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Uncertainty in climate science:
opportunities for reframing the debate
Myles Allen
Department of Physics, University of Oxford
[email protected]
trillionthtonne.org
University of Oxford
What they were able to agree on
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“…recognizing the scientific view that the increase
of global temperature should be below 2 degrees
Celcius…”
“…deep cuts in global emissions are required … to
hold the increase in global temperature below 2
degrees Celsius.”
– Copenhagen Accord, 2010
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Why 2oC? Vulnerability of critical components of
the global climate system
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Lenton and Schellnhuber (2007)
University of Oxford
Vulnerability versus target of 2oC above preindustrial temperatures (<1.5oC above present)
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Lenton and Schellnhuber (2007)
University of Oxford
And the not-so-good news: the impact of
national pledges following Copenhagen
Rogelj et al, 2010
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How not to avoid dangerous climate change
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Desperate search for the
“scientific case” that 2oC means
– Annex 1 emissions must drop by
25-40% by 2020, or
– Long-term concentrations must
stabilise at 350-450ppm.
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There is none: not because these
targets are too ambitious, but
because the problem is ill-posed.
Kyoto/Copenhagen vision of
scientifically-determined
emission and/or concentration
targets has become part of the
problem.
Kyoto and Wallace’s Technotrousers: Prins & Rayner, 2008
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Asking a different question: the story of the
trillionth tonne of carbon
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Generate idealised CO2
emission scenarios varying:
– Initial rate of exponential
growth
– Year in which growth begins
to slow
– Rate of turnaround.
– Maximum rate of decline.
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Simulate response using
simple coupled climate
carbon-cycle models.
Identify properties of
emission scenarios that
determine peak warming.
University of Oxford
Cumulative emissions of carbon dioxide are the
principal determinant of dangerous climate change
From Allen et al, Nature, 2009
& see also Meinshausen et al, Nature, 2009
& Solomon et al, PNAS, 2