Effects of Airborne Particles on Climate:

an Expert Elicitation

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

7 March 2006

2

Overview

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

Expert Elicitation• Design• Results

Lessons Learned

3

Overview

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

Expert Elicitation• Design• Results

Lessons Learned

4

Earth’s Energy Balance

Sunlight (Shortwave, visible radiation)

235 Watts per square meter (W/m2)

Heat (Longwave, infrared radiation)

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

Red: darker clouds (large droplets)

Green: brighter clouds (small droplets)

Blue: clear sky

Power plant

Lead smelter

Port

Oil refineries

Rosenfeld, Science (2000)

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Aerosols and Clouds

Clean Air

Polluted Air

Aerosol Particles

Cloud Droplets

Brighter, more

persistent clouds

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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”

13

Overview

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

Expert Elicitation• Design• Results

Lessons Learned

14

Source: IPCC Third Assessment Report

Indirect effect(s):

•TAR figure shows “brightness” effect only

•“lifetime” effect potentially comparable

•discussion buried in text

Direct effect(s):

•best understood

•divided by aerosol type

Semi-direct effect(s):

•not shown on TAR figure

•postulated in 2000

•discussed in text but no global estimate given

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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

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

FT global average temperature

change

global average radiative forcing

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Forcing (W m-2)

Tem

per

atu

re C

han

ge

(K)

Aerosols and Climate Uncertainty

High sensitivity

Low sensitivity

GHG forcing

20th century T increase

Aerosol + GHG forcing

??

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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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NH/SH mixing

intra-hemispheric

mixing

Challenges

Need to characterize particle• mass/number concentration• size distribution: ~10 nm to 10 m• chemical composition: >hundreds compounds• mixing state• interactions with clouds

Highly variable in space and time:

centurydecadalannualdaily monthlyhourly

Mean CO2

residenceMean

aerosol

residence

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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 parts1. Direct: scattering/absorption of sunlight

2. Semi-direct: change in clouds as absorbing aerosols heat atmosphere

3. Cloud brightness (first indirect): smaller droplets → brighter clouds

4. Cloud lifetime (second indirect): smaller droplets → less precipitation

5. Total: net effect of above at top-of-atmosphere

6. Surface: net effect of above at surface

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Elicitation Methodology

For each part/effect:a) list top factors contributing to uncertaintiesb) estimate radiative forcing probability distributions

upper/lower bounds “counterfactual” question 5/95% confidence intervals 25/75% confidence intervals best estimate

c) 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

•Best understood

•Responses broadly consistent with IPCC TAR

•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?

•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

•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?

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

0

+1

-1

-2

-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Surface Forcing

-4

-5

-6

-7

-9

-8

-10

NA

NA

to -12

NA

?

?

16 17 18 19 20 21

NA

NA

NA

NA

22 23 24

N+A

4 4 4 4 5 6 6 5 6 6 74 2 1 0 2 2 5 3

ExpertLevel of expertise 0 - 7

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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