Chapter 15 Climate Fluctuations and Global Change

Have you ever been in a building where one room feels much colder, or warmer,

than the others? To explain why this room is colder than the others are we would

investigate the differences in the energy gains and losses of the rooms. The energy

budget of a room depends on many things including how many windows it has, the

direction the windows are facing, the floors and walls, and the heating and ventilation

systems. The number of windows and the direction they face play a role in how much

sunshine enters the room. Heat losses out windows can be large depending on how well

the window frame is insulated and whether the windows are single or multiple paned.

The condition of the walls and floor also play an important aspect of the heat budget of

the room. If the walls are poorly insulated they will be cold and, even if the air

temperature is 70F, the room will feel cold because of the energy imbalances felt by the

human body. The air circulation will also play a role in the climate of the room. A strong

blower, like a fan, would circulate air, and heat, in the room, while poor circulation may

not spread heated air evenly throughout the room. These factors all related to energy

exchanges, determine the climate of a room.

Earth's climate is also determined by the circulation patterns of the air and how

heat is transferred.

Climate is controlled by complex interactions among the individual components

of the Earth/sun system. The basic parameters that change our climate are solar energy

input, the state of the atmosphere, the hydrological cycle and the biosphere, including

human activities. It is the general opinion of the scientific community that a global

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climate change will occur in your lifetime. This chapter uses specific examples to

explore the underlying causes of regional and global climate change.

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Observations of Global Warming

Weather conditions change both rapidly and slowly. In less than an hour, a

thunderstorm can change a bright sunny day into a dark rainy one. Temperature and

precipitation also vary from one year to the next. There is convincing evidence that

climate varies naturally on time scales from interannual variations to changes that occur

over millions of years. Over the last decade, the global average temperature has increased

(Figure 15.1). Is this change a natural fluctuation in our climate, or is it a result of human

activities in the last hundred and fifty years or so? Natural variations in climate make it

difficult to distinguish long-term trends caused by humans. While a few warm winters

and hot summers do not mean global warming, the observed warming trend over the last

two decades is indicative of a global change.

This trend in global warming has resulted in debate about what measures might be

taken to mitigate this warming. Most of these measures involve changing human

activities, such as reducing the burning of fossil fuels and deforestation. Humans

currently burn about 6 billion tons of fossil carbon per year, with the rate of burning

increasing about 2 percent per year. The oceans and forests absorb only about half the

carbon dioxide emitted into the atmosphere. Because of the potential impact on society,

global warming is frequent topic in news reports. Sometimes these reports suggest that

scientists are arguing whether greenhouse gases will change the climate. In most

scientific discussions, the issue is not whether greenhouse gases will induce a climate

change. The issue is what the effects will be and how these can be detected best.

There are several factors affecting our climate. These factors combine to cause

fluctuations on many different time scales (Table 15.1). Factors that cause long-term

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fluctuations in climate include changes in the sun’s output of energy, the Earth's orbit

about the sun, and changes in ocean/atmospheric circulation. Fluctuations on a shorter

time scale can be caused by changes in clouds and water vapor, and increased

concentration of greenhouse gases due to human activities. To address the issue of global

warming, this chapter provides examples of climate fluctuations caused by natural

phenomena as well as those due to human activities. The chapter also provides a

foundation to answer questions that will arise about climate change in the future.

Table 15.1 Cause of Climate fluctuations and approximate time lines.

Cause of Climate Change Number of Years

Human effects on land surface 1-100

Human effects on atmosphere 1-100

Volcanism 1-1000

Solar variability 10-1000

Air-sea interaction 1-100,000

Orbital variations 10,000-100,000

Plate Tectonics 100,000-100,000,000

Earth's orbit

Solar radiation is the source of energy that drives the dynamics of the Earth’s

atmosphere and oceans, therefore its climates. Changes in the output of the sun's

radiation affect Earth's climate (Box 15.1). Any change in a planet's orbit about the sun

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can also lead to a climate change. Variations in Earth's orbital movements around the sun

generate very slow climate cycles. Called the Milankovitch cycle, these variations affect

the amount of solar energy the Earth receives and where on the planet that solar energy is

absorbed. Changes in this energy distribution affect climate.

The Milankovitch Theory proposed by Milutin Milankovitch in the 1930s,

attributes climate change to natural variation in the sun-earth astronomical relationships.

There are three independent cycles, which combine to produce variations in Earth's orbit

around the sun and consequently affect the distribution of solar energy on the planet.

These cycles are referred to as the eccentricity, obliquity, and precession of the Earth's

orbit.

The earth orbital eccentricity describes changes in the shape of the earth's orbit

around the sun. Figure 15.2 shows that Earth's orbit around the sun varies from a near

circular orbit to an ellipse. The more circular the orbit, the more uniform the sun's rays on

Earth, because the variations in distance from the sun are smaller. An increase in the

eccentricity results in an increase in insolation in the summer hemisphere and a decrease

in the winter hemisphere. This tends to amplify the season cycle in the high and

midlatitude regions. To go from a near circular orbit to the most extreme ellipsoid takes

about 100,000 years. Currently, the orbital changes that are occurring are making the

Earth's orbit more elliptical.

Changes in Earth's tilt angle, or obliquity, (Figure 15.3) determines how different

the seasons will be in a given hemisphere. The angle of inclination of Earth (see Chapter

2) is presently 23.5 off the perpendicular. The larger the angle the greater the differences

in the seasonal weather of middle and high latitude locations. Summers will be hotter

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and winters colder when the axis of tilt is large. Over a 41,000 year period this angle

varies between 22 and 24.5. The Earth's current tilt is about mid-way between these

extremes.

The third orbital parameter describes the Earth's wobble as it revolves around the

sun. If you spin a top, you will notice that it wobbles in the sense that the spin axis rotates

with time (Figure 15.4). The same happens with Earth. The wobble is known as the

precession of Earth's axis. Because of this wobbling, the time when the earth is closest to

the sun advances by about 025' a year, or a period of 27,000 years. Today, Earth's axis of

rotation is pointed towards Polaris, also known as the North Star. Because of precession,

13,500 years from know it will be pointing toward Vega. The precession is a very slow

motion that determines the time of the equinoxes (Figure 15.5). The precession and the

time of aphelion and perihelion define the differences between winter and summer

condition, or the seasonality. Warmer summers and colder winters will result in the

Northern Hemisphere when the axis of rotation is pointed towards the sun during

perihelion. In this case, the seasonality of the Southern Hemisphere would be decreased.

These cycles are the main factor behind the onset and retreat of the ice ages.

Major ice sheets occur every 100,000 years. Superimposed on this are smaller ice

advances every 41,000 and 23,000 years. These periods coincide well with the natural

variations of Earth's orbit and orientation to the sun (Figure 15.6). While the

Milankovitch cycles may initiate global climate change, these cycles alone cannot fully

account for the observed variations in temperatures. This 10% variation in the solar

energy gains due to variations in Earth's orbit is not enough to explain the 10C (18F)

observed swings in temperature. The conclusion is that orbital changes are not the only

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forces responsible of the ice ages. An additional factor maybe changes in CO2

concentration.

