stephen e. schwartz€¦ · forcing / od e w m-2 aod-1-83 -29 2.86 forcing per optical depth rasool...

DEEP INSIGHTS FROM SIMPLE MODELSStephen E. Schwartz

Upton NY USA

Stephen Schneider LectureGlobal Environmental Change Section

American Geophysical UnionWashington DC

December 13, 2018

https://www.bnl.gov/envsci/schwartz/ [email protected]

WHAT IS A MODEL?A model is a mathematical construct that tells us the

consequences of what we know or assume.

WHAT MAKES A SIMPLE MODEL USEFUL?Has small number of parameters. Constrained by

observations and/or theoretical understanding.

Is transparent (e.g., see fluxes through all processes).

Provides insight. Illuminates more complex situations.

Yields readily interpretable quantities (e.g., time constants).Gives the “gist” of the situation rather than “exact”

reproduction of observations.

Allows examination of the consequences of changing parameters. “What if?” scenarios, unrealistic situations.

Gives the right answer. Right for the wrong reason.

Readily allows uncertainty estimation.

THREE EXAMPLESAerosol radiative forcing

Transient climate sensitivity: The “Cold Turkey” experiment

Adjustment time of anthropogenic CO2

An increase by only a factor of 4 in global aerosol background concentration may be sufficient to reduce the surface temperature by as much as 3.5°K.

An increase by only a factor of 4 in global aerosol background concentration may be sufficient to reduce the surface temperature by as much as 3.5°K.

It is found that even an increase by a factor of 8 in the amount of CO2, which is highly unlikely in the next several thousand years, will produce an increase in the surface temperature of less than 2˚K.

2

CLEAR-SKY GLOBAL SHORTWAVE FLUX AND FORCING Dependence on aerosol optical depth

Rasool and Schneider, Science, 1971, and replotted as forcing

-80

-60

-40

-20

0

Forc

ing,

W m

-2

1.00.80.60.40.20.0Optical depth

Slope: -86 W m-2 OD-1(Cloud-free)

0 = 0.99

Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree.

DIRECT RADIATIVE FORCING BY ANTHROPOGENIC SULFATE AEROSOLGeophysics

AerosolMicrophysics

Column BurdenAtmospheric Chemistry

Aerosol Optical Depth, t

ΔF F T A R f Q Y AR T SO SO SO SO= − − ⋅ ⋅⎛

⎝⎜

⎞

⎠⎟−−

−

−- 12

RHMW

MWc sSO

S

2 21 142 2 4

242

42( )( ) ( ) ταβ

ΔFR is the global-average shortwave radiative forcing due to the aerosol, W m-2

FT is the solar constant, W m-2

Ac is the fractional cloud cover

T is the fraction of incident irradiance transmitted by the atmosphere above the aerosol

Rs is the albedo of the underlying surface

β is upward fraction of the radiation scattered by the aerosol,

αSO42− is the scattering efficiency of sulfate and associated cations at a reference low RH, m2 (g SO4

2-)-1

ƒ(RH) accounts for the relative increase in scattering due to relative humidity

QSO2 is the source strength of anthropogenic SO2 , g S yr -1

YSO42− is the fractional yield of emitted SO2 that reacts to produce sulfate aerosol

MW is the molecular weight

τSO42− is the sulfate lifetime in the atmosphere, yr

A is the area of Earth, m2

Penner et al., BAMS, 1994Factor of 5

I believe there is little foundation for the expectation that comprehensive modeling alone can provide a basis for reducing uncertainty in estimates of Faer or that somehow these models encapsulate uncertainty in understanding.

[T]heoretical justification is given for the simple model used to interpret the historical aerosol forcing.

aer

More than 20 years ago Charlson et al. (1992) used simple physical arguments to raise the specter of a relatively large but negative (–2.3 W m–2) radiative forcing by tropospheric aerosols resulting from human activities.

