Chapter 21: Global Climate Change

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Chapter 21 Global Climate Change

• Mian Liu 2007 Fall

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The climate system

• The climate system includes the
• Atmosphere
• Hydrosphere
• Geosphere
• Biosphere
• Cryosphere (ice and snow)

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How is climate change detected?

• Techniques for analyzing Earths climate history
• Seafloor sediments numbers and types of organic
  remains are indicative of past sea-surface
  temperatures

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How is climate change detected?

• Techniques for analyzing Earths climate history
• Oxygen isotope analysis ratio of 18O/ 16O in
  shells of microorganisms reflect past temperatures

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How is climate change detected?

• Techniques for analyzing Earths climate history
• Growth of tree rings
• Pollen contained in sediment and coral reefs
• corals
• Information found in historical documents

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Some atmospheric basics

• Composition of the atmosphere
• Air is a mixture of many discrete gases
• Composition varies over time and distances
• After water vapor, dust, and other variable
  components are removed, nitrogen and oxygen make
  up 99 of the clean dry air
• CO2, although present in minute amounts
  (0.0338), greatly influences the heating of the
  atmosphere

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Composition of Earths atmosphere
Figure 21.9
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Some atmospheric basics

• Composition of the atmosphere
• Two important variable components of air are
  water vapor and aerosols
• Similar to CO2, water traps heat from in Earth
• Aerosols (tiny solid and liquid particles)
  provide surfaces for condensation and also are
  good absorbers and reflectors

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Some atmospheric basics

• Energy from the Sun
• Solar energy propagates in form of
  electromagnetic waves

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Some atmospheric basics

• Some basic laws governing radiation
• All objects emit radiant energy
• Hotter objects radiate more total energy than do
  colder objects
• The hotter the radiating body, the shorter the
  wavelengths of maximum radiation
• Gases are selective absorbers and emitters

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Some atmospheric basics

• The fate of incoming solar energy
• Approximately 50 of the solar energy that
  strikes the atmosphere reaches Earths surface
• 30 is reflected back to space
• 20 is absorbed by clouds and the atmospheres
  gases

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Some atmospheric basics

• The greenhouse effect
• Earth re-radiates the absorbed solar energy to
  outer space
• in the form of longwave infrared radiation
• Atmospheric gases, primarily H2O and CO2, are
  efficient absorbers of longwave radiation
• This selective absorption, called the greenhouse
  effect, warms up the atmosphere

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The greenhouse effect
Figure 21.12
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Natural causes of climate change

• Plate tectonics
• Variations in Earth orbit eccentricity,
  obliquity, and precession
• Volcanic activity
• Changes in the Suns output associated with
  sunspots

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Plate tectonics?
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Volcanic activity
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Mt. Tambora

• The volcano is approximately 60 km wide at sea
  level.
• Prior to the 1815 eruption it rose to about
  4,000m above sea level.
• It's current height as a result of the 1815
  eruption is 2,850.
• It's caldera is about 6km in diameter and 1,110m
  deep.
• The 1815 eruption sent about 150 cubic kilometers
  of ash into the atmosphere.
• The 1815 eruption has been assigned a VEI 7 or
  about 100 times more powerful than the 1980
  eruption of Mount St Helens.

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The top-left graphic from SAGE II data shows a
relatively aerosol-free atmosphere before the
eruption. The top-right graphic reveals that
aerosols in the tropics increased by almost a
factor of 100 immediately following the eruption.
The bottom-left graphic shows that aerosols had
spread into the Earths mid-latitudes three
months later. The bottom-right graphic
illustrates how volcanic aerosols slowly
decreased in the atmosphere over several years.
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Human impact on global climate

• Humans have been modifying the environment for
  thousands of years
• Ground cover has been altered by
• Fire
• Overgrazing
• Results in modification of climatological factors
  such as reflectivity, evaporation rates, and
  surface winds

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Human impact on global climate

• Humans have been modifying the environment for
  thousands of years
• Addition of carbon dioxide and other trace gases
  to the atmosphere are likely contributing to
  global climate change
• Trace gases include methane, nitrous oxide, and
  chlorofluorocarbons