Analysis of air trapped in bubbles in ice sheets shows that CO2 levels are lower

during colder glacial periods than during interglacial periods. These changes in CO2

levels are likely due to changes in biological activity and would enhance the changes

caused by orbital and positional variations. These additional effects are considered

feedback mechanisms.

Feedback Mechanisms

A positive feedback mechanism is one that

enhances an existing trend of a change in climate. It is

through positive feedback mechanisms that small

changes can lead to large ones. For example, the amount of water vapor in the

atmosphere is a positive feedback mechanism involved in climate change. As the air

temperature warms there is increased evaporation from surface waters, resulting in higher

atmospheric water content. Water vapor is a greenhouse gas. So, the atmosphere warms

even further, causing more water vapor to evaporate into the atmosphere, enhancing the

warming, and so on. While human emissions of CO2 are attributed to the current trends

in global warming, it is the water vapor feedback that causes most of the warming.

Another example of a positive feedback mechanism is the ice-albedo temperature

feedback. Ice sheets have a high albedo and affect climate by reflecting more sunlight

than other types of surfaces, such as bare ground and areas covered by vegetation. This

reduces the amount of sun's energy that can warm the planet’s surface. All things being

equal, the more ice the cooler the Earth. Ice-covered areas could expand when the

Feedbacks occur when one change leads to some other change, which can act to either reinforce (positive feedback) or inhibit (negative feedback), the original change.

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atmosphere along the margins of established ice regions cool. The increased area of ice

reflects even more sunlight and further reduces the amount of solar energy absorbed by

the surface. This reduction in solar energy gains causes a further cooling and results in

the formation of more ice, and so on. This is a positive feedback loop; more ice causes a

cooling which reduces the temperature so that more ice develops, leading to a further

cooling (Figure 15.7). Correspondingly, a retreat of an ice sheet is also a positive

feedback since it would cause a warming, due to the lower albedo, and lead to a warming

and a further retreat of the ice sheet.

A negative feedback mechanism mitigates an existing trend of climate change.

A simple example of a negative feedback mechanism involves the influence of CO2

concentration on plant photosynthetic rates. In the process of photosynthesis plants use

carbon dioxide and water to make sugar. An environment rich in CO2 accelerates the

growth of many plant species. This is a negative feedback: increasing CO2

concentrations allow plants to grow faster and thereby increase the overall photosynthetic

rate which removes increased amounts of CO2 from the atmosphere. So, by a negative

feedback loop more carbon dioxide results in its decrease.

Of course, there are other limiting factors in a plant's ability to respond to an

enriched CO2 environment, such as a lack of water nutrients. Also, insects and pests

might also enjoy the warmer environment associated with high concentrations of CO2

which can reduce the number of plants. Feedback processes can quickly become

complicated even in the simplest climate change scenario!

In many of the discussions that follow, we will focus on primary processes that

can lead to a change in climate. We will limit discussion of secondary processes

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associated with positive or negative feedback. These primary processes include changes

in amounts and concentrations of aerosols, greenhouse gases, ocean circulation patterns,

and surface properties.

Aerosols

As you learned in Chapter 1, aerosols are small solid particles and liquid droplets

(excluding cloud droplets and precipitation.) Aerosols are also called particulates.

Aerosols typically range in size from 0.1 to 100 microns. The period at the end of this

sentence is about 10 microns in diameter. Natural processes, such as volcanic eruptions

or anthropogenic processes (human activities), such as automobile exhausts, form

aerosols. There are primary and secondary sources of aerosols. Primary sources are those

processes that directly emit aerosols into the atmosphere. Examples are dust storms,

automobile emissions, volcanic eruptions, and smoke from agricultural fires. In the case

of secondary sources, aerosols form as a result of chemical reactions in the atmosphere.

An example are biogenic aerosols that form from sulfur-bearing gases, primarily

dimethyl sulfide (DMS), generated by ocean biology. Most aerosols are generated by

natural process (Figure 15.8).

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Aerosols sometimes have notable effects on the condition of the atmosphere.

Large concentrations can be extremely hazardous. In large concentrations, such as in

dust storms, (Figure 15.9) dust presents a health hazard, suffocates livestock, and

severely limits visibility causing hazardous traffic conditions. Heavy air pollution poses a

health hazard. An extreme case of this occurred on December 5-9, 1953 when an

estimated 3500 to 4000 people died in London, England. Over this five day period

stagnant moist air combined with the smoke from

burning of low-quality coal to produce a lethal mixture

of fog and smoke. This combination of smoke and fog

produced a pollution event called smog. Today, smog is a generic term used to indicate

polluted air (Box 15.2).

Increases in aerosol concentration can occur naturally, as in volcanic eruptions, or

due to human activities, as in the manufacture of chemicals and particles from burning

fossil fuels. Regardless of how aerosols enter the atmosphere they can modify the energy

balance of a region and affect climate by changing the radiation budget of the planet. The

general climatic impact of most aerosols is to reduce the solar radiation reaching the

surface by scattering radiation out to space. The type of aerosol determines the degree to

which it impacts climate.

Volcanic Eruptions

Chapter 3 introduced the concept of how volcanic eruptions can affect climate.

Debris from Mt. Tambora (8S latitude, 118E longitude) (Figure 15.10) resulted in the

year without a summer. Large quantities of ash, dust, and sulfur dioxide can be injected

into the stratosphere during a violent eruption. Since the stratosphere is stable due to the

Smog originally a combination of smoke and fog, this term is now used to describe mixtures of pollutants in the atmosphere.

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increase in temperature with altitude, volcanic debris stays in the stratosphere for a

couple of years. The long residence time of volcanic debris modifies the energy balance

of the planet. The amount of cooling due to a volcanic eruption is determined by:

The force of the eruption. For a global impact, the debris from the volcanoes must be

injected into the stratosphere where it can remain for many months. A large, forceful

eruption can put dust into the atmosphere.

The amount of sulfur dioxide (SO2) in the volcanic plume. In the stratosphere, SO2

combines with water vapor to make tiny particles of sulfuric acid. The particles

reflect solar radiation back to space, reducing the amount of solar energy at the

surface.

Latitude and winds in the stratosphere. Stratospheric winds at the time of the eruption

and the latitude of the volcano determine how the volcanic plume will spread. For

global affect on weather, the sulfuric acid particles have to spread over the globe.

Not all volcanic eruptions result in a cooling of the earth. Here we will compare

three recent volcanic eruptions: Mt. St. Helens, El Chichón, and Mt. Pinatubo. The

eruption of Mt. St. Helens in the state of Washington in May of 1980 produced little, if

any, effect on global temperatures. While a violent eruption, most of the force of the

eruption was horizontal, resulting in relatively little debris being injected into the

stratosphere. The emissions from Mt. St. Helens primarily stayed in the troposphere and

the ash and dust settled quickly to the ground.

El Chichón, Mexico erupted in 1982. El Chichón was a much less violent eruption

than Mt. St. Helens; however, debris was injected into the stratosphere. The global spread

of El Chichón’s stratospheric debris was primarily limited to between 5 N and 40 N

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latitude. Estimates of the global cooling due to the eruption of El Chichón are

approximately 0.3 to 0.5 C. El Chichón also had much more sulfur dioxide in its plume

than Mount St. Helens. The cooling effects of the El Chichón eruption may have also

been hidden by the warming caused by El Niño (Chapter 8).

