Effects of Airborne Particles on Climate: an Expert Elicitation

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Effects of Airborne Particles on Climate an
Expert Elicitation

• M. Granger Morgan, Peter J. Adams, and David W.
  Keith
• 7 March 2006

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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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Earths Energy Balance
Heat (Longwave, infrared radiation)
Sunlight (Shortwave, visible radiation)
235 Watts per square meter (W/m2)
235 Watts per square meter (W/m2)
Perturbations to energy balance are known as
radiative forcings
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Radiative Forcings

• Shortwave (incoming) or longwave (outgoing)
• Both positive (warming) and negative (cooling)
• Computed at various altitudes
• Top-of-atmosphere (TOA) most useful metric for
  global average temperature
• Surface useful metric for evaporation / changes
  to hydrological cycle

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Source IPCC Third Assessment Report
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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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Aerosols Scattering Sunlight
Dust and smoke over Australia (Terra)
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Aerosols Absorbing Sunlight
Kuwaiti oil fires
photo courtesy of Jay Apt (via Steve Schwartz)
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Aerosols and Clouds
AVHRR satellite false color image
Power plant
Lead smelter
Port
Oil refineries
Red darker clouds (large droplets) Green
brighter clouds (small droplets) Blue clear
sky
Rosenfeld, Science (2000)
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Aerosols and Clouds
Aerosol Particles
Cloud Droplets
Clean Air
Brighter, more persistent clouds
Polluted Air
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How direct is direct?

• Direct effect scattering/absorbing sunlight
• Semi-direct effect
• aerosol absorption heats atmospheric layer
• warmer air ? lower relative humidity ? less/no
  cloud
• Indirect effect modifying cloud properties
• brightness (first) effect
• lifetime (second) effect

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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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• Indirect effect(s)
• TAR figure shows brightness effect only
• lifetime effect potentially comparable
• discussion buried in text

• Semi-direct effect(s)
• not shown on TAR figure
• postulated in 2000
• discussed in text but no global estimate given

• Direct effect(s)
• best understood
• divided by aerosol type

Source IPCC Third Assessment Report
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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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Climate Change Uncertainty

• Climate sensitivity is a key parameter
• l is climate sensitivity
• 0.3 to 1 C per W/m2
• 1.5 - 4.5 C for doubling of CO2
• In climate models, representation of cloud
  feedback is largest source of uncertainty
• In retrospective studies, knowledge of aerosol
  forcing is lacking

global average temperature change
global average radiative forcing
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Aerosols and Climate Uncertainty
Aerosol GHG forcing
High sensitivity
GHG forcing
??
Low sensitivity
20th century T increase
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Aerosols and Climate Uncertainty

• Uncertainty in aerosol forcing makes testing
  climate models against 20th century temperature
  record almost meaningless
• Nevertheless all climate models do this test and
  claim good agreement as validation of their
  model
• Aerosol forcing is a tunable parameter
• High sensitivity models ? Strong aerosol cooling
• Low sensitivity models ? Weak aerosol cooling

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Challenges

• Need to characterize particle
• mass/number concentration
• size distribution 10 nm to 10 mm
• chemical composition gthundreds compounds
• mixing state
• interactions with clouds
• Highly variable in space and time

intra-hemispheric mixing
Mean CO2 residence
Mean aerosol residence
NH/SH mixing
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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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Expert Elicitation

• Granger Morgan unofficially invited by IPCC to
  survey expert opinion
• Not intended to replace peer-reviewed scientific
  studies in literature
• Usefulness
• reveal agreement/disagreement between experts
• little systematic work on uncertainty in aerosol
  forcing

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Elicitation Methodology

• Administered by mail
• 52 experts invited from broad base of expertise
  types
• Aerosols, clouds, and climate
• Modeling, experimental
• Global to micro scale
• 29 agreed
• 2 said they lacked expertise
• 3 did not complete
• 24 useable responses
• Participants acknowledged but responses are
  anonymous

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Elicitation Methodology

• Six parts
• Direct scattering/absorption of sunlight
• Semi-direct change in clouds as absorbing
  aerosols heat atmosphere
• Cloud brightness (first indirect) smaller
  droplets ? brighter clouds
• Cloud lifetime (second indirect) smaller
  droplets ? less precipitation
• Total net effect of above at top-of-atmosphere
• Surface net effect of above at surface

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Elicitation Methodology

• For each part/effect
• list top factors contributing to uncertainties
• estimate radiative forcing probability
  distributions
• upper/lower bounds
• counterfactual question
• 5/95 confidence intervals
• 25/75 confidence intervals
• best estimate
• probability uncertainty will (in 20 years)
• increase
• shrink by 0-50
• shrink by 50-80
• shrink more than 80

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Overview

• Background
• Radiative forcing
• Aerosol (airborne particles) climate effects
• Previous assessments (IPCC TAR)
• Aerosols and climate uncertainty
• Expert Elicitation
• Design
• Results
• Lessons Learned

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• Best understood
• Responses broadly consistent with IPCC TAR

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• One respondent semi-direct effect is positive
  by definition
• Absorbing aerosols above marine stratocumulus
  increase reflectivity via dynamical effects
  still semi-direct?
• Forcing or feedback?

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• Most experts mostly in 0 to -2 W m-2 range of
  IPCC TAR
• A minority suggest possible effects of -3 to -4 W
  m-2

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• Omitted from IPCC TAR
• Many reflect conventional wisdom of 0 to -2 W
  m-2
• Significant minority give wider uncertainties
• Believers in positive an enlightened minority?

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• Forward modeling estimate forcing based on
  aerosol physics
• Reverse modeling estimate aerosol forcing as
  that needed to match historical temperature trends

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Conclusions

• IPCC TAR assessment ok for what was reported
• Significant uncertainties (cloud lifetime and
  semi-direct) unreported
• Field is not mature new physical mechanisms
  being uncovered/studied, significant chances of
  uncertainty increasing
• Terminology is ambiguous (as well as confusing)
• Lines between forcings and feedbacks blurred
• Aerosols are part of the (irreducible?) climate
  uncertainty