–2

Quantity Symbol Unit Charlson Stevens Charlson/Stevens

SO2 Source QSO2 Tg SO2 yr-1 180 130 1.38

SO42- Yield Y --- 0.4 0.62 0.65

SO42- Res Time TSO42- days 8 3.8 2.11

SO42- Burden BSO42- (gSO42-) m-2 0.046 0.031 1.48

Scat effic K m2 (gSO42-)-1 8.5 11.3 0.75

SO42- OD SO42- --- 0.039 0.035 1.14

Forcing / OD E W m-2 AOD-1 -83 -29 2.86

Clear-sky fract C --- 0.4 0.6 0.67

Forcing F W m-2 -1.3 -0.60 2.17

COMPARE CHARLSON et al. (1992) AND STEVENS (2017)ESTIMATES OF GLOBAL ANTHRO SULFATE FORCING

Rasool & Schneider (71) -86

Quantity Symbol Unit Charlson Stevens Charlson/Stevens

Forcing / OD E W m-2 AOD-1 -83 -29 2.86

FORCING PER OPTICAL DEPTH

Rasool & Schneider (71) -86

-100

-80

-60

-40

-20

0

Forc

ing

per o

ptic

al d

epth

, W m

-2/O

D55

0

1000800600400200 Radius, nm

0 = 1Rs = 0.15

550 = 0.20

BNL3 UIUC Oslo Oslo1 ULille1 BNL4

CSU Dalhousie2 Dalhousie3 NASA Ames ULille Streamer1 Streamer2 BNL1 BNL2

LMD/UW – Sunray

Dalhousie UKMO UMD

100

Based on Boucher, Schwartz, et al., JGR, 98

Model intercomparison shows lower forcing efficiency than earlier estimates.

Ammonium sulfate, RH = 80%

-27 ± 16%

-55 ± 19%-40 ± 18%

1 σ

INSIGHTS FROM THIS EXAMPLE

Sulfate direct aerosol forcing still remains highly uncertain (factor of 2).

A simple model can point the way to improved quantification.

Expressing forcing as product of factors allows examination of individual factors, quantification of uncertainties.

EQUILIBRIUM CLIMATE SENSITIVITYT = SeqF

T , change in global mean surface temperature, KF, forcing, W m-2

Seq , “equilibrium” climate sensitivity, K / (W m-2)Commonly given as 2 = F2 CO2Seq

A model that has outlived its usefulnessT , ~3 K

It is suggested that the nature of the transient response is a major uncertainty in characterizing the CO2 problem and that study of this topic should become a major priority for future research.

2

TWO COMPARTMENT ENERGY BALANCE MODEL

Deep OceanLarge Heat CapacityLong Time Constant

SW LWAtmosphereUpper Ocean

F T∆

U

L

U

T∆ U T∆ L–( )

CUdTU

dt= F TU ( TU TL )

CLdTL

dt= ( TU TL )

κ

κ

κ

Upper Compartment

LowerCompartment

Refs: Schneider, 81; Gregory, 00; Schwartz, 08, 12, 18; Held, 10; Geoffroy, 13

Flow of heat into large, deep compartment acts in parallel to emitted LW radiation to decrease temperature of upper compartment until deep compartment fills up.

MODEL PARAMETERSStr = ( + ) 1 Most important, most

uncertain, needs to be determined

Transient climate sensitivity parameter;decadal to century response

Seq = 1“Equilibrium” climate sensitivity parameter; millennial response

Less important, decadal to century, Also quite uncertain

s =CU+

Short time constant Important. 5 – 10 years on various grounds

l =CL1+

1Long time constant ~500 years; unimportanton century timescale

CLIMATE SYSTEM RESPONSE TO RAMPED FORCING

Multidecadal Millennial

On multidecadal time scale upper compartment temperature ΔT is in quasi steady state with forcing F, with slight lag (one time constant). Supports determination of transient sensitivity as Str = Tsfc / FApply to forcing and temperature change over the instrumental record.

10

0F, W

m-2 F = t

= 0.01 W m-2 yr-10.5

0.0F, W

m-2 F = t

= 0.01 W m-2 yr-1

0.5

0.0

T/F,

K/(W

m-2

)10008006004002000

Time, yr

Upper Compartment Lower Compartment

Str

Seq0.5

0.0

T/F,

K/(W

m-2

)