Mt. Pinatubo was this century’s most violent eruption (Figure 15.11), injecting

over 25 million tons of sulfur dioxide into the stratosphere. Once in the stratosphere, tiny

sulfuric acid particles formed and rapidly spread all over the globe. These tiny particles

reflected solar energy back to space and resulted in a cooling of the globally averaged

surface temperature by approximately 0.6C (1F). The NASA Earth Radiation Budget

Experiment program measured the effect of the Mt. Pinatubo’s stratospheric aerosol on

the radiation balance of the planet and for the first time provided firm scientific evidence

that volcanic eruptions cool the Earth.

Figure 15.12 shows the departure of the average global air temperature between

1970 and 2000) from the 1951-1980 average temperature. Mount Pinatubo erupted in

June 1991. This provided a unique opportunity to evaluate long term weather predictions

by putting the aerosols into the models and predicting the global mean temperature. The

weather models predicted that after about one year, the average hemispheric temperatures

should decrease by about .2 to 0.5 C. By July 1992 the global mean temperature had

decreased by approximately 0.5 C. During this time there was also an El Niño event that

lasted from 1990 and 1995, so the cooling due to Mt Pinatubo might even have been

greater, where it not for the warming associated with El Niño.

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Aerosols generated by human activities

The atmosphere is a complex mixture of gases and aerosols. Human activities

modify the air we breathe (Box 15.2) and the climate we live in. The atmospheric

concentration of sulfur dioxide has increased over the last century due to human-related

activities. Sulfur dioxide (SO2) is an industrial by-product that arises from the burning of

sulfur. The major source of SO2 from human activities is the burning of coal that

contains sulfur. SO2 can be a hazard to health. SO2 is important for climate change as it

can produce aerosols that scatter solar radiation back to space, causing a cooling of the

planet.

Acid Deposition

Air pollution from industrial areas can

become acidic and carried downwind for many

miles. When these acids settle on the ground, they

can damage plants and aquatic life. This settling may occur as dry particles (dry

deposition) or as rain, snow, or fog (wet deposition). When in the form of rain, the acid

deposition is referred to as Acid Rain.

The acidity or alkalinity of a solution is measured using the pH scale. pH levels

range from 0 to 14, with 0 being extremely acidic. The scale is logarithmic, so a unit

change in pH represents a tenfold change in acidity. A pH of 7 is neutral. Normal

precipitation has a slightly acidic pH value of approximately 5.5. Acid rain forms due to

the increased levels of sulfur dioxide and oxides of nitrogen that enter the atmosphere due

to burning of high-sulfur content coal. These gases dissolve in the cloud drops, making

Acid Deposition refers to the falling of acids and acid-forming compounds from the atmosphere to Earth's surface.

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the precipitation more acidic by a factor of ten, since one unit of change in pH means a

ten-fold change in acidity.

Industrial sources and petroleum-powered vehicles emit massive quantities of

sulfur dioxide and nitrogen oxides into the atmosphere. The United States alone emits

approximately 40 million tons per year. These chemicals undergo complex changes

when in the atmosphere. Some get dissolved in raindrops, snow, or fog particles and

produce weak solutions of sulfuric and nitric acids. As these acids fall to the ground they

can accumulate in lakes and can affect ecosystems. For example, there has been a decline

in the health of coniferous forests in Appalachian Mountains from North Carolina to New

England.

In North America, acid rain is primarily in the Northeast and Canada, downwind

of sources of sulfur dioxide and nitrogen oxides. Figure (15.13) represents the pH level of

precipitation measured over the United States and Canada. Notice the low values of pH

east of the Mississippi, with the lowest pH values (most acidic) occurring in northeastern

United States and Canada. The situation is improving with the 1990 Clean Air Act

amendments that mandate reductions in sulfur and nitrogen acid-forming compounds

over the next 50 years.

Acid rain is an example of how human activities that emit gases into the

atmosphere can impact the environment. Output of greenhouse gases is another example

of how human activities impact the environment.

Greenhouse Gases and Clouds

Greenhouse gases are important ingredients that determine the radiative energy

balance of the planet and contribute greatly to determining Earth's average temperature.

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The primary greenhouse gases are water vapor, CO2, methane (CH4), and CFCs

(chlorofluorohydrocarbons). Human activities are changing the concentration of these

gases. The concentration of CO2 is increasing at a rate of approximately 0.4 percent per

year. Since the beginning of the Industrial Revolution in the 18th century, concentrations

have increased by 25%. Methane has doubled over the same time period. The current

rate of increase in methane is about 1% per year and CFCs have increased at an even

greater rate.

In the long term, Earth must emit energy to space at the same rate at which it

absorbs solar energy. As discussed in Chapter 2, the atmosphere and surface absorb solar

radiation and the rest is reflected back to space. Earth emits radiation to space in infrared

wavelengths. Greenhouse gases prevent some of this radiation from escaping into space.

As human activities increase the amount of greenhouse gases, the global energy balance

of the Earth system appears to be perturbed. To maintain an energy balance, Earth's

climate must adjust to get rid of the extra energy trapped by anthropogenic greenhouse

gases. Global warming increases the amount of energy Earth radiates to space.

Water vapor and clouds play a vital role in preserving the balance between

incoming and outgoing energy. More water vapor molecules exist in warm air than in

cold air. Water vapor is a strong greenhouse gas. Water vapor is transparent to solar

radiation while being extremely effective at absorbing terrestrial radiation emitted from

the earth's surface. Increased water vapor concentration leads to warmer atmospheres

and a further increase in atmospheric water vapor. Human activities add little water

vapor to the troposphere, at least directly. Water vapor increases are a result of internal

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controls of the climate system. In addition to warming, increased water vapor amounts

may enhance cloud cover that can induce a global cooling.

It is difficult to predict the effect of changes in cloudiness on climate. Different

cloud types affect the climate in different ways. High thin cirrus can lead to a warming,

similar to the greenhouse warming. Thick stratocumulus cause a cooling by reflecting

large amounts of solar energy back to space. Satellite observations indicate that the

average global cloud distribution causes a net cooling of the planet. However, we don't

know which cloud types might predominate in a different climate scenario.

Aerosols can also affect climate by causing changes in cloud radiative properties.

Aerosols serve as cloud condensation nuclei (CCN - Chapter 5). When more CCN are

present, more droplets will form in the cloud, and they will be smaller. This makes the

cloud more reflective, further reducing the solar energy reaching the surface.

Observations of cloud droplets over the Atlantic Ocean downwind of Northeast North

America, tend to be composed of drops that are smaller than similar clouds that are in a

pristine environment. Evidence of this indirect aerosol effect is seen in satellite images of

ship-tracks (Figure 15.15). Effluents from the ship engines rise upward into the cloud

and serve as CCN. This increases the number of cloud droplets in the cloud, making it

appear brighter. Smaller particles in high concentrations also make it less likely the cloud

will yield precipitation. In addition to changing cloud properties, human activities may

directly change cloud amount (Box 15.3)

The effect of aerosols on the radiation budget through their impact on clouds is

called an indirect aerosol effect on climate. The aerosols indirectly affect climate by

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changing cloud properties, such as cloud amount or particle size, that then changes the

energy budget of the region.

