Transcript Ch15Pres - Leornian.org

AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 15
Climate and Climate
Change
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Case-in-Point
 The consensus of scientific opinion is that the
present global warming trend is largely
anthropogenic in origin
– Human activity, principally the combustion of fossil fuels,
is responsible for the build-up of carbon dioxide and
other infrared-absorbing gases in the atmosphere
– The amount of anthropogenic CO2 already emitted into
the atmosphere ensures a magnitude of warming that will
cause an unacceptable rise in sea level in some localities
– The 2007 IPCC Fourth Assessment Report estimates
that msl will be 0.2 to 0.6 m higher than now by the year
2100
 An example of an island nation that is particularly vulnerable to
rising sea level is the Maldives
– Migration may be their only alternative
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Driving Question
 How and why does climate change?
– Climate changes over a broad range of time
scales
 Years, decades, centuries, millennia
– Many forces working together are responsible for
climate change
 Variability in available solar energy
 Volcanic eruptions
 Changes in the Earth’s surface properties
 Human activities
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Earth’s Climate System
 Climate: The weather of some locality
averaged over some time period plus
extremes in weather
– Varies spatially and temporally
Described quantitatively in terms of:
 Normals
 Means
 Extremes
 Weather elements including wind, temperature and
precipitation
NOAA’s National Climatic Data Center (NCDC)
 Collects (from NWS), processes and archives data
 Makes available to users
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Earth’s Climate System
 The Climatic Norm
Average value of some climatic element
– Encompasses the total variation in the climate
record, including both averages plus extremes
– Computed averages of weather elements over a
30-year time period
 Adjusted every 10 years to add the latest decade and
drop the oldest
 In the U.S. 30-year averages only done for
temperature, precipitation and air pressure
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Earth’s Climate System
 Climatic Anomalies
– Departures from long-term climatic averages
 Positive anomalies: above long-term averages
 Negative anomalies: below long-term averages
Precipitation anomalies usually form more complex
patterns than temperature anomalies
 Due to greater spatial differences in precipitation arising from:
– Variability of storm tracks
– Almost random distribution of convective showers
Middle and high latitudes have a geographic nonuniformity of climatic anomalies
 Due to prevailing westerly wave pattern
Geographic non-uniformity also applies to trends in
climate
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Climate Boundary Conditions

Boundary conditions are imposed on
climate due to:
Latitude
Elevation
Topography
Proximity to large bodies of water
Earth’s surface characteristics
Long-term average atmospheric circulation
Prevailing ocean circulation
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Climate Boundary Conditions
 Latitude
Incoming solar radiation and length of daylight vary
Earth’s surface temperature responds to these variations
 Elevation
Temperature drops with increasing elevation
 Topography
Affects the distribution of clouds and precipitation
 Proximity to large bodies of water
Important in storage and exchange of heat, water, and greenhouse
gases
 Earth’s surface characteristics
Ocean vs. Land, vegetative cover, semi-permanent snow and ice
Influence the amount of incident solar radiation that is converted to
heat
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Climate Boundary Conditions
 Atmospheric Circulation
– Encompasses influence of all
weather systems
– Less regular and less predictable
than other facts
 Prevailing Ocean Circulation
– Influences radiational heating and
cooling of the planet
– Primary control of the amount of solar
radiation absorbed at the Earth’s
surface
– Ocean is the main source of the most
important greenhouse gas (water
vapor), major regulator of CO2
– Transfers heat from lower to higher
© AMS latitudes
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Global Climate Patterns
 Temperature
Latitude of highest mean annual surface temperature
(heat equator) located about 10° north of the
geographical equator
Heat equator is located in the Northern Hemisphere
 Polar regions have different radiational characteristics
Arctic is warmer than Antarctic: Arctic is mostly ocean,
Antarctic is ice sheets with a higher albedo
 Northern Hemisphere has more land then the
Southern Hemisphere
Land warms much faster than water
 Ocean circulation transports more warm water to the
Northern Hemisphere then the Southern Hemisphere
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Global Climate Patterns
 Temperature, continued
– Systematic patterns appear in January/July
temperatures
 Isotherms tend to parallel latitude circles
– Isotherms shift north and south from January to
July following the Sun
 Greater shift over land
– Steeper isothermal gradients mean more
vigorous circulation and stormier weather in the
winter hemisphere
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Global Climate Patterns
Mean sea-level air temperature for January in °C
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Global Climate Patterns
Mean sea-level air temperature for July in °C
