Climate Sensitivity, IPCC, and Future Climate Projections

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Climate Sensitivity, IPCC, and Future Climate
Projections
The IPCC AR4 2006 Report
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Take away ideas

• What is Radiative Forcing?
• What is Climate Sensitivity?
• Who what is the IPCC?
• IPCC Scenarios and Projections
• How much will climate warm due to future
  emissions?
• How does this future warming compare to the
  Earths geologic past?

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What is Radiative Forcing?

• Radiative forcing An imposed change in Earths
  radiative energy balance. Measured in Watts per
  square meter (W/m2)
• Radiative because these factors change the
  balance between incoming solar radiation and
  outgoing infrared radiation within the Earths
  atmosphere. This radiative balance sets the
  Earths surface temperature.
• Forcing indicates that Earths radiative
  balance is being pushed away from its normal
  state.
• Examples Solar variability, volcanic emissions,
  greenhouse gases, ozone, changes in ice cover
  (albedo), land use changes.

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Earths temperature with no greenhouse
effectTeffective 254.8K (-18C)

• Set Sunlight Earthlight
• Solve for Tearth
• Eqn. 3.1 in Archer Chapter 3

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Cool Science Using Earthshine to measure
albedo changes

• Scientists look at earthshine - the earths
  sunlight reflection on the new moon surface.
• Changes in earthshine used to estimate changes
  in Earth albedo.
• Volcanic eruption higher albedo more
  Earthshine.

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Volcanic eruption can change albedo by 1

• ? 30 on average
• Teffective 254.8K
• Increase ? to 31
• New Teffective 253.9K
• or -1C cooler due a volcanic eruption

Recalling I (1-a) s T4
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Adding an atmosphere
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Greenhouse gases are selective absorbersof
outgoing long wavelength radiation (Earthlight)
Spectrum of IR light emitted from earth to
space
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Water Vapor Molecule (H2O)Vibrational modes
H2O stretch
H2O bend
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Carbon Dioxide Molecule (CO2)Vibrational mode
(15µm)
CO2 bend
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Natural CO2 radiative forcing
Makes Earth habitable Pre-Industrial CO2 level
of 280 ppm Increases surface temperature from
-18C (effective temperature) to 15C (Water
vapor is also important)
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CO2 Band SaturationMore CO2 warms the Earth
less and less
10 ppm
No CO2
Notice the CO2 absorption band
1000 ppm
100 ppm
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CO2 and surface warming just due to radiation
changes - no feedbacks

• About 1C per 100 ppm
• Pre-Industrial 280 ppm
• Today 390 ppm
• So, w/o feedbacks 1.2C
• With feedbacks 3C
• (Feedbacks include water vapor and sea ice
  changes)

Temp (K)
CO2 ppm
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Atmospheric CO2
CO2 (ppm)
CO2 has increased by about 40  Long term
average growth rate is 1.4 per year  Last
decade growth rate is 2.0 per year
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All Radiative Forcing factors (1750-2005)
Sum 1.6 W/m2
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Radiative Forcing Contributions

• GHGs warm (CO2, CH4, N2O)
• H2O (vapor) warms
• Tropospheric O3 warms, Strat O3 cools
• Human and natural Aerosols cool
• Solar irradiance warms
• Net Effect 1.6 W/m2

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But how much warmer will it get?

• Climate Sensitivity Amount of warming for a
  given radiative forcing
• Depends not only on CO2 but also related climate
  changes such as water vapor, clouds, sea ice,
  aerosols
• Average is 0.7 Kelvins / (W/m2)

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Natural and Human RF Forcing
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Intergovernmental Panel on Climate Change (IPCC)

• Established by the UN and World Meteorological
  Organization in 1988.
• Role of the IPCC to assess the scientific,
  technical, and socio-economic information
  relevant for the understanding of the risks of
  human-induced climate change.
• Assessments based on published and peer-reviewed
  literature

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IPCC Reports

• 2,500 of the worlds leading climate scientists
  and technical experts contribute to reports
• Provides comprehensive and balanced assessments
  of climate change science, impacts, and
  adaptation and mitigation options
• Extensive peer-review and governmental review
  ensures scientific credibility and policy
  relevance
• 1st report (FAR 1990)
• 2nd Report (SAR 1995)
• 3rd Report (TAR 2001)
• 4th Report (AR4 2007)
• Projections of future climate based on differing
  emissions scenarios (future population, economy,
  technology, regional-global political systems)

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IPCC Assessment report foci
1990
1995
2001
2007
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IPCC ReportsWriting, Review, Publication
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IPCC AR4 (2007)
1) GW Basis, 2) Impacts, 3) Mitigation
AR4 Volume 1 has the GW story - what did it say?
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An aside Skeptics on the offensive

• ExMo Offered 10,000 to scientists and economists
  to discredit / challenge the IPCC AR4 report.
• Skeptic IPCC report published at same time,
  similar layout/appearance.
• Sen. Inhofe (R, OK) Climate Hoax.
• Michael Crighton Climate of Fear conspiracy.
• Rep. Joe Barton (R, TX) Senate hearing /
  intimidation of climate scientists (2006).

