Re-examination of the emission paths
considering uncertaintyof the climate sensitivity
Yoichi Kaya
Research Institute of Innovativetechnology for the Earth ( RITE )2015.2.27
1

Contents
1.Back ground of the issue: difficulty in achieving
2 degree target
2.Climate sensitivity and its uncertainty
climate model and energy balance model
3. Change in climate sensitivity and its effect on
cumulative CO2 emission for achieving 2 degree
target4. The impact of change in climate sensitivity on theemission paths
5. Conclusion
2New target of EU and the difficulty inachieving the target
2015.2.25 Nikkei newspaper says:
EU fixed the new long term target such that
GHG emission should be reduced by 60% whencompared with the level in 2010.
Difficulty in achieving the above target
present emission: developing c. 6 vs. advanced c.4
Following the above target requires developing c. to reduce
their emission by half by 2050, even if advanced c.
reduce their emission by 80%.
According to OECD forecast: energy demand of developing c.
will be doubled by 2050
→ How do we fill this gap?
3

2 ℃ target and overshoot scenarios
Scenarios of IPCC AR-5 WG3 for 2℃ target
2100: 653 scenarios of 480～720ppm
235 scenarios : negative CO2 emission
before 2100 (see the next figure )negative CO2 emission: afforestation
BECCS
the issue: whether the negative emission as
above will be able to be realized4

Source: Fuss,S.et al, Nature Climate Change .October 2014p.851

5

Measures for negative emission of CO2- Capture of CO2 in the air and its storage1. Afforestation: storage of CO2 in forests
2.BECCS(BioEnergy Carbon Capture and Storage)
grasses－harvesting ーburningーcapture of CO2ーstorage in underground( Notice that trees are not utilized for BECCS, as
trees store CO2 within themselves. Cutting
trees for CCS is meaningless for CO2 capturefrom the air. )6

Areas required
for afforestation and BECCS
1.

Base data of CO2 absorption ( Whittaker et al.)temperate forest:18.3 ton CO2/ha/year
temperate grass field: 8.4 ton CO2/ha/year
2. CO2 to be absorbed
3Gton CO2 /year ( most frequently used in theIPCC scenarios : 1/10 of annual global emission )3. Areas required for absorption of CO2
temperate forest: 170 Mha/year ( ¼ of Australia )BECCS : 380 Mha/year ( ½ of Australia)
・・・too large in practice( Present global deforestation is several Mha, much less thanthe above number )7How can we do then?
The above discussions tell how difficult toachieve 2℃ target. How shall we do then?
Idea 1: Adoption of higher temperature riseas the target ( already mentioned in
the same seminar of last year )Idea 2: Re-examination of science of climatechange→ the issue of climate sensitivity8

The proposal by J.Curry in Wall Street
Journal in Oct 13, 2014
1. The climate sensitivity evaluated by climate
models seems too high. Our evaluation
indicated much lower climate sensitivity.
2. We are not outlier. There have been publishedmore than a dozen of papers of similar character.
3. Lower climate sensitivity indicates that we havemore time for decarbonization of the economy
than expected in the past.
* J.Curry: Professor of Georgia Inst. of technology,President of Climate Forecast Applications Network.
9Climate sensitivity
ECS(equilibrium climate sensitivity)
The final value of rise in global temperaturewhen CO2 concentration in the air doubles
TCR(transient climate response)rise in temperature when CO2 concentration
doubles with the speed of 1% per year

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Methods for evaluatingclimate sensitivity

11

FIG.：IPCC AR-5 Evaluation of ECS by various methods
Source: IPCC AR-5 WG1,TS,TFE6,Fig.1

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Fig. Recent results mainly by use of energy balance models
Source: Michaels,P.J.et al,2014.913

Estimation results: ECS
band estimates（ likely )IPCC AR4

2.0 ~ 4.5 KAR5
Average of median
1)IPCC AR5 WG,TS Fig.1

1.5 ~ 4.5 K

2)Michaels,P.J.etal,2014.9

best estimate

3.0 K
cannot bedetermined
Average
Instrumental
2.6 K
Climate model
3.2 K
mostlyobservation data
2.0K
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Fig. A simple estimation of ECS―The basis of energy balance method?－ΔＴ℃

ECS＜2.2～2.5?

2.0

Effect of GHG
other than CO2～TCR

Past data
1.00.85

Change due to CO2

In case of Lewis
and Curry＊:
ECS～TCR+0.3℃

0.30
future280 320(1880) (1960)

400(2012)

560＊Lewis,N.&J.A.Curry,Climate Dynamics,Springer,Sept.2014

CO2ppm15

Climate sensitivity in IPCC AR-5
１．ＷＧ１
The temperature rise vs. cumulative CO2 emission curve
has been made by climate models of which ECS are relatively
high. The change in lower limit of band estimate of ECS wasthen not utilized in the evaluation of the relation between thetemperature rise and cumulative CO2 emission.
２．ＷＧ３１）RCP’s climate model are MAGICC in which ECS of 3 degrees
has been utilized.
２）Most of other models quoted in WG3 report utilized the bestestimate of AR4 which is 3 K .
16

