IPCC WG3 Symposium, Tokyo
November 29, 2017

Toward better understandings of uncertainties
in climate change countermeasures anddevelopment of risk management strategies

Keigo Akimoto,Systems Analysis Group
Research Institute of Innovative Technology for the Earth (RITE)History of climate sensitivity judgment by IPCC and thesensitivity employed in the scenario assessments of theIPCC WG3 AR5

2Equilibrium climate sensitivity
Likely range (“best estimate” or “mostlikely value”)

Before IPCC WG1 AR4Same “likely” range
1.54.5C (2.5C)

IPCC WG1 AR42.04.5C (3.0C)

IPCC WG1 AR51.54.5C (no consensus)

Global mean temperature estimations for the long-term
scenarios in the IPCC WG3 AR5 (employing MAGICC)

2.0ｰ4.5C（3.0C）[Based on the AR4][The related descriptions of the SPM of WG1 AR5]

Likely in the range 1.5 C to 4.5 C (high confidence)
Extremely unlikely less than 1 C (high confidence)
Very unlikely greater than 6 C (medium confidence)
No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines ofevidence and studies.

 The equilibrium climate sensitivity, which corresponds to global mean temperature increase inequilibrium when GHG concentration doubles, is still greatly uncertain.
 AR5 WG1 judged the likely range of climate sensitivity to be 1.54.5 C, in which the bottom range
was changed to a smaller number than that in the AR4, based not only on CMIP5 (AOGCM) results but
also other study results.
 AR5 WG3 adopted the climate sensitivity of AR4, which has the likely range of 2.04.5 C with the best
estimate of 3.0 C, for temperature rise estimates of long-term emission scenarios.

Global CO2 emission profiles toward 2300
for the 2 C targets

350
Business as Usual (BaU)(the middle socioeconomic scenario)

CO2 emissionis [GtCO2/yr]

40

Submitted NDCs

30+2℃ stabilization under climate
sensitivity of 2.5℃ (Noexceedance of 580 ppm)

20+2℃ stabilization under climate
sensitivity of 3.0℃ (Noexceedance of 500 ppm)
450 ppm CO2eq stabilization

100-1020102050
2060

21002110

2300
2160

2210

2260

Submitted NDCs
Estimated by RITE using
MAGICC and DNE21+- The global CO2 emissions should be nearly zero for a long-term period in the far future inany pathway to achieve temperature stabilization.
- On the other hand, the allowable global CO2 emissions toward the middle of this century
have a wide range according to the uncertainties in climate sensitivity (or achievingprobability) even when the temperature target level is determined as a 2 C. We should use
this flexibility to develop several kinds of innovative technologies and societies.

Marginal CO2 abatement costs (Carbon prices)
for the 2 C target

4

SSP: “Shared Socioeconomic Pathways”

SSP2 (Middle of the Road)

SSP1 (Sustainability)+2C stab.
under
climatesensitivityof 2.5C+2C stab.
under
climatesensitivity of3.0C

450 ppm
CO2eq stab.
(climatesensitivity of3.4C)+2C stab.
under
climatesensitivityof 2.5C+2C stab.
under
climatesensitivityof 3.0C

450 ppm
CO2eq stab.
(climatesensitivity of3.4C)

2050

12135

604

14117

518

2100

408

427

457134140143

Unit: $/tCO2 (real price); Uniform carbon prices among all nations are assumed.
Source) estimated by RITE DNE21+-The marginal abatement costs (carbon prices) for the 2 C target are huge even under
the global least cost measures (uniform carbon prices) except in the case of low
climate sensitivity (2.5 C) and by 2050.
The carbon price in SSP1 that energy demands in the end-use sectors are much smaller
than in SSP2 is much lower than that in SSP2.
Technological and social innovations are definitely required for the 2 C target to beachieved in harmony with other SDGs. (Newly emerging technologies such as AI, IoTetc. will induce social changes which may lower the energy demand.)CO2 marginal abatement costs of the NDCs
5

Average of 2025-2030
2030 (2025 for the U.S.)【World GDP loss due to mitigation】
NDCs:0.38%; the global least cost：0.06%
The least cost (equal marginal abatement costs)：6$/tCO2
Switzerland

380

Japan

378

EU28210

Canada

166

Korea

144

New Zealand95

United States

85

NorwaySource: J. Aldy et al., Nature Climate Change, 201670

East Europe (Non-EU member)

58

Thailand54

Australia

33

Mexico27

Kazakhstan

14

Belarus12

Russia4South Africa

1

Turkey

0

India

0

Ukraine

0

China

00

50

100

150200

250

300

350

400

CO2 marginal abatement cost ($/tCO2)

Source: K. Akimoto et al., Evol. Inst. Econ. Rev., 2016

- The estimated marginal abatement costs of NDCs are largely different among countries,
and the mitigation costs are much larger than those under the least cost measures due tosuch large difference in marginal abatement costs.
- The difference will induce carbon leakages, and the leakages will reduce the effectivenessof global emission reductions.

