of global total
annual emissions
Tonnes of GHG
per capita
 China 23.6 5.1
 United States 17.9 16.9
 India 5.5 1.4
 Russia 5.3 10.8
 Japan 3.8 8.6
 Germany 2.6 9.2
 Iran 1.8 7.3
 Canada 1.8 15.4
 South Korea 1.8 10.6
 United Kingdom 1.6 7.5

Cumulative

The C-Story of Human Civilization by PIK
Top-10 cumulative energy-related CO
2
emitters between 1850 and 2008
Country % of world
total
Metric tonnes
CO
2
per person
 United States 28.5 1,132.7
 China 9.36 85.4
 Russia 7.95 677.2
 Germany 6.78 998.9
 United Kingdom 5.73 1,127.8
 Japan 3.88 367
 France 2.73 514.9
 India 2.52 26.7
 Canada 2.17 789.2
 Ukraine 2.13 556.4

Embedded emissions

One way of attributing greenhouse gas (GHG) emissions is to measure the embedded emissions (also referred to as "embodied emissions") of goods that are being consumed. Emissions are usually measured according to production, rather than consumption.[136] For example, in the main international treaty on climate change (the UNFCCC), countries report on emissions produced within their borders, e.g., the emissions produced from burning fossil fuels.[129]:179[137]:1 Under a production-based accounting of emissions, embedded emissions on imported goods are attributed to the exporting, rather than the importing, country. Under a consumption-based accounting of emissions, embedded emissions on imported goods are attributed to the importing country, rather than the exporting, country.

Davis and Caldeira (2010)[137]:4 found that a substantial proportion of CO
2
emissions are traded internationally. The net effect of trade was to export emissions from China and other emerging markets to consumers in the US, Japan, and Western Europe. Based on annual emissions data from the year 2004, and on a per-capita consumption basis, the top-5 emitting countries were found to be (in tCO
2
per person, per year): Luxembourg (34.7), the US (22.0), Singapore (20.2), Australia (16.7), and Canada (16.6).[137]:5 Carbon Trust research revealed that approximately 25% of all CO
2
emissions from human activities 'flow' (i.e., are imported or exported) from one country to another. Major developed economies were found to be typically net importers of embodied carbon emissions—with UK consumption emissions 34% higher than production emissions, and Germany (29%), Japan (19%) and the US (13%) also significant net importers of embodied emissions.[138]

Effect of policy

Governments have taken action to reduce greenhouse gas emissions (climate change mitigation). Assessments of policy effectiveness have included work by the Intergovernmental Panel on Climate Change,[139] International Energy Agency,[140][141] and United Nations Environment Programme.[142] Policies implemented by governments have included[143][144][145] national and regional targets to reduce emissions, promoting energy efficiency, and support for renewable energy such as Solar energy as an effective use of renewable energy because solar uses energy from the sun and does not release pollutants into the air.

Countries and regions listed in Annex I of the United Nations Framework Convention on Climate Change (UNFCCC) (i.e., the OECD and former planned economies of the Soviet Union) are required to submit periodic assessments to the UNFCCC of actions they are taking to address climate change.[145]:3 Analysis by the UNFCCC (2011)[145]:8 suggested that policies and measures undertaken by Annex I Parties may have produced emission savings of 1.5 thousand Tg CO
2
-eq
in the year 2010, with most savings made in the energy sector. The projected emissions saving of 1.5 thousand Tg CO
2
-eq is measured against a hypothetical "baseline" of Annex I emissions, i.e., projected Annex I emissions in the absence of policies and measures. The total projected Annex I saving of 1.5 thousand CO
2
-eq does not include emissions savings in seven of the Annex I Parties.[145]:8

Projections

A wide range of projections of future emissions have been produced.[146] Rogner et al. (2007)[147] assessed the scientific literature on greenhouse gas projections. Rogner et al. (2007)[114] concluded that unless energy policies changed substantially, the world would continue to depend on fossil fuels until 2025–2030. Projections suggest that more than 80% of the world's energy will come from fossil fuels. This conclusion was based on "much evidence" and "high agreement" in the literature.[114] Projected annual energy-related CO
2
emissions in 2030 were 40–110% higher than in 2000, with two-thirds of the increase originating in developing countries.[114] Projected annual per capita emissions in developed country regions remained substantially lower (2.8–5.1 tonnes CO
2
) than those in developed country regions (9.6–15.1 tonnes CO
2
).[148] Projections consistently showed increase in annual world emissions of "Kyoto" gases,[149] measured in CO
2
-equivalent
) of 25–90% by 2030, compared to 2000.[114]

