Greenhouse gas | Wikipedia audio article

A greenhouse gas is a gas that absorbs and
emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse
effect. 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), rather than the present average of
15 °C (59 °F). 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 40% increase in the atmospheric concentration of carbon dioxide (CO2), 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. 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).Should greenhouse gas emissions continue at their rate in 2017,
Earth’s surface temperature could exceed historical values as early as 2047, with potentially
harmful effects on ecosystems, biodiversity and human livelihoods. 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.==Gases in Earth’s atmosphere=====
Greenhouse gases===Greenhouse gases are those that absorb and
emit infrared radiation in the wavelength range emitted by Earth. In order, the most
abundant greenhouse gases in Earth’s atmosphere are: Water vapor (H2O)
Carbon dioxide (CO2) Methane (CH4)
Nitrous oxide (N2O) Ozone (O3)
Chlorofluorocarbons (CFCs) Hydrofluorocarbons (incl. HCFCs and HFCs)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). 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 CO2
was about 0.45. The annual airborne fraction increased at a rate of 0.25 ± 0.21% per year
over the period 1959–2006.===Non-greenhouse gases===
The major atmospheric constituents, nitrogen (N2), oxygen (O2), and argon (Ar), are not
greenhouse gases because molecules containing two atoms of the same element such as N2 and
O2 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 heterodiatomic molecules containing atoms of different elements
such as carbon monoxide (CO) or hydrogen chloride (HCl) do absorb infrared radiation, although
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===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 CO2 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 CO2 (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 CO2, which requires reactions with the OH radical, produces an instantaneous
reduction in radiative absorption and emission since CO2 is a weaker greenhouse gas than
methane. However, the oxidations of CO and CH4 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.Methane has indirect effects in addition to forming
CO2. 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 CO2 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 CO2.
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 CH4 increases as well as producing
stratospheric water vapor.===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.==Impacts on the overall greenhouse effect
==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 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) argue that the contribution to
climate change from methane is at least double previous estimates as a result of this effect.When
ranked by their direct contribution to the greenhouse effect, the most important are:
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.===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. In addition,
some gases, such as methane, are known to have large indirect effects that are still
being quantified.===Atmospheric lifetime===
Aside from water vapor, which has a residence time of about nine days, major greenhouse
gases are well mixed and take many years to leave the atmosphere. 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) defines the lifetime τ {\displaystyle \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 τ {\displaystyle \tau }
can be defined as the ratio of the mass m {\displaystyle 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 o
u t {\displaystyle F_{out}}
), chemical loss of X
( L {\displaystyle L}
), and deposition of X
( D {\displaystyle D}
) (all in kg/s): τ
=m F o
u t +
L +
D {\displaystyle \tau={\frac {m}{F_{out}+L+D}}}
. If output of this gas into the box ceased,
then after time τ {\displaystyle \tau }
, 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.
The atmospheric lifetime of CO2 is estimated of the order of 30–95 years.
This figure accounts for CO2 molecules being removed from the atmosphere by mixing into
the ocean, photosynthesis, and other processes. However, this excludes the balancing fluxes
of CO2 into the atmosphere from the geological reservoirs, which have slower characteristic
rates. Although more than half of the CO2 emitted is removed from the atmosphere within
a century, some fraction (about 20%) of emitted CO2 remains in the atmosphere for many thousands
of years. Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes
than CO2, e.g. N2O has a mean atmospheric lifetime of 121 years.===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. Earth’s surface temperature
depends on this balance between incoming and outgoing energy. If this energy balance is
shifted, Earth’s surface becomes warmer or cooler, leading to a variety of changes in
global climate.A number of natural and man-made mechanisms can affect the global energy balance
and force changes in Earth’s climate. Greenhouse gases are one such mechanism. 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. 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. Radiative forcing quantifies the effect of factors that
influence Earth’s energy balance, including changes in the concentrations of greenhouse
gases. Positive radiative forcing leads to warming by increasing the net incoming energy,
whereas negative radiative forcing leads to cooling.===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 CO2 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 CO2 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. A 2014 analysis, however, states that although methane’s
initial impact is about 100 times greater than that of CO2, 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. The decrease in GWP at longer
times is because methane is degraded to water and CO2 through chemical reactions in the
atmosphere. Examples of the atmospheric lifetime and GWP
relative to CO2 for several greenhouse gases are given in the following table: The use of CFC-12 (except some essential uses)
has been phased out due to its ozone depleting properties. The phasing-out of less active
HCFC-compounds will be completed in 2030.==Natural and anthropogenic sources==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.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”. 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 Ice cores provide evidence for greenhouse
gas concentration variations over the past 800,000 years (see the following section).
