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Radiation & Greenhouse Gases

Spring 2012, Lecture 4

Radiation• Radiation describes a process in which energetic

particles or waves travel through a medium or space

• Radiation is often referred to as electromagnetic radiation (EMR)

• It comprises both electric and magnetic field components

• These components oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation

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Classification of radiation

• Electromagnetic radiation is classified into several types according to the frequency, or alternatively its related property wavelength, of the wave

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Frequency

• Frequency is the number of occurrences of a repeating event per unit time.

• The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency

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Frequency Diagram

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• Sinusoidal waves of various frequencies

• Bottom waves have higher frequencies, and shorter periods, than those above

• Horizontal axis represents time

Wavelength

• Wavelength of a sinusoidal wave is the spatial period of the wave, the distance over which the wave's shape repeats

• It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings

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Wavelength Diagram

• Wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests, or troughs, or corresponding zero crossings as shown

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Relationship of Frequency and Wavelength

• λ = c/ν c is the speed of light in a vacuum, a

fundamental constant of nature (cm/s) ν is the frequency, measured in cycles per

second, now called hertz (cycles/sec) λ is the wavelength (cm/cycle)

c = 29,979,245,800 cm/sec (29.979 x 109 cm/sec)

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Radiation Classification Types• Classification types include (in order of increasing

frequency and decreasing wavelength) Radio waves Microwaves Infrared radiation Visible light Ultraviolet radiation X-rays Gamma rays

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Visible Light• Visible light, as perceived by the human

eye, lies between approximately 400 to 700 nanometers

What is a Greenhouse?

• A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls

• Greenhouses heat up because incoming visible solar radiation from the sun is absorbed by plants, soil, and other things inside the building

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Greenhouse Physics - 1

• Glass is transparent to this radiation

• The warmed structures and plants inside the greenhouse re-radiate this energy in the infra-red, to which glass is partly opaque, and that energy is trapped inside the glasshouse

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Greenhouse Physics - 2

• Although there is some heat loss due to conduction, there is a net increase in energy (and therefore temperature) inside the greenhouse

• Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall

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Incoming Radiation

• Some trace gases are known as “greenhouse" gases because they function like the glass in a greenhouse Incoming radiation strikes the earth and some is

absorbed, with the remainder being reflected The absorbed radiation heats the earth, which

results in the earth reradiating energy, in the infrared portion of the spectrum

Temperature Scales

• Scientists (and most of the world) uses the Celsius temperature scale, a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death

• From 1744 until 1954, 0°C was defined as the freezing point of water and 100°C was defined as the boiling point of water, both at a pressure of one standard atmosphere

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Definition of Celsius Scale

• By international agreement, the unit "degree Celsius" and the Celsius scale are currently defined by two different points: absolute zero, and the triple point of VSMOW (specially prepared water)

• This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature (symbol: K)

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Relation to Absolute Scale

• Absolute zero, the hypothetical but unattainable temperature at which matter exhibits zero entropy, is defined as being precisely 0 K and −273.15°C

• The temperature value of the triple point of water is defined as being precisely 273.16 K and 0.01°C The triple point is where water, ice, and water vapor

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Blackbody

• A theoretical object that absorbs all electromagnetic radiation that falls on it

• Since no light is reflected, the object is perfectly black at absolute zero (0 K)

• As a blackbody is heated above 0 K, it begins to emit radiation

• The wavelength of the emitted radiation decreases as temperature increases

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Infrared Blanket

• If the earth's atmosphere were transparent to infrared radiation, the earth would lose heat rapidly and would have a low average temperature

• This temperature would be about 254 K, or about -19°C

• Although life might survive at these temperatures, it would be difficult and life on earth would likely be much different from life as we know it

• Fortunately, some gases in the earth's atmosphere absorb some outgoing infrared radiation

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Greenhouse Gas Properties

• Greenhouse gases absorb infrared radiation that corresponds to the vibrational and rotational energy levels of their bonds

• Normally these gases have three or more atoms (they are “polyatomic”)

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Major Atmospheric Gases

• The major gases in the atmosphere, nitrogen, oxygen, and argon, are either mono- or diatomic

• They do not have any appreciable absorption in the infrared

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Polyatomic Gases

• The most abundant polyatomic gas is water

• Water is the most important greenhouse gas in the sense that it accounts for the major portion of the natural greenhouse effect

• Water is followed by carbon dioxide (CO2) and methane (CH4)

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Absorption of IR

• IR radiation absorbed by polyatomic molecules excites rotational and vibrational states and raises the molecules to a higher energy state

• They return to the ground state by radiating IR radiation in all directions

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An IR Blanket

• Some of this radiation is directed at the ground and will likely be reabsorbed by the ground

