The Greenhouse Effect

The Greenhouse Effect

  • Introductory chemistry topics used:

    • Combustion
    • Sunlight wavelength regions (UV, visible, IR)
    • Absorption spectra
    • ppm, ppb, ppt concentration scale for gases
    • CFCs and their replacements
    • Tropospheric ozone
    • Aerosols

  • Introduction

    • The greenhouse effect is predicted to significantly affect climates.
    • Greenhouse warming and global warming refer to the expected increase in average global air temperatures due to the buildup of carbon dioxide and other greenhouse gases.
    • Most atmospheric scientists believe global warming has been underway, contributing to a two-thirds of a Celsius degree increase since 1860.
    • Global warming is considered a crucial worldwide environmental problem, with potential positive and negative effects.
    • Unlike stratospheric ozone depletion, global warming's effects are not universally convincing.
    • The extent and timing of future temperature increases are uncertain.
    • Understanding the driving factors is crucial for avoiding potential catastrophes from rapid climate change.

Mechanism of the Greenhouse Effect

  • Earth’s Energy Source

    • The Earth is kept warm by energy from the Sun, which radiates energy as light.
    • The Sun behaves like a blackbody, emitting and absorbing light efficiently.
    • The wavelength of maximum energy emission by a blackbody decreases with increasing Kelvin temperature. The relationship is expressed as: \lambda_{peak} = \frac{2897}{T}
      • \lambda_{peak} is the peak wavelength in micrometers.
      • T is the Kelvin temperature.
    • The Sun's surface temperature is approximately 5800 K, resulting in \lambda_{peak} of about 0.50 \mu m, which is in the visible region.
    • The maximum observed solar output occurs in the visible light range (0.40 to 0.75 μm).
    • The Earth receives infrared light (IR) in the 0.75–4 μm region from the Sun.
    • Slightly over half of the energy received at the top of the Earth’s atmosphere from the Sun is IR, and most of the remainder is visible light.
    • The Earth also receives ultraviolet light (UV), which has wavelengths less than 0.4 μm, and is a minor component of sunlight.

  • Energy Absorption and Reflection

    • About 50% of incoming sunlight is absorbed by water bodies, soil, vegetation, buildings, etc.
    • 20% is absorbed by water droplets in air and by molecular gases, such as UV by stratospheric ozone (O3) and diatomic oxygen (O2), and IR by carbon dioxide (CO_2) and water vapor.
    • A small amount of sunlight is absorbed by suspended particulates of black soot.
    • The remaining 30% is reflected back into space by clouds, suspended particles, ice, snow, sand, and other reflecting bodies.
    • The fraction of sunlight reflected back into space by an object is called its albedo, which is about 0.30 for the Earth overall.
    • Clouds are good reflectors, with albedos ranging from 0.4 to 0.8.
    • Snow and ice are also highly reflecting surfaces, whereas bare soil and bodies of water are poor reflectors.
    • Melting sea ice increases the fraction of sunlight absorbed, decreasing the Earth’s overall albedo

Historical Temperature Trends

  • Temperature Reconstruction

    • Average surface temperature trends over the past 2000 years show a consistently downward trend until the Industrial Revolution.
    • The warming of the climate during the twentieth century contrasts with the gradual cooling trend in the previous 900 years, producing a “hockey stick” shape.

  • Twentieth-Century Trends

    • Air temperature did not increase continuously throughout the twentieth century.
    • A significant warming trend occurred in the 1910–1940 period, due mainly to a lack of volcanic activity and a slight increase in the intensity of sunlight.
    • Some cooling followed over the next three decades, due mainly to aerosols resulting from increased volcanic activity.
    • A sustained warming period has been ongoing from about 1970 to the present, attributed almost entirely to anthropogenic influences.
    • The 2000–2010 period was the hottest at least since 1850, with 2005 and 2010 being the warmest years on record.

  • Warming Rate Increase

    • The rate of warming has increased over the past century and a half.
    • The rates of warming for various periods ending in 2005 are:
      • 1856–2005: 0.045°C per decade
      • 1906–2005: 0.074°C per decade
      • 1956–2005: 0.128°C per decade
      • 1981–2005: 0.177°C per decade

Earth’s Energy Emissions and the Greenhouse Effect

  • Energy Emission

    • The Earth emits energy as infrared light.
    • The Earth’s emission peaks at about 13 μm and consists of infrared light having wavelengths starting at about 5 μm.
    • The 5–100-μm range is called the thermal infrared region.
    • Infrared light is emitted both at the Earth’s surface and by its atmosphere.

