OVERSHOOT PATHWAYS TO CO2
CONCENTRATION STABILIZATION[Formerly ‘THE EFFECT OF CARBON-CYCLE CLIMATEFEEDBACKS ON THE EMISSIONS REQUIREMENTS FOR CO2
STABILIZATION’]

Tom Wigley,
National Center for Atmospheric Research,Boulder, CO 80307, USA
wigley@ucar.edu
Presented at:
Workshop on GHG Stabilization Scenarios
Tsukuba, Japan
Jan. 23, 2004

SUMMARY• Description of revised carbon cycle model• Production of new stabilization profiles accounting forclimate feedbacks• Emissions requirements to follow new profiles• Quantification of the effect of climate feedbacks onthese requirements• Alternative pathways to concentration stabilizationBACKGROUND:(1) The first CO2 stabilization profiles that attempted to accountrealistically for economic constraints were the WRE profiles (Wigley,Richels and Edmonds, Nature 379, 240–243, 1996).

(2) These profiles assumed that concentrations could, initially, only
depart slowly from a baseline, no-climate-policy scenario.
(3) They also assumed that the date for the beginning of asignificant departure was later for higher stabilization targets.
(4) Emissions requirements were determined using an inverse
carbon cycle model.
(5) The original carbon cycle was calibrated to reflect the state of
the science in 1995/6, and did not account for climate-related
feedbacks on the carbon cycle.

ORIGINAL (WRE) STABILIZATION PROFILES
800
WRE750750

CO2 CONCENTRATION (ppm)

700

BASELINE
WRE650650600

CONST EFOSS(2000)
WRE550550500

Stabilization points

WRE450450400Departure points

350
1990 2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210 2230 2250
YEAR

REVISED CARBON-CYCLE MODELQUESTION: How does the revised MAGICC carbon
cycle model compare with the other two models used inthe IPCC TAR?

Table 1: Comparison of climate feedbacks: 2100
concentrations (ppm) for the IS92a emissions scenario.

No climatefeedbacks

With climatefeedbacks

Increase due
to climatefeedbacks

Bern

651706

55

ISAM

682

723

41

MAGICC

67571540

MODEL

CONSTRUCTING NEW STABILIZATION PROFILES
Changes are required because …..
• there are new baseline no-policy scenarios,• there are improved carbon cycle models, and• these models now account for climate feedbacks on thecarbon cycle.

SUMMARY OF NEW CONCENTRATION PROFILES
800750750
P50 BASELINE

CO2 CONCENTRATION (ppm)

700
650

6506005505505004504504003503503002000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300

YEAR

REVISED EMISSIONS REQUIREMENTS FOR STABILIZATION
18P50 BASELINE
16FOSSIL CO2 EMISSIONS (GtC/yr)

14750126501085506450423500-22000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300

YEAR

QUANTIFYING THE CLIMATE FEEDBACKINFLUENCE
P50 SCENARIO: WITH (FB) AND WITHOUT (NFB) CLIMATE FEEDBACKS
750700E N D Y E A R C O2 C ON C E N T R A T ION (p p m

Climate feedbacks leadto higher concentrations
for any given emissions
scenario

FB

650

600

550

NFB
500

450

400

350
2000

….. hence …..

2010

2020

2030

2040

2050

2060

2070

2080

YEAR

For a given concentration profile, climate feedbacks
lead to lower emissions requirements

2090

2100

EFFECT OF CLIMATE FEEDBACKS ON EMISSIONS (WRE450)
18

P50 BASELINE

450 ppmF O S S IL C O 2 E M IS S IO N S (G tC /y r)

16
14
12108

OLD WRE

6NFB

4FB

2020002020 2040

2060

2080 2100

2120

2140 2160

YEAR

2180

2200 2220

2240

2260 2280

2300

EFFECT OF CLIMATE FEEDBACKS ON EMISSIONS (WRE550)
18550 ppm

P50 BASELINEF O S S IL C O 2 E M IS S IO N S (G tC /y r)

161412NFB

10OLD WRE

864FB (2 cases)

202000

2020

2040

2060

2080

2100

2120

2140

2160

YEAR

2180

2200

2220

2240

2260

2280

2300

SUMMARY OF FEEDBACK EFFECTNFB EMISSIONS minus FB EMISSIONS FOR DIFFERENT STABILIZATION TARGETS
3.5

F O S S IL E M IS S IO N S D IF F E R E N C E (G tC /y r)