CO2 marginal abatement cost for the U.S, EU and Japanconsidering several kinds of policy constraints
CO2 marginal abatement cost ($/CO2)

500

II-c
I-d

400

III-d
III-c

I-c

300
II-b
II-a

200

III-b
III-a

100

I-b
I-a0

I. US
I-a: -26%; the least costI-b: -28%; the least cost
I-c: -26%; power sector
according to CPP
I-d: -28%; power sector
according to CPP* CPP: Clean Power Plan

Source: estimated by RITE DNE21+

II. EU
II-a: the least costII-b: Brexit (-40% for UK)
II-c: splitting into ETS andnon-ETS sectors

III. Japan
III-a: the least cost undernuclear of maximum 20%
III-b: the least cost undernuclear of maximum 15%
III-c: following the NDC
including the energy mix(nuclear of 20%)
III-d: following the NDC
including the energy mix
but nuclear of 15%- It is not easy to achieve the least cost measures because there are several kinds of social andpolitical constraints in each nation.
- The mitigation costs constrained by other policies can be much higher than those under theleast cost measures.

6Image of standard scenario by models and realworld scenarios for deep cuts
Model world:
Ordinary technology progress
Carbonprice

Carbon price/
Marginal abatement cost

Realistic world requirement:
Innovations stimulated & implemented
Carbonprice

Baseline scenario
CO2emission

Implicit carbon price/
Marginal abatement cost

Baseline
scenario
CO2emissionIntervention scenario

7Intervention scenario

By technology
and social
innovations

Explicit high carbon prices such as over 100$/tCO2 in real price are unlikely in a real world.
Technology and social innovations resulting in low (implicit or explicit) carbon prices(including coordination of secondary energy prices) are key for deep emission cuts to beimplemented.

Issues of IPCC
 Basically, the IPCC assessment reports are written

relying on scientifically reviewed articles.
 In addition, the principle of IPCC is “not policy

prescriptive but policy relevant”.
 Under such conditions, the IPCC reports have thedilemma that they are valuable in practice but not easily
compatible with the realistic solutions considering
economic, social and political constraints in the realworld.

8付録

The Assumed Scenarios for Obtaining the Emission
Pathways Meeting the 2 C and 1.5 C targets
C.S. likely: 2.0-4.5C,
most likely:3.0Cby IPCC AR4 (=WG3 AR5)
Category byconcentration in2100 (ppmCO2eq)

Global GHG
emissionsSub-category(relative to2010)[0] <430[1] 450 (430-480)

in 2050

Temperaturein 2100 (C,relative to1850-1900)

Only a limited number of studies exist.
(There are no scenarios in the AR5 DB.)―-72～-41%

1.5～1.7℃(1.0～2.8)[2a] No exceedanceof 530 ppm CO2eq-57～-42%1.7～1.9℃(1.2～2.9)[2] 500 (480-530)

C.S. likely: 1.5-4.5C,
Most likely: 2.5Cby IPCC WG1 AR5+ TAR

Probability of notexceeding the temp.
rise over 21st century(relative to 1850-1900)*

Probability of notexceeding the temp.
rise over 21st century(relative to 1850-1900)*

1.5℃

1.5℃

2.0℃

50%以上*-55～-25%

1.8～2.0℃(1.2～3.3)[2a] No exceedanceof 580 ppm CO2eq-47～-19%2.0～2.2℃(1.4～3.6)[3] 550 (530-580)

2.0℃

66%以上
66%以上

50%以上
[2b] Exceedance of530 ppm CO2eq

1050%以上

66%以上50%以上[2b] Exceedance of580 ppm CO2eqSource) IPCC AR5; * simply estimated by RITE

-16～+7%2.1～2.3℃(1.4～3.6)

Huge costs are estimated for achieving the 2 C target
Corresponding to 2 C targetSource) IPCC WG3 AR5

11Corresponding to 2 C target- According to the IPCC AR5, the CO2 marginal abatement costs(carbon prices) for the 430-530 ppm CO2eq (which areconsistent with the 2 C target) are about 1000-3000 $/tCO2 (2575 percentile) and 150-8000 $/tCO2 (full range) in 2100.
- About 25% of the analyzed scenarios estimate global GDP
losses of over 10%.
- The feasibility of such scenarios should be carefully examinedin terms of various real world constraints.