Relative CO
2
emission from various fuels

One liter of gasoline, when used as a fuel, produces 2.32 kg (about 1300 liters or 1.3 cubic meters) of carbon dioxide, a greenhouse gas. One US gallon produces 19.4 lb (1,291.5 gallons or 172.65 cubic feet)[150][151][152]

Mass of carbon dioxide emitted per quantity of energy for various fuels[153]
Fuel name CO
2

emitted
(lbs/106 Btu)
CO
2

emitted
(g/MJ)
CO
2

emitted
(g/kWh)
Natural gas 117 50.30 181.08
Liquefied petroleum gas 139 59.76 215.14
Propane 139 59.76 215.14
Aviation gasoline 153 65.78 236.81
Automobile gasoline 156 67.07 241.45
Kerosene 159 68.36 246.10
Fuel oil 161 69.22 249.19
Tires/tire derived fuel 189 81.26 292.54
Wood and wood waste 195 83.83 301.79
Coal (bituminous) 205 88.13 317.27
Coal (sub-bituminous) 213 91.57 329.65
Coal (lignite) 215 92.43 332.75
Petroleum coke 225 96.73 348.23
Tar-sand Bitumen
Coal (anthracite) 227 97.59 351.32

Life-cycle greenhouse-gas emissions of energy sources

A 2011 IPCC report included a literature review of numerous energy sources' total life cycle CO
2
emissions. Below are the CO
2
emission values that fell at the 50th percentile of all studies surveyed.[154]

Lifecycle greenhouse gas emissions by electricity source.
Technology Description 50th percentile
(g CO
2
/kWhe)
Hydroelectric reservoir 4
Ocean Energy wave and tidal 8
Wind onshore 12
Nuclear various generation II reactor types 16
Biomass various 18
Solar thermal parabolic trough 22
Geothermal hot dry rock 45
Solar PV Polycrystalline silicon 46
Natural gas various combined cycle turbines without scrubbing 469
Coal various generator types without scrubbing 1001

Removal from the atmosphere

Natural processes

Greenhouse gases can be removed from the atmosphere by various processes, as a consequence of:

  • a physical change (condensation and precipitation remove water vapor from the atmosphere).
  • a chemical reaction within the atmosphere. For example, methane is oxidized by reaction with naturally occurring hydroxyl radical, OH· and degraded to CO
    2
    and water vapor (CO
    2
    from the oxidation of methane is not included in the methane Global warming potential). Other chemical reactions include solution and solid phase chemistry occurring in atmospheric aerosols.
  • a physical exchange between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans.
  • a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for CO
    2
    , which is reduced by photosynthesis of plants, and which, after dissolving in the oceans, reacts to form carbonic acid and bicarbonate and carbonate ions (see ocean acidification).
  • a photochemical change. Halocarbons are dissociated by UV light releasing Cl· and F· as free radicals in the stratosphere with harmful effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

Negative emissions

A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analysed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as bio-energy with carbon capture and storage[155][156][157] and carbon dioxide air capture,[157] or to the soil as in the case with biochar.[157] The IPCC has pointed out that many long-term climate scenario models require large-scale manmade negative emissions to avoid serious climate change.[158]

History of scientific research

In the late 19th century scientists experimentally discovered that N
2
and O
2
do not absorb infrared radiation (called, at that time, "dark radiation"), while water (both as true vapor and condensed in the form of microscopic droplets suspended in clouds) and CO
2
and other poly-atomic gaseous molecules do absorb infrared radiation. In the early 20th century researchers realized that greenhouse gases in the atmosphere made Earth's overall temperature higher than it would be without them. During the late 20th century, a scientific consensus evolved that increasing concentrations of greenhouse gases in the atmosphere cause a substantial rise in global temperatures and changes to other parts of the climate system,[159] with consequences for the environment and for human health.

