stephen e. schwartz€¦ · forcing / od e w m-2 aod-1-83 -29 2.86 forcing per optical depth rasool...
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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.
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.
THREE EXAMPLESAerosol radiative forcing
Transient climate sensitivity: The “Cold Turkey” experiment
Adjustment time of anthropogenic CO2
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 Total forcing, W m
-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
3 Total forcing, W m
-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
SCALING OF TOTAL FORCING TIME SERIES
TO OBSERVED TEMPERATURE RECORD TO OBTAIN TRANSIENT CLIMATE SENSITIVITY
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 Total forcing, W m
-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
3 Total forcing, W m
-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)
SCALING OF TOTAL FORCING TIME SERIES
TO OBSERVED TEMPERATURE RECORD TO OBTAIN TRANSIENT CLIMATE SENSITIVITY
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
100806040200Time after cessation, yr
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
100806040200Time after cessation, yr
70200
500
τCO2, yr
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0F,
W m
-2
100806040200Time after cessation, yr
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
100806040200Time after cessation, yr
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
100806040200Time after cessation, yr
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
INSIGHTS FROM THIS EXAMPLE
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
THREE EXAMPLESAerosol radiative forcing
Transient climate sensitivity: The “Cold Turkey” experiment
Adjustment time of anthropogenic CO2
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
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
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
Adjustment time (60 yr) is much shorter than prior values.
2
This Study
INSIGHTS FROM THIS EXAMPLE
. . .
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.