36507502.555024501.513500.5020002050

21002150

2200
YEAR

225023002350

2400

EFFECT OF CLIMATE FEEDBACKS ON
CUMULATIVE CO2 EMISSIONSEFFECT OF CLIMATE FEEDBACKS ON CUMULATIVE EMISSIONS (550ppm)
2000
NFB

CUMULATIVE FOSSIL CO2 EMISSIONS (GtC)

1800
1600
OLD WRE550

1400
FB
1200
1000
800
600
40020002000

2050

21002150
YEAR

220022502300

RATIO NFB/FB CUMULATIVE EMISSIONS : DIFFERENT STABILIZATION TARGETS
1.05
1750

F O S S I L E M IS S I O N S R A T IO (N F B /F B )0.950.9

6500.85

5500.80.750.7

4500.650.60.55350

0.50.452000

2050

21002150

2200
YEAR

2250

2300

2350

2400

ALTERNATIVE PATHWAYS TO STABILIZATION :OVERSHOOT POSSIBILITIES• Except for the 350 ppm stabilization case, all WREprofiles assume monotonic increases in concentration.

• What if we allow the profile to go above the stabilizationlevel and then decline?

550 AND 650 ppm STABILIZATION PLUS 550 ppm OVERSHOOT CASES
700

WRE650E N D Y E A R C O 2 C O N C E N T R A T IO N ( p p m650

OV2600
OV1550
WRE550500450400350
2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300
YEAR

550 AND 650 ppm STABILIZATION PLUS 550 ppm OVERSHOOT CASES
12WRE650

11F O S S IL C O 2 E M IS S IO N S (G t C /y r)

109876

WRE5505432OV11OV202000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300
YEAR

CUMULATIVE EMISSIONS
2000
1900

WRE6501800
CUMULATIVE FOSSIL CO2 EMISSIONS (GtC

1700
1600
1500

OV21400
1300

OV11200
1100
1000

WRE550900
800
700600500
400
300200100020002050

2100

2150

2200

2250

2300

2350

2400

YEAROvershoot cases allow more emissions for 100+ years, but have very similar
asymptotic cumulative emissions. In general, total allowed emissions
depends on the stabilization level, but not on the path to stabilization.

TEMPERATURE AND SEA LEVELCONSEQUENCES OF OVERSHOOTPROFILES

550 AND 650 ppm STABILIZATION PLUS 550 ppm OVERSHOOT CASES
3.5

G L O B A L -M E A N T E M P E R A T U R E C H A N G E (d e g C

WRE6503OV2OV12.5
WRE55021.5

1Maximum additional warming= 0.2 to 0.3 degC0.502000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300
YEAR

550 AND 650 ppm STABILIZATION PLUS 550 ppm OVERSHOOT CASES80WRE650
OV2GLOBAL-M E AN S E A LE V E L RIS E (cm7060
OV15040

WRE5503020Maximum additional sea-level
rise = 4 cm.
1002000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300
YEAR

SUMMARY OF OVERSHOOT RESULTS FOR
DIFFERENT STABILIZATION LEVELS

350, 450, 550 & 650 ppm

CONCENTRATION PROFILES, FOSSIL CO2EMISSIONS, CUMULATIVE FOSSIL CO2 EMISSIONS,GLOBAL-MEAN TEMPERATURE & SEA LEVEL RISE(out to 2400)

CONCENTRATION PROJECTIONS FOR STABILIZATION PROFILES
750

ENDYEAR CO2 CONCENTRATION (ppm)

70065065060055055050045045040035035030020002050

21002150

2200
YEAR

2250

2300

2350

2400

FOSSIL EMISSIONS FOR STABILIZATION PROFILES
131211650

FOSSIL CO2 EMISSIONS (GtC/yr)

10955087450654350

3210-1-22000

2050

2100

2150

2200
YEAR

2250

2300

2350

2400

CUMULATIVE FOSSIL EMISSIONS FOR STABILIZATION PROFILES
22002000
CUMULATIVE FOSSIL CO2 EMISSIONS (GtC6501800
1600
550140012001000450800600
40035020002000

