April 22, 2015

Interpretation of IPCC AR5 Scenarios under
better understanding of climate sensitivity
assessments, and the implications of
INDCs in terms of 2 C target
Systems Analysis Group
Research Institute of Innovative Technology
for the Earth (RITE)
Contact to:E-mail: sysinfo@rite.or.jp

Introduction2

 Several countries including the U.S., EU have submitted their INDCs (Intended

Nationally Determined Contributions) by the end of March, 2015.
 There are discussions on the emission gap between the expected global

emissions by the INDCs and a political target, “2 C target”, in which theglobal mean temperature change should be below 2 C increase relative topreindustrial level.

 Under the important discussions for the global emission reductions beyond

2020, it is important to understand that the scientific knowledge require theemission pathways for the 2 C target appropriately. The incorrectunderstandings of the scientific knowledge will mislead the climate change
negotiations toward COP21 and the collapse.
 This paper summarized the scientific knowledge on climate sensitivity withuncertainties in the IPCC AR5, and evaluated the allowable cumulativeemissions, CO2 concentration levels, and emission pathways.
 In addition, the expected global emission by the INDCs submitted by the endof March was estimated, and was compared with the emission pathways forthe 2 C target with the uncertainty of climate sensitivity.
Also refer to:
Keigo Akimoto, Discussion paper providing to the advisory committee for the INDCs, December 5, 2014 (in Japanese)
http://www.meti.go.jp/committee/sankoushin/sangyougijutsu/chikyu_kankyo/yakusoku_souan_wg/pdf/003_s02_00.pdf
Mitsutsune Yamaguchi & Yoichi Kaya, Nikkei-Keizai-kyoshitsu, Nikkei Newspaper, April 3, 2015 (in Japanese)

History of climate sensitivity judgment by IPCC and thesensitivity employed in the scenario assessments of the
IPCC WG3 AR5

3Equilibrium climate sensitivity
Likely range (“best estimate” or “mostlikely value”)

Before IPCC WG1 AR4

1.54.5C (2.5C)

IPCC WG1 AR4

2.04.5C (3.0C)

Global mean temperature estimations for the long-term
scenarios in the IPCC WG3 AR4 (employing MAGICC)

No estimates with probability(3.0C)IPCC WG1 AR5

1.5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 is 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 CIMP5 (AOGCM) results but
also other study results.
 However, AR5 WG3 adopted the climate sensitivity of AR4, which has the likely range of 2.04.5 Cwith the best estimate of 3.0 C, for temperature rise estimates of long-term emission scenarios.

Equilibrium climate sensitivity estimates andthe IPCC WG1 expert judgment

4[ Evaluations in the IPCC WG1 AR5 ][ A sample estimate after IPCC WG1 AR5]
- Lewis and Curry (2014):
1.252.45 C [1783% (66%) range]；
1.054.05 C [ 595% (90%) range];
best estimate : 1.64 C

Estimates from AOGCMs
The model comparison project CMIP5: 2.04.5Cwith the mean value of 3.2 C(CMIP3 (for AR4):, 2.14.4C; mean: 3.2 C)

 The expert judgment of the AR5 WG1 was

Source：IPCC WG1 AR5, TS

1.54.5C considering broad studies including
instrumental and palaeoclimate studies. The lower
boundary is smaller than that of the AR4 and theestimates of CMIP5.

The climate sensitivity of MAGICC model which was
employed for the temperature change estimations in the longterm scenarios of IPCC WG3 AR5
WG3 AR5 employed a simple climate change model
MAGCC for the temperature estimates for long-term
scenarios.

5

The AR5 checked the differences between
MAGICC and CMIP5

For the probability estimationsof temperature, 600 model runs
were conducted assuming theprobability density function ofequilibrium climate sensitivity
which is consistent with theAR4 insight.

Source：J. Rogelj et al., 2012

Source：IPCC WG3 AR5, Ch.6

 WG3 AR5 employed the climate sensitivity of AR4 (likely range: 2.0ｰ4.5 C, best estimate: 3.0 C) for

estimating the temperature of long-term scenarios. This is almost consistent with the CMIP5 results
but is inconsistent with the new judgment of WG1 AR5 considering other studies.
 Therefore, when the latest judgment for climate sensitivity of WG1 AR5 is employed, the temperaturerise estimates of WG3 scenarios can be smaller than those shown in the WG3 AR5.

Points should be recognized (1)6 IPCC WG1 AR5 judged the likely range of climate

sensitivity to be 1.5ｰ4.5C, in which the bottom range waschanged to a smaller number than that in the AR4 (thelikely range of 2.0ｰ4.5C; the best estimate of 3.0C).
 However, the calculations of global mean temperaturechange for long-term scenarios in the WG3 AR5 adopted
the climate sensitivity of AR4.
 The calculations of global mean temperature change forlong-term scenarios in the WG3 AR4 adopted the ”bestestimate” or “most likely value” of climate sensitivity of
3.0C. This also means that the achieving probability oftemperature targets were adopted to be >50% (notadopting >66%).