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Increasing CO2 levels in Earths atmosphere
Figure 21.17
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Figure TS.2. The concentrations and radiative
forcing by (a) carbon dioxide (CO2), (b) methane
(CH4), (c) nitrous oxide (N2O) and (d) the rate
of change in their combined radiative forcing
over the last 20,000 years reconstructed from
antarctic and Greenland ice and fi rn data
(symbols) and direct atmospheric measurements
(panels a,b,c, red lines). The grey bars show the
reconstructed ranges of natural variability for
the past 650,000 years. The rate of change in
radiative forcing (panel d, black line) has been
computed from spline fi ts to the concentration
data. The width of the age spread in the ice data
varies from about 20 years for sites with a high
accumulation of snow such as Law Dome,
Antarctica, to about 200 years for
low-accumulation sites such as Dome C,
Antarctica. The arrow shows the peak in the rate
of change in radiative forcing that would result
if the anthropogenic signals of CO2, CH4, and N2O
had been smoothed corresponding to conditions at
the low-accumulation Dome C site. The negative
rate of change in forcing around 1600 shown in
the higher-resolution inset in panel d results
from a CO2 decrease of about 10 ppm in the Law
Dome record. Figure 6.4
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IPCCs Summary

• Changes in the atmosphere, cryosphere and ocean
• show unequivocally that the world is warming.
• Both land surface air temperatures and SSTs show
• warming. In both hemispheres, land regions have
• warmed at a faster rate than the oceans in the
  past
• few decades, consistent with the much greater
  thermal
• inertia of the oceans.
• The warming of the climate is consistent with
• observed increases in the number of daily warm
• extremes, reductions in the number of daily cold
• extremes and reductions in the number of frost
  days at
• mid-latitudes.
• Surface air temperature trends since 1979 are now
• consistent with those at higher altitudes.

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IPCCs Summary (II)

• Changes in temperature are broadly consistent
• with the observed nearly worldwide shrinkage of
  the
• cryosphere.
• Observations of sea level rise since 1993 are
• consistent with observed changes in ocean heat
• content and the cryosphere.
• Observations are consistent with physical
• understanding regarding the expected linkage
• between water vapour and temperature, and with
• intensifi cation of precipitation events in a
  warmer
• world.

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Climate-feedback mechanisms

• When any component in the climate system is
  altered, scientists must consider many possible
  outcomes
• These possible outcomes are called
  climate-feedback mechanisms

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Climate-feedback mechanisms

• Climate-feedback mechanisms
• Positive-feedback mechanisms changes that
  reinforce the initial change
• Example warmer surface temperatures cause an
  increase in evaporation, which further increases
  temperatures as water vapor absorbs more
  radiation

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Climate-feedback mechanisms

• Climate-feedback mechanisms
• Negative-feedback mechanisms produce results
  that are the opposite of the initial change and
  tend to offset it
• Example negative effect of increased cloud
  cover on the amount of solar energy available to
  heat the atmosphere

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How aerosols influence climate

• Global climate is affected by human activities
  that contribute to the atmospheres aerosol
  content
• Produce a cooling effect by reflecting sunlight
  back to space
• The effect on todays climate is determined by
  the amount emitted over the course of a few weeks
• By contrast, carbon dioxide remains for much
  longer spans and influences climate for many
  decades

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Some possible consequences of global warming

• Altering the distribution of the worlds water
  resources
• Probable rise in sea level
• Greater intensity of tropical cyclones
• Changes in the extent of Arctic sea ice and
  permafrost

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Table 21.1
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Figure TS.23. (a) Global mean surface temperature
anomalies relative to the period 1901 to 1950, as
observed (black line) and as obtained from
simulations with both anthropogenic and natural
forcings. The thick red curve shows the
multi-model ensemble mean and the thin yellow
curves show the individual simulations. Vertical
grey lines indicate the timing of major volcanic
events. (b) As in (a), except that the simulated
global mean temperature anomalies are for
natural forcings only. The thick blue curve
shows the multi-model ensemble mean and the
thin lighter blue curves show individual
simulations. Each simulation was sampled so that
coverage corresponds to that of the
observations. Figure 9.5
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Some possible consequences of global warming

• Due to complexity of the climate system, not all
  future shifts can be foreseen
• Sudden unexpected changes in climate are possible
• A constant state of change is very likely

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End of Chapter 21