2050

2100

2150

2200
YEAR

2250

2300

2350

2400

RATE OF CHANGE OF EMISSIONS FOR STABILIZATION PROFILES
1.61.4RATE OF CHANGE OF EMISSIONS (GtC/decade

1.2
10.8
0.6
0.4
0.2
0-0.2
-0.4
-0.6
-0.8
-1-1.2650

-1.4

350

550

-1.6
-1.8

450-2-2.2
2000

2020

2040

2060

2080

2100
YEAR

2120

2140

2160

2180

2200

TEMPERATURE PROJECTIONS FOR STABILIZATION PROFILES
4GLOBAL-MEAN TEMPERATURE CHANGE (degC6503.555032.5

4502350

1.5

10.502000

2050

2100

2150

2200
YEAR

2250

23002350

2400

RATE OF CHANGE OF TEMPERATURE FOR STABILIZATION PROFILES
0.32

RATE OF TEMPERATURE CHANGE (degC/decade)

0.3
0.28
650

0.26
5500.240.22

4500.2
0.18

350

0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
02000

2010

2020

2030

2040

2050
YEAR

2060

2070

2080

2090

2100

SEA LEVEL PROJECTIONS FOR STABILIZATION PROFILES
90650
80

GLOBAL-MEAN SEA LEVEL RISE (cm550
70
45060

50
350
40

30

20

1002000

2050

2100

2150

2200
YEAR

2250

23002350

2400

CONCLUSIONS•For a given emissions scenario, climate feedbacks lead to concentration
increases 10-20% larger than without feedbacks (even larger with some models!)• Climate feedbacks lead to substantially lower emissions requirements tomeet any given stabilization target• The percentage reduction in cumulative emissions is larger for lowerstabilization targets• Overshoot pathways …..
• delay the time when emissions must begin to decrease by about 10 years• can be constructed to both delay and not increase (dE/dt)max• allow much larger near-term (100+ years) cumulative emissions• lead to small increases in the magnitude of future climate andsea-level change, and small increases in the rates of change• It seems likely that overshoot pathways would reduce mitigation costs much
more than they would increase climate-change damages – unless there arenonlinearities that lead to much larger damages if thresholds are passed

ADDITIONAL MATERIAL ON CHOOSING A
STBILIZATION TARGETWHAT SHOULD THE
STABILIZATION TARGET BE?
(What does ‘dangerous
interference’ mean?)INPUT PDF FOR GLOBAL WARMING LIMIT (from 2000)

PROBABILITY DENSITY (degC**-1)

INPUT PDFs : CO2STABILIZATIONCONCENTRATION IS
CONTROLLED BYWARMING LIMIT,
CLIMATE SENSITIVITY
AND NON-CO2 FORCING

0.60.50.40.30.20.100123456GLOBAL WARMING LIMIT (degC)

INPUT PDF FOR CLIMATE SENSITIVITYINPUT PDF FOR NON-CO2 FORCING
0.70.5PROBABILITY DENSITY (m**2/W)

PROBABILITY DENSITY (degC**-1)

0.60.4

0.3

0.2

0.1

0.5

0.4

0.3

0.20.10001234

CLIMATE SENSITIVITY, ∆T2x (degC)

56-1012NON-CO2 FORCING (W/m**2)

34CO2 CONCENTRATION STABILIZATION TARGET
0.0018Median (615ppm)

0.0016
HIGH SENSITIVITY
HIGH NON-CO2
FORCINGPRO BABIL IT Y DENSIT Y (p p m**-1)

0.0014LOW WARMING
LIMIT

0.00120.001

0.0008LOW SENSITIVITY
LOW NON-CO2
FORCING

0.0006

HIGH WARMING LIMIT

0.00040.0002

10%00100

2003004005006007008009001000 1100

CO2 CONCENTRATION (ppm)

1200 1300

1400 1500

C O N C L U S IO N
• T h e C O 2 s ta b iliz a tio n ta r g e t d e p e n d s o n w h a t isju d g e d to b e ‘d a n g e r o u s in te r fe r e n c e ’, th e c lim a tes e n s itiv ity , a n d fo r c in g fr o m n o n - C O 2 s o u r c e s .
• T h e r e is a p r o b a b ility o f a r o u n d 1 0 % th a t th eta r g e t c o u ld b e b e lo w th e c u r r e n t le v e l.