The IPCC “official” explanation for the allowable
cumulative emissions for 2C target

7

This presentation slide indicates that it is required to be limit of 2900 GtCO 2 of cumulativeemissions in order to achieve +2 C target, and that the rest of allowable emissions is only 1000
GtCO2 because 1900 GtCO2 has been already emitted by 2011. This explanation is conducted by
the IPCC bureaus, but the IPCC reports do not provide such a simple outcome (will be discussed
in the following slides).

The relationship between cumulative emissions andglobal mean temperature change (1/2)

8This figure is estimated by the
CMIP5 models (the equilibrium
climate sensitivity of CMIP5: 2ｰ
4.5 C; mean: 3.2 C)
The solid lines in the figure show
the average value of CMIP5 for
RCPs, which means the results inthe case of >50%.
>33% ⇒900 GtC (3300 GtCO2)【1500 GtCO2】>50% ⇒820 GtC (3010 GtCO2)【1300 GtCO2】>66% ⇒790 GtC (2900 GtCO2)【1000 GtCO2】(The numbers in 【】 show the allowable
cumulative emissions after 2011.)

The discussions for the allowableglobal emissions beyond 2020
based on the rest of the budgetof 1000 GtCO2 are misleading.
Source) IPCC WG1 AR5, TS, TFE.8, Figure 1

Points should be recognized (2)9 IPCC bureaus make the presentation that the rest ofallowable cumulative emissions is about 1000 GtCO2 toachieve the 2C target. However, the number is calculatedby the AOGCMs that the range of climate sensitivity is 2.0ｰ
4.5C and the mean value is 3.0C which is consistent withthose of AR4 (but not with the AR5) and in the case ofachieving probability of the 2C target of >66%.
 As pointed out in the (1), the climate sensitivity in the AR5

was changed to be lower than that in the AR4 considering
the other types of estimates than AOGCMs.
 In addition, the figures in the IPCC AR5 for the relationshipbetween cumulative emissions and temperature changes
employed the case mainly of >50% probability.

The allowable cumulative emissions expected
when the climate sensitivity is lower by 0.5 C

10

Estimates based on CMIP5:
2ｰ4.5C (mean: 3.2C)

Estimates by
MAGICC:
2.0ｰ4.5C(median: 3.0C)
Note: The ranges in thetable are generated bydifferences in non-CO2
GHG emission scenarios.

IPCC Synthesis report, Table 2.2
The difference is 750 Gt for the temperature difference of 0.5 C.

Although there are differences between temperature estimate and equilibrium
climate sensitivity, a rough estimation can be conducted: 1300-550=750 GtCO2.
According to this estimate, the rest of the allowable cumulative emission will be
about 2000 GtCO2 even for the 2 C target, when the achieving probability of50%> and the climate sensitivity of 2.5 C are employed. (This is consistent with the
cumulative emissions of 530-580 ppm scenarios provided by the IPCC WG3 AR5.)

Differences in the allowable cumulative emissions to 2100 for
2 C target between the two climate sensitivity of 3.0 and 2.5 C

11

80000

Difference in cumulative emissionsof ‘overshoot’ scenarios for 2C in
2100 under C.S. of 3.0C and 2.5C:
910GtCO270000

GHG emissions (MtCO2eq./yr)

60000

Difference in cumulative emissionsof concentration stabilization
scenarios for 2C target under C.S.
of 3.0C and 2.5C: 720GtCO2

50000

40000

30000

20000

10000

0
1990

2.0℃ stabilization; climate sensitivity of 2.5℃
(no exceedance of 580 ppm)
2.0℃ stabilization; climate sensitivity of 3.0℃
(below 500 ppm)
2.0℃ in 2100; climate sensitivity of 3.0℃
(exceedance of 530 ppm)
2.0℃ in 2100; climate sensitivity of 2.5℃
(exceedance of 580 ppm)
2000

2010

2020

2030

2040

2050

2060

2070

2080

2090

2100

Note: GHG emissions are positive in the figure but CO2 emissions in the second half of this century are negative for the 2 C target under the climate sensitivity of 3 C.
Source: estimated by RITE

The previous slide indicates that the allowable cumulative emissions of about 750 GtCO2 increase if theclimate sensitivity is changed from 3.0 C to 2.5 C, according to the IPCC estimates (The figure and table
which is estimated based on the CIMP5 results). In addition, according to the emission pathways by RITE
and MAGICC model estimates, the allowable cumulative emissions will be about 720ｰ910 GtCO2. The twodifferent kinds of estimations are almost consistent with each other.

