Atmospheric lifetime

Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO₂ and 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.^[51] 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.^[52][53][54] 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.^[55] 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.^[56] This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.^[56][57][58]

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.^[59] 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,^[60] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO₂ variability.^[61][62] 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 by about 36% to 380 ppm, or 100 ppm over modern pre-industrial levels. The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973.; however the next 50 ppm increase took place in about 33 years, from 1973 to 2006.^[63]

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.^[64]

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.^[65]

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.^[68] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,^[69] 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.^[70]

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.^[71] Future warming is projected to have a range of impacts, including sea level rise,^[72] increased frequencies and severities of some extreme weather events,^[72] loss of biodiversity,^[73] and regional changes in agricultural productivity.^[73]

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.^[70]
• livestock enteric fermentation and manure management,^[74] paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane.
• use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
• agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide () concentrations.

The seven sources of CO₂ from fossil fuel combustion are (with percentage contributions for 2000–2004):^[75]

Carbon dioxide, methane, nitrous oxide () and three groups of fluorinated gases (sulfur hexafluoride (), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major anthropogenic greenhouse gases,^[77]:147[78] and are regulated under the Kyoto Protocol international treaty, which came into force in 2005.^[79] Emissions limitations specified in the Kyoto Protocol expire in 2012.^[79] The Cancún agreement, agreed in 2010, includes voluntary pledges made by 76 countries to control emissions.^[80] At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions.^[80]

Although CFCs are greenhouse gases, they are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the media.

Sectors

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.^[81]

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.^[16] Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass at in saturated air at about 32 °C.(see Relative humidity#other important facts) ^[82]

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 and CO₂.^[83] 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.^[84]

Direct greenhouse gas emissions

Between the period 1970 to 2004, GHG emissions (measured in CO₂-equivalent)^[85] 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.^[86][87] Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO₂-equivalent.^[88]:15 These emissions include CO₂ from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other GHGs covered by the Kyoto Protocol.

At present, the two primary sources of CO₂ emissions are from burning coal used for electricity generation and petroleum used for motor transport.

Regional and national attribution of emissions

There are several different ways of measuring GHG emissions, for example, see World Bank (2010)^[89]:362 for tables of national emissions data. Some variables that have been reported^[90] 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 GHGs: Contribution of a given GHG 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).^[91] This 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.^[90]

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 GHGs (IEA, 2007, p. 199).^[92]

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.(Holtz-Eakin, 1995, pp.;85;101).^[93]

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).^[77]: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.^[89]:370 Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–107).^[91]

Greenhouse gas intensity and land-use change

The first figure shown opposite is based on data from the World Resources Institute, and shows a measurement of GHG emissions for the year 2000 according to greenhouse gas intensity and land-use change. Herzog et al. (2006, p. 3) defined greenhouse gas intensity as GHG emissions divided by economic output.^[94] GHG intensities are subject to uncertainty over whether they are calculated using market exchange rates (MER) or purchasing power parity (PPP) (Banuri et al., 1996, p. 96).^[91] Calculations based on MER suggest large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

Land-use change, e.g., the clearing of forests for agricultural use, can affect the concentration of GHGs in the atmosphere by altering how much carbon flows out of the atmosphere into carbon sinks.^[95] Accounting for land-use change can be understood as an attempt to measure “net” emissions, i.e., gross emissions from all GHG sources minus the removal of emissions from the atmosphere by carbon sinks (Banuri et al., 1996, pp. 92–93).^[91]

There are substantial uncertainties in the measurement of net carbon emissions.^[96] Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri et al., 1996, p. 93).^[91] For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

Cumulative and historical emissions

Cumulative anthropogenic (i.e., human-emitted) emissions of CO₂ from fossil fuel use are a major cause of global warming,^[97] and give some indication of which countries have contributed most to human-induced climate change.^[98]:15

The table above to the left is based on Banuri et al. (1996, p. 94).^[91] 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)^[91] 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).^[91] 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 1890–2007.^{[99]:179–180} 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%.^{[99]:179–180}

Changes since a particular base year

Between 1970–2004, global growth in annual CO₂ emissions was driven by North America, Asia, and the Middle East.^[100] 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.^[75] In comparison, methane has not increased appreciably, and by 0.25% y⁻¹.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming.^{[98]:17–18[101]} 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.^[98]: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 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.^[77]: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 USA).^[102] Other countries with fast growing emissions are South Korea, Iran, and Australia. On the other hand, annual per capita emissions of the EU-15 and the USA are gradually decreasing over time.^[102] Emissions in Russia and the Ukraine have decreased fastest since 1990 due to economic restructuring in these countries.^[103]

