Transcript chapt04_lecture Getis 13e(2).

Introduction to
Geography
Arthur Getis, Judith Getis, &
Jerome D. Fellmann
Physical Geography:
Weather and Climate
Chapter 4
Overview
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Introduction
Air Temperature
Air Pressure and Winds
Ocean Currents
Moisture in the Atmosphere
Climate Regions
Climatic Change
Introduction
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Weather
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Climate
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State of the atmosphere at a given time and place
Long-term average weather conditions in a place
Geographers analyze differences in weather
and climate from place to place to understand
how they affect human occupance of the earth
Introduction
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Troposphere is of particular concern
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Atmospheric layer closest to the earth
Contains virtually all of the air, clouds, and
precipitation of the earth
Air Temperature
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Why do temperatures vary from place to place?
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Need to understand how heat accumulates on the
earth’s surface
Solar energy is transformed into heat primarily at
the earth’s surface and secondarily in the
atmosphere
Insolation (amount of incoming solar radiation)
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Solar radiation received at the earth’s surface
 Determined by angle of the sun’s rays and number
of daylight hours
Air Temperature
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Angle of sun’s rays, number of daylight hours
and these modifying variables determine the
temperature at any given location:
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Amount of water vapor in the air
Cloud cover
Nature of the surface of the earth (land or water)
Elevation
Degree and direction of air movement
Air Temperature
Earth’s Movements
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Rotation
Revolution
Air Temperature
Earth Inclination
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Axis of the earth tilts at ≈ 23.5°
Location of highest incoming solar radiation
varies during the year
Summer Solstice in Northern Hemisphere
(about June 21)
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Northern Hemisphere tilted toward the sun
Vertical rays of the sun at noon are at 23.5° N
Air Temperature
Earth Inclination
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Winter Solstice in the Northern Hemisphere
(about December 21)
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Northern Hemisphere tilted away from the sun
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Vertical rays of the sun at noon are at 23.5° S
Spring and fall equinoxes in the Northern
Hemisphere (about March 21 and
September 21, respectively)
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Vertical rays of the sun at noon are at the equator
Air Temperature
Earth Inclination
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Causes variation in length of days and nights
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At the equator
 12 hours of light each day of the year
Inside the Arctic Circle and Antarctic Circle
 24 hours of daylight/darkness on solstices
Angle of the sun’s rays above the surface of the
earth
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The larger the angle, the more energy available
Air Temperature: Reflection
and Radiation Emission
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Clouds in the atmosphere and light colored
surfaces on the earth reflect 32% of incoming
solar energy back to outer space
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Radiation cycle
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Shortwave solar radiation is absorbed by the surface
of the earth, thereby heating the surface. Then
radiation is emitted from the earth into the
atmosphere in the form of longwave radiation.
Earth’s atmosphere is mainly heated by terrestrial
radiation.
Air Temperature: Reflection
and Radiation Emission
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Daily cycle of temperature
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Air temperature rises when incoming solar radiation
exceeds energy lost through reflection and terrestrial
radiation
Air temperature drops when energy lost through
reflection and terrestrial radiation exceeds incoming
solar radiation
Air Temperature: Reflection
and Radiation Emission
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Water heats and cools more slowly than land
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Marine environment
 Cooler summers, warmer winters
Land heats and cools more rapidly than water
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Continental environment
 Hotter summers, colder winters
Air Temperature
The Lapse Rate
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Temperature generally decreases as altitude
increases in the troposphere
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Lapse rate
 Average of 3.5° F per 1000 feet (6.4° C per 1000 m)
Temperature inversion
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Cooler air trapped below warmer air
 Contributes to smog problems
Air Pressure and Winds
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How do differences in air pressure from place to
place affect weather conditions?