Relationship between cumulative emissions andCO2 concentration and temperature change

12

Ellipses show the
relationship between the
cumulative emissions andCO2 concentration and the
expected temperaturechange by the medianestimates (>50% probability)of MAGICC with the most
likely value of climate
sensitivity of 3.0 C.

The 530-580 ppm range
will be also expected tobe below +2 C if the best
estimate of climate
sensitivity is 2.5C which
is 0.5 C lower than 3.0C.

The allowable cumulativeemissions will increase by 750
GtCO2 if the best estimate ofclimate sensitivity is 2.5Cwhich is 0.5 C lower than 3.0C.

The 2900 GtCO2 corresponds
to 430-480 ppm, and 2C with
>66% probability.

The 3000 GtCO2 corresponds
to 480-530 ppm, and 2C with>50% probability.

Source: modified by RITE
based on Figure SPM.5 of
the Synthesis Report of
IPCC AR5

Probabilities to achieve 2C target and the global
emission allowance in 2050
Estimated by a simple climatechange model MAGICC

Long-term scenarios summarized by IPCC WG3 AR5
CO2eq
concentration
Subcategories
in 2100 (ppm
CO2eq)<430
450 (430-480)

500 (480-530)

550 (530-580)(580-650)

RCP

CO2eq
emissions in
2050 relative
to 2010

Probability of exceeding the2100
target temperaturestemperature(C, relative
2.0C
3.0C
to 1850-1900) 1.5C

13

Equilibrium climate sensitivity isbased on that of the AR4 (the likely
range: 2.0ｰ4.5 C, best estimate: 3.0
C)
Likelihood ofstaying below 2C

Only a limited number of individual model studies have explored levels below 430 ppm CO2eq―

RCP2.6-72～-41%1.5～1.7℃(1.0～2.8)
1.7～1.9℃(1.2～2.9)

49-86%

12-37%

1-3%

80-87%

32-40%

3-4%

Exceedance of
530 ppm CO2eq-57～-42%

Exceedanceof
530 ppm CO2eq-55～-25%

1.8～2.0℃(1.2～3.3)

88-96%

39-61%

4-10%

No exceedance of
580 ppm CO2eq-47～-19%2.0～2.2℃(1.4～3.6)

93-95%

54-70%

8-13%

Exceedance of
580 ppm CO2eq-16～+7%2.1～2.3℃(1.4～3.6)

95-99%

66-84%

8-19%―-38～+24%

2.3～2.6℃(1.5～4.2)
2.6～2.9℃(1.8～4.5)
3.1～3.7℃(2.1～5.8)
4.1～4.8℃(2.8～7.8)96100%
99100%
100100%100100%74-93%

14-35%

88-95%

26-43%

RCP4.5(650-720)―(720-1000)―

RCP6.0+18～+54%>1000―

RCP8.5+52～+95%-11～+17%97100%
100100%>66% (the C.S.
is derived fromthe CMIP5)>50% (the C.S.
is derived fromthe CMIP5)

Probably,>66% (the C.S. isderived from the
WG1’s latest
insight: 1.5ｰ4.5 C)
Probably,>50% (the C.S. isderived from the
WG1’s latest
insight: 1.5ｰ4.5 C)

Note: IPCC AR4 only shows thetemperature estimates of the best estimate
which is almost consistent with theachievability of >50%.

55-83%
92-98%

 For the 2 C target with the likelihood of >66%, the global emission in 2100 should be reduced by 72ｰ42%, but withthe likelihood of >50% (which is consistent with the AR4 estimates using the best estimate of climate sensitivity) theemission should be reduced by >25%.
 The temperatures and the likelihoods were estimated by MAGICC in which the climate sensitivity was employedbased on the AR4 insight (2.0ｰ4.5 C, best estimate: 3.0 C). When the latest insight of WG1 AR5 (1.5ｰ4.5 C) is
employed, the 530-580 ppm category scenarios also meet the 2 C target with >50% likelihood. In this case, the global
emissions in 2050 are -72 to +7% relative to 2010.

Points should be recognized (3)14 The allowable cumulative emissions for achieving the 2C target

will increase by 750 GtCO2 if the best estimate of climate
sensitivity is 2.5C which is 0.5C lower than 3.0C.
 Based on the estimations, the rest of the allowable cumulativeemissions for achieving the 2C target will be around 2000
GtCO2, which is almost double compared with 1000 GtCO2
which is explained by the IPCC bureaus.
 The concentration of 530ｰ580 ppm CO2eq. also expect toachieve the 2C target with probability of >50%.
 The corresponding global GHG emissions in 2050 are -47ｰ+7%

relative to 2010. Some people interpret that the IPCC reports
concluded the 2℃ target = 430ｰ480 ppm CO2eq.= -72ｰ-41% in
2050 relative to 2010. However, this is not necessarily correct,
and rather the IPCC reports should be interpreted in the above
for the precise understandings.