Both CO2 and CH4 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 CO2 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 CO2 levels were likely 10 times higher than now.
Indeed, higher CO2 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. The spread of land plants is thought to have
reduced CO2 concentrations during the late Devonian, and plant activities as both sources
and sinks of CO2 have since been important in providing stabilising feedbacks.
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 CO2 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. 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 CO2 per year, whereas
humans contribute 29 billion tons of CO2 each year.===Ice cores===
Measurements from Antarctic ice cores show that before industrial emissions started
atmospheric CO2 mole fractions were about 280 parts per million (ppm), and stayed between
260 and 280 during the preceding ten thousand years. 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, though others have argued that
these findings more likely reflect calibration or contamination problems rather than actual
CO2 variability. 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
===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.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.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.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==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. Natural sources
of carbon dioxide are more than 20 times greater than sources due to human activity, 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.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. Future warming is projected to have a range of impacts, including
sea level rise, increased frequencies and severities of some extreme weather events,
loss of biodiversity, and regional changes in agricultural productivity.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 CO2 emissions.
livestock enteric fermentation and manure management, paddy rice farming, land use and
wetland changes, man-made lakes, 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 (N2O) concentrations.The seven sources
of CO2 from fossil fuel combustion are (with percentage contributions for 2000–2004):
Carbon dioxide, methane, nitrous oxide (N2O) and three groups of fluorinated gases (sulfur
hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major
anthropogenic greenhouse gases, and are regulated under the Kyoto Protocol international treaty,
which came into force in 2005. Emissions limitations specified in the Kyoto Protocol expired in
2012. The Cancún agreement, agreed on in 2010, includes voluntary pledges made by 76
countries to control emissions. At the time of the agreement, these 76 countries were
collectively responsible for 85% of annual global emissions.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.===Sectors=======
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 CO2. 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.====Trucking and haulage====
The trucking and haulage industry plays a part in production of CO2, contributing around
20% of the UK’s total carbon emissions a year, with only the energy industry having a larger
impact at around 39%. 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. 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 (PET) produced—the type
of plastic most commonly used for beverage bottles, the transportation produce greenhouse
gases also. 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.From
the other side, if it is placed in a landfill, it becomes a carbon sink although biodegradable
plastics have caused methane emissions. 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 packag is single use, of course .==
Role of water vapor==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. 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. 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. (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 CH4 and CO2. 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.==Direct greenhouse gas emissions==
Between the period 1970 to 2004, greenhouse gas emissions (measured in CO2-equivalent)
increased at an average rate of 1.6% per year, with CO2 emissions from the use of fossil
fuels growing at a rate of 1.9% per year. Total anthropogenic emissions at the end of
2009 were estimated at 49.5 gigatonnes CO2-equivalent. These emissions include CO2 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 CO2 emissions
is the burning of coal, natural gas, and petroleum for electricity and heat.===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) for tables of national emissions data. Some variables that have been reported 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
CO2 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).