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Air pressure = weight of the atmosphere
Normal sea level pressure = 14.7 pounds or 6.7
kilograms
Air pressure is highest closest to the earth’s
surface
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Air Pressure and Winds
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Temperature and air pressure relationship (for
equal amounts of cold and hot air):
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Cold air is denser than warm air: high pressure
Warm air is lighter than cold air: low pressure
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Air pressure is measured by a barometer in
inches of mercury or millibars
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Air pressure at a given location changes as
surfaces heat or cool
Air Pressure and Winds
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Zones of high and low air pressure exist in
earth’s atmosphere
Pressure gradient force
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Causes air to flow from high to low pressure areas
Wind
 Velocity is in direct proportion to pressure
differences
Convection system
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Circulatory movement of rising warm air and
descending cool air
Air Pressure and Winds
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Convectional wind effects due to differential
heating and cooling of land and water:
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Land sea breezes
 Day: from sea to land
 Night: from land to sea
Mountain and valley breezes
 Day: from valley to mountains
 Night: from mountains to valley
Air Pressure and Winds
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Coriolis effect
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Result of Earth’s rotation
Earth rotates in a counterclockwise direction when
viewed from the position of the North Pole
Earth rotates in a clockwise direction when viewed
from the position of the South Pole
Apparent deflection relative to the earth’s surface
 Northern Hemisphere: wind veers toward the right
Southern Hemisphere: wind veers toward the left
Coriolis effect and pressure gradient force produce
spiral wind patterns
Air Pressure and Winds
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Frictional effect of Earth’s surface on movement
of wind
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Decreases wind speed
Changes wind direction
Air Pressure and Winds
Global Air-Circulation Pattern
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Equatorial low pressure
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Subtropical high pressure
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Warm air rises and moves away from Equatorial
region
About 30° N and 30° S of the equator
Cold air aloft sinks
When sinking air reaches Earth’s surface it moves
away from the region of high pressure, thereby
creating the trade winds and the westerlies
Trade winds
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In the tropics
Air Pressure and Winds
Global Air-Circulation Pattern
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Westerlies
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In the midlatitudes
Subpolar low
Polar easterlies
Polar high
Air Pressure and Winds
Global Air-Circulation Pattern
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Jet streams
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Belts of strong winds in the upper atmosphere
 Flow from west to east in an undulating path
 Bring cold air equator ward and warm air pole ward
Guide the movement of weather systems
More pronounced in the winter than in the summer
Air Pressure and Winds
Global Air-Circulation Pattern
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Monsoon
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Wind system that reverses direction seasonally
 Produces wet and dry seasons
Significant effect on parts of southern and eastern Asia
 Farm economy is dependent upon summer monsoon
rains
 Negative impact if rainfall arrives late or rainfall is
less or more than optimum
 May cause disastrous flooding and loss of lives
Ocean Currents
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Surface ocean currents correspond roughly to
global wind direction patterns
Movement due to winds and differences in
water density
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Direction affected by Coriolis effect
Direction also influenced by landmasses and shape
of ocean basins
North Atlantic drift
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Warm current
Winters in northern Europe are warmer than
expected for its latitude
Ocean Currents
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Ocean currents affect precipitation on adjacent
land areas
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Cold currents
 Dry conditions (coastal deserts)
Warm currents
 Moist conditions
Moisture in the Atmosphere
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Ascending air expands and cools
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Less able to hold water vapor
Supersaturated air
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Water vapor condenses around condensation nuclei
Moisture in the Atmosphere
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Clouds
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Consist of rain droplets or ice crystals supported by
upward movements of air
 Droplets may coalesce and fall as rain
 Temperatures below freezing - formation of snow
Form and altitude of cloud depends on:
 Water vapor content, temperature, wind movement
Moisture in the Atmosphere
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Clouds (continued)
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Types of clouds:
 Cumulus
 Stratus
 Cirrus
 Cumulonimbus
Moisture in the Atmosphere
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Relative humidity
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Percentage measure of the moisture content of the air
 Amount present relative to the maximum that can
exist at the current temperature
Dew point
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Temperature at which condensation forms
Moisture in the Atmosphere
Types of Precipitation
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Convectional precipitation
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Heated, moisture-laden air rises and then cools
below the dew point
Summer in tropical and continental climates
Orographic precipitation
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Warm, moisture-laden air is forced to rise over hills
or mountains and is thereby cooled
 Windward side
 Receives a great deal of precipitation
 Leeward side
 Very often dry (rain shadow)
Moisture in the Atmosphere
Types of Precipitation
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Cyclonic (frontal) precipitation
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Common to the midlatitudes
In the tropics – originator of hurricanes and typhoons
Occurs where cool and warm air masses meet
Air mass
 Body of air with similar temperature, pressure, and
humidity characteristics throughout
 Forms over a source region
Front
 Zone of separation between two air masses
Moisture in the Atmosphere
Storms
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Midlatitude Cyclone
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Masses of air circulating about a region of low
atmospheric pressure
Counterclockwise rotation in Northern Hemisphere
Warm air moves up and over cold air along a front
Can develop into a storm
Moisture in the Atmosphere
Storms
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Hurricane
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Typhoon