Evaluated INDCs(The submitted countries by the end of March 2015 and China)
INDCs(in the end of March 2015)15

Supplementary explanations on theevaluations in this study

2628% reduction in GHG by 2025 relative toUnited States
2005

17% reduction in GHG by 2020 relative to 2005;
3738% reduction in GHG by 2030 relative to 2005,
which is assumed by using a linear interpolation
between 2025 and 2050 targets of 80% reduction

EU28

40% reduction in GHG by 2030 relative to 1990

24% reduction in GHG by 2020 relative to 1990
(Reference scenario 2013 in “EU energy, transport and
GHG emissions trends to 2050”)

Norway40% reduction in GHG by 2030 relative to 1990

3040% reduction in GHG by 2020 relative to 1990

Switzerland

50% reduction in GHG by 2030 relative to 1990
(By 2025, 35% reduction in GHG relative to 1990 2030% reduction in GHG by 2020 relative to 1990
is anticipated.)

Russia2530% reduction in GHG by 2030 relative to
1990

Mexico2540% reduction in GHG + black carbon by
2030 compared to BaU levels (2236% reduction A net emissions peak starting from 2026
in GHG by 2030 compared to BaU levels)China

4045% reduction in CO2 intensity per unit of GDP by
No submission
2020 relative to 2005;(However, China expressed net emissions peak
GHG emission in 2030 is assumed to be 15.016.5
starting from 2030.)
GtCO eq.

1525% reduction in GHG by 2020 relative to 19902Note) Gabon submitted her INDC by the end of March 2015, but this study does not evaluate her INDC.

The relationship between climate sensitivity and global
emission pathways for 2C target, and outlook of INDCs

Deeper emission
reductions should be
realized throughtechnology
innovations although
they are uncertain.

Larger emission reductions
should be realized throughpeer-reviews in the PDCA cycle.

80

70

GHG emissions (GtCO2-eq./yr)

1660+2 C with around
50% probability

50AR5
530-580 ppm(+7 -47%)

40

30
Actual emissions20

AR5
480-530 ppm(-25 -57%)

Emissions under current policies and measures
INDCs submitted by March 31 assumed to be implemented + China's pledge

AR5 430-480 ppm(-41 -72%)

Below +2.0 ºC through 2100 under climate sensitivity of 2.5 ºC; temporary overshoot of 580 ppm

10

The probability
achieving below+2C increases.
On the other
hand, unrealistic
measures withhigh costs haveto be expected.

Below +2.0 ºC in 2100 under climate sensitivity of 3.0 ºC; temporary overshoot of 530 ppm
Below +2.0 ºC through 2100 under climate sensitivity of 3.0 ºC; below 500 ppm through 2100

0
1990

2000

2010

2020

2030

2040

2050

Estimated by RITE

There are large gaps between the expected global emission under current INDCs
and the emission pathway to 2C target under climate sensitivity of 3.0C.
However, the INDCs are consistent with 2C target if climate sensitivity is 2.5C.

The framework of theprocesses to induce
Future emission reductions
are more important than thelevels of INDCs decided in
COP21.

Conclusion

17

Climate sensitivity has still large uncertainty. Equilibrium climate sensitivity waschanged to 1.5–4.5 C in the AR5 from 2.0–4.5 C with 3.0 C of best estimate in the
AR4.

However, the temperature rise estimates for the WG3 long-term scenarios were
estimated based on the AR4 insight on the climate sensitivity. Therefore, thetemperatures will be smaller than those shown in the WG3 AR5 when the climate
sensitivity based on the AR5 is employed.

The estimates of the allowable cumulative emissions for 2 C target provided by
IPCC are based on the CMIP5 results, whose the climate sensitivity evaluations
correspond to those of AR4. When the new judgment of the climate sensitivity of
AR5 is employed, the allowable cumulative emissions are also much bigger.

Even when the long-term target of 2 C persists, it is not necessarily correct thatthe remaining allowable cumulative emission is 1000GtCO2 and the global emission
in 2050 is required to reduce by 40 to 70% relative to 2010. These numbers change
greatly when the climate sensitivity is changed just by 0.5 C. Therefore, thenumbers should be taken flexibly to avoid the deadlock of the negotiations toward
COP21. The acceptance of flexibility is considered more scientific.

The expected global emission from the INDCs submitted by the end of March 2015
and the pledge of China has large gaps from the emission pathways for 2 C under
the climate sensitivity of 3 C, but they are almost within the allowable pathway for
2 C target under the sensitivity of 2.5 C.