• Other rays are directed sideways, or upward

• These rays will likely encounter other greenhouse gas molecules before escaping from the atmosphere

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Absorption

• Each gas adsorbs at a discrete set of wavelengths

• Combinations of gases are more effective at absorbing across the electromagnetic spectrum than any single gas

• Some gases are much more effective than others

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Atmospheric Windows

• Amount of absorption at different wavelengths in the atmosphere, presented in terms of the half-absorption altitude

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Half-Absorption Altitude

• Half-absorption altitude is defined to be the altitude in the atmosphere (measured from the Earth's surface) where 1/2 of the radiation of a given wavelength incident on the upper atmosphere has been absorbed

• Windows correspond to those regions where the half-absorption altitude is very small

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Effect of Concentration

• As the concentration increases, so does absorption

• Eventually, the absorption is nearly 100% (saturation)

• Further increases in concentration have little effect on absorption

• The next several slides illustrates the effect for C2F6

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C2F6 Transmissivity vs. Wavenumber and Concentration

Wavenumber (k) is defined as

where λ is the wavelength

• Animation made from a sequence of still images (double-click to play)

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Wavenumber vs. Wavelength

• Diagram shows the relationship between wavelength in microns and the wavenumber in cm-1

• Note this is an inverse relationship• Visible light is between 0.4 to 0.7 microns• IR is > 0.7 microns

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Blackbody Radiation vs. C2F6

Absorption

• Diagram shows the affect of increasing temperature on the absorbance of IR radiation for one Freon compound

NOTE: TheX-axis is mislabeled; it is actually wavenumbers (cm-1)

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IR Absorption

• The program (text above) demonstrates how four greenhouse gases actually absorb IR radiation

• The incoming radiation, of the correct frequency, excites the moles as shown

• Click the text to run the program - you will probably get a warning about a possible virus, but the program is known to be virus free

IR Gas Spectra

Heat Absorption by CO2

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Human_produced_CO2_is_a_greenhouse_gas__and_thus__causing_global_warming

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Natural Greenhouse Effect

• The atmosphere naturally contains carbon dioxide, methane and nitrous oxide

• These gases--together with water vapor--create the natural greenhouse effect

• They trap some of the sun's energy and keep the Earth warm enough to sustain life

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Cumulative Absorption

• The animation shows the absorbance of carbon dioxide, methane, water, and nitrous oxide

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Enhanced Greenhouse Effect

• An increase in the natural process of the greenhouse effect, brought about by human activities, whereby greenhouse gases such as carbon dioxide, methane, chlorofluorocarbons and nitrous oxide are being released into the atmosphere at a far greater rate than would occur through natural processes

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Carbon dioxide

• Each year we add more than 30 billion tons of carbon dioxide to the air mainly by: Burning fossil fuels Cutting down and burning trees

• Deforestation accounts for about 20 percent of the carbon dioxide increase from human activities

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Deforestation

• Until 50 years ago most of the carbon dioxide from deforestation was released from temperate zones

• Now tropical deforestation is the largest source

• Tropical forests are being burned and cut for farming, mining and raising cattle

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Automobile Exhaust

• Burning One Gallon of Gasoline Generates 22 Pounds of Carbon Dioxide

• When gasoline is burned, the carbon in it combines with oxygen in the air to form carbon dioxide

• Because the oxygen adds weight, the newly formed carbon dioxide weighs more than the original unburned fuel

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Cars and Population Increase

• There are over 1 billion motor vehicles in the world today

• If present trends continue, the number of cars on Earth will reach 2.5 billion cars by 2050

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Coal

• It takes a pound of coal to generate the electricity to light a 100-watt bulb for 10 hours

• For every pound of coal we burn, nearly three pounds of carbon dioxide go into the atmosphere

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Good Pollution?

• Burning coal and other fossil fuels also releases sulfur dioxide, polluting the air and forming a haze that blocks sunlight

• Haze cools the climate, partly masking the greenhouse effect

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Methane

• Generated naturally by bacteria that break down organic matter in wetlands and in the guts of termites and some other animals

• It also escapes from natural gas deposits• Methane gas escapes from garbage landfills and

open dumps• It also leaks out during mining, extraction and

transportation of coal, oil and natural gas

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Adding Methane

• Each year we add 350 to 500 million tons of methane to the air mainly by: Raising livestock Coal mining and drilling for oil and natural gas Rice cultivation Disposing of garbage in landfills Burning forests and fields

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New Source of Methane

• In 2006, research has shown that permafrost melting in the arctic is releasing methane trapped in formerly frozen sediments