See also

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Bibliography

External links

Carbon dioxide emissions

Emission intensity

An emission intensity (also carbon intensity, C.I.) is the emission rate of a given pollutant relative to the intensity of a specific activity, or an industrial production process; for example grams of carbon dioxide released per megajoule of energy produced, or the ratio of greenhouse gas emissions produced to gross domestic product (GDP). Emission intensities are used to derive estimates of air pollutant or greenhouse gas emissions based on the amount of fuel combusted, the number of animals in animal husbandry, on industrial production levels, distances traveled or similar activity data. Emission intensities may also be used to compare the environmental impact of different fuels or activities. In some case the related terms emission factor and carbon intensity are used interchangeably. The jargon used can be different, for different fields/industrial sectors; normally the term "carbon" excludes other pollutants, such as particulate emissions. One commonly used figure is carbon intensity per kilowatt-hour (CIPK), which is used to compare emissions from different sources of electrical power.

Global Warming Solutions Act of 2006

The Global Warming Solutions Act of 2006, or Assembly Bill (AB) 32, is a California State Law that fights global warming by establishing a comprehensive program to reduce greenhouse gas emissions from all sources throughout the state. AB 32 was authored by then-Assembly member Fran Pavley and Assembly Speaker Fabian Nunez (D-Los Angeles) and signed into law by Governor Arnold Schwarzenegger on September 27, 2006.

On June 1, 2005, Governor Schwarzenegger signed an executive order known as Executive Order S-3-05 which established greenhouse gas emissions targets for the state. The executive order required the state to reduce its greenhouse gas emissions levels to 2000 levels by 2010, to 1990 levels by 2020, and to a level 80% below 1990 levels by 2050. However, to implement this measure, the California Air Resources Board (CARB) needed authority from the legislature. The California State Legislature passed the Global Warming Solutions Act to address this issue and gave the CARB authority to implement the program.

AB 32 requires the California Air Resources Board (CARB or ARB) to develop regulations and market mechanisms to reduce California's greenhouse gas emissions to 1990 levels by the year of 2020, representing approximately a 30% reduction statewide, with mandatory caps beginning in 2012 for significant emissions sources. The bill also allows the Governor to suspend the emissions caps for up to a year in case of emergency or significant economic harm.

The State of California leads the nation in energy efficiency standards and plays a lead role in environmental protection, but is also the 12th largest emitter of carbon worldwide. Greenhouse gas emissions are defined in the bill to include all of the following: carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons. These are the same greenhouse gases listed in Annex A of the Kyoto Protocol.

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 speciesThus, 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.

Glossary of climate change

This article serves as a glossary of climate change terms. It lists terms that are related to global warming.

Greenhouse Gases Observing Satellite

The Greenhouse Gases Observing Satellite (GOSat), also known as Ibuki (Japanese: いぶき, Hepburn: Ibuki, meaning "breath"), is an Earth observation satellite and the world's first satellite dedicated to greenhouse-gas-monitoring. It measures the densities of carbon dioxide and methane from 56,000 locations on the Earth's atmosphere. The GOSAT was developed by the Japan Aerospace Exploration Agency (JAXA) and launched on 23 January 2009, from the Tanegashima Space Center. Japan's Ministry of the Environment, and the National Institute for Environmental Studies (NIES) use the data to track gases causing the greenhouse effect, and share the data with NASA and other international scientific organizations.

Greenhouse gas emissions by Australia

Australia has one of the highest per capita emissions of carbon dioxide in the world, with 0.3% of the world's population it produces 1.3% of the world's greenhouse gases. It was 18.3 tonnes per year per person and the 11th highest in the world per capita in 2009. Australia uses principally coal power (70%) for electricity, with the remainder mainly gas, with no nuclear, low levels of hydro power, and low, but increasing, levels of solar, wind and wave power.

Greenhouse gas emissions by the United Kingdom

According to official statistics, there has been a reduction in domestic greenhouse gas emissions in the United Kingdom. These emissions are caused primarily by primary energy consumption. If indirect emissions are accounted for, however, research suggests that UK emissions may have increased since 1990, due largely to manufacture of short-term consumer items overseas.Carbon dioxide (CO2) and other greenhouse gases continue to drive global warming and ocean acidification. As of May 2019 not yet made into law, however the government Committee on Climate Change (CCC) has recommended carbon neutrality by 2050 and the Energy and Climate Intelligence Unit (ECIU) has said it would be affordable.