50403020100Time, yr

Upper Compartment Lower Compartment Str

Seq

6

0

T, K

F Seq Upper Cmpt F Str Lower Cmpt

ses, jgr, 2018

0.2

0.0

T, K F Seq

F Str Upper Compartment Lower Compartment

,

5-95%Confidence

Range

2.29

3.33

1.13

2.82

-0.09

-1.88

-0.90

Tropospheric Aerosol

0.401.13

0.070.050.05

Black carbon on snow + contrailsStratospheric H O2Tropospheric O3

Other well mixed greenhouse gases1.69

-0.15

-0.45-0.45

-0.05 Stratospheric O3

Laki

Tambora

CosiguinaKrakatau

Agung

El ChichónPinatubo

Unknown

SantaMaria

Year

TotalAnthro

Modified from IPCC AR5, 2013, Fig. 8.18

CLIMATE FORCINGS OVER THE ANTHROPOCENE

Cotopaxi

Aerosol

magnitude Low

High

forcing

Best

Aerosol forcing and uncertainty, 1750 – 2011.Three time series of aerosol forcing corresponding to Low, Best, and High estimates of aerosol forcing magnitude.

,

Laki

Tambora

CosiguinaKrakatau

Agung

El Chichón

Unknown

SantaMaria

Year

CLIMATE FORCINGS OVER THE ANTHROPOCENE

Cotopaxi

Three time series of total forcing corresponding to Low, Best, and High estimates of aerosol forcing magnitude.

Total forcing accounting for uncertainty in aerosol forcing.

Aerosol forcingmagnitude

Low Best High

Pinatubo

Scale of forcing is adjusted tomatch forcing curve to temperature record.

Black curve denotes observed temperature record.

Green curve denotes best estimate of total forcing.

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

cha

nge,

K

-3

-2

-1

0

1

2

3 Total forcing, W m

-2

Best

Cotopaxi

Krakatau

SantaMaria

AgungPinatubo

ElChichón

0.35

Referenceperiod

Transient sensitivity is ratio oftemperature change to forcing.

2000197519501925190018751850

Transient climate sensitivity, K/(W m-2 )

SCALING OF TOTAL FORCING TIME SERIES

TO OBSERVED TEMPERATURE RECORD TO OBTAIN TRANSIENT CLIMATE SENSITIVITY

Aerosol forcing magnitudeand climate sensitivity

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

cha

nge,

K

-2

-1

0

1


-2

High0.55

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

cha

nge,

K-3

-2

-1

0

1

2


-2

Best

Cotopaxi

Krakatau

SantaMaria

AgungPinatubo

ElChichón

0.35

-1.0

-0.5

0.0

0.5

1.0Te

mpe

ratu

re c

hang

e, K

2000197519501925190018751850

-3-2-10123

Total forcing, W m

-2

Low3.8

-3.8

0.27Observed temperature record

is closely matched by quitedifferent forcings.

Black curves denote observed temperature record.

RGB curves denote estimated total forcing.

Referenceperiod



Transient climate sensitivityK/(W m-2)

Aerosol forcing magnitudeand climate sensitivity

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

cha

nge,

K

-2

-1

0

1


-2

High0.55

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

cha

nge,

K-3

-2

-1

0

1

2


-2

Best

Cotopaxi

Krakatau

SantaMaria

AgungPinatubo

ElChichón

0.35

-1.0

-0.5

0.0

0.5

1.0Te

mpe

ratu

re c

hang

e, K

2000197519501925190018751850

-3-2-10123

Total forcing, W m

-2

Low3.8

-3.8

0.27

Purple curves denote temperaturerecord calculated with model.

Black curves denote observed temperature record.

RGB curves denote estimated total forcing.

Observed temperature record is accurately reproduced for differing forcings compensated by differing sensitivities.

EQUIFINALITY

Referenceperiod

Transient climate sensitivityK/(W m-2)



ses, jgr, 18

-1.0

-0.5

0.0

0.5

1.0

Tem

pera

ture

ano

mal

y, K

2000197519501925190018751850

Sensitivity and aerosol forcingHighBestLow

1.5 HadCRUT4

GISTEMPNOAA MLOSTCMIP5 mean

Observed (black); Simulated, CMIP5 (thin colors); CMIP5 mean (cyan)GLOBAL MEAN SURFACE TEMPERATURE CHANGE

Simulated, two-compartment model (thick colors)

Two-compartment energy-balance model compares well with CMIP5 models. The three Forcing-Sensitivity pairs do comparably well over the time record.

Modified fromAR5 (2013)Fig. 9.8a

Abrupt cessation of fossil fuel combustion.

Limiting case for phasing out fossil fuels.

