Greenhouse gas emission targets for limiting global warming to 2CMalte Meinshausen (Potsdam Institute for

Climate Impact Research)

Overview

• Nature study

“Greenhouse Gas targets

for staying below 2C”.Meinshausen, M., Meinshausen, N.,

Hare, W., Raper, S. C. B., Frieler, K.,

Knutti, R., Frame, D. J. & Allen, M.

• Implications for

emitting CO2 from

fossil fuels.

• Implications for the Copenhagen

agreement, peaking by 2015.

Introduction

• More than 100 countries call for limiting warming to maximally 2C or lower. (Most of them, i.e. Small Island States and LDCs: 1.5C target)

• How much emission reductions necessary for staying below 2C warming?

• Reflecting current scientific uncertainties across carbon cycle and climate response, as well as radiative forcing.

Cumulative emissions

• Cumulative emissions up to the point of maximal

temperatures are key.

• If you increase your emissions afterwards again, then of

course you can further increase global temperature

levels.

• In the lower mitigation scenarios, temperatures do peak

in the early part of the second half of the 21st century.

• Thus, the cumulative emissions in the first half of the

21st century is a necessary, not a sufficient condition for

staying below 2C.

What is left for 2000 to 2050?

1000 GtCO2 = 1 Trillion tonnes CO2 = 0.27 TtC

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Key findings: 1 Trillion tonnes CO2

• If probability of exceeding 2C shall be limited to

25%: 1 Trillion Tonnes of CO2 (=0.27 TtC)

between 2000 and 2050.

• A third of that budget we used already in past

nine years.

• If we continue to emit fossil fuels at current

rates, by 2030 we will have run out of this

budget.

What does the Trillion tonnes CO2 budget mean?

• From a purely physical point of view, it does not matter

when emissions occur.

• We cannot afford to emit the CO2 from proven

recoverable reserves (in fact, only 1/4. Resources being even far larger)

• The budget for 2C is very tight, but manageable (Lower

mitigation scenarios do indicate the technological challenges and moderate economic costs –

See e.g. IPCC RCP3-PD vetting process).

• The carbon budget is the yardstick against which to

measure the appropriateness of 2020 and 2050 targets.

What does it not mean?

• … that we can delay action as long as there is

some of the carbon budget left.

• … that we should bet on future technological

advancements (CCS, free-air capture etc.)

• … that “when”-flexibility would allow us to defer

the global peak.

We had flexibility to choose from multiple pathways

tens years ago, not any more. (See WRE, Nature 1996 and

subsequent discussion).

Q: When would global emissions need

to peak?

Kyoto-2GHGs: 1750 GtCO2eq

Total CO2: ~1250 GtCO2

nearly 40% risk of exceeding 2C

Conclusion

• We exhausted the options for

multiple pathways: global emissions

have to peak by 2015, before 2020.

Other studies, a tool & Further Reading

“2C Check” tool and more background information and data

available on www.primap.org Nature 2C paper

Meinshausen, M., Meinshausen, N., Hare, W., Raper, S. C. B.,

Frieler, K., Knutti, R., Frame, D. J. & Allen, M. Greenhouse gas

emission targets for limiting global warming to 2°C. Nature, doi:

10.1038/nature08017 (2009).

Allen et al. 2009 Cumulative CO2 emissions determine peak

warming; 1TtC over anthropocene will induce 2C as best

estimate.

Allen, M. R., Frame, D. J., Frieler, K., Hare, W., Huntingford, C.,

Jones, C., Knutti, R., Lowe, J., Meinshausen, M., Meinshausen,

N. & Raper, S. The exit strategy: Emission targets must be

placed in the context of a cumulative carbon budget if we are to

avoid dangerous climate change. Nature Reports Climate

Change, doi:10.1038/climate.2009.38 (2009).

Matthews, D. et al. (2009) Nature.

See more comments, essays and News & Views pieces in 30th April

issue of Nature www.primap.org.