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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. Overview. Background Radiative forcing Aerosol (airborne particles) climate effects Previous assessments (IPCC TAR) Aerosols and climate uncertainty

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effects of airborne particles on climate an expert elicitation

Effects of Airborne Particles on Climate: an Expert Elicitation

M. Granger Morgan, Peter J. Adams, and David W. Keith

7 March 2006

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

2

overview3
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

3

earth s energy balance
Earth’s 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”

4

radiative forcings
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

5

slide6
6

Source: IPCC Third Assessment Report

overview7
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

7

aerosols scattering sunlight
Aerosols Scattering Sunlight

Dust and smoke over Australia (Terra)

8

aerosols absorbing sunlight
Aerosols Absorbing Sunlight

Kuwaiti oil fires

9

photo courtesy of Jay Apt (via Steve Schwartz)

aerosols and clouds
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

10

Rosenfeld, Science (2000)

aerosols and clouds11
Aerosols and Clouds

Aerosol Particles

Cloud Droplets

Clean Air

Brighter, more persistent clouds

Polluted Air

11

how direct is direct
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”

12

overview13
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

13

slide14

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

14

Source: IPCC Third Assessment Report

overview15
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

15

climate change uncertainty
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

16

aerosols and climate uncertainty
Aerosols and Climate Uncertainty

Aerosol + GHG forcing

GHG forcing

High sensitivity

??

Low sensitivity

20th century T increase

17

aerosols and climate uncertainty18
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

18

challenges

hourly

daily

monthly

annual

decadal

century

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

intra-hemispheric mixing

Mean aerosolresidence

Mean CO2 residence

NH/SH mixing

19

overview20
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

20

expert elicitation
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

21

elicitation methodology
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

22

elicitation methodology23
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

23

elicitation methodology24
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%

24

overview25
Overview
  • Background
    • Radiative forcing
    • Aerosol (airborne particles) climate effects
    • Previous assessments (IPCC TAR)
    • Aerosols and climate uncertainty
  • Expert Elicitation
    • Design
    • Results
  • Lessons Learned

25

slide26

Best understood

  • Responses broadly consistent with IPCC TAR
slide27

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?
slide28

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
slide29

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?
slide30

“Forward” modeling: estimate forcing based on aerosol physics

  • “Reverse” modeling: estimate aerosol forcing as that needed to match historical temperature trends
conclusions
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

32