Ozone

In addition to strongly absorbing radiation at ultraviolet wavelengths (Chapters 1

and 2), ozone also absorbs and emits electromagnetic radiation at wavelengths in the

vicinity of 9.6 microns. So, Ozone is a greenhouse gas; however, its impact on our health

is far more important than absorption of terrestrial radiation. The impact of ozone on

health is determined by whether changes in ozone are occurring in the troposphere or

stratosphere.

Ozone primarily occurs in the stratosphere though some ozone, approximately

10% of the total amount, exists in the troposphere. The maximum ozone concentration is

between 20 and 25 kilometers (about 12 to 15 miles) above the surface. The layer of

maximum ozone concentration in the stratosphere is referred to as the ozone layer. The

altitude of the ozone layer varies with latitude. Stratospheric ozone is beneficial to life

because it absorbs ultraviolet radiation coming from the sun that is biologically

damaging. Absorption of UV energy heats the atmosphere and is responsible for the

temperature inversion observed in the stratosphere. First we will re-visit stratospheric

ozone hole and then discuss changes in tropospheric ozone concentrations.

Stratospheric Ozone Hole

As you learned in Chapter 1, ozone (O3) is produced by the combination of three

oxygen atoms (O). The concentration of atmospheric ozone is small, approximately 3

molecules of ozone for every ten million air molecules. Through absorption of ultraviolet

radiation (UV), ozone plays a fundamental role in the radiation budget and the dynamics

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of life on earth. Absorption of UV energy causes a heating that produces the increasing

temperature with altitude, a characteristic feature of the stratosphere. Ozone absorption of

UV also keeps this harmful radiation from reaching the surface. Reduction in amounts of

ozone can lead to increase amounts of biologically damaging UV-B radiation (radiation

with wavelengths of .2 to .32 microns) at the surface. Increased amounts of UV-B can

lead to incidents of cataracts and skin cancers known as melanoma. For every 1 percent

increase in UVB there is likely to be a 2 percent increase in the risk for contracting

melanoma.

The total amount of ozone in an atmospheric column at a given location is

measured in Dobson Units (DU), named after Dobson who developed methods of

measuring atmospheric ozone from the ground. Dobson units represent how thick a layer

of ozone from the surface to the top of the atmosphere would be if it all existed at 0C and

the average surface pressure. 300 Dobson Units is 0.3 cm thick or approximately the

thickness of a dime.

Observations of the monthly average total column ozone amounts show a

minimum amount over the Southern Hemisphere in spring. During winter, the amounts of

ozone over the South Pole region remain fairly constant. A decline in ozone is seen in

September and a minimum amount of ozone is observed in October. After October,

ozone levels begin to increase. Why does the minimum occur in October?

The winter atmosphere above Antarctica is very cold. These cold temperatures

result from the high altitude of the Antarctic continent and the resulting energy losses due

to emission to space of longwave radiation. Temperatures in the stratosphere can be less

than -90C (-130F)! In addition, the cold temperatures result in a temperature gradient

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between the Southern Hemisphere polar and the midlatitudes. The temperature gradients

result in pressure gradient that, in combination with the Coriolis force produces a belt of

strong stratospheric winds that encircle the South Pole region. The strong westerly winds,

referred to as the polar vortex, prevent the transport of warm equatorial air to the polar

latitudes. This isolation between the south polar regions and midlatitudes and tropical

regions helps keep the stratospheric air very cold. These cold temperatures cause water

vapor and some nitrogen compounds to condense and form clouds. These polar

stratospheric clouds, or PSCs, are composed of ice and frozen nitrogen particles and form

in air temperatures colder than approximately -80C or -112F. PSCs begin to form during

June, and dissipate in October, the Antarctic spring.

Chapter 1 mentioned that Cl is an important atom that contributes to the

destruction of ozone. In the winter time chemical reactions on the surface of the particles

composing PSC result in chemical reactions that bind the Cl into the PSC. During the

spring, when sunlight again shines on the Antarctic stratosphere, chlorine atoms are

freed and ozone is rapidly depleted. Destruction is so rapid over the South Pole region in

the Southern Hemisphere springtime (e.g., October) that it has been termed a "hole in the

ozone layer." Observations of ozone concentrations over Antarctica during October

reveal a decreasing trend since 1975 (Figure 15.16). Observations of total ozone amounts

in the month of October measured over Halley Bay, Antarctica (76 degrees south

latitude) demonstrate this trend.

There is a strong year-to-year variation in the development and size of the

Antarctic ozone hole. This is demonstrated by plotting the October monthly average

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ozone amounts over the Antarctic during the periods 1980-1991 (Figure 15.17). Notice

the reduced ozone amounts in 1987, 1989, 1990 and 1991.

Why does the ozone hole appear more often over the South Pole than the North

Pole? Stratospheric clouds composed of ice particles exist over Antarctica but not over

the mid-latitudes or tropical regions. This is because the Arctic stratosphere does not get

as cold as the air over Antarctica. As a result, while ozone depletion of 15-20% have been

observed in certain regions of the Arctic stratosphere, the development of an ozone hole

has not been observed.

Tropospheric ozone

Ozone is a chemically active molecule and is considered a corrosive gas. High

levels of ozone can damage plant and animal tissues when they come into contact with

ozone due to chemical reactions. When atmospheric ozone is present near the surface it is

a pollutant. High concentrations of ozone reduce crop production and are detrimental to

human health. Thus, while decreased amounts of stratospheric ozone are dangerous to

human health, decreased amounts of tropospheric ozone near the surface are beneficial!

Ozone is a major component of photochemical smog, a type of air pollution that

forms during sunny days when vehicular traffic is congested. In the presence of sunlight,

oxides of nitrogen from engine exhaust and hydrocarbons react to form a noxious mixture

of aerosols and gases. This mixture includes ozone, formaldehyde, and PAN

(peroxyacetyl nitrates). Exposure to high concentrations of ozone irritates the eyes, nose

and throat, and causes coughing, chest pain, and shortness of breath. Ozone also

aggravates diseases such as asthma and bronchitis.

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Ocean-feedbacks

There are two important reasons why the oceans are critical to understanding

climate fluctuations. First, oceans occupy 70% of the Earth's surface and are the major

source of atmospheric water vapor. Water vapor is intimately related to heat exchanges,

and heat exchanges between the atmosphere and the large ocean surface determines

atmospheric conditions. Second, the oceans have a large heat capacity. That is,

compared to most other substances, it takes a relatively large amount of energy to change

the temperature of water. This prevents extreme variation in global temperatures.

There are some obvious relationships between the atmosphere and ocean.

Chapter 8 discussed these intimate relationships in terms of El Niño, La Niña, and

hurricanes. Another relationship exists between water temperature and sea level. If the

oceans warm, they expand causing a rise in sea level. Increases in atmospheric

temperatures may also melt glaciers back into the oceans, increasing the sea level. If the

West Antarctic ice sheet melted, sea level would rise by 5 m or more. There is evidence

that the global sea level has risen by 20 cm (7.9 inches) during the last 100 years.

Changes in sea level can change climate patterns by changing ocean circulation patterns.