Greenhouse gas emissions by the United States

According to the U.S. Energy Information Industry (EIA), the United States produced 5.14 billion metric tons of carbon-dioxide equivalent greenhouse gas (GHG) emissions in 2017, the lowest since the early 1990s. From year to year, emissions rise and fall due to changes in the economy, the price of fuel and other factors. The US Environmental Protection Agency attributed recent decreases to a reduction in emissions from fossil fuel combustion, which was a result of multiple factors including switching from coal to natural gas consumption in the electric power sector; warmer winter conditions that reduced demand for heating fuel in the residential and commercial sectors; and a slight decrease in electricity demand.While the Bush administration opted against Kyoto-type policies to reduce emissions, the Obama administration and various state, local, and regional governments have attempted to adopt some Kyoto Protocol goals on a local basis. For example, the Regional Greenhouse Gas Initiative (RGGI) founded in January 2007 is a state-level emissions capping and trading program by nine northeastern U.S. states. In December 2009 President Obama set a target for reducing U.S. greenhouse gas (GHG) emissions in the range of 17% below 2005 levels by 2020.The U.S. State Department offered a nation-level perspective in the Fourth US Climate Action Report (USCAR) to the United Nations Framework Convention on Climate Change, including measures to address climate change. The report showed that the country was on track to achieve President Bush's goal of reducing greenhouse gas emissions (per unit of gross domestic product) by 18% from 2002 to 2012. Over that same period, actual GHG emissions were projected to increase by 11%. The report estimated that in 2006, U.S. GHG emissions decreased 1.5% from 2005 to 7,075.6 million tonnes of carbon dioxide equivalent. This was an increase of 15.1% from the 1990 levels of 6,146.7 million tonnes (or 0.9% annual increase), and an increase of 1.4% from the 2000 levels of 6,978.4 million tonnes. By 2012 GHG emissions were projected to increase to more than 7,709 million tonnes of carbon dioxide equivalent, which would be 26% above 1990 levels.

Individual and political action on climate change

Individual and political action on climate change can take many forms. Many actions aim to build social and political support to limit, and subsequently reduce, the concentration of greenhouse gases (GHGs) in the atmosphere, with the goal of mitigating climate change. Other actions seek to address the ethical and moral aspects of climate justice, especially with regard to the anticipated unequal impacts of climate change adaptation.

The effects of climate over the course of the past century have made worldwide negative impacts on the globe. The individual actions of countries and cities within these countries have been making significant efforts to offset the amount of carbon emitted into the atmosphere by promoting renewable energy and cleaner overall practices. As countries develop more strategies to promote clean and renewable energy and ways of living, places within the United States such as California are making reforms. In 2016, the state of California passed Senate Bill 32, which strengthens the need to withhold the state from emitting excess carbon. Although Donald Trump has tried remove the United States from the Paris Agreement, other states such as New York have been creating greener spaces by installing more solar panels and creating 'green building' which mitigate pollution in an effort to make New York city 'cleaner'. Countries are making large efforts to fight and reduce the effects of climate change; however, in order to see improvements, more countries with large CO2 emissions such as China and India will need to reform and cut emissions by large percentages.

Intergovernmental Panel on Climate Change

The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations, dedicated to providing the world with an objective, scientific view of climate change, its natural, political and economic impacts and risks, and possible response options.It was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), and later endorsed by the United Nations General Assembly. Membership is open to all members of the WMO and UN.

The IPCC produces reports that contribute to the work of the United Nations Framework Convention on Climate Change (UNFCCC), the main international treaty on climate change. The objective of the UNFCCC is to "stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic (human-induced) interference with the climate system". The IPCC's Fifth Assessment Report was a critical scientific input into the UNFCCC's Paris Agreement in 2015.IPCC reports cover the "scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation." The IPCC does not carry out original research, nor does it monitor climate or related phenomena itself. Rather, it assesses published literature including peer-reviewed and non-peer-reviewed sources. However, the IPCC can be said to stimulate research in climate science. Chapters of IPCC reports often close with sections on limitations and knowledge or research gaps, and the announcement of an IPCC special report can catalyse research activity in that area.

