Global Warming

1
Global Warming

• The name given to the increase in the average
  temperature of the Earths near surface
  temperature and oceans that has occurred since
  the middle of the 20th century.
• In the graph to the right, panel a gives the
  Earth's surface temperature is shown year by year
  (red bars) and approximately decade by decade
  (black line, panel b gives the year by year (blue
  curve) and 50 year average (black curve)
  variations of the average surface temperature of
  the Northern Hemisphere for the past 1000 years
  have been reconstructed from "proxy" data
  calibrated against thermometer data.

2
Greenhouse effect

• Main culprit behind global warming
• Sunlight enters the atmosphere and warms the
  Earth (UV, visible, IR)
• At night, Earth radiates heat energy it gained
  during the day(mostly at IR wavelengths)
• Not all of this energy escapes the
  atmosphere-some of it is absorbed and re-emitted
  back into the atmosphere, warming the atmosphere
  and the Earth.
• Not necessarily a bad thing-without it the Earth
  would be cold and life (a least as we know it)
  would not be possible

3
Venus

• Hottest planet in the solar system
• But not the closest to the sun
• Atmosphere is 96.5 carbon dioxide
• Venus once had immense amounts of water, but no
  ozone layer-the UV radiation broke up the water
  molecules
• Unfortunate, water absorbs carbon dioxide wand
  mitigates the greenhouse effect
• Resulted in an extreme greenhouse effect

4
Greenhouse effect-What could happen

• Global temperatures rise until boiling point of
  water is reached
• As water boils away, it becomes water vapor in
  the atmosphere, accelerating the heat rise
  (called positive feedback).
• At a few hundred degrees celsius, carbon dioxide
  would be released from the rocks (sublimation),
  accelerating the heating again
• Eventually the Earth would stabilize at surface
  temperatures similar to Venus (T860F)

5
Greenhouse Gasses in order of importance

• water vapor
• carbon dioxide
• methane
• nitrous oxide
• ozone
• CFCs

6
Changes in greenhouse gas concentrations

• Historical variations can be tracked via analysis
  of ice cores
• Ice core - a core sample (a cylindrical section
  of a naturally occurring medium consistent enough
  to hold a layered structure) from the
  accumulation of snow and ice over many years that
  have re-crystallized and have trapped air bubbles
  from previous time periods.
• The composition of these ice cores provides a
  picture of the climate at the time.
• Record for over 800,000 years

Gas Preindustrial Level
Current Level Increase since 1750 Carbon
dioxide 280 ppm 387ppm
104 ppm Methane 700 ppb
1,745 ppb 1,045 ppb Nitrous
oxide 270 ppb 314 ppb
44 ppb CFC-12
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What do the ice cores tell us?

• Natural variations occur in the greenhouse gas
  concentrations

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Post industrial revolution
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Man made sources of greenhouse gasses

• Also called Anthropogenic, which designates an
  effect or object resulting from human activity
• burning of fossil fuels and deforestation
  leading to higher carbon dioxide concentrations.
• Account for one third of total anthropogenic CO2
  emissions.
• livestock enteric fermentation and manure
  management, paddy rice farming, land use and
  wetland changes, pipeline losses, and covered
  vented landfill emissions leading to higher
  methane atmospheric concentrations. Many of the
  newer style fully vented septic systems that
  enhance and target the fermentation process also
  are sources of atmospheric methane.
• use of chlorofluorocarbons (CFCs) in
  refrigeration systems, and use of CFCs and halons
  in fire suppression systems and manufacturing
  processes.
• agricultural activities, including the use of
  fertilizers, that lead to higher nitrous oxide
  (N2O) concentrations.

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What are we seeing?

• Since 1860-1900, global surface temperatures have
  increased by 1.35 F
• Urban heat island effect only accounts for 0.02 C
  rise
• Since 1979 land T have increased twice as fast as
  ocean Temperatures
• Temperatures on the lower troposphere have have
  increased by 0.62 between 1979 and 2000.

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Hold on, could it be the sun?

• No overall increase in solar brightness in over
  1000 years
• Solar cycles do cause small variations in
  brightness, but not enough to account for what
  has been seen
• No its not the sun

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Feedback-making the problem worse (or better)

• Positive feedback-when the warming induces
  further warming
• Negative feedback when the warming induces a
  cooling

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Sources of Feed back

• Positive
• Water vapor
• Clouds
• Ice albedo
• Arctic methane release
• Reduced carbon dioxide absorption in the oceans

• Negative
• Lapse rate

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Negative feedback

• Lapse Rate
• Increased heating means increased IR emission
• Global warming reduces the rate of Temperature
  decrease with height, which means more long
  wavelength radiation will be emitted by the upper
  atmosphere
• This will weaken the greenhouse effect

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Positive feedback

• Water vapor
• Warming increases the amount of water vapor in
  the atmosphere, which increases the warming since
  water vapor is also a greenhouse gas
• Clouds
• Act as a blanket, reflect IR radiation downward
  toward the surface
• Ice albedo
• Melting ice reveals land and water, both reflect
  less light than ice-more warmth is absorbed
  increasing warming
• Arctic methane release
• Warming releases sources of carbon dioxide
• Reduce carbon dioxide absorption in oceans
• Warm waters favor the growth of plankton rather
  than diatoms-diatoms are more efficient carbon
  dioxide absorbers

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Global dimming

• An effect that has been counteracting some of
  global warming from about 1960 forward
• Aerosols produced by volcanoes and pollutants
  such as sulfur dioxide reflect incoming sunlight
• Soot suspended in the atmosphere, it can absorb
  solar radiation and heat the atmosphere, but cool
  the surface

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What are we seeing

• Besides the increases in Temperature
• Worlds glaciers are melting
• Arctic sea ice is reducing in both extent ( 9
  reduction in area per decade) and thickness
  (15-40 in thickness in the last 30 years)
• Ocean levels are rising-both due to melting of
  Antarctic ice and thermal expansion of sea water
• Longer growing seasons
• Thawing of permafrost in Alaska
• Coral reef bleaching-whitening of reefs due to
  increased temperature
• Earlier plant flowering
• Earlier bird arrivals
• Shifting of animal ranges poleward
• More frequent EL Nino-warming of the Pacific
  ocean surface temperatures-causes changes in
  local weather patterns

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Global warming-the debate

• The fact that the Earth is warming is not a
  matter of debate, the evidence is clear
• The fact that global carbon dioxide levels have
  increased is also clear
• The debate centers around how much of it is
  caused by human involvement vs a natural cycle
• Think about it in terms of the steps in the
  scientific method

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Global warming in the context of the scientific
method

• Observations-Earths temperature is warming
• Hypothesis-due to a man-made increase in green
  house gases
• Testing the hypothesis-many scientists hold
  different views and interpret data differently,
  but the consensus is that it is due to increases
  in greenhouse gasses from man made sources.
• Until the data is incontrovertible, there will
  always be naysayers. Thats ok, they keep us
  honest and push the method forward
• Lots of examples like this in modern science
  (evolution, big bang, cosmological
  interpretations of galaxy redshifts)