If oceanic temperature changes are occurring due to changes in atmospheric

temperatures, it could take decades to observe these changes. If the oceans warm, it

could modify atmospheric pressure distributions and wind patterns.

In addition to serving as a heat reservoir, oceans moderate atmospheric

temperatures by removing CO2 from the atmosphere. Table 16.2 lists the amount of CO2,

N2 and O2 in the atmosphere and oceans. The amount of CO2 in the oceans is 500 times

more than that in the atmosphere! We do not know how much more CO2 the oceans can

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absorb. If they can absorb more atmospheric CO2, they will modify the global warming.

However, water can hold less gas at higher temperatures. If the oceans warm up, some of

the currently dissolved CO2 would diffuse out to the atmosphere, increasing atmospheric

concentrations. Predictions about climate change need to account for changes in

temperatures of the Earth’s oceans.

Ocean currents transfer heat from one place to another. Global-scale ocean

circulation patterns transport heat poleward. As discussed in Chapter 8, surface currents

in the Atlantic transport heat poleward. These currents cool as they move northward and

sink in the North Atlantic. They then flow southward at great depths to Antarctica Figure

15.18 demonstrates how North Atlantic Deep Waters are circulated through the ocean.

This "conveyer belt" transports about 20 times more water than all of the world's rivers

do in about 1000 years. The surface water in the North Atlantic near Greenland and

Iceland is cooled through interactions with the atmosphere. The water sinks and flows

south around Africa and northward in the Indian and Pacific Oceans. In the Indian and

North Pacific Oceans the water mixes to the surface and enters the surface layer current,

eventually bringing warm surface water to the Atlantic Ocean. .

A global warming might affect the energy transport of the ocean by changing

water temperatures in North Atlantic. There is convincing evidence that these flow

patterns can change abruptly. Changes in ocean circulation are found in a marine record

that appears at nearly the same time as swings in the Greenland air temperature.

Evidence of past climates indicates that these changes could trigger a change in

atmospheric temperature of more than 5C (9F) over a period of less than half a century!

For example, the Younger Dryas cold period (Chapter 14) occurred at a time when the

22

North Atlantic Deep Water circulation slowed. There is concern that a modest global

warming can cause a rapid change in the ocean circulation pattern over the next century.

Table 15.2 Gases in the ocean and atmosphere.Percentage by volume in

AtmospherePercentage by volume in

OceanNitrogen 78.03 47.50

Oxygen 20.99 36.00

Carbon Dioxide 0.03 15.10

Changes land surfaces

As discussed in Chapters 2 and 3, the Earth's surface has a direct effect on

regional weather and climate. Large-scale changes in land use, due to urbanization,

deforestation, and farming, affect the surface of Earth, which can cause climate changes.

These, in turn, affect the biosphere. The biosphere is comprised of all of Earth's living

organisms. The biosphere plays an important role in the climate. It helps to regulate the

carbon cycle, the hydrological cycle and the heat budget of the planet. Climate changes

are linked to changing land-vegetation patterns.

The influence of climate on vegetation is straightforward. Indeed, a climate

classification system can be based on the types of plants of a region. Plants also influence

the climate they live in. Examples of how changes in the surface, caused by a

combination of human activity and natural variations in weather, impact on the condition

of the ecosystem include the Dust Bowl, desertification, and urbanization. First we

consider the Dust Bowl.

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Dust Bowl

The climate of a region includes year to year variations in the weather. This is

why we cannot attribute a single warm winter to global warming. We have to look at

trends over many years. Sometimes abnormal weather persists for a few years or even a

decade, after which the variations return to near normal. We can think of these shifts in

weather as a climate fluctuation. The Dust Bowl of the United States is an example of a

climate fluctuation.

The Dust Bowl of the 1930's removed unprotected topsoil from productive

farmland. It resulted from a combination of years of drought combined with a misuse of

land as grasslands that had been plowed for wheat in the 1920s were abandoned or

returned to grazing. At its greatest extent the Dust Bowl region covered 25,000 sq. mi.

(64,750 sq. km) of the southwestern Great Plains (Figure 15.19). Millions of hectares of

farmland went to waste, and hundreds of thousands of people were forced to leave their

homes. It was an ecological disaster, but was not a long-term change in the climate.

Plotting the June, July, and August average temperature and rainfall of Topeka,

Kansas can illustrate the climate fluctuations associated with the Dust Bowl for the 100-

year period between 1890 and 1990 (Figure 15.20). Notice the large variations in the

observed year-to-year temperature and rainfall. In the mid 1930s there are a few years

where there is a warm departure in summer temperature as well as a dry spell. There is no

simple explanation of the cause of these hot and dry summers. While these are probably a

natural fluctuation in the climate system, inappropriate land use could have enhanced the

departures.

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Prior to World War I farmers plowed up the natural grasses to make way for

farmland. This set the stage for the Dust Bowl years. A severe drought occurred in the

1930s. While the natural grasses had adapted to extended dry periods, the lack of

precipitation denuded the surface of the agricultural plants. As the land dried up, the

winds blew the topsoil away because of the lack of vegetation. The winds could

sometimes generate dust storms that would darken the skies.

The Dust Bowl lasted about a decade. Since the Dust Bowl, new cultivation

methods designed for dryland ecosystems were developed which has reduced the impact

of subsequent droughts. During the mid-1930's shelter belts where planted to stabilize the

soil. A shelterbelt consists of plants, often a mixture of conifer and deciduous tress, in

rows perpendicular to the prevailing wind direction. As winds blow through the trees,

friction increases and the wind is reduced downwind for a distance of about 25 times the

height of the trees composing the belt (Figure 15.21). When properly planted, a

shelterbelt keeps the wind from lifting the soil and transporting it downwind. Irrigation,

replacing grasslands, contour farming, and other conservation measures are now widely

used to mitigate the impact of droughts in this region.

Desertification

Desertification is the process of reducing

the productive potential of arid or semiarid land

due to a combination of climate change and human

mismanagement. Practices that leave the land susceptible to desertification include: over

grazing, deforestation without reforestation, and farming on land with unsuitable terrain

or soil. The consequences of desertification include a magnification of the drought, a

Desertification refers to the spreading of a desert region due to a combination of climate change and human impacts on the land.

25

decline in the standard of living, and even famine. The semi-arid fringes of the Sahara

Desert are vulnerable to desertification.

The Sahara Desert is the largest land desert in the world. Just south of this desert,

between 14N and 18 N is the Sahel. Below the Sahel are grasslands and then tropical

forests. The Sahel, or Sub-Sahara, is a semi-arid region (climate type Bsh) with

pronounced wet and dry seasons. The amount of rainfall is variable from year to year and

from region to region. In winter, the Sahel is hot and dry. The Intertropical Convergence

Zone (ITCZ) brings precipitation as summer approaches and the band of convective

precipitation moves northward. The year-to-year variation in precipitation depends on

the movement of the ITCZ and varies considerably.

In the early 1960's, rainfall was plenty and the nomadic people who live in this

region of the world found ample grazing land for cattle and goats. Herds grew in number

and so did the human population. In 1968 the ITCZ did not bring the needed rains as far

north, marking the beginning of a severe drought that lasted into the 1980s The drought,

in concert with overgrazing, turned large regions of pasture into a wasteland. The

drought peaked in 1973 when rainfall totals where more than half the long-term average,

and extended into the 1980s. The Sahara desert moved southward into the Sahel and a

famine ensued that took the lives of more than 100,000 people and affected more than 2

million.