Thousands of scientists and other experts contribute on a voluntary basis to writing and reviewing reports, which are then reviewed by governments. IPCC reports contain a "Summary for Policymakers", which is subject to line-by-line approval by delegates from all participating governments. Typically, this involves the governments of more than 120 countries.The IPCC provides an internationally accepted authority on climate change, producing reports which have the agreement of leading climate scientists and the consensus of participating governments. The 2007 Nobel Peace Prize was shared, between the IPCC and Al Gore.Following the election of a new Bureau in 2015, the IPCC embarked on its sixth assessment cycle. Besides the Sixth Assessment Report, to be completed in 2022, the IPCC released the Special Report on Global Warming of 1.5 °C in October 2018, will release an update to its 2006 Guidelines for National Greenhouse Gas Inventories—the 2019 Refinement—in May 2019, and will deliver two further special reports in 2019: the Special Report on the Ocean and Cryosphere in a Changing Climate, and Climate Change and Land. This makes the sixth assessment cycle the most ambitious in the IPCC's 30-year history. The IPCC also decided to prepare a special report on cities and climate change in the seventh assessment cycle, and held a conference in March 2018 to stimulate research in this area.

Joint Implementation

Joint implementation (JI) is one of three flexibility mechanisms set out in the Kyoto Protocol to help countries with binding greenhouse gas emissions targets (the Annex I countries) meet their treaty obligations. Under Article 6, any Annex I country can invest in a project to reduce greenhouse gas emissions in any other Annex I country (referred to as a "Joint Implementation Project") as an alternative to reducing emissions domestically. In this way countries can lower the costs of complying with their Kyoto targets by investing in projects where reducing emissions may be cheaper and applying the resulting Emission Reduction Units (ERUs) towards their commitment goal.

A JI project might involve, for example, replacing a coal-fired power plant with a more efficient combined heat and power plant. Most JI projects are expected to take place in the economies in transition (the EIT Parties) noted in Annex B of the Kyoto Protocol. Currently Russia and Ukraine are slated to host the greatest number of JI projects.Unlike the case of the Clean Development Mechanism, the JI has caused less concern of spurious emission reductions, as the JI project, in contrast to the CDM project, takes place in a country which has a commitment to reduce emissions under the Kyoto Protocol.

The process of receiving credit for JI projects is somewhat complex. Emission reduction projects are awarded credits called Emission Reduction Units (ERUs), which represents an emission reduction equivalent to one tonne of CO2 equivalent. The ERUs come from the host country's pool of assigned emissions credits, known as Assigned Amount Units, or AAUs. Each Annex I party has a predetermined amount of AAUs, calculated on the basis of its 1990 greenhouse gas emission levels. By requiring JI credits to come from a host country's pool of AAUs, the Kyoto Protocol ensures that the total amount of emissions credits among Annex I parties does not change for the duration of the Kyoto Protocol's first commitment period.

Life-cycle greenhouse-gas emissions of energy sources

Measurement of life-cycle greenhouse gas emissions involves calculating the global-warming potential of electrical energy sources through life-cycle assessment of each energy source. The findings are presented in units of global warming potential per unit of electrical energy generated by that source. The scale uses the global warming potential unit, the carbon dioxide equivalent (CO2e), and the unit of electrical energy, the kilowatt hour (kWh). The goal of such assessments is to cover the full life of the source, from material and fuel mining through construction to operation and waste management.

In 2014, the Intergovernmental Panel on Climate Change harmonized the carbon dioxide equivalent (CO2e) findings of the major electricity generating sources in use worldwide. This was done by analyzing the findings of hundreds of individual scientific papers assessing each energy source.For all technologies, advances in efficiency, and therefore reductions in CO2e since the time of publication, have not been included. For example, the total life cycle emissions from wind power may have lessened since publication. Similarly, due to the time frame over which the studies were conducted, nuclear Generation II reactor's CO2e results are presented and not the global warming potential of Generation III reactors, presently under construction in the United States and China. Other limitations of the data include: a) missing life cycle phases, and, b) uncertainty as to where to define the cut-off point in the global warming potential of an energy source. The latter is important in assessing a combined electrical grid in the real world, rather than the established practice of simply assessing the energy source in isolation.

List of U.S. states by carbon dioxide emissions

This is a list of U.S. states by carbon dioxide emissions due to human activity. The data presented below from the US Environmental Protection Agency and the US Energy Information Administration corresponds to emissions in 2014.

List of countries by greenhouse gas emissions

This is a list of countries by total greenhouse gas (GHG) annual emissions in 2014. It is based on data for carbon dioxide, methane, nitrous oxide, perfluorocarbon, hydrofluorocarbon, and sulfur hexafluoride emissions compiled by the World Resources Institute. The emissions data shown below do not include land-use change and forestry.

Because individual countries vary vastly in size and population, consider referring to List of countries by greenhouse gas emissions per capita which may be more relevant.

