Future climate projections, Climate Sensitivity, and the "dangerous climate change"

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Future climate projections, Climate Sensitivity,
and the "dangerous climate change"
The IPCC AR4 2006 Report
V1003 - Science and Society
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Take away ideas

• Who What is the IPCC?
• What is Radiative Forcng?
• How much will climate warm from GHG emissions?
• Whats in the pipeline? Why?
• What is Climate Sensitivity?
• IPCC Scenarios and Projections
• How much CO2 emission is too much?

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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)

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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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Atmospheric CO2
CO2 (ppm)
CO2 has increased by about 30  Long term
average growth rate is 1.4 per year  Last
decade growth rate is 2.0 per year What is
current CO2 level? When will CO2 double?
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What is Radiative Forcing?

• Radiative forcing is a measure of how the energy
  balance of the Earth-atmosphere system is
  influenced when factors that affect climate are
  altered.
• Radiative because these factors change the
  balance between incoming solar radiation and
  outgoing infrared radiation within the Earths
  atmosphere. This radiative balance controls the
  Earths surface temperature.
• Forcing to indicate that Earths radiative
  balance is being pushed away from its normal
  state.
• Measured in Watts per square meter (W/m2)

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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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Current RF is equivalent to 1-2 mini-lights on
every square meter of the planet
Today 1.6 W/m2 Committed 1.6 0.7 2.3
W/m2 By 2100 6 to 7 W/m2
Jim Hansen 2004
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Natural and Human RF Forcing
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Why is there additional warming already in the
pipeline?

• The Earths energy balance, isnt.
• Earth isnt at equilibrium.
• 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 C per W/m2 - But very uncertain
• Current climate 0.8C for 1.6 W/m2
• Future climate (2xCO2) _3_C for _6_ W/m2

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IPCC Emission Scenarios

• The A1 storyline and scenario family describes a
  future world of very rapid economic growth, low
  population growth, and the rapid introduction of
  new and more efficient technologies. Major
  underlying themes are convergence among regions,
  capacity building, and increased cultural and
  social interactions, with a substantial reduction
  in regional differences in per capita income.
• The A2 storyline and scenario family describes a
  very heterogeneous world. The underlying theme is
  self-reliance and preservation of local
  identities. Fertility patterns across regions
  converge very slowly, which results in high
  population growth. Economic development is
  primarily regionally oriented.
• The B1 storyline and scenario family describes a
  convergent world with the same low population
  growth 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. Global
  solutions to economic, social, and environmental
  sustainability, including improved equity, but
  without additional climate initiatives.
• The B2 storyline and scenario family describes a
  world in which the emphasis is on local solutions
  to economic, social, and environmental
  sustainability. It is a world with moderate
  population growth, intermediate levels of
  economic development, and less rapid and more
  diverse technological change than in the B1 and
  A1 storylines. While the scenario is also
  oriented toward environmental protection and
  social equity, it focuses on local and regional
  levels.

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Estimating the Future

• Can estimate future emissions using some
  assumptions.
• Need to know Climate Sensitivity.
• Use historical ?T and ?RF to estimate CS.
• Use physical models (Climate Models) to calculate
  effects of GHGs and their feedbacks on global
  climate.
• 23 different models.

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SRES - Special Report Emissions Scenarios
2xCO2
450 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 (2xCO2) Radiative Imbalance is 6 to 7
  W/m2

Longwave forcing (GHG)
Shortwave forcing (Sun)
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IPCC AR4 Climate Projections
2xCO2 3C 2.0 - 4.5 C range
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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? A subjective call but
  can be estimated.
• 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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Why Dangerous?

• Ice-albedo feedback
• Ice-sea-level feedback
• Salinity-Ocean circulation feedback
• Glacier-water supply link
• Hydrological shifts
• unknowns

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What CO2 level is this?

• Depends of Climate Sensitivity!
• Current estimate for this level is 450 ppm.
• Best guess is about 3010 years from now.

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Some perspective

• 450 ppm 570 Gt carbon
• Weve already emitted 300 Gt
• So only about 300 Gt more until the limit. At
  the current emission rate, about 30-40 years.
• Business-as-usual scenario 1600 Gt by 2100.
• Emission reductions prudent.

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QUIZ

• Why is Radiative Forcing?
• What is Climate Sensitivity?
• Why do they matter?