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Severe tropical cyclone with winds exceeding 74 mph
Forms in low-pressure zone over warm waters; e.g., in
the Atlantic, Caribbean, or Gulf of Mexico
Name for hurricanes in the western Pacific Ocean
Structure of hurricane
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Eye
Eye wall
Surge
Concentric belts of rising air
Moisture in the Atmosphere
Storms
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Blizzard
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Heavy snow and high winds
Tornado
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Most violent of all storms
Smallest storm
Common in Central U.S. in spring and fall
Spawned in huge cumulonimbus clouds
Funnel-shaped cloud of whirling winds that spins at
speeds as high as 300 mph
Enhanced Fujita Scale
Waterspout
Climate Regions
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Climate of an area is a generalization based on
daily and seasonal weather conditions
Two most important elements that differentiate
weather conditions are temperature and
precipitation
Climate Regions
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Köppen climate system
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Based on temperature, precipitation, and natural
vegetation criteria
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Six broad categories
 A: tropical
 B: dry
 C: mild midlatitude
 D: midlatitude with severely cold winters
 E: polar
 H: highland
Climate Regions
Tropical Climates (A)
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Generally found between Tropics of Cancer
and Capricorn
Tropical rainforest (Af)
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Found in equatorial low pressure zone; also along
coasts extending away from the equator
High temperatures and daily convectional rainfall all
year
Tall, dense forests
Climate Regions
Tropical Climates (A)
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Tropical savanna (Aw)
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To the north and south of rain forests
High temperatures
Heavy convectional rainfall in summer, dry winters
Forests to grasslands
Tropical monsoon (Am)
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Significant increase in rainfall when summer
monsoon winds bring water-laden air
Dense forests
Climate Regions
Dryland Climates (B)
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Hot deserts (BWh)
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Found in subtropical high pressure zone
Considerable sunshine, high temperatures
Very little precipitation
Shrubs in gravelly or sandy environments
E.g., Sahara, Arabian, Australian and Kalahari
Deserts
Climate Regions
Dryland Climates (B)
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Midlatitude deserts and semideserts (BWk)
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Warm/hot summers and cold winters
Some convectional or frontal rainfall in summer,
some snowfall in winter
Extremely dry areas known as cold deserts
Moderately dry lands known as steppes
 Among most naturally fertile soils in the world
 Vegetation: grasslands, desert shrubs
 Known for hot, dry summers and biting winter
winds which sometimes bring blizzards
Climate Regions
Humid Midlatitude Climates (C)
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Mediterranean (Cs)
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Located in transition zone between subtropical highs
and the westerlies
Aside from Mediterranean region itself, generally
found on the western coasts of continents in the
middle latitudes; e.g., Southern California, tip of
South Africa, western Australia and central Chile
Warm/hot summers and mild/cool winters
Dry summer, frontal precipitation in winter
Vegetation
 Shrubs
 Small deciduous trees
Climate Regions
Humid Midlatitude Climates (C)
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Humid subtropical (Cfa)
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Located on eastern coasts on continents
Hot, moist summers and moderate, moist winters
Convectional summer showers, winter cyclonic
storms
Deciduous and coniferous forests
May be affected by hurricanes
Climate Regions
Humid Midlatitude Climates (C)
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Marine west coast (Cfb)
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Prevailing winds from the sea
Moderate temperatures in both summer and winter
Frontal and orographic precipitation
Deciduous and coniferous forests
Climate Regions
Humid Continental Climates (D)
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Cyclonic storms are more responsible for rainfall
than are convectional showers
Colder winters, shorter summers
Prevailing winds from land
Deciduous and coniferous forests
Location:
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Northern and central US
Southern Canada
Most of European portion of Russia
Northern China
Climate Regions: Subarctic
Climates (Dfc, Dfd, Dwb)
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Subarctic
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Cool/cold, short summers and very cold winters
Coniferous forest to mosses and lichens
Tundra
 Treeless area between the Arctic tree line and the
permanently ice-covered zone
Location
 Alaska
 Northern Canada
 Northern Russia
Climate Regions
Arctic Climates (E)
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Arctic
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Ice cap near the poles
Extremely cold with light precipitation
Antarctica and Greenland are icy deserts
Climate Regions
Highland Climates (H)
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Lower temperatures than lowlands at the same
latitude
Variety of conditions based on:
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Elevation
Prevailing winds
Orientation of slope relative to the sun
Valley, slope, or peak
Ruggedness
Climatic Change
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Long-term climatic change
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Significant variations over geologic time
 Ice ages
 Medieval warm period and “little ice age”
May be due to variations in: shape of Earth’s orbit, tilt of
the axis, gyration of the rotation axis
Short-term climatic change
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Natural processes
 Volcanic eruptions, oceanic circulation, sunspot activity
Human processes
 Enhanced greenhouse effect
Climatic Change
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Greenhouse effect
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Certain gases in the atmosphere function as an
insulating barrier, trapping infrared radiation
Global warming
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Caused by human activities that have increased the
amount of greenhouse gases in the atmosphere
 Carbon dioxide: burning fossil fuels, deforestation
 Methane: natural gas and coal mining, agriculture
and livestock, swamps, landfills
 Nitrous oxides: motor vehicles, industry, fertilizers
 Chlorofluorocarbons: industrial chemicals
Climatic Change
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Evidence of global warming (continued)
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20th century was the warmest in 600 years
 Average surface temp rose over 1° F during the
century
Winter temps in the Arctic have risen about 7° F since
the 1950s
 Loss of Arctic ice cap
Glaciers are thinning and retreating
Climatic Change
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Consequences of global warming include:
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Rising sea levels
Changes in temperature and precipitation patterns
 Impact on soils, vegetation, agriculture