Uncertainties in implementation of common metrics

Jolene Cook, 3 April 2012, Bonn

With thanks to: Chris Huntingford (CEH), Laila Gohar (Met Office Hadley Centre), Nick Howarth (Oxford Univ.), Myles Allen (Oxford Univ.), Alex Lorenz (Oxford Univ.), Keith Shine (Reading Univ.), Bill Collins (Met Office Hadley Centre), Jason Lowe (Met Office Hadley Centre), David Lee (Tau Scientific)

Uncertainties in current knowledge and use of metrics

• Physical- e.g. climate sensitivity, radiative impact

• Structural- e.g. impact of different lifetimes, choice of time horizon, relationship with

end impact

• Economic- e.g. choice of discount rate, potential for perverse incentives

• Political- e.g. winners and losers, ease of communication, stability

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Value choices affect metrics

• Policy goals, e.g. 2°C, emission trading, reporting and monitoring

• What’s included in the basket of gases

• Revisions due to new evidence, e.g. SAR to AR4.

• Choice of metric, e.g. GWP, GTP or other?

• Choice of time horizon

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Value choices affect metrics

• Policy goals, e.g. 2°C, emission trading

• What’s included in the basket of gases

• Revisions due to new evidence, e.g. SAR to AR4

• Choice of metric, e.g. GWP, GTP or other?

• Choice of time horizon

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Value choices affect metrics

• Policy goals, e.g. 2°C, emission trading

• What’s included in the basket of gases

• Revisions due to new evidence, e.g. SAR to AR4

• Choice of metric, e.g. GWP, GTP or other?

• Choice of time horizon

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Impact of new evidence:updating GWP values

IPCC SAR IPCC AR4

CO2 1 1

CH4 21 25

N2O 310 298

HFC23 11,700 14,800

SF6 23,900 22,800

Other HFCs 140 – 9,200 124 – 9,810

Source: IPCC 2007;

0

10

20

30

40

50

60

1990 2008 1990 2008 1990 2008 1990 2008

High

N2O

CH4

CO2

SAR AR4

26.4% 26.2%

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Impact of new evidence

Global total CO2e emissions per year

(GWP100)

Impact of new evidence

0

2

4

6

8

10

12

1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008

High F gases

N2O

CH4

CO2

160% 155%

8.6% 8.6%

8.2% 8.5%

7.9% 6.5%

No change

Smaller increase

No significant effect

Smaller decrease

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China EU-27USA BrazilSAR AR4 SAR AR4 SAR AR4 SAR AR4

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Value choices affect metrics

• Policy goals, e.g. 2°C, emission trading

• What’s included in the basket of gases

• Revisions due to new evidence, e.g. SAR to AR4

• Choice of metric, e.g. GWP, GTP or other?

• Choice of time horizon

Impact of choice of metric

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GWP100 GTP100

CO2 1 1

CH4 25 4

N2O 298 265

HFC23 14,800 16,000

SF6 22,800 28,000

Other HFCs 124 – 12,200 18 – 15,100

Source: IPCC 2007; Fuglestvedt et al. 2010

0

10

20

30

40

50

60

1990 2008 1990 2008 1990 2008 1990 2008

High F Gases

N2O

CH4

CO226% 27%

Global total CO2e emissions per year

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Impact of choice of metric

GWP100 GTP100

0

2

4

6

8

10

12

1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008 1990 2008

High F gasesN2OCH4 CO2

8.2% 7.2%

160% 193%

8.5% 7.8 %

Larger increase

Smaller increase

Smaller decrease

Stronger decrease7.9% 16%

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China USA EU27 Brazil

GWP GTP GWP GTP GWP GTP GWP GTP

Impact of choice of metric

Value choices affect metrics

• Policy goals, e.g. 2°C, emission trading

• What’s included in the basket of gases

• Revisions due to new evidence, e.g. SAR to AR4

• Choice of metric, e.g. GWP, GTP or other?

• Choice of time horizon

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0

10

20

30

40

50

60

70

80

0 100 200 300 400 500

Time Horizon (years)

Met

ric v

alue

GWP

GTP

Impact of choice of time horizon:e.g. methane

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Data source: IPCC, 2007; Fuglestvedt, et al. 2010

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0

10

20

30

40

50

60

70

20 years 100 years 20 years 100 years

High F gasesN2O

CH4

CO2

Impact of choice of time horizon

GWP GTP

Conclusions

• Both GWP and GTP require value choices.

• Large sensitivity to choice of time horizon.

• Important to consult IPCC Fifth Assessment Report related to status of scientific findings related to metrics

• Next step: consider economic implications.

• Any value in changing metric when impact on other areas unknown?

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