  • Energy Release Rate

    • The rate of energy release increases in proportion to the fourth power of its Kelvin temperature: rate of energy release = k T^4, where k is a proportionality constant.
    • Doubling the absolute temperature increases the rate at which a body releases energy sixteenfold (2^4).
    • For contemporary surface conditions of planet Earth, a one-degree rise in temperature would increase the rate of energy release by 1.3%.

  • Absorption and Re-emission

    • Some gases in air absorb thermal infrared light at characteristic wavelengths.
    • After absorption, the IR photon may be re-emitted or the energy redistributed as heat among molecules.
    • The direction of the photon is completely random.
    • Some thermal IR is redirected back toward the Earth’s surface and reabsorbed.
    • The atmosphere is fairly opaque to infrared, in contrast to its near transparency to sunlight.

  • Greenhouse Effect

    • The phenomenon of interception of outgoing IR by atmospheric constituents and its dissipation as heat to increase the temperature of the atmosphere is called the greenhouse effect.
    • It is responsible for the average temperature at the Earth’s surface and the air close to it being about 15°C rather than about -18°C.
    • Increasing the concentration of the trace gases in air that absorb thermal infrared light would result in the absorption and conversion to heat of an even greater fraction of the outgoing thermal infrared energy, thereby increasing the average surface temperature well beyond 15°C.
    • This phenomenon is sometimes referred to as the enhanced greenhouse effect (or artificial global warming) to distinguish its effects from the greenhouse effects that have been operating naturally for millennia.
    • The principal constituents of the atmosphere—N2, O2, and Ar—are incapable of absorbing infrared light.
    • The atmospheric gases that in the past have produced most of the greenhouse warming are water vapor (responsible for about two-thirds of the effect) and carbon dioxide (responsible for about one-quarter).
    • The absence of water vapor and of clouds in the dry air of desert areas leads to low nighttime temperatures there since so little of the outgoing IR is redirected back to the surface.

A Very Simple Model of the Greenhouse Effect

  • Energy Balance

    • The temperature of an Earth that had no greenhouse gases in its air but was balanced with respect to incoming and outgoing energy is calculated to be -18°C, or 255 K.
    • The rate of energy emission from such a planet would be k (255)^4.
    • The rate of energy input from the Sun is also k (255)^4.
    • About 60% of the energy emitted as infrared light is eventually transmitted into space.

  • Temperature Calculation

    • Based on this model, the Earth's temperature can be calculated using: 0.6 kT^4 = k (255)^4
    • Solving for T gives: T = \frac{255}{0.6^{0.25}}
    • This results in: T = 290 K, which is -17°C, an increase of 35 degrees by the operation of the natural greenhouse effect.

  • Atmospheric Layers

    • In reality, very little of the IR emitted at or near the Earth’s surface escapes into space. Rather it is absorbed by the air close to the ground, and then re-emitted.
    • The IR from the air close to the ground that is emitted upward is mainly absorbed by the next layer of air, which is heated by it, though to a lesser extent than is the layer underneath, and is partially re-emitted.
    • The temperature at the top of the troposphere, from which the emitted IR reaches outer space, is only -18°C.

Earth’s Energy Balance

  • Energy Fluxes

    • A total of 342 watts per square meter (W m^{-2}) are present in sunlight outside the Earth’s atmosphere.
    • Of this, 235 W m^{-2} are absorbed by the atmosphere and the surface.
    • If 390 W m^{-2} are emitted from the surface, 155 W m^{-2} of IR does not escape into space.

  • Stratospheric Cooling

    • An increase in CO_2 concentration is predicted to cause a cooling of the stratosphere.
    • More outgoing thermal IR is absorbed at low altitudes (the troposphere), so less is left over to be absorbed by and warm the gases in the stratosphere.
    • At stratospheric temperatures CO_2 emits more thermal IR upward to space and downward to the troposphere than it absorbs as photons, so increasing its concentration cools the stratosphere.
    • The observed cooling of the stratosphere has been taken to be a signal that the greenhouse effect is indeed undergoing enhancement.

Molecular Vibrations: Energy Absorption by Greenhouse Gases

  • Vibrational Motion

    • Light is most likely to be absorbed by a molecule when its frequency almost exactly matches the frequency of an internal motion within the molecule.
    • For frequencies in the infrared region, the relevant internal motions are the vibrations of the molecule’s atoms relative to each other.