Rainfall has returned to the region but not to the levels of the 1950s and 1960s. It

is possible that this change in regional precipitation is a natural variation, Indeed, the

southern boundary of the Sahara shifts position north and south by 100 km (60 miles).

However, the drought may also be enhanced due to a bio-geophysical positive feedback

26

mechanism The Sahel lies below the descending branch of the Hadley during its dry

season. Sinking air warms and is thus an energy gain for the atmosphere. With a

reduction in rainfall there is less vegetation. The reduced vegetation results in an

increase in surface albedo, reducing the energy gains of the surface The energy gains of

the atmosphere are also reduced as there is less transfer of heat from the surface to the

atmosphere. To make up for this reduced energy gains from the surface, the atmosphere

subsides. The sinking motion of the atmosphere compresses the air and warms and dries

the air. This warming and drying associated with the sinking motion of the atmosphere

further enhances desert conditions. Thus, the reduced precipitation of a semi-desert

region is a positive feedback mechanism. The drought reduces vegetation amount

resulting in small energy gains resulting in subsidence that enhances the drought.

Urban Heat Island

It is a well-known fact, and has been for some time, that cities are generally

warmer than the surrounding, rural areas. Cities are called urban heat islands. The reason

the city is warmer than the country comes down to a difference between the energy gains

and losses of each region. Urban heat islands are a good model with which to explore

how changing the energy balance of a region can affect its temperature, its macroclimate.

There are a number of factors that

contribute to the relative warmth of cities, heat

from engines, thermal properties of buildings, and

evaporation of water. The heat produced by heating and cooling city buildings and

running planes, trains, buses and automobiles contribute to the warmer city temperatures.

Urban heat island refers to the increased temperatures of urban areas compared to a city’s rural surroundings.

27

Heat generated by these objects eventually makes its way into the atmosphere, adding as

much as one-third of the heat received from solar energy.

The thermal properties of buildings and roads are also important in defining the

urban heat island. Asphalt, brick, and concrete retain heat better than natural surfaces.

Buildings, roads, and other structures add heat to the air throughout the night and thus

reduce the nighttime cooling of the air, so that the maximum temperature difference

between the city and surroundings occurs during the night. The canyon shape of the tall

buildings and the narrow space between them magnifies the longwave energy gains.

During the day solar energy is trapped by multiple reflections off the many closely

spaced, tall buildings reducing heat losses by longwave radiation (Figure 15.22).

Pollution in the city's air also modifies the absorption of longwave and shortwave

radiation of the atmosphere.

Evaporation of water may also play a role in defining the magnitude of the urban

heat island. During the day in rural areas, the solar energy absorbed near the ground

evaporates water from the vegetation and soil. Thus, while there is a net solar energy

gain, heating is lessened to some degree by evaporative cooling during

evapotranspiration. In cities, where there is less vegetation, the buildings, streets, and

sidewalks absorb the majority of solar energy input.

The urban heat island is clearly evident in statistical tables of surface air

temperatures (Figure 15.23). The warmer temperatures of urban areas are also apparent in

cloud-free satellite images. Figure 15.24 is a satellite infrared image of radiative energy

exiting the atmosphere. The image is similar to what you see on the television weather

but with finer details. At this wavelength, the satellite instrument is measuring the

28

amount of radiant energy emitted by the surface and the tops of clouds, which is

proportional to the temperature of the emitting body. The warmer the body, the greater

the amount of radiant energy emitted. White portions of the image represent cold objects

(e.g., cloud tops) and dark regions are warm areas. A map is shown to help orient you to

the geography of the region. Notice that on this day in April, the land is warmer (it

appears darker in the image) than the Great Lakes and the Atlantic Ocean. Urban heat

islands appear on the image as “dark blemishes.”

Climate Modeling

Climate results from a balance between interacting physical processes that occur

in the oceans, atmosphere, land, or biosphere. Figure 15.25 is a simple illustration of the

complexity of our climate system. . A change in any of these processes illustrated in this

figure could have an impact on the climate system. To assess how human activities might

impact future climate we must understand how these physical processes interact to

produce today's climate. Separating the human impact from any natural fluctuation is a

major scientific challenge. To accomplish this requires careful monitoring of trends, such

as those shown in Figure 15.1. Observations are important, but they will never be

perfect. We are not able to measure everything, everywhere, all the time. To synthesize

the measurements requires us to combine our observations with appropriate methods to

forecast climate change.

An important tool for forecasting climate

changes is the Global Climate Model, or GCM.

GCMs solve mathematical equations that express

physical laws, such as the conservation of energy (Chapter 2) and Newton's Laws of

Global Climate Model (GCM) refers to a computer program that calculates global climate using mathematical equations derived from physical principles.

29

Motion (Chapter 6), and all the relationships discussed throughout this book. All these

equations cannot be solved exactly, so computers use approximate solutions using finite

differences. The roots of today's GCMs go back to the methods first developed by L. F.

Richardson as discussed in Chapter 13.

Different models make different approximations to finding these solutions. So,

models tend to differ in their predictions. Accurate GCMs must include the behavior of

the oceans and the biosphere as well as atmospheric processes. The interactions between

the atmosphere, biosphere, oceans and land, as discussed throughout this book, are what

make climate prediction so difficult. However, the ability of GCMs to account for

current climate conditions as well as past climates lends confidence to predictions by

GCMs. As with today's weather models, climate models continue to improve and are

essential in understanding changes in our atmosphere.

Models are also valuable in identifying the fingerprints of global climate change.

The impact of volcanic eruptions on changes in surface temperature was confirmed with

observations and model predications of surface temperature after the eruption of Mt.

Pinatubo. Until recently there have been limited observations of climate changes in the

polar regions. Climate models have indicated that polar regions are very sensitive to a

global climate warming. As a result, new efforts have been implemented to study the

climate of the poles and document current changes. Models are also very valuable in

understanding feedbacks and helping us to separate natural variations from human

activities.

A model can be used to determine how sensitive climate is to a given process.

For example, if we want to understand how a change in cloud cover impacts climate, we

30

can "force" a change in cloud amount in a climate model and analyze the model's

response to this simulation. Or, we can fix the amount of cloud and change the vertical

distribution of clouds. The climate model is the atmospheric scientists' laboratory to run

controlled experiments! Such an experiment is shown in Figure 15.26?. The observed

trend in global average surface temperature between 1860 and 1990 is shown. The dotted

line shows the prediction of the global change using a GCM that allows for increases in

greenhouse gases only. In the second experiment, the GCM allows for increases

greenhouse gases and increases in sulfate aerosols. A comparison of these two runs with

observations demonstrates the importance of sulfate aerosols in offsetting the greenhouse

warming.

