environment_matters

Greenhouse Effect

Greenhouse gases are components of the atmosphere that contribute to the Greenhouse effect . Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities add to the levels of most of these naturally occurring gases.

The "Greenhouse effect"
The greenhouse effect was discovered in 1824 by Joseph Fourier and first quantitatively investigated in 1896 by Svante Arrhenius.

When sunlight reaches the surface of earth, some of it is absorbed and warms the earth. Because the Earth's surface is much cooler than the sun, it radiates energy at much longer wavelengths than the sun (see black body radiation and Wien's displacement law). Some energy in these longer wavelengths is absorbed by greenhouse gases in the atmosphere before it can be lost to space. The absorption of this longwave radiant energy warms the atmosphere (the atmosphere also is warmed by transfer of sensible and latent heat from the surface). Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the "greenhouse effect." The term is in fact a misnomer, as this process is not the primary mechanism that warms greenhouses.

The major natural greenhouse gases are water vapour, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect , because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) Other greenhouse gases include, but are not limited to, nitrous oxide , sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons (see IPCC list of greenhouse gases).

The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. This is because homonuclear diatomic molecules such as N₂ and O₂ neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light.

It is worth noting that late 19th century scientists experimentally discovered that N₂ and O₂ did not absorb infrared radiation (called, at that time, "dark radiation") and that CO₂ and many other gases did absorb such radiation. It was recognized in the early 20th century that the known major greenhouse gases in the atmosphere did cause the earth's temperature to be higher than it would have been without the greenhouse gases.

Anthropogenic greenhouse gases
Global anthropogenic greenhouse gas emissions broken down into 8 different sectors for the year 2000.

The concentrations of several greenhouse gases have increased over time. Human activity increases the greenhouse effect primarily through release of carbon dioxide, but human influences on other greenhouse gases can also be important. Some of the main sources of greenhouse gases due to human activity include:

Burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations; livestock and 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 major 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. Greenhouse gas emissions from industry, transportation and agriculture are very likely the main cause of recently observed global warming.

Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gasses (sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the subject of the Kyoto Protocol, which entered into force in 2005.

CFCs, although greenhouse gasses, 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.

The role of water vapor
Increasing water vapor at Boulder, Colorado.

Water vapor is a naturally occurring greenhouse gas and accounts for the largest percentage of the greenhouse effect. Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at very local scales.

In climate models an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapor content of the troposphere, with approximately constant relative humidity. The increased water vapor in turn leads to an increase in the greenhouse effect and thus a further increase in temperature; the increase in temperature leads to still further increase in atmospheric water vapor; and the feedback cycle continues until equilibrium is reached. Thus water vapor acts as a positive feedback to the forcing provided by human-released greenhouse gases such as CO₂ (but has never, so far, acted on Earth as part of a runaway feedback). Changes in water vapor may also have indirect effects via cloud formation.

Intergovernmental Panel on Climate Change (IPCC) IPCC Third Assessment Report chapter lead author Michael Mann considers citing "the role of water vapor as a greenhouse gas" to be "extremely misleading" as water vapor can not be controlled by humans. The IPCC report has discussed water vapor feedback in more detail.

Increase of greenhouse gases
Measurements from Antarctic ice cores show that just before industrial emissions began, atmospheric CO₂ levels were about 280 parts per million by volume (ppm; the units µL/L are occasionally used and are identical to parts per million by volume). From the same ice cores it appears that CO₂ concentrations stayed between 260 and 280 ppm during the preceding 10,000 years. Studies using evidence from stomata of fossilized leaves suggest greater variability, with CO₂ levels above 300 ppm during the period 7,000-10,000 years ago, though others have argued that these findings more likely reflect calibration/contamination problems rather than actual CO₂ variability.

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. The concentration of CO₂ has increased by about 100 ppm (i.e., from 280 ppm to 380 ppm). The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; the next 50 ppm increase took place in about 33 years, from 1973 to 2006.

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Atmospheric Concentration of Carbon Dioxide

As of January 2007, the earth's atmospheric CO₂ concentration is about 0.0383% by volume (383 ppmv) or 0.0582% by weight. This represents about 2.996×10¹² tonnes, and is estimated to be 105 ppm (37.77%) above the pre-industrial average.

Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared with the southern hemisphere, there is an annual fluctuation of up to 6 ppmv (± 3 ppmv), peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest.

Despite its small concentration, CO₂ is a very important component of Earth's atmosphere, because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode) and enhances the greenhouse effect. The three vibrational modes of carbon dioxide: (a) symmetric, (b) asymmetric stretching; (c) bending. In (a), there is no change in dipole moment, thus interaction with photons is impossible, while in (b) and (c) there is optical activity.

The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity; this was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year. Volcanic releases are about 1% of the amount which is released by human activities.

Since the start of the Industrial Revolution, the atmospheric CO₂ concentration has increased by approximately 110 ppmv or about 40%, most of it released since 1945. Monthly measurements taken at Mauna Loa since 1958 show an increase from 316 ppmv in that year to 376 ppmv in 2003, an overall increase of 60 ppmv during the 44-year history of the measurements. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO₂; deforestation is the second major cause. Around 24 billion tonnes of CO₂ are released from fossil fuels per year worldwide, equivalent to about 6 billion tonnes of carbon.

In 1997, Indonesian peat fires may have released 13% – 40% as much carbon as fossil fuel burning does. Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO₂ remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO₂ to the increase in atmospheric CO₂ is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.

Increased amounts of CO₂ in the atmosphere tend to enhance the greenhouse effect and thus contribute to global warming. The effect of combustion-produced carbon dioxide on climate is called the Callendar effect.

Variation in the past
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ levels were about 260 – 280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years before the present. During this time, the atmospheric carbon dioxide concentration has varied between 180 – 210 µL/L during ice ages, increasing to 280 – 300 µL/L during warmer interglacials. The data can be accessed here.

Some studies have disputed the claim of stable CO₂ levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al. argued that CO₂ levels during the period 7 – 10 kyr ago were significantly higher (~300 µL/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO₂ measurements.

Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.

On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO₂ concentrations (>3,000 ppmv) between 600 and 400 Myr BP and between 200 and 150 Myr BP. On long timescales, atmospheric CO₂ content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO₂. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO₂ response to emissions over the next hundred years. In more recent times, atmospheric CO₂ concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 ppmv by about 20 Myr BP. Low CO₂ concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP. Present carbon dioxide levels are likely higher now than at any time during the past 20 million years. During this period however atmospheric CO₂ concentration has been lower than in preceding history.

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26 May 2007
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