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Why is there additional warming already in the
pipeline?

• The Earth is in radiative imbalance due to
  greenhouse gases some warming has happened, more
  to come.
• Earth isnt at equilibrium, due mainly to the
  long response time of the oceans.
• Imbalance is 0.7 W/m2 (or about 0.4C), adds to
  existing 1.6 W/m2
• This addl warming is there even if we shut off
  emissions today.

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What is Climate Sensitivity?

• The equilibrium temperature change (in C) for
  given radiative forcing (in W/m2) change.
• Estimates based on basic radiative equations as
  well as climate model
• CS is about 0.5 to 0.8 C per W/m2 - But very
  uncertain
• Current climate 1C for 1.6 W/m2
• Future climate (2xCO2) _3_C for _6_ W/m2

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IPCC Emission Scenarios

• A1 storyline and scenario family
• a future world of very rapid economic growth,
  global population that peaks in mid-century and
  declines thereafter, and rapid introduction of
  new and more efficient technologies.
• A2 storyline and scenario family
• a very heterogeneous world with continuously
  increasing global population and regionally
  oriented economic growth that is more fragmented
  and slower than in other storylines.
• B1 storyline and scenario family
• a convergent world with the same global
  population as in the A1 storyline but with rapid
  changes in economic structures toward a service
  and information economy, with reductions in
  material intensity, and the introduction of clean
  and resource-efficient technologies.
• B2 storyline and scenario family
• a world in which the emphasis is on local
  solutions to economic, social, and environmental
  sustainability, with continuously increasing
  population (lower than A2) and intermediate
  economic development

http//sedac.ciesin.columbia.edu/ddc/sres/
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IPCC Scenarios
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How are we doing so far?
Current emissions are higher than even the
largest growth scenario (A1)
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Estimating Future Climate ChangesComputer
Climate Models

• Earth is divided up into 3-D grid boxes.
• Climate models are systems of differential
  equations based on the basic laws of physics,
  fluid motion, and chemistry.
• Models calculate winds, heat transfer, radiation,
  relative humidity, and surface hydrology within
  each grid and evaluate interactions with
  neighboring points.

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SRES - Special Report Emissions Scenarios
2xCO2
550 ppm
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Current and 2xCO2 RF
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Radiative ForcingProjections

• Current (2005) forcing is 1.6 W/m2
• Committed Forcing is 1.6 0.7 2.3 W/m2
• By 2100 Radiative Imbalance is 6 to 7 W/m2

Longwave forcing (GHG)
Shortwave forcing (Sun)
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IPCC AR4 Climate Projections
A2 scenario warmest B1 Scenario least warm All
scenarios WARMER
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Dangerous Climate Change

• A future level of warming sufficiently great to
  push climate to a tipping point.
• What value should this be? Can estimate from the
  geologic record.
• Level 2C above modern temperatures
• Why? Warmer than any time in last three million
  years (Pliocene) when world was considerably
  warmer than today.

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What are Tipping Points?

• When pushed beyond a certain threshold, climate
  cannot return to its prior state.
• Feedbacks accelerate climate toward tipping
  points

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Why Dangerous?

• Examples of Tipping point feedback processes
• Ice-albedo feedback
• Ice-sea-level feedback
• Salinity-Ocean circulation feedback
• Ocean temperatures - Drought
• The possibility of unknown feedbacks

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What CO2 level is Dangerous?

• Depends of Climate Sensitivity!
• Current estimate for this level is 2C, or 450
  ppm.
• Today 390 ppm, 2 ppm/year growth rate.

About 3010 years from now.
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When was the last time CO2 was near 600 - 1000
ppm?
cold

• a long, long time ago...
• 25-35 million years.

warm
Data source Zachos et al., 2001
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35 million years ago
Today
Ice free
Arctic
Ice free
Antarctic
6-8C warmer Sea level was 60 meters
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When was the last time CO2 levels were near 400
ppm?

• a long time ago...
• 3-4 million years.

fossil beaver den in the arctic
Arctic was 10-15C warmer Global Temps 2C
warmer Sea level was 20m higher
Data source Tedford and Harrington, 2003