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.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).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.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). 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. Per capita emissions may be based on historical or annual emissions
(Banuri et al., 1996, pp. 106–07).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.===From land-use change===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. 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).There are substantial uncertainties in the measurement of net carbon
emissions. Additionally, there is controversy over how carbon sinks should be allocated
between different regions and over time (Banuri et al., 1996, p. 93). 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===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. Emission
intensities may be calculated using market exchange rates (MER) or purchasing power parity
(PPP) (Banuri et al., 1996, p. 96). 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 anthropogenic (i.e., human-emitted)
emissions of CO2 from fossil fuel use are a major cause of global warming, and give
some indication of which countries have contributed most to human-induced climate change.
The table above to the left is based on Banuri et al. (1996, p. 94). Overall, developed countries
accounted for 83.8% of industrial CO2 emissions over this time period, and 67.8% of total
CO2 emissions. Developing countries accounted for industrial CO2 emissions of 16.2% over
this time period, and 32.2% of total CO2 emissions. The estimate of total CO2 emissions includes
biotic carbon emissions, mainly from deforestation. Banuri et al. (1996, p. 94) 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). 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 CO2 emissions between 1890
and 2007. 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%.===Changes since a particular base year===Between 1970 and 2004, global growth in annual
CO2 emissions was driven by North America, Asia, and the Middle East. The sharp acceleration
in CO2 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. In comparison, methane has not increased appreciably, and
N2O by 0.25% y−1. Using different base years for measuring emissions
has an effect on estimates of national contributions to global warming. 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. Within the G8 group of countries, it is most significant
for the UK, France and Germany. These countries have a long history of CO2 emissions (see
the section on Cumulative and historical emissions).===Annual emissions===Annual per capita emissions in the industrialized
countries are typically as much as ten times the average in developing countries. 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). 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. Emissions in Russia and Ukraine have decreased
fastest since 1990 due to economic restructuring in these countries.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%.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.===Top emitter countries=======
Annual====In 2009, the annual top ten emitting countries
accounted for about two-thirds of the world’s annual energy-related CO2 emissions.====Cumulative=======
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. 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. 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) found that a substantial proportion of CO2 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 tCO2 per person, per year): Luxembourg (34.7), the US (22.0), Singapore (20.2), Australia
(16.7), and Canada (16.6). Carbon Trust research revealed that approximately 25% of all CO2
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.===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, International Energy Agency, and United
Nations Environment Programme. Policies implemented by governments have included 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. Analysis by the UNFCCC (2011) suggested that policies and measures
undertaken by Annex I Parties may have produced emission savings of 1.5 thousand Tg CO2-eq
in the year 2010, with most savings made in the energy sector. The projected emissions
saving of 1.5 thousand Tg CO2-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 CO2-eq does not include emissions savings
in seven of the Annex I Parties.===Projections===A wide range of projections of future emissions
have been produced. Rogner et al. (2007) assessed the scientific literature on greenhouse gas
projections. Rogner et al. (2007) 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. Projected annual energy-related
CO2 emissions in 2030 were 40–110% higher than in 2000, with two-thirds of the increase
originating in developing countries. Projected annual per capita emissions in developed country
regions remained substantially lower (2.8–5.1 tonnes CO2) than those in developed country
regions (9.6–15.1 tonnes CO2). Projections consistently showed increase in annual world
emissions of “Kyoto” gases, measured in CO2-equivalent) of 25–90% by 2030, compared to 2000.===Relative CO2 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)==Life-cycle greenhouse-gas emissions of
energy sources==A literature review of numerous energy sources
CO2 emissions by the IPCC in 2011, found that, the CO2 emission value that fell within the
50th percentile of all total life cycle emissions studies conducted was as follows.==Removal from the atmosphere (“sinks”)=====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 CO2 and water vapor (CO2 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 CO2, 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 and carbon dioxide air capture, or to the soil as in the case with
biochar. The IPCC has pointed out that many long-term climate scenario models require
large-scale manmade negative emissions to avoid serious climate change.==History of scientific research==
In the late 19th century scientists experimentally discovered that N2 and O2 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 CO2 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, with consequences
for the environment and for human health.==See also

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