Some thoughts on global climate risks
and the Paris goals
Seita Emori
National Institute for Environmental Studies
Head, Climate Risk Assessment Section
Follow us! “Social Dialogue and Co-production Office"@NIES@taiwa_kankyo(in Japanese)@taiwa.kankyo
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Long-term goal of climate change policy“… holding the increase in the global average temperature
to well below 2oC above preindustrial levels and pursuingefforts to limit the temperature increase to 1.5oC above
preindustrial levels”
Global Mean
Surface Air Temperature
Change (℃)(Framework Convention on Climate Change COP21 Paris Agreements,2015)2oCriseo1.5 Crise

Scenario withno action(RCP8.5)
Scenario thatachieves agoal of ‘below
2oC’ (RCP2.6)
Based on IPCC WG1 AR5 2

Simulation ofTemperature Change
By MIROC5 Climate Model(AORI/NIES/JAMSTEC/MEXT)

Scenario with no action(RCP8.5)

Scenario to achieve‘below 2℃’(RCP2.6)

3

oEmission pathway to meet the 2 C goal“…achieve a balance between anthropogenic emissions bysources and removals by sinks of greenhouse gases in thesecond half of this century”
120100[Gt-CO2/year]

Anthropogenic CO2 emissions

UNFCCC COP21 Paris Agreement (2015)

8060

Present40
200-201850

1900

1950

Compiled by Masahiro Sugiyama and Shinichiro Asayama
based on various IPCC material

2000

2050

2100
4

“Temperature goal”： well below 2oC, pursue 1.5oC“Emission goal”: net zero GHG emission in 2nd half of 21C

1. Uncertainties in the connection between Temperatureand Emission goals
2. Risk treatment strategy in the face of uncertainties– Pursuing the emission goal
– Reducing climate uncertainties– Discussing options left in the case that thetemperature goal doesn’t appear to be met
3. Toward achieving the emission goal (discourses inJapan)
5Connection between temperature and emission goals(1) Climate uncertainties (RCP2.6 as an example)
The connection has a range due to uncertainty in “climatesensitivity”
temperature

人為起源CO2排出量12010080emission

60
40
2001850-201900

1950

2000

2050

2100

If RCP2.6 (one of pathways to meet emission goal) is realized…,• Likely below 2oC (~66%)• More unlikely than likely below 1.5oC
6Connection between temperature and emission goals(2) When in the 2nd half of 21C is net zero achieved?
Global mean temperatureochange sincesincepreindust.
preindustrial
Warming(°C) ( C)GHG
emission(GtCO2eq/yr)
GHGemissions(GtCO2eq/year)

70

3.5

60

3.0

50

2.5

40
30

2.0

20

1.5

10

1.0
0.5
2000

(Climate Sensitivity=3oC)
2020

2040

2060

2080

2100

2120

2140

0-10
2000

2020

2040

2060

2080

2100

2120

2140

• Peak temperature significantly depends on when the net zero
emission is achieved.
• “Emission peaks as soon as possible” is naturally an importantcondition.
(Tanaka and O’Neill, Nat Clim Change, accepted)

7Connection between temperature and emission goals(3) net zero GHG or CO2 emissions?
Global mean temperatureochange sincesincepreindust.
preindustrial
Warming(°C) ( C)

3.0

GHGemission(GtCO
2eq/yr)
GHGemissions(GtCO
2eq/year)70

CS=4.5oC

60

2.550oCS=3 C

2.0

40
30

1.5
1.0
0.5
2000

20

CS=1.5oC

1002020

2040

2060

2080

2100

2120

2140

-102000

2020

2040

2060

2080

2100

2120

2140

• If temperature doesn’t overshoot, GHG emission doesn’t have tobecome net zero.
• Net zero anthropogenic CO2 emission (+minimum essential emissionof other GHGs) is more consistent with stabilization (whether thetemperature goal is met then depends on climate sensitivity).
(Tanaka and O’Neill, Nat Clim Change, accepted)

8Connection between temperature and emission goals(3) net zero GHG or CO2 emissions?
Global mean temperaturechange since preindustrial (oC)

GHG emission (GtCO2eq/yr)

Analysis of IPCC AR5 scenario database• In the scenarios that meet 2oC goal without temperature decline(green), GHG emission doesn’t fall to zero.
• In many of the scenarios with temperature decline (gray), GHG
emission falls to zero or negative.
(Tanaka and O’Neill, Nat Clim Change, accepted)