Lower bound to expected increase in global temperature if emissions continue.

THE “COLD TURKEY”EXPERIMENT

Assume abrupt cessation of anthropogenic sources of CO2 and aerosols. What is the forcing? 2

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

F, W

m-2

100806040200Time after cessation, yr

High

Best

Low

Climate sensitivity andaerosol forcing magnitude

TroposphericAerosolForcing

+0.09

+0.90

+1.88

Cessation of negative aerosol forcing results in step-function positive increase in forcing.The magnitude of this forcing is highly uncertain.

AEROSOL FORCING CHANGE AFTER ABRUPT CESSATION OF EMISSIONS

DECAY OF EXCESS ATMOSPHERIC CO2 AFTER CESSATION OF EMISIONS

Calculated and redrawn from recent publications Abrupt Cessation

Convolution of IRF

1.0

0.9

0.8

0.7

0.6

CO2 a

tmos

pher

ic fra

ctio

n

403020100Time after cessation, years

F, W m

-2

70

100

200

500-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.68

0

Matthews 08 2000 PgC Plattner 08 CLIMBER2 Cao 10 Matthews 08 500 PgC Zickfeld 12 Solomon 09 Knutti 12 Gillett 11 Frölicher 10 Hare 06

Allen 09 Zickfeld 13 Ensemble RCP6

Joos 13 Bern3D LPJ Ref Matthews 08 Pulse 2000 PgC Joos 13 CLIMBER2 LPJ Joos 13 Multimodel mean Matthews 08 Pulse 500 PgC Joos 13 NCAR CSM1.4 Joos 13 MPI ESM

Adjustment times,years

Current estimates vary by an order of magnitude!

2

ses, jgr, 18

-1.4

-1.0

0.0


70200

500

τCO2, yrSources of anthropogenic CO2 → 02

CO2 FORCING CHANGE AFTER ABRUPT CESSATION OF EMISSIONS

F, W

m-2

Forcing of incremental CO2 is fairly certain.2Rate of decrease of excess CO2 following cessation of emissions is quite uncertain.

2

2

-1.4

-1.0

0.0


70200

500

τCO2, yr

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0F,

W m

-2


Decreasing CO2High

Best

Low

Climate sensitivity andaerosol forcing magnitude

70

200

70

200

70200

500

500

500

Constant CO2τCO2, yr

Sources of anthropogenic CO2 → 02

TOTAL FORCING CHANGE AFTER ABRUPT CESSATION OF EMISSIONS

Total forcing change is uncertain even in sign. If aerosol forcing is large, would be positive for over a century. If aerosol forcing is small, would go negative very quickly (5 – 15 years).

F, W

m-2

Sources of anthropogenic CO2 → 02

TEMPERATURE CHANGE AFTER ABRUPT CESSATION

If CO2 emissions abruptly halted (and aerosols held constant), temperature would increase slightly and level off or decrease, depending on sensitivity and CO2 decay rate.

1.0

0.5

0.0

-0.5

ΔT

rela

tive

to c

essa

tion,

K


2.5

2.0

1.5

1.0

0.5ΔT relative to preindustrial, K

500 200 70

τCO2, yr ∞

Climate sensitivityHigh Best Low

-0.6

∞

1.5

2

2

Infinite-time valuesat constant CO2

Sources of anthropogenic CO2 → 02 Sources of anthro aerosols → 0Sources of anthropogenic CO2 → 02

TEMPERATURE CHANGE AFTER ABRUPT CESSATION

Temperature change is likewise uncertain even in sign. If aerosol forcing is large, ΔT would be large and positive for over a century. If aerosol forcing is small, ΔT might go negative in 20 – 100 yr.

1.5

1.0

0.5

0.0

-0.5100806040200

Time after cessation, yr

2.5

2.0

1.5

1.0

0.5

ΔT relative to preindustrial, K ∞ 500

200 70

τCO2, yr

-0.6

Climate sensitivity and aerosol forcing magnitude

High Best Low

ΔT

rela

tive

to c

essa

tion,

K1.0

0.5

0.0

-0.5

ΔT

rela

tive

to c

essa

tion,

K


2.5

2.0

1.5

1.0

0.5ΔT relative to preindustrial, K

500 200 70

τCO2, yr ∞

Climate sensitivityHigh Best Low

-0.6

∞

1.5


The simple two-compartment model matches the historical temperature record “pretty well” for quite different forcings compensated by quite different transient sensitivities: Equifinality.