Climate models have been used to assess how increased amounts of CO2 impact

on climate by allowing us to increase the amount of CO2 in the model atmosphere and

have the GCM predict the atmosphere's response. A typical experiment used by climate

modelers is to increase the amount of greenhouse gases in the atmosphere to those levels

expected by the year 2050 and see how the model responds to these changes. Models

predict a warming, though the degree of warming varies with the model used (Figure

15.26). This range can be thought as representing an uncertainty in our understanding of

the atmosphere's response to changes. GCMs predict that the global mean temperature by

the year 2100will be warmer than today by 1C (2F) to 4.5C (8F). It was through this

type of modeling study that we learned the importance of water vapor as a greenhouse

gas. GCMs are also critical in deciphering the impact of cloud-feedbacks on climate.

GCMs consistently predict that the warming would be greater at the poles than the

tropics, and that continents will warm more than oceans. The effects of this warming for

31

humanity depend on the speed of the warming. Changes in temperature and precipitation

may cause agricultural zones to move northward. Adapting to a rapid shift in agricultural

zones could take decades, while an adaptation by natural ecosystems may take centuries.

GCMs are certainly in need of improvement. For example, most GCMs are only

now learning how to properly include the role of oceans. Models also do not accurately

portray local conditions. It is likely that future climate change and fluctuations will result

from a complex interaction of differing factors--GCMs will help us unravel the

relationship between cause and effect. Finally, GCM simulations of climate change due

to anthropogenic activities indicate what to look for in observations of the atmosphere.

While variations exist, many GCMs predict consistent qualitative changes (Figure 15.27)

in the global distributions of temperature and the hydrological cycle. For example, while

the troposphere is expected to warm, the stratosphere is predicted to cool. Temperature

changes in the troposphere are expected to be greatest in the Northern Hemisphere

winter. In addition, the land nighttime air temperatures are expected to rise faster than

the daytime temperatures. Recent analysis of temperature observations supports these

predictions.

32

Summary

The forces and dynamics that produce Earth's climates are complex. Climates are

related to latitudinal differences in energy gains and energy losses. If the energy gains or

losses of a region are slightly modified, its climate can undergo a series of changes.

Recent observations indicate that the Earth is warming. Human activities may

play a role in this warming as it is accompanied by rapidly increasing concentration of

atmospheric greenhouse gases. Greenhouse gases play a crucial role in determining the

Earth's climate by affecting the energy budget of the planet. Burning of fossil fuels is

increasing the amount of carbon dioxide in the atmosphere. Coal mining, intensive

agriculture and leaky natural-gas lines yield higher methane concentrations. Methane is a

greenhouse gas. CFCs, used in refrigerants and propellants in spray cans as well as many

industrial processes, are greenhouse gases that also severely impact the protective ozone

layer in the atmosphere..

Increases in any greenhouse gas can lead to a global warming. This warming, in

the absences of other changes, increases evaporation that in turn increases the amount of

water in the atmosphere. Since water vapor is also a greenhouse gas, the warming is

enhanced.

Clouds can lead to a cooling or a warming, depending on the type of cloud. This

is because clouds have opposite effects on the solar and terrestrial radiation budgets.

Clouds tend to reduce the amount of solar radiation absorbed by increasing the albedo of

the planet. In the longwave radiation, clouds increase the energy gains of the planet by

reducing the amount of energy emitted to space. It is difficult to determine what will be

the final outcome of cloud feedbacks on climate changes, since it depends on changes on

33

the amount of cloud, the type of cloud, as well as the size of the cloud droplets that the

cloud is composed of.

Natural temperature fluctuations occur on several time-scales and space-scales.

Changes in the Earth's orbit around the sun are responsible for some climate fluctuations.

The Milankovitch cycles combine to produce variation in the solar radiation received by

Earth that correspond to the major ice ages discussed in Chapter 14. Volcanic eruptions

can also cause climate changes. The recent eruption of Mt. Pinatubo resulted in a global

cooling of 0.5C the year after its eruption. Natural variations complicate our ability to

separate natural climate fluctuations from those caused by human activities.

Unambiguous detection of climate change is therefore a slow process that must be done

carefully and involve detailed comparisons of observations with climate model

predictions.

Climate variations are governed by changes in the atmosphere, but also by

changes in the ocean, cryosphere, and the biosphere. The interaction between climate

and a change in the land are complex. However, these interactions are sometimes

measurable. For example, the expansion of a city modifies the energy and water budgets

of the previously rural region. These changes have resulted in cities being warmer than

the surrounding regions. As another example, severe overgrazing by cattle and sheep

denude the land of vegetation. The loss of vegetation can greatly affect

evapotranspiration and the heat budget of the surface, giving way to a permanent desert.

In addition, removal of vegetation exposes the topsoil to erosion, as in the Dust Bowl of

the U.S. Great Plains in the 1930s.

34

Predicting future climate trends over the next hundred years is a difficult task.

Global Climate Models help us to better understand and predict climate fluctuations and

changes by incorporating mathematical models that represent the physics, chemistry, and

biology of the Earth. There are different models with varying complexity. Climate

simulations using these models indicate that the Earth is warming, and will continue to

warm over the next 50 years. Based on the results of different GCMs, the amount of this

warming is uncertain. Predictions continue to be refined as we improve our

understanding of the atmosphere and its relationship to the oceans and the biosphere.

35

Terminology

You should understand all of the following terms. Use the glossary and this Chapter to

improve your understanding of these terms.

Acid rain

Carbon Monoxide

Desertification

Dust Bowl

Eccentricity

Global Circulation Model

Milankovitch cycles

Negative Feedback

Nitrogen Dioxide

Obliquity

Positive Feedback

Precession

Shelterbelt

Sulfur Dioxide

Urban heat island

36

Review Questions

1. Why are the average temperatures of cities often greater than the surrounding rural

region?

2. Are there other differences in weather between the city and its surroundings?

3. How do the oceans influence climate?

4. Do you think a Dust Bowl can occur again?

5. What is a Global Climate Model?

6. What is an Urban Heat Island?

7. If you live in a large city, compare the observations of temperature in the city with

those of the surrounding regions for a month. Is there any difference?

8. What is the most important gas that contributes to the greenhouse warming?

9. What is the difference between a positive feedback and a negative feedback?

10. What is desertification?

11. Explain why increasing the amount of stratus clouds over the oceans would result in a

net radiative energy loss.

12. Explain why increasing the amount of cirrus clouds over the hot deserts would result

in a net radiative energy gain.

13. What are the Milankovitch cycles and how do they cause variations in climate.

14. Provide two examples of a positive and negative feedback.

15. Why do thin cirrus tend to cause a warming while stratus clouds cause a cooling?

37

16. Describe how a slight increase in the horizontal extent of glaciers could cause the

glaciers to grow?

17. Why are climate models useful for understanding climate change?

18. Why do some volcanoes cause a cooling of the earth while others have no impact?

19. Why would increasing levels of ozone in the troposphere be of concern to health

officials?

20. What are advantages and disadvantages of using global climate models to predict

future climate changes?

Web Activities

Simple climate models

The Earth radiation budget experiment

Forming Contrails

Practice multiple choice exam

Practice true/false exam

38

Box 15.1 Variations in Solar Output

Recent measurements of the energy output of the sun indicate that the output

varies slightly. The sun's energy output is correlated to sunspot activity. Sunspots are

magnetic storms on the sun that appear as dark spots on the sun's surface (include figure

of sunspot). Since sunspots are magnetic storms, the sun's magnetic field also varies with

sunspot activity. Like magnets, sunspots have a polarity. The polarity of the sunspots

reverses every 11 years, so the sun's magnetic cycle is 22 years.

