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TitleGreen House Effect Seminar
TagsGlobal Warming Greenhouse Effect Greenhouse Gas Particulates Chlorofluorocarbon
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Document Text Contents
Page 1

Contents

1. Introduction
2. Greenhouse Effect
3. Greenhouse Gases
4. Radiative Forcing of Climate Change
5. Non Greenhouse Gas Radiative Forcing
6. Climate Models
7. Predictions of Future Climate
8. Conclusions
Glossary
Bibliography
Biographical Sketches
To cite this chapter


Summary


Global warming is an important environmental issue which is rapidly becoming a part
of popular culture. This paper provides an account of the science associated with this
important issue. Historical evidence for past climate change is discussed. The difference
between weather and climate is highlighted. The physics of the greenhouse effect and
the concept of greenhouse gases are presented. The concepts of radiative forcing of
climate change and global warming potential as measures of the absolute and relative
strengths of greenhouse gases are discussed. Global warming, the enhancement of the
natural greenhouse effect caused by emissions associated with human activities of
greenhouse gases such as carbon dioxide, methane, nitrous oxide, and halogenated
compounds (e.g. CFCs and SF6), is described. Techniques used to model past, current,
and future climate are discussed. The models are based upon fundamental well
established scientific principals and incorporate the current understanding of the
complex feedback and couplings between the atmosphere, hydrosphere, and biosphere.


Projections of future global climate change from state-of-the-art computer models are
given. With our current level of scientific understanding we expect that over the next
century the world will warm substantially.

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1. Introduction

Weather and climate have a profound impact on living organisms on the planet.
Ecological systems have evolved over geological time scales to suit the prevailing
climate. The past 10 to 20 years have brought disturbing evidence that human activities
may cause significant changes in future global climate. "Global Warming" is now an
issue known to hundreds of millions of people across the world. We provide herein an
overview of the current state of knowledge concerning greenhouse gases and global
warming. For detailed scientific reviews of the subject with numerous references to the
latest technical articles the reader should consult the comprehensive IPCC reports on the
subject listed in the Bibliography. For a thought provoking historical perspective we
recommend the reader consult the articles on the greenhouse effect published by Fourier
in 1827 and Arrhenius in 1896. Our purpose here is to address the following questions:
"Is global climate changing?", "To what degree are human activities responsible for
climate change?", "What is the state of the science?", "What will future global climate
be like?" and "What should we do?".


At this point it is germane to note the difference between weather and climate. Weather
is the state of the atmosphere (temperature, humidity, precipitation, wind, cloud cover,
etc.) in a particular location at a particular time; it fluctuates greatly and is notoriously
difficult to predict. Climate is the time-averaged weather in a given geographical region.
Climate is a temporal and spatial average and is consequentially much more predictable
than weather. Thus, the average temperature during a given month in a particular area
(climate) can be predicted with some confidence, however, the temperature at a given
time and location (weather) is much more difficult to predict. Climate varies from
month to month, season to season, and year to year. Statistically significant changes in
climate occurring over a time scale of decades or longer constitute "climate change".


Climate change is not a new phenomenon in the Earth's history. The geological record
shows that when viewed over a time scale of thousands of years the climate is in a state
of more or less continual change with major ice ages occurring approximately every
100 000 years. Regular occurrence of ice ages has an astronomical origin associated
with subtle changes in the separation and relative orientation of the Earth and the Sun.
Three regular variations occur as the Earth orbits around the Sun. First, the Earth's orbit
is slightly elliptical and the eccentricity (degree to which it differs from circular) varies
with a period of approximately 100 000 years. Second, the Earth's rotational axis is
tilted with respect to its orbital axis, the tilt oscillates between 21.6o and 24.5o with a
period of approximately 41 000 years (currently the tilt is 23.5o). Third, the month when
the Earth is at perihelion (closest approach) with respect to the Sun changes with a
period of approximately 23 000 years. The three regular variations give rise to
latitudinal and seasonal variations of solar radiation intensity (up to 10% variation in
polar regions in summer time) which provides an astronomical forcing of climate
leading to ice ages in which the global average temperature is believed to be
approximately 5-10 oC cooler than at present. Currently, we are in a period between two

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Figure 5. Global mean radiative forcing of climate for the year 2000, relative to 1750.

(Reproduced with permission from IPCC)

It is believed that the levels of ozone in the troposphere have increased by 30-40% since
1750 due to increased emission of organic compounds and NOx. This increased
concentration of tropospheric ozone has contributed a positive radiative forcing of 0.35
W m-2 (see Figure 5). The forcing associated with tropospheric ozone varies
substantially by region and season and will respond quickly to changes in emissions of
ozone forming compounds.