9Connection between temperature and emission goals
Summary and discussion• The connection has a range due to climate uncertainties(temperature goal is a “moving target”).
• If temperature doesn’t overshoot, GHG emission doesn’t
have to become net zero (CO2 emission doesn’t have tobe negative).
• If the Paris emission goal is modified to be “net zero CO2emission” (+ minimum essential emissions of otherGHGs) in the 2nd half of 21C, its implication for society isclearer (decarbonization and not more than that) and thegoal is easier to pursue.
• However, whether the temperature goal is met depends
on climate sensitivity and how soon the net zero CO2emission is met.
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Risk treatment strategy in the face ofuncertainties
1. Pursuing the “modified” emission goal
achieving net zero CO2 as soon as possible

2. Reducing climate uncertainties… expected even without improved scientific understanding

3. Discussing options left in the case that thetemperature goal doesn’t appear to be meta. Accepting/adapting to a warmer world
b. Boosting mitigation (negative emission)c. Climate geoengineering (solar radiation management)
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Reducing climate uncertainties
Prediction of “reducing uncertainties by future observation”
with combined ASK method and pseudo-observation approach
Allen, Stott and Kettleborough (ASK) method(Allen et al., 2000; Stott and Kettleborough 2002)(Eq. 1) Hist Obs = (Model’s Hist run – Internal Var)β + Internal Var(Eq. 2) Future ∆T = (∆T in model – Internal Var)β + Internal Var• Adjusting future projections considering that a model will
over/underestimate future change at the same rate as the model
over/underestimated a historical trend against observation.
• How much uncertainties can be constrained at a certain point in the future
can be estimated by regarding one of model projections as a pseudoobservation.

(Shiogama et al. 2016)12

Increase in accuracy (%)
for the prediction of ∆T in 2090s

Reduction of uncertainties by accumulated data forfuture development of climate change
Upper limit due to natural variability

2050年には、不確実性幅~70%of uncertaintiesを約7割低減できる。
can be reduced by 2050

Results could
be wrong
before 2039

The year by which data is accumulated(Shiogama et al. 2016)13

As climate uncertainties are getting reduced, aprospect that the temperature goal won’t be met
could be obtained at a point in time at a certain
probability.
(In the case that net zero CO2 emission can’t be achieved
early enough for actual climate sensitivity)-> Options left in that case include:a. Accepting/adapting to a warmer world
b. Boosting mitigation (negative emission)c. Climate geoengineering (solar radiation management)or any combination of the above three.
(Emori et al. 2018)14

Risks associated with each optiona. Accepting/adapting to a warmer world––

Severe climate change impacts and adaptation costs
Increasing possibility of crossing any tipping point

b. Boosting mitigation (negative emission)––

Severe mitigation costs
Potential side-effects of mitigation (e.g., conflict over land
between massive bio-energy production and food/ecosystem)c. Climate geoengineering (solar radiation management)––

Potential side-effects of SRM
Governance failure

Shouldn’t we start seriously discussing which risks to take?
(Emori et al. 2018)15

Conclusion(1/2)
1. The Paris temperature goal is a “moving target” due toclimate uncertainties. We can’t plan and follow anemission path that “surely” leads to its achievement.
2. There are a bunch of studies on scenarios likely (~66%)
to achieve the temperature goal. But little has been
discussed from a risk management perspective on whatif the “remaining 33%” is materialized.
3. The modified emission goal (net zero anthropogenic
CO2 emission) has a clear implication for society(decarbonization and not more than that). How about
pursuing to achieve this goal as soon as possible anddeal with uncertainties separately?
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Conclusion(2/2)
4. Reducing climate uncertainties is one of the keys for
obtaining a prospect of achieving the temperature goalor otherwise. Uncertainties will be reduced due toaccumulation of data for future development of climatechange, even without improved scientific understanding.
5. Any of the options left when we realize that temperature
goal won’t be met entails potentially disastrous risks.
We should start discussing scientific and social aspectsof those risks to prepare to make a judgment of whichrisks to take.

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What is the key to “solve” the climate problem?
Technologytension

High efficiency (coal) fire
Nuclear

Efficiency improvement-><- Concerns about lock-in<- Concerns about accidents, waste…

Innovative technologies
Needs more development ->(too expensive and variable)

Social reform<- Concerns about realizability

Could be deployed more
Renewables <(inhibited by conventional system)

Energy saving

Concerns about economic ->depression, unemployment, …

<- Lifestyle change

Moving away from
Material growth18

What is the key to “solve” the climate problem?
Technologytension

Conventional technologicaldevelopment doesn't seem to work

Just appealing for social ideals
doesn’t seem to change societyfusion?
Innovation(ride share apps,driverless cars)

Business model
Industrial structure(manufacturing -> data industry?)

Social reformIdeals(ex. Too many cars
are produced!)

Life style(purchasing a car-> mobility service)19