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Global Climate Patterns
 Precipitation
Variability in patterns due to:
 Topography
 Distribution of land and sea
 Planetary-scale circulation
Intertropical Convergence Zone (ITCZ)
 Rainfall in the adjacent belt poleward to about 20° latitude
depends on seasonal shifts of the ITCZ
Subtropical anticyclones
 Dominate climate all year between 20° to 35° N and S
Prevailing wind patterns
 Between about 35° and 40° latitude, the prevailing westerlies
and subtropical anticyclones govern precipitation
Precipitation generally declines poleward of about 40° latitude
as lower temperatures reduce the amount of precipitable water
Tendency in continental interiors for more precipitation in the
summer
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Global Climate Patterns
Mean annual precipitation (rain plus melted snow) in millimeters (mm)
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Global Climate Patterns
 Climate classification
– Classification schemes group climates
according to:
 Meteorological basis of climate
Asks why climate types occur where they do
 Environmental effects of climate
Infers the type of climate from environmental indicators
such as the distribution of indigenous vegetation
Numerical climate classification schemes also
employed that utilize statistical techniques
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The Climate Record
 Climate varies from place to place and with time
 Reconstructions of past climate based on
historical documents, and longer-term geological
and biological evidence
 Geological Time
General conclusions made regarding climate over
geological time
Geological past subdivided using geologic time scale
Plate tectonics complicates climate reconstruction by
moving continents and opening and closing ocean
basins
 Continental drift
 Alters the course of heat-transporting surface ocean currents
and the ocean conveyor-belt system
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Geologic time
scale
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The Climate Record
About 225 million years
ago, the super-continent
Pangaea began to split
apart into separate
continents that slowly
drifted apart. Ocean
basins opened, and
eventually continents
reached their present
positions. Continental drift
is responsible for climate
changes operating over
hundreds of millions of
years
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The Climate Record
 Geologic time, continued
570 million years ago, between Proterozoic and
Phanerozoic Eons: abrupt climatic changes between
extreme cold and tropical heat. Global warming.
Mesozoic Era, from about 245 to 70 million years ago,
remained relatively warm
Global mean temperature rose 3 - 4°C between Triassic
and Jurassic Periods
Creaceous Period saw temperatures 6 - 8°C higher then
now
Cenozoic Era, 55 million years ago, methane released by
deep sea sediments enhanced the greenhouse effect
 40 million years ago saw shift toward colder, drier and
more variable climate
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The Climate Record
 Past Two Million Years
– Plate tectonics not a major factor
– Climate has favored the development of glacial ice sheets
 Glacial theory
– Pleistocene Ice age began 1.7 million years ago, and
culminated about 10,500 years ago
 The Pleistocene included glacial and non-glacial events
 Ice sheets caused sea level to drop by 113 to 135 m (370 to
443 ft)
 Land bridge was exposed linking Siberia and Alaska
 During interglacial episodes, ice sheets thinned and retreated,
sometimes disappeared entirely
 During glacial episodes temperatures were cooler than today,
but cooling was not geographically uniform
– Polar amplification: an increase in the magnitude of a
climatic change with increasing latitude
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The Climate Record
A. Variation in global
glacial ice volume
from the present
back to about 600
million years ago
B. Temperature
variation over the
past 160,000 years
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The Climate Record
Extent of glacial
ice cover over
North America
about 20,000 to
18,000 years
ago, the time of
the last glacial
maximum
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The Climate Record
 Past Two Million Years, continued
Fluctuations between major glacial and interglacial climatic
episodes over the past 600,000 years
Last major glacial climatic episode began about 27,000 years
ago, reaching its peak about 18,000 to 20,000 years ago
Present interglacial known as the Holocene
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The Climate Record
 Past Two Million Years, continued
Over the past 1000 years, there has been a Medieval
Warm Period (about 950 to 1250 CE) followed by a
cooling known as the Little Ice Age (about 1400 to 1850
CE)
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The Climate Record
 Instrument-Based Temperature Trends
Invention of weather instruments and establishment of weather
observing networks made climate record much more detailed
and dependable
– Highest confidence is in temperature records dating from the late
1800s with the birth of national weather services along with the
founding of the International Meteorological Organization (WMO
today)
– Examination of temperature trends over the past 140 years or so is
instructive as to climate variability and climate change
 General consensus in the scientific community holds that an overall