Change in ECS
in the following investigation
1. The lower limit of band estimate of ECS was lowered by 0.5℃.
２．While the best estimate of ECS evaluated from climate models is3℃, the average of medians of ECS evaluated by energy balance
models is
2.0～2.6℃.
３．From IPCC 1st report to 3rd report, ECS was thought
to be 1.5～4.5℃, and the best estimate is 2.5℃*.
*IPCC 2nd report, WG1,p.34,1995,Cambridge Univ.Press⇒ Set the best estimate of ECS to be 2.5℃,and evaluate its impacts on emissions.
Point 1. How much is the rest of cumulative emission of CO2
for 2℃ target ( how much easier than when ECS=3℃?)
2. Changes in realizability of emission path ( marginal cost, etc.)17

60

ECS=2.5℃
Increase in cumulative
CO2 emission

GHG排出量 [GtCO2eq/yr]

50
402.0℃安定化_気候感度2.5℃

302.0℃安定化_気候感度3.0℃

ECS=3℃
201001990

2040

2090

Fig. Global GHG emission-２℃target case 18

Fig. Rise in global temperature and cumulative CO2 emission

Source: IPCC AR5 WG1, SPM, 201319

Remaining cumulative CO2 emission- for 2℃ rise ECS2.5℃

Remaining amount

Already spent

既消費量
残余量

ECS3.0℃

01000

2000

Note: Achieving probability of 66%

3000

4000
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Changes in emissionable amount of CO2 when
ECS changes from ３℃ to２．５℃
１．Remaining cumulative emissionable CO2
ECS ３℃
1,000Gt CO2 (>66%probability）
remaining time＝remaining amount/annual emission～３０ｙ
ECS 2.5℃ 1,800Gt CO2
remaining time ～ ６０y (almost twice )２．Improvement in realizability of emission paths（to be shown )21

800GｔCO２Fig. Rise in temp. and cumulative CO2 emission
Source: IPCC AR-5 Synthesis Rep.RFIg.SPM.5 (b)-5 Synthesis Rep. SPM, Fig.SPM.5(b)TIたいきた

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Change in ECS ～Change in target rise in temperature
1. ＥＣＳ ３℃→2.5℃
Increase in remaining cumulative CO2 emissionis almost the same as in the case of
ECS 3℃ andthe target of rise in temp. of 2.5℃
(final CO2 concentration ５３０～５８０ｐｐｍ)
2. The above means that lowering in ECS is almost
equal to rise in the temperature target.
23Changes in emission paths when
ECS=3.0℃→2.5℃
１．Ｗｏｒｌｄ
Ｗｈａｔ ｃｈａｎｇｅ ｗｉｌｌ ｈａｐｐｅｎ ｉｎ ｍａｒｇｉｎａｌ
ｃｏｓｔ ｏｆ ＧＨＧ ｒｅｄｕｃｔｉｏｎ？
２．Ｖａｒｉｏｕｓ ｒｅｇｉｏｎｓ
If Developed countries ( monotonous reduction)
China.India.Brazil（peaking in 2030）
Other developing c.（peaking in 2050）
what will be emission paths and marginal costs?

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CO2限界削減費用 [$/tCO2 in US 2000 price]

350
300

ECS=3℃

250

2.0℃安定化_気候感度2.5℃
200
150

2.0℃安定化_気候感度3.0℃

100

ECS=2.5℃

5002010

2020

2030

2040

2050

Fig. Marginal cost of GHG reduction
ーin case of globally monotonous action ー25ab

GHG emission

25160

FUTURE

140

20

DC-a15DC-b10IC

5

GHG marginal abatement cost($/t CO2eq)

GHG emissions (Gt CO2eq/yr)

PASTMarginal cost

120100IC80
6040DC-aDC-b200
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

02010 2015 2020 2025 2030 2035 2040 2045 2050

Fig. Region oriented approach: 2℃ target, ECS =2.5 ℃
ＩＣ：developed countries
ＤＣ－a：China.India.Brazil （peaking 2030)
ＤＣ－b: Other developing countries ( peaking 2050)
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GHG emissions of the world and developedcountries in 2050 - 2 degree target Base year: presentworld

developedcountries

Climate sensitivity=3℃

５０% reduction

80% reduction

Climate sensitivity= 2.5℃

the same aspresent value

50% reduction27

Summary
1. It is hard to achieve 2℃ target by measures including CO2 absorption.
It is recommended to consider more practical strategy.
2. There is a possibility that the climate sensitivity is lower than the value
evaluated in the past. With that climate sensitivity we may realize more
realistic strategy for climate change.
3. Lowering of ECS by o.5 ℃ has the following large effects on emission.
1) For the same target of the global temperature riseremaining cumulative CO2 amount will be almost doubled.
2)The marginal costs of GHG reduction of all over the world
will be largely reduced .

Therefore we earnestly recommend to do efforts for reducing
as much uncertainties of climate sensitivity as possible.

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