List of countries by greenhouse gas emissions per capita

This is a list of countries by total greenhouse gas (GHG) emissions per capita by year. It is based on data for carbon dioxide, methane, nitrous oxide, perfluorocarbon, hydrofluorocarbon, and sulfur hexafluoride emissions compiled by the World Resources Institute, divided by the population estimate by the United Nations (for July 1) of the same year. The emissions data do not include land-use change and forestry.

Low-carbon diet

A low-carbon diet refers to making lifestyle choices to reduce the greenhouse gas emissions (GHGe) resulting from consumption decisions. It is estimated that the U.S. food system is responsible for at least 20 percent of U.S. greenhouse gases. This estimate may be low, as it counts only direct sources of GHGe. Indirect sources, such as demand for products from other countries, are often not counted. A low-carbon diet minimizes the emissions released from the production, packaging, processing, transport, preparation and waste of food. Major tenets of a low-carbon diet include eating less industrial meat and dairy, eating less industrially produced food in general, eating food grown locally and seasonally, eating less processed and packaged foods and reducing waste from food by proper portion size, recycling or composting.

Post–Kyoto Protocol negotiations on greenhouse gas emissions

Post-Kyoto negotiations refers to high level talks attempting to address global warming by limiting greenhouse gas emissions. Generally part of the United Nations Framework Convention on Climate Change (UNFCCC), these talks concern the period after the first "commitment period" of the Kyoto Protocol, which expired at the end of 2012. Negotiations have been mandated by the adoption of the Bali Road Map and Decision 1/CP.13 ("The Bali Action Plan").

UNFCCC negotiations are conducted within two subsidiary bodies, the Ad Hoc Working Group on Long-term Cooperative Action under the Convention (AWG-LCA) and the Ad Hoc Working Group on Further Commitments for Annex I Parties under the Kyoto Protocol (AWG-KP) and were expected to culminate in the United Nations Climate Change Conference taking place in December 2009 in Copenhagen (COP-15); negotiations are supported by a number of external processes, including the G8 process, a number of regional meetings and the Major Economies Forum on Energy and Climate that was launched by US President Barack Obama in March 2009. High level talks were held at the meeting of the G8+5 Climate Change Dialogue in February 2007 and at a number of subsequent G8 meetings, most recently leading to the adoption of the G8 leaders declaration "Responsible Leadership for a Sustainable Future" during the G8 summit in L´Aquila, Italy, in July 2009.

Regional Greenhouse Gas Initiative

'The Regional Greenhouse Gas Initiative (RGGI, pronounced "Reggie") is the first mandatory market based program in the United States to reduce greenhouse gas emissions. RGGI is a cooperative effort among the states of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont to cap and reduce carbon dioxide (CO2) emissions from the power sector. RGGI compliance obligations apply to fossil-fueled power plants 25MW and larger within the ten-state region.RGGI establishes a regional cap on the amount of CO2 pollution that power plants can emit by issuing a limited number of tradable CO2 allowances. Each allowance represents an authorization for a regulated power plant to emit one short ton of CO2. Individual CO2 budget trading programs in each RGGI state together create a regional market for CO2 allowances.The RGGI states distribute over 90 percent of allowances through quarterly auctions. These allowance auctions generate proceeds, which participating states are able to invest in strategic energy and consumer benefit programs. Programs funded through RGGI have included energy efficiency, clean and renewable energy, greenhouse gas abatement, and direct bill assistance.

An initial milestone program's development occurred in 2005, when seven states signed a Memorandum of Understanding (MOU) announcing an agreement to implement RGGI. The RGGI states then established individual CO2 budget trading programs, based on the RGGI Model Rule. The first pre-compliance RGGI auction took place in September 2008, and the program became effective on January 1, 2009. The RGGI program is currently in its fourth three-year compliance period, which began January 1, 2018.

Reservoir

A reservoir (from French réservoir – a "tank") is, most commonly, an enlarged natural or artificial lake, pond or impoundment created using a dam or lock to store water.

Reservoirs can be created in a number of ways, including controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, through excavation, or building any number of retaining walls or levees.

Defined as a storage space for fluids, reservoirs may hold water or gasses, including hydrocarbons. Tank reservoirs store these in ground-level, elevated, or buried tanks. Tank reservoirs for water are also called cisterns. Most underground reservoirs are used to store liquids, principally either water or petroleum, below ground.

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