• Permafrost melting is the result of global warming

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Potential Source of Methane

• A great deal of natural gas is trapped as a solid clathrate complex

• These methane clathrates are found in bands under the coastal sediments offshore from continents in a number of areas

• They represent a possibly large new source of energy

• Utilization of this resource may resource in large releases of methane

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Animals and Methane

• Bacteria in the gut of cattle break down the food these animals eat, converting some of it to methane gas

• Cattle can belch up to a half-pound of methane a day

• Sheep, goats, buffalo and camels also belch methane

• Rapidly growing world population produces greater demand for meat and dairy products

• Number of cattle has doubled in the past 40 years

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Methane Increase

• The methane we release today could still trap heat more than a decade from now, because it stays in the atmosphere that long

• Each molecule of methane traps heat more than 20 times more effectively than a carbon dioxide molecule

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Global Carbon Flux

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Nitrous Oxide

• Nitrous oxide is released naturally from oceans and by bacteria in soils

• Each year we add 7 to 13 million tons of nitrous oxide to the atmosphere mainly by: Using nitrogen-based fertilizers Disposing of human and animal wastes Automobile exhausts

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Chemical Fertilizers

• U.S. consumption of nitrogen-based fertilizer use increased more than 450% from 1960 to 2008, but only increased 2% from 1998-2008

• Nitrogen fertilizers provide nutrients for crops

• When these fertilizers break down in the soil, nitrous oxide is released into the air

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Sewage

• Human and animal wastes release nitrous oxide

• Sewage treatment plants may be a major source of this gas

• More research is needed

• Animal feed lots also contribute

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Nitrous Oxide Increase• Nitrous oxide in the

atmosphere has risen by more than 16 percent since 1750

• The nitrous oxide we release today could still trap heat more than a century from now because it remains in the atmosphere that long

• Data has not been updated by EPA since 1998A nitrous oxide molecule

traps heat about 21 times more effectively than a carbon dioxide molecule

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Fluorocarbons

• Fluorocarbons come almost entirely from human activities

• They are manufactured by humans for refrigeration and other uses

• Some fluorocarbons, including chlorofluorocarbons (CFCs), are being phased out because they damage the Earth's protective ozone layer

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Fluorocarbons, cont.

• HCFCs (hydrochlorofluorocarbons) destroy ozone, although less than CFCs - they are also being phased out

• Newer fluorocarbons, including hydrofluorocarbons (HFCs), do not harm the ozone layer but still trap heat in the atmosphere, adding to the greenhouse effect

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Effect of Greenhouse Gases

• Chart shows how much warming could be caused by each of the gases that human activities release

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• Concentrations of principal anthropogenic greenhouse gases in the industrial era

• Black curves denote measurements of in situ atmospheric samples collected in recent years

• Points denote concentrations determined from air bubbles trapped in polar ice sheets using ice cores obtained in Antarctica (blue) or Greenland (yellow); red curves denote fits to these points

Rates of Increase

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Relative Effectiveness Of

Greenhouse Gases

• Some gases cause much larger greenhouse effects than others

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A Global Problem

• An increase in global temperature would bring changes to the entire planet, and therefore to every nation

• This makes it an international issue which needs worldwide study and responses

• Individual countries are each responsible for their own greenhouse gas production

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International Agreements

• Over 150 countries signed the United Nations Framework Convention on Climate Change at the first Earth Summit held in Rio de Janeiro in 1992

• This agreement set up a process which enabled governments to meet regularly to discuss action to avert extreme climate change

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Rio de Janeiro

• As a result of subsequent talks, all developed countries were asked to reduce their greenhouse gas emissions to 1990 levels

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Kyoto, Japan

• In December 1997, a conference of governments held in Japan attempted to reach legally binding agreements about what each country should do

• Now known as the Kyoto Protocol, a protocol to the International Framework Convention on Climate Change, with the objective of reducing greenhouse gases in an effort to prevent anthropogenic climate change

Kyoto Protocol Objective

• The protocol seeks to stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system

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Specific Goals• Industrialized countries will reduce their collective

emissions of greenhouse gases by 5.2% compared to the year 1990 (but note that, compared to the emissions levels that would be expected by 2010 without the Protocol, this limitation represents a 29% cut).

• The goal is to lower overall emissions of six greenhouse gases - carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, and perfluorocarbons - averaged over the period of 2008-2012.

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Specific Goals

• National limitations: 8% reductions for the European Union and

some others 7% for the US 6% for Japan 0% for Russia permitted increases of 8% for Australia and

10% for Iceland

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Kyoto Protocol Signatory Nations

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• Green = signed protocol• Gray = undecided• Red = no intention of signing

Current Effectiveness

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