4. Radiative Forcing of Climate Change


Net radiation is defined as the difference between the solar radiation absorbed by the
Earth-atmosphere system and the longwave radiation emitted by the Earth-atmosphere
system to space. Net radiation influences the Earth's climate because it determines the
energy available for heating the atmosphere, ocean and land. Hence net radiation
influences the seasonal variation of rainfall and the strength of the global circulation
patterns. When greenhouse gases increase in the atmosphere on account of human
activities, the radiative balance of the Earth is altered. The greenhouse gases absorb the
longwave radiation emitted by the Earth but are transparent to the radiation coming
from the sun. Hence the increase in greenhouse gases causes an increase in the net
radiation at the top of the atmosphere. The change in net radiation at the tropopause
caused by changes in greenhouse gas or aerosol concentrations is called radiative
forcing. Radiative forcing can be calculated accurately if the temperature profile and the
concentration of the greenhouse gases in the atmosphere is known. Radiative forcing
depends upon how strongly a greenhouse gas absorbs radiation and the location of its
absorption bands. As discussed in section 2 and illustrated in Figure 3, the radiation
emitted by the Earth-atmosphere system is at its maximum around in the wavelength
region 10-12 microns (1 micron = 10-6 meter). If the absorption band of a greenhouse
gas is located in this region then it will tend to have a high radiative forcing. On the
other hand, if the absorption lies in a region in which there is already strong absorption

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by greenhouse gases presently in the atmosphere (for example at around 15 μm where
CO2 absorbs, see Figure 3) then it will have a low radiative forcing. This later effect
explains why increases in atmospheric CO2 levels have a radiative forcing effect which
are small (on a molecule for molecule basis) relative to the effect of other greenhouse
gases. During the calculation of radiative forcing one must account for the overlap
between the absorption bands of different greenhouse gases. Some gases, such as CFC-
12 (CCl2F2), can cause direct as well as indirect radiative forcing. The direct radiative
forcing of CFC-12 is positive since it has an absorption band in the infrared region. The
indirect radiative forcing associated with CFC-12 is negative because emission of CFC-
12 leads to the loss of stratospheric ozone with a resulting cooling effect (see
Atmospheric Chemistry and Air Pollution).


How do we compare the radiative impact of different greenhouse gases released by
human beings? Chlorofluorocarbons absorb much more longwave radiation than carbon
dioxide on a per molecule basis. The change in atmospheric abundance of
chlorofluorocarbons has, however, been much smaller than that of carbon dioxide. To
compare the effect of different gases with different abundance and different capability
to absorb longwave radiation it is most appropriate to compare their impact on net
radiation at the tropopause (boundary between the troposphere and stratosphere, located
at approximately 10-15 km). Since pre-industrial times the amount of carbon dioxide in
the atmosphere has increased from 278 ppm to 370 ppm and resulted in a radiative
forcing change of approximately 1.46 W m-2. The amount of methane in the atmosphere
has increased from 700 ppb (in pre-industrial times) to 1700 ppb at present and has
resulted in a radiative forcing change of 0.47 W m-2. On the other hand, the amount of
CFC-12 has increased from 0 to 0.5 ppb since pre-industrial times and resulted in a
radiative forcing change of 0.14 W m-2. Radiative forcing is a measure of the effect of
different greenhouse gases or aerosols on radiative balance at the tropopause. Note that
the radiative forcing due to increased carbon dioxide is three times as large as that
caused by the increase in methane and eleven times as large as that due to CFC-12.
Hence, it can be inferred that carbon dioxide is more important than methane or CFC-12
as regards its impact on global warming. It is reasonable to conclude that attempts to
significantly reduce global warming should include a reduction of carbon dioxide
emissions from human activities. To evaluate the long-term impact of emission of
greenhouse gases we need information about the residence times of greenhouse gases
emitted on account of human activities. A gas with a long residence time will have a
greater impact on future climate than a gas with a short residence time even if the
radiative forcing of the two gases is the same. The atmospheric lifetime of carbon
dioxide is around 100 years while that of methane is around 12 years. To assist policy
makers to understand the potential impact on climate of different greenhouse gases, the
concept of Global Warming Potential (GWP) has been introduced. This concept
compares the potential impact of different greenhouse gases using carbon dioxide as a
reference greenhouse gas. The use of carbon dioxide as a reference greenhouse gas is
logical since it is the most important greenhouse gas associated with human activities.
Global Warming Potential (GWP) is defined as the ratio of the time integrated radiative
forcing from instantaneous release of one kilogram of a substance relative to that of one
kilogram of carbon dioxide (CO2) and can be defined as:

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Chlorofluorocarbons: Industrially made compounds containing chlorine, fluorine,

and carbon. Commonly known as CFCs, these non-
flammable, non-toxic compounds found widespread use as
refrigerants, foam blowing agents, aerosol propellants, and
solvents prior to the recognition of their damaging effect on
stratospheric ozone.

Climate: Time-averaged weather in a given geographical region.
Climate change: Statistically significant change in climate occurring over a

time scale of decades or longer.
Climate models: Complex computer programs incorporating mathematical

descriptions of climatically relevant processes in the
atmosphere, hydrosphere, and biosphere and their interactions

Global warming: Warming of climate by enhanced greenhouse effect resulting
from an increase in atmospheric concentration of greenhouse
gases from anthropogenic emissions.