  • Types of Molecular Vibrations

    • The simplest vibrational motion in a molecule is the oscillatory motion of two bonded atoms X and Y relative to each other, called a bond-stretching vibration.
    • The exact frequency of the oscillatory motion depends primarily upon the type of bond.
    • The stretching frequency of carbon–fluorine bonds occurs within the thermal infrared range (4 to 50 μm), and thus any molecules in the atmosphere with C!F bonds will absorb outgoing thermal IR light and enhance the greenhouse effect.
    • The other relevant type of vibration is an oscillation in the distance between two atoms X and Z bonded to a common atom Y but not bonded to each other. Such motion alters the XYZ bond angle from its average
      value, and is called a bending vibration.
    • The frequencies of many types of bending vibrations in most organic molecules occur within the thermal infrared region.

  • Dipole Moment

    • If infrared light is to be absorbed by a molecule during a vibration, there must be a difference in the position in the molecule between its center of positive charge—that of its nuclei—and the center of negative
      charge—that of its electron “cloud,”—at some point during the motion.
    • More compactly stated, in order to absorb IR light, the molecule must have a dipole moment during some stage of the vibration.
    • The centers of charge coincide in free atoms and (by definition) in homonuclear diatomic molecules like O2 and N2, so these molecules have dipole moments of zero at all times in their stretching vibration.
    • For carbon dioxide, during the vibratory motion in which both CO lengths lengthen and shorten simultaneously, i.e., synchronously, there is at no time any difference in position between the centers of positive and negative charges.

  • However, in the antisymmetric stretch vibration in CO_2, the contraction of one CO bond occurs when the other is expanding, or vice versa, so that during the motion the centers of charge no longer necessarily coincide.

    • Molecules with three or more atoms generally have some vibrations that absorb IR, since even if their average shape is highly symmetric with a zero dipole moment, they undergo some vibrations that reduce this symmetry and produce a nonzero dipole moment.

The Major Greenhouse Gases

  • Carbon Dioxide: Absorption of Infrared Light

    • The absorption of light by a molecule occurs most efficiently when the frequencies of the light and of one of its vibrations match almost exactly.
    • Light of somewhat lower or somewhat higher frequency than that of the vibration is absorbed by a collection of molecules because there is also a change in the energy associated with the rotation (tumbling) of the molecule about its internal axes.
    • For CO_2, the maximum absorption of light in the thermal infrared range occurs at a wavelength of 15.0 μm, which corresponds to a frequency of 2 \times 10^{13} cycles per second (Hertz).
    • Carbon dioxide also strongly absorbs IR light having a wavelength of 4.26 μm, which corresponds to the 7 \times 10^{13} cycles per second (Hertz) frequency of the antisymmetric OCO stretch vibration.

  • Energy Calculation

    • It's is possible to calculate the energy absorbed when CO_2 absorbs infrared light. To calculate the energy, knowledge of the Avogadro's number 6.02 \times 10^{23} is required.
    • The carbon dioxide molecules that are now present in air collectively absorb about half of the outgoing thermal infrared light having wavelengths in the 14–16-μm region, together with a sizable portion of that in
      the 12–14- and 16–18-μm regions.
    • Increasing further the CO_2 concentration in the atmosphere will prevent more of the remaining IR from escaping, especially in the shoulder regions around 15 μm, and hence will further warm the air.

  • Carbon Dioxide: Past Concentration and Emission Trends

    • Measurements of air trapped in ice-core samples from Antarctica indicate that the atmospheric concentration of carbon dioxide in preindustrial times (i.e., before about 1750) was about 280 ppm.
    • The concentration had increased by almost 40%, to 390 ppm, by 2010.
    • Seasonal fluctuations in CO2 concentrations are due to the spurt in the growth of vegetation in the spring and summer, which removes CO2 from air, and the vegetation decay cycle in fall and winter, which
      increases it.
    • CO2 concentration by plant photosynthesis: CO2 + H2O \rightarrow O2 + polymeric CH_2O
    • Much of the considerable increase in anthropogenic contributions to the increase in carbon dioxide concentration in air is due to the combustion of fossil fuels—chiefly coal, oil, and natural gas.
    • A significant amount of carbon dioxide is added to the atmosphere when forests are cleared and the wood burned in order to provide land for agricultural use.
    • Overall, deforestation accounts for about one-quarter of the annual anthropogenic release of CO_2, the other three-quarters originating mainly with the combustion of fossil fuels.

  • Carbon Dioxide: Atmospheric Lifetime

    • The lifetime for a carbon dioxide molecule emitted into the atmosphere is a complex quantity since, in contrast to most gases, it is not decomposed chemically or photochemically.
    • The only permanent sink for CO_2 is dissolution in the deep waters of the ocean and/or precipitation there as insoluble calcium carbonate.
    • Although the oceans will ultimately dissolve much of the increased CO_2 now in the air, the time scale associated with this permanent sink is very long, hundreds of years.
    • In effect, the atmosphere rids itself of almost half of any new carbon dioxide within a decade or two but requires a much longer period of time to dispose of the rest.
    • It is commonly quoted as taking 50–200 years for the carbon dioxide level to adjust to its new equilibrium concentration if a source of it increases.