global-scale warming trend has prevailed since the end of the Little Ice
Age
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Lessons of the Climate Past
 Climate is inherently variable over a broad spectrum
of time scales ranging from years to decades, to
centuries, to millennia
 Variations in climate are geographically non-uniform
in both sign (direction) and magnitude
Some areas may experience warming while others cooling
 Climate change may consist of a long-term trend in
various climate elements and/or a change in the
frequency of extreme weather events
 Climate change tends to be abrupt rather than
gradual
Abrupt means change is shorter than duration of episodes
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Lessons of the Climate Past
 Only a few cyclical variations
can be discerned from the
long-term climate record
Regular cycles: diurnal and
seasonal variations, incoming
solar radiation
Quasi-regular variations: El Niño,
Holocene millennial-scale
fluctuations, major glacialinterglacial shifts
 Climate change impacts
society
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Causes of Climate Change
 Match a possible cause (or forcing) with a specific climatic
oscillation (or response) based on similar periods of
oscillation
Example: El Niño may account for climate shifts lasting several
months
 Global energy budget
Changes in energy input/output will change the planet’s climate
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Causes of Climate Change
 Climate and Solar Variability
Earth’s climate system can be altered by
fluctuations in the Sun’s energy output,
sunspots, or regular variations in Earth’s
orbital parameters
 A sunspot is a dark blotch on the face
of the Sun that develops where an
intense magnetic field suppresses the
flow of gases transporting heat from the
Sun’s interior
 Typically last for a few days, total
number varies systematically
 Satellite monitoring reveals that Sun’s
energy output varies directly with
sunspot number
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Causes of Climate Change
Variation in mean annual sunspot number
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Causes of Climate
Change
 Earth’s Orbit and Climate
Change
– Milankovitch Cycles
 Regular variations in the
precession and tilt of Earth’s
rotational axis and the
eccentricity of its orbit around
the Sun
 Caused by gravitational
influences exerted on the Earth
by other large planets, the
moon and sun
 Tilt of Earth changes from 22.1°
to 24.5° over about 41,000
years
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Milankovitch Cycles
Past and future
variations in the
Milankovitch
cycles
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Causes of Climate Change
 Volcanoes and Climate Change
– Only explosive eruptions rich in sulfur
dioxide (SO2) are likely to impact global or
hemispheric climate for a few years at the
most
– When combining with water vapor, droplets
of sulfuric acid (H2SO4) and sulfate
particles form; together called sulfurous
aerosols
 Can remain in stratosphere for many
months
 Collectively thicken the stratospheric
aerosol veil
 Absorb both incoming solar and outgoing
infrared radiation, warming the lower
stratosphere
 In the presence of chlorine destroy ozone
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Volcanoes and Climate Change
Temperature anomalies in Fahrenheit degrees in the
Midwest during June, July, and August 1992
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Causes of Climate Change
 Volcanoes and Climate
Change
– The combination of stratospheric
warming and ozone depletion
strengthens the circumpolar
vortex
 This is associated with a nonuniform change in surface
temperatures
A violent sulfur-rich eruption is
unlikely to lower the mean
hemispheric or global
temperature by more then 1 °C
 Local and regional
temperature change may be
greater
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Large scale cooling followed
massive volcanic eruptions that
emitted SO2 into the stratosphere
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Causes of Climate Change
 Earth’s Surface Properties and Climate Change
Earth’s surface, comprised of approximately 71% ocean
water, is the prime absorber of solar radiation
Changes in the physical properties of water or land
surfaces, or relative distribution of ocean, land or ice
may affect Earth’s radiation budget and climate
Variations in snow cover
 Snow has refrigerating
effect on atmosphere
 Reflects 80% of solar radiation
reducing solar heating
 Emits infrared radiation,
radiating heat to space
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Causes of Climate Change
 Earth’s Surface Properties and Climate Change,
continued
Changes in sea ice or glacial ice coverage have longerlasting impacts on climate
 Ice is much more reflective of incident solar radiation than
ocean or snow-free land
Changes in sea surface temperature and ocean
circulation
 SST patterns can exert a strong influence on the location of
major features of the atmosphere’s planetary scale circulation
 Ocean circulation transports heat throughout the world; changes
in circulation patterns can cause changes in climate
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Causes of Climate Change
 Human Activity and Climate Change
– Modification of the landscape
 Clearing forests, urbanization
 Alters radiative properties of the Earth’s surface
– Combustion of fossil fuels
 Alters concentrations of gases and aerosols
 Rising concentrations of CO2 and other infrared-absorbing gases