Aerosol forcing over the Anthropocene and transient climate sensitivity are quite uncertain but coupled: large forcing, large sensitivity.

Transient sensitivity is a useful model; more useful than “equilibrium sensitivity.”

. . .

The two-compartment model can be readily used to examine consequences of abrupt cessation of emissions. Within current uncertainty in forcing, the committed increase in global temperature over a decade would range from minimal to 1.3 K. This has major implications, for example whether or not 2 K temperature increase above preindustrial can be achieved.

INSIGHTS FROM THIS EXAMPLEcont’d

Uncertainty is an uncomfortable position...

But certainty is an absurd one.

– Voltaire



Convolution of IRF

1.0

0.9

0.8

0.7

0.6

CO2 a

tmos

pher

ic fra

ctio

n


F, W m

-2

70

100

200

500-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.68

0





Current estimates vary by an order of magnitude!

2

ses, jgr, 18

PreindustrialAnthropogenic

perturbation9.9 ± 0.51.4 ± 0.7

3700 – 420

Surface sediment150

119.4120

Respiration

Gross primaryproductivity Land

sinkLand use change 70.670

Stock, Pg CFlux, Pg C yr -1

Mixed-layer ocean

1.8 ± 0.6

Annual change, Pg C yr -1

906.1+ 32.2 +0.6100 m

3583 m

32.0 29.8

2032 + 73

Deep ocean35,917 + 128

44.750

5.3 ± 1.855.149.8

Concentration, µmol kg-1

2250 + 9

0.2

3.6 ± 1.3

+1.5

0.6Ftm

Sd

FmaFam

FmdFdm Fpc

Sm

QluFtaFat

QffFat

Fam Fma

Fmd Fdm

Marine biota3

3700 – 422Fossil fuels & cement

+2.1

Total ocean36,823 + 160

+5.5 ± 0.8589.4 + 269.2AtmosphereSa

2300 – 228 + 219

Vegetation, soil and detritusSt

Surface sediment150

± 0.6

CO2 STOCKS, FLUXES, AND ANNUAL CHANGE2

ses, in prep.modified (considerably) from AR4 (2007), Fig. 7.3

after Sarmiento & Gruber, Phys. Today (2002)

Preindustrial

3700 – 420

Surface sediment150

0.6

Stock, Pg CFlux, Pg C yr -1

Mixed-layer ocean906.1

100 m

3583 m

2032

Deep ocean35,917

5.3

5.35.3

Concentration, µmol kg-1

2250

0.6Ftm

Sd

Fma,net

Fdm,net

Fpc

Sm

Q lu Qffkatkam kma

kmd kdm

Marine biota

3700 Fossil fuels & cement

Total ocean36,823

589.4 AtmosphereSa

2300

Vegetation, soil and detritusSt

at,netF0.6

TRANSFER COEFFICIENTS FOR ANTHRO CO22

kam = Fam,pi / Sa,pi; global mean deposition velocity

kmdzm = kdmzd = vp; global mean piston velocity, 5.5 m yr-1

+0.6

+1.5

+5.5

k by differenceat Q tot - dSa/dt( )/Sa,ant ] 2016= [ - dSm/dt - dSd/dt

Annual change, Pg C yr -1

from obs’d global heat uptake rate

all gasesacid dissocchemistryKam = (dSa/dSm)eq , a known function of Sa, 5–10kma = kamKam;

Transfer coefficients, yr-1-1

0.0130.12 0.6–1.2

0.055 0.0015

Anthropogenicsources

HISTORICAL GLOBAL ANTHRO CO2 EMISSION2

Boden, Houghton, tabulated by Le Quéré et al., ESSD, 18

12

10

8

6

4

2

0

CO

2 so

urce

, Pg

yr-1

200019501900185018001750Year

Total Fossil + cement Land use change

(Linear ramp 1750-1850)

1-sigma

MODELED CO2 MIXING RATIO2Comparison with measurements

440

420

400

380

360

340

320

300

280

CO

2, pp

m

2100200019001800 Year

278

Model does “pretty good job.”Does not capture the “flattening” of CO2 in the 1940’s.2