  • Carbon Dioxide: Inputs and Outputs

    • The upper layers of the ocean absorbed tens of gigatonnes of carbon and released about 10% less than this quantity, giving a net average absorption by this principal sink of 2.3 Gt.
    • Because overall less than half the anthropogenic CO_2 emissions are quickly removed, over the short and medium terms the gas continues to accumulate in the atmosphere, in this period by 4.1 Gt annually.
    • The increase in growth rate of certain types of trees due to the increased concentration of carbon dioxide in the air is called CO_2 fertilization.

  • Water Vapor: Its Infrared Absorption and Role in Feedback

    • Water molecules absorb thermal IR light through their H!O!H bending vibration; the peak in the spectrum for this absorption occurs at about 6.3 μm.
    • Water is the most important greenhouse gas in the Earth’s atmosphere.
    • The rate at which water evaporates and the maximum amount of water vapor that an air mass can hold both increase sharply with increasing temperature.
    • Positive feedback is the operation of a phenomenon that produces a result that itself further amplifies the result.

The Atmospheric Window

  • The Atmospheric Window

    • As a result of absorption of IR light of other wavelengths, mainly by carbon dioxide, methane, and water, it is essentially only infrared light from 8 to 13 μm that escapes the atmosphere efficiently.
    • Since light of these wavelengths passes unimpeded, this portion of the spectrum is called the atmospheric window.

  • Additional Warming

    • The injection into the atmosphere, even in trace amounts, of gases that can absorb thermal IR light will lead to additional global warming.
    • Of special concern are pollutant gases that absorb thermal IR in the atmospheric window region.
    • The additional global warming produced by carbon dioxide is logarithmically related to the increases in its concentration.
    • The warming produced by trace gases is linearly proportional to their concentration increases.

Other Greenhouse Gases

  • Methane: Absorption and Sinks

    • After carbon dioxide and water, methane, CH_4, is the next most important greenhouse gas.
    • H!C!H bond-angle-bending vibrations absorb at 7.7 μm, near the edge of the thermal IR window; consequently atmospheric methane absorbs IR in this region. In contrast to the century-long lifetime of carbon dioxide emissions, molecules of methane in air have an average lifetime of slightly less than a decade.
    • The dominant sink for atmospheric methane, accounting for almost 90% of its loss from air, is its reaction with molecules of hydroxyl, OH, the very reactive free radical gas present in air in tiny concentration: CH4 + OH \rightarrow CH3 + H_2O
    • Stratospheric water vapor acts as a significant greenhouse gas.
    • Per molecule, increasing the amount of methane in air causes a much larger warming effect than does adding more carbon dioxide.
    • To date, methane is estimated to have produced about one-third as much global warming as has carbon dioxide.

  • Methane: Emission Sources

    • About 70% of current methane emissions are anthropogenic in origin and originate from several different source types, including energy production, agriculture, and waste disposal.
    • Most of the natural sources involve plant decay, as do some of the anthropogenic emissions.
    • Most of the methane produced from plant decay results from the process of anaerobic decomposition.
    • Wetlands are the largest natural source of methane emissions.
    • The combined global warming effect of the methane and carbon dioxide produced by a large, shallow reservoir created to generate hydroelectric power can, for many years, exceed the carbon dioxide that would have been emitted if a coal-fired power plant used to generate the same amount of electrical power.
    • The anaerobic decomposition of the organic matter in garbage in landfills (garbage dumps) is another important source of methane emissions to air.
    • Ruminant animals produce huge amounts of methane as a by-product in their stomachs when they digest the cellulose in their food.
    • Methane is released into air when natural gas pipelines leak, when coal is mined and the CH4 trapped within it is released into the air, and when the gases dissolved in crude oil are released.

  • Methane: Concentration Trends and Possible Future Increases

    • Historically (i.e., before 1750), the methane concentration in air was approximately constant at about 0.75 ppm, i.e., 750 ppb.
    • It has more than doubled since preindustrial times, to about 1.8 ppm.
    • Measurements of the methane levels in the air of various cities have indicated that much of the loss from pipelines in the past occurred in eastern Europe.
    • Some scientists have speculated that the rate of release of methane into air could greatly increase in the future as a consequence of temperature rises from the enhanced greenhouse effect.
    • There is much methane currently immobilized in the permafrost of far northern regions; it was produced from the decay of plant materials