The number and size of the spots reaches a maximum every 11 years. Sunspot

activity peaked in 1980 and 1991 and is expected to peak again in 2002. Recently, a

minimum number of sunspots occurred in 1975, 1986 and 1997. Large numbers of

sunspots reduces the solar output by only 0.1%. This change in total solar energy output

is not very large, particularly in terms of modifying the weather . There is however,

anecdotal and empirical evidence that links the sunspot cycle with Earth's climate. For

example, a period of reduced sunspot activity was observed during 1645 to 1710. During

this time there were few, if any sunspots. This is period is called the Maunder Minimum

and it occurred at the same time as the Little Ice Age. There are also fluctuations in

weather patterns of the Northern Hemisphere that coincide to an approximate 22-year

cycle. For example, there is a periodic 20-year drought in the Great Plains of the United

States. So, any correlation between sunspot number and climate fluctuations is likely to

include some feedback process. As of yet, scientific explanations of the 22-year

correlation are have not been fully proven.

The peak in sunspot activity affects the solar wind that bombards our upper

atmosphere with high-energy particles. At a peak of sunspot cycle the upper atmosphere

39

can reach temperature of1225C (2240F), whereas during a sunspot minimum the

temperature is only 225C (440F). It is not fully understood how these upper

atmospheric changes can affect the troposphere.

40

Box 15.2 Atmospheric Pollutants

Air pollution is defined as airborne solids, liquids, or gases that, when in high

concentrations, threaten the lives of people and animals, harm plants, or threaten

ecosystems.. Pollutants can arise from human activities (anthropogenic sources) or from

natural sources (such as, dust storms and volcanic eruptions).

Human activities can both enhance and degrade our quality of life. Air pollution

consists of the undesirable gases and particulates emitted into the atmosphere by humans.

Humans create two basic types of air pollution, primary pollutants and secondary

pollutants. Primary air pollutants are those directly emitted from a source. Examples are

carbon monoxide from automobile engines or smoke from burning rainforests.

Secondary pollutants form as a result of chemicals produced by humans that subsequently

react in the atmosphere with harmful results. Examples are tropospheric ozone and acid

rain.

Carbon monoxide (CO), a colorless and odorless gas, is an example of a primary

pollutant. CO is very toxic as it disrupts how red blood cells absorb oxygen. As a result,

inhalation of CO reduces the body's ability to provide oxygen to the body. CO gas is also

a byproduct of incomplete combustion. Automobiles are a source of CO. In cities, the CO

levels can approach unsafe levels in confined areas, such as garages and tunnels. CO is

also very dangerous in the home where it can be produced by heating devices that are not

operating properly.

Secondary pollutants, can develop from harmless chemicals that are emitted

directly into the atmosphere, but become a noxious gas or particulate after chemically

combing with other atmospheric constituents.

41

Human activities produce oxides of sulfur, in particular sulfur dioxide (SO2) and

sulfur trioxide (SO3). These sulfur oxide compounds are released into the atmosphere

primarily through the burning of fossil fuels that contain sulfur. Levels can become large

when the activities concentrate the compounds over a small region that allows the

pollutant to reach high levels, such as urban and industrial areas.

SO2 is a highly corrosive gas that irritates the human respiratory system. SO3 is

an important secondary pollutant as it readily combines with water vapor to form droplets

of sulfuric acid (H2SO4). This combination can result in and acid fog, or acid

precipitation (i.e. acid rain or acid snow). Acid fog is particularly dangerous to people, as

it can be easily inhaled. Acid precipitation affects the environment in falls on, depositing

the sulfuric acid into the lakes, streams, ground water, and the soil. Over time, the

surface water system becomes acidic and can become threatening to vegetation and

animal life. Acidification can be high enough to render lakes barren of fish. This is a

particular problem in the eastern United States, and eastern Canada. The problem is even

more severe in Norway and Sweden where an estimated 6500 lakes are essentially

lifeless. Much of the sulfur that comprises the acidic rain is produced upwind of the

affected region.

Oxides of nitrogen, in particular nitric oxide (NO) and nitrogen dioxide (NO2) are

two important air pollutants. NO is a byproduct of high temperature combustion, such as

in automobile engines and electric power generation. Indeed, atmospheric concentrations

of NO2 in urban areas are well correlated with the density of vehicular traffic. NO is a

very reactive gas and quickly forms NO2. NO2 is also a toxic gas that is also emitted by

42

automobile engines. High concentrations of NO2 give polluted air its reddish-brown

color. In high concentrations, oxides of nitrogen cause serious pulmonary problems.

Hyrdrocarbons are compounds made of hydrogen and carbon atoms. Example

hydrocarbons are methane, butane and propane, which can occur as either a gas or

particulate. Hydrocarbons are also called volatile organic compounds (VOC).

Hyrdrocarbons do not appear hazardous in themselves, but , during the day, they can

combine with nitrogen oxides and oxygen to produce photochemical smog.

Air pollution is not a modern age problem. Cave dwellers undoubtedly had to

deal with local air quality problems such as smoke from fires. The Hopi Indians had to

deal with SO2 emitted from burning coal to make pottery. England has long had a

problem with coal burning. Infant mortality in the late 18th century was alarming due to

the burning of high sulfur-content coal. Air quality was so poor, that it has been estimated

that half the children died before their 2nd birthday

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Box 15.3 Contrails

The white condensation trails left behind jet aircraft are called contrails

(CONdensation TRAILS). Contrails form when hot humid air from jet exhaust mixes

with environmental air of low vapor pressure and low temperature. The mixing is a result

of turbulence generated by the engine exhaust. Cloud formation by a mixing process is

similar to the cloud you see when you exhale in cold air and "see your breath". The figure

below represents how saturation vapor pressure varies as a function of temperature. The

blue line is the saturation vapor pressure for ice as a function of temperature (in degrees

Kelvin). Air parcels in the region labeled saturated will form a cloud. Imagine two

parcels of air, A and B as located on the diagram. Both parcels are unsaturated. If B

represents the engine exhaust, then as it mixes with the environment (parcel A) its

temperature and corresponding vapor pressure will follow the dotted line. Where this

dotted line intersects the blue line is were the parcel becomes saturated and a cloud

forms.

If you are attentive to contrail formation and duration, you will notice that they

can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long

a contrail remains intact, depends on the humidity structure and winds of the upper

troposphere. If the atmosphere is near saturation, the contrail may exist for several hours.

On the other hand, if the atmosphere is dry then as the contrail mixes with the

environment it dissipates. Contrails are a concern in climate studies , because increased

jet aircraft traffic may result in an increase in cloud cover. It has been estimated that in

certain heavy air-traffic corridors, cloud cover has increased by as much as 20%. An

increase in cloud amount changes the region's radiation balance. For example, solar

44

energy reaching the surface may be reduced, resulting in surface cooling. They also

reduce the terrestrial energy losses of the planet, resulting in a warming. Jet exhaust also

plays a role in modifying the chemistry of the upper troposphere and lower stratosphere.

45