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%.^[102]

Top emitters

Annual

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related CO₂ emissions.^[104]

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.^[107] 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.^{[99]:179[108]: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)^[108]:4 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).^[108]:5 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 USA (13%) also significant net importers of embodied emissions.^[109]

Effect of policy

Governments have taken action to reduce GHG emissions (climate change mitigation). Assessments of policy effectiveness have included work by the Intergovernmental Panel on Climate Change,^[110] International Energy Agency,^[111][112] and United Nations Environment Programme.^[113] Policies implemented by governments have included^{[114][115][116]} national and regional targets to reduce emissions, promoting energy efficiency, and support for renewable energy.

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.^[116]:3 Analysis by the UNFCCC (2011)^[116]:8 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.^[116]:8

Projections

A wide range of projections of future GHG emissions have been produced.^[117] Rogner et al. (2007)^[118] assessed the scientific literature on GHG projections. Rogner et al. (2007)^[86] 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.^[86] 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.^[86] 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).^[119] Projections consistently showed increase in annual world GHG emissions (the "Kyoto" gases,^[120] measured in CO₂-equivalent) of 25–90% by 2030, compared to 2000.^[86]

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)^{[121][122][123]}

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.^[125]

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^{[126][127][128]} and carbon dioxide air capture,^[128] or to the soil as in the case with biochar.^[128] The IPCC has pointed out that many long-term climate scenario models require large scale manmade negative emissions to avoid serious climate change.^[129]

History of scientific research

Late 19th century scientists experimentally discovered that and do not absorb infrared radiation (called, at that time, "dark radiation") while, on the contrary, water, both as true vapor and condensed in the form of microscopic droplets suspended in clouds, as well as CO₂ and other poly-atomic gaseous molecules, do absorb infrared radiation. It was recognized in the early 20th century that greenhouse gases in the atmosphere made the 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 are causing a substantial rise in global temperatures and changes to other parts of the climate system,^[130] with consequences for the environment and for human health.

See also

Notes and references

Bibliography

• (pb: 0-521-01495-6)
• (pb: 978-0-521-70596-7)

• Zhou, Yiqin (2011). Compar[ison of] Fresh or Ensiled Fodders (e.g., Grass, Legume, Corn) on the Production of Greenhouse Gases Following Enteric Fermentation in Beef Cattle. Rouyn-Noranda, Qué.: Université du Québec en Abitibi-Témiscamingue. N.B.: Research report.

External links

• DMOZ
• NOAA
• Atmospheric spectra of GHGs and other trace gases
• How Much Greenhouse Gas Does the United States Emit?
• Grist article on convenient summary from various sources incl IPCC of greenhouse gas emissions * *
• Summary of Greenhouse gas emissions at Google Docs
• Greenhouse Gases
• Greenhouse Gases Sources, Levels, Study results — University of Michigan; eia.doe.gov findings
• EM-1 | Greenhouse gas emissions research project

Carbon dioxide emissions

• The Guardian February 2011
• International Energy Annual: Reserves
• International Energy Annual 2003: Carbon Dioxide Emissions
• International Energy Annual 2003: Notes and Sources for Table H.1co2 (Metric tons of carbon dioxide can be converted to metric tons of carbon equivalent by multiplying by 12/44)
• Textbook on Eddy Covariance Measurements of Gas Emissions
• Trends in Atmospheric Carbon Dioxide at NOAA
• NOAA Paleoclimatology Program — Vostok Ice Core
• NOAA CMDL CCGG — Interactive Atmospheric Data Visualization NOAA CO₂ data
• Carbon Dioxide Information Analysis Center (CDIAC)
  • Carbon Dioxide Information Analysis Centre FAQ Includes links to Carbon Dioxide statistics
• Little Green Data Book 2007, World Bank. Lists CO₂ statistics by country, including per capita and by country income class.
• Database of carbon emissions of power plants
• NASA's Orbiting Carbon Observatory
• The Carbon Bag: the carbon dioxide emission of a typical British home

Methane emissions

• BBC News — Thawing Siberian bogs are releasing more methane
• Textbook on Eddy Covariance Measurements of Gas Emissions