enhance greenhouse effect, contributing to global warming
 Rapid rise in CO2 with start of Industrial Revolution
– Rise of other infrared-absorbing gases (e.g., methane and
nitrous oxide) could also enhance greenhouse effect
 Methane is a product of organic decay in the absence of oxygen
 Nitrous oxide increase likely due to industrial air pollution
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Anthropogenic vs. Natural Forcing of Climate
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The Climate Future
 Global Climate Models (GCMs)
– Simulate Earth’s climate systems
– Use mathematical equations that describe physical
interactions among the various components of the climate
system
– Differ from numerical models in that GCMs predict broad
regions of expected positive and negative temperature and
precipitation anomalies and mean locations of circulation
features
– For example, used to predict potential climatic impacts of
rising levels of greenhouse gases using boundary
conditions
– GCMs are in need of considerable refinement
 May miss important feedback processes
 Need a greater resolution
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The Climate Future
 Search for Cycles and Analogs
– Climate records an alternative approach to predicting
climate future
– Instrument-based and reconstructed climate record
probed in search of regular cycles that might be
extended into the future, and analogs that show how the
climate in specific regions responds to global-scale
climate change
 Identification of any statistically significant periodicities or trends
in the climate record would be a powerful tool in climate
forecasting
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The Climate Future
 Enhanced Greenhouse Effect and Global Warming
Over the next 10,000 - 20,000 years, Milankovitch
cycles favor return of Ice Age conditions
Rising concentrations of greenhouse gases are likely to
cause global warming to continue throughout this
century
– Climate models predict that over the subsequent 20
years, the global mean annual temperature will increase
at an average rate of about 0.2 Celsius degrees per
decade
 Even if greenhouse gas emissions were to stabilize at present
levels, global warming would likely continue well beyond the
21st century
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Potential Impacts of Global Climate
Change
 Shrinking Glaciers and Rising
Sea Level
– Persistence of the current global
warming trend appears likely to
cause sea level to rise in
response to melting of land-based
polar ice sheets and mountain
glaciers, coupled with thermal
expansion of seawater
– Amplification of the warming trend
at higher latitudes would threaten
the ice sheets of Antarctica and
Greenland
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 Melting could cause a
considerable rise in sea level
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Potential Impacts of Global Climate Change
 Shrinking Glaciers and Rising
Sea Level, continued
– The shrinkage of mountain
glaciers and ice fields whose
seasonal would be a major
concern, as people, livestock, and
crops rely on the season runoff for
fresh water
– Thermal expansion of warming
seawater will be a greater
contributor to mean sea level rise
than melting of land-based
glaciers during the 21st century
– Higher mean sea level would
accelerate coastal erosion,
inundate wetlands, estuaries, and
islands, and make coastal zones
more vulnerable to storm surges
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Grinnell Glacier
1938
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1981
1998
2006
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Potential Impacts of Global Climate
Change
 Arctic Environment
– Shrinkage of Arctic sea ice is likely to
trigger an ice-albedo feedback
mechanism that would accelerate
melting of sea ice and amplify
warming
– As sea ice cover shrinks, the greater
area of ice-free ocean waters absorbs
more solar radiation, sea-surface
temperatures rise, and more ice melts
– Less sea ice cover on the Arctic
Ocean is likely to increase the
humidity of the overlying air leading to
more cloudiness
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Sea Ice in the Northern Hemisphere
Polar Region
September 2008
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March 2009
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Potential Impacts of Global Climate
Change
 Other Impacts
– Higher temperatures and more frequent drought may
affect food production
– Possible concern for an increase in hurricane intensity
 High SST only one factor contributing to tropical cyclone
formation
 Improved models of the climate system are needed to better
assess the effect of warming on hurricane activity
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Potential Impacts of Global Climate
Change
 Ocean Acidification
– CO2 that is absorbed by the ocean participates in
chemical reactions that increase the acidity (lowers the
pH) of ocean waters
 Potentially harmful to marine organisms that use carbonate ions
(CO3-2) to build calcium carbonate (CaCO3) shells or skeletons
– Marine organisms that are particularly vulnerable to
ocean acidification are coccolithophorids, foraminifera
(phytoplanktonic organisms), and pteropods (small
marine snails)
 All important food source in marine food webs
 Corals which filter plankton from ocean water and secrete
calcium carbonate are also vulnerable
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