Global warming
potential:

Ratio of the time integrated radiative forcing of a given mass
of a well mixed GHG relative to that of the same mass of
another well mixed GHG (typically CO2) over a specified
time horizon. Global warming potential is a useful index to
compare the radiative effects of different gases.

Greenhouse effect: Greenhouse gases absorb infrared radiation passing through
atmosphere thereby impeding the flow of heat from the
Earth's surface into space. In the early 1800s this effect was
compared to that of the glass panes which keep greenhouses
warmer than their surroundings – hence "greenhouse effect".
As a result of the natural greenhouse effect the Earth's surface
is approximately 33 oC warmer than would be the case in the
absence of greenhouse gases.

Greenhouse gas: Gas in the atmosphere that absorbs and emits at wavelengths
within the spectrum of infrared radiation emitted by the
Earth's surface (terrestrial radiation).

Gt: 109 tonnes
Halons: Compounds containing carbon, bromine, and chlorine which

are commonly used as fire fighting agents.
Hydrofluorocarbons: Compounds containing hydrogen, fluorine, and carbon

(HFCs), often used as CFC replacements.
Hydrosphere: The component of the global climate system comprising

liquid water; oceans, lakes, seas, rivers, underground water
systems, etc.

Infrared radiation
(IR):

Portion of the electromagnetic spectrum that extends from the
long wavelength (red) end of the visible spectrum to the
microwave region. Invisible to the eye, it produces a sensation
of warmth on the skin. IR radiation has frequencies below
those of red (infra is Latin for below).

IPCC: Intergovernmental Panel on Climate Change (see http:
/www.ipcc.ch/).

Nitrous oxide (N2O): Also known as dinitrogen monoxide or laughing gas, an
anesthetic. Natural sources include emissions from land and

http://www.ipcc.ch/)

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ocean; anthropogenic sources include biomass burning, fossil
fuel combustion, and industrial processes.

Ozone (O3): Triatomic allotrope of oxygen present in troposphere and
stratosphere.

ppb: Parts per billion.
ppm: Parts per million.
Radiative Forcing: Change in net vertical irradiance (units of W m-2) at the

tropopause caused by internal (e.g. change in GHG or aerosol
concentrations) or external change (e.g. change in solar flux)
in the climate system.

Solar constant: Flux of solar radiation arriving at Earth (approximately 1370
W m-2).

Solar radiation: Electromagnetic radiation emitted from the sun.
Terrestrial radiation: Infrared radiation from the Earth's surface.
Tg: 1012 g (109 kg).
Tonne: 1000 kg.
Troposphere: Lowest region of the atmosphere (up to approximately 10-15

km) characterized by decreasing temperature with increasing
altitude.

Tropopause: Boundary between the troposphere and stratosphere, located
at an altitude of approximately 10-15 km.

Weather: State of the atmosphere (temperature, humidity, precipitation,
wind, cloud cover, etc.) in a particular location at a particular
time.


Bibliography


Arrhenius, S., (1896). On the influence of carbonic acid in the air on the temperature of the ground, Phil.
Mag., S. 41, 237-277. [First assessment of the likely climatic impact of doubling atmospheric CO2 levels
due to fossil fuel combustion]



Fourier, J., (1827). Memoire sur les Temperatures du Globe Terrestre et des Escapes Planetaires, Mem.
Aca. Sci. Inst. Fr., 7, 569-604. [First account of the greenhouse effect]



Houghton, J., (1997). Global Warming: The Complete Briefing, Cambridge University Press (1997).
[Overview of global warming and its impacts]



http://www.epa.gov/globalwarming// [United States Environmental Protection Agency site with
information on climate change and its impacts]



http://www.giss.nasa.gov/ [National Aeronautics and Space Administration site with a wealth of global
warming datasets and images.



http:/www.ipcc.ch/ [Home page for Intergovernmental Panel on Climate Change]


http://www.met-office.gov.uk/research/hadleycenter/ [General information about climate modeling and
results (including animated movies) from climate models at Hadley Center]

http://www.pewclimate.org/ [Non governmental organization site for global warming information]

Intergovernmental Panel on Climate Change, (1996). Climate Change 1995: The Science of Climate
Change, Cambridge University Press (1996). [Comprehensive account of the state of scientific
understanding of climate change as of 1996 with numerous references]



Intergovernmental Panel on Climate Change, (2001). Climate Change 2001: The Scientific Basis,
Cambridge University Press. [Comprehensive account of the current state of scientific understanding of
climate change with numerous references]

http://www.epa.gov/globalwarming/
http://www.giss.nasa.gov/
http://www.ipcc.ch/
http://www.met-office.gov.uk/research/hadleycenter/
http://www.pewclimate.org/

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