Model Law Dome, Cape Grim Global CO2 measmts

Etheridge 96, 01Dlugokencky 17Keeling

Abrupt cessation of emissions leads to rapid decrease of CO2. 2

MODELED CO2 MIXING RATIO2

440

420

400

380

360

340

320

300

280

CO

2, pp

m

2100200019001800 Year

Model Law Dome, Cape Grim Global CO2 measmts Exponential decay fit = 60.4 yr

278

Decay of excess CO2 is well fit by exponential. 2

“Cold turkey” experiment: Abrupt cessation of emissions

NEAR EQUILIBRIUM BETWEEN ATMOSPHERE AND OCEAN MIXED LAYER

40353025201510

50

-5

Mix

ed la

yer s

tock

, Pg

C

2100200019001800Year

Anthropogenic Mixed-Layer Stock

Equilibrium Modeled Difference

Time constant for equilibration of atmosphere and ocean mixed layer is short, ~ 1 yr.

These two compartments are thus in near equilibrium and are usefully considered a single compartment.

Usefully defined as sum of anthro atmospheric and mixed-layer stocks upon sum of net fluxes to deep ocean and terrestrial biosphere.

Virtually constant (~ 60 yr) over Anthropocene, increasing to ~ 80 yr as deep ocean fills up.

ADJUSTMENT TIME FOR DECREASE OF ANTHROPOGENIC CO22

Agrees with τ from decay following abrupt cessation.

100

80

60

40

20Rem

oval

tim

e co

nsta

nt

, yr

2100200019001800Year

=Sa,ant+Sm,ant

Fat +Fmd,net

Abrupt cessationof emissions

Assumed terrestrial sink with constant transfer coefficient is supported by agreement of measured and modeled atmospheric sink rate over entire simulation.

7

6

5

4

3

2

1

0

-1Atm

os s

ink

rate

, Pg

C y

r-1

400380360340320300280CO2, ppm

ModelLaw Dome, Cape Grim Global CO2 measmts

Uncertainty fromuncertainty in emission

and variation in atmospheric growth rate

ATMOSPHERIC SINK: Q - dSa/dta

MODELED CO2 MIXING RATIO2

Solution: Decrease emission to equal present sinks into deep ocean and terrestrial biosphere (~ half of present emission).

“Warm turkey” experiment440

420

400

380

360

340

320

300

280

CO

2, pp

m

2100200019001800

278

YearObjective: Stabilize CO2 at present value.2

Model Law Dome, Cape Grim Global CO2 measmts



Convolution of IRF

1.0

0.9

0.8

0.7

0.6

CO2 a

tmos

pher

ic fra

ctio

n


F, W m

-2

70

100

200

500-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.68

0





Adjustment time (60 yr) is much shorter than prior values.

2

This Study


. . .

The historical CO2 budget can be accurately represented by a three- (or two-) compartment model with independently determined, observationally based transfer coefficients.

2

The adjustment time of excess atmospheric CO2 is found to be about 60 years. If emissions were abruptly halted, excess CO2 would decrease with half-life of about 42 years.

22

Atmospheric CO2 could be stabilized at present value by halving current emissions.

2

The adjustment time found here is much shorter than most present estimates.

This would be good news for strategies to meet climate change targets.

INSIGHTS FROM THIS EXAMPLE cont’d

On the one hand, as scientists we are ethically bound to the scientific method, in effect promising to tell the truth, the whole truth, and nothing but - which means that we must include all the doubts, the caveats, the ifs, ands, and buts. On the other hand, we are not just scientists but human beings as well. And like most people we'd like to see the world a better place, which in this context translates into our working to reduce the risk of potentially disastrous climatic change. That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have. To do that we need to get some broadbased support, to capture the public's imagination.

STEVE SCHNEIDER ONSCIENTIST AS ADVOCATE

THE ROLE OF THE SCIENTIST MY OWN VIEW

Our highest obligation as scientists is to the truth as we understand it. We must report our findings and our understanding honestly, accurately, and fully, including the uncertainties and their implications.

We should express our understanding in the simplest terms possible.

This extends to the societal implications of our findings. We should convey these implications without exaggeration. To do otherwise is to undermine public trust in the scientific enterprise.

stephen e. schwartz€¦ · forcing / od e w m-2 aod-1-83 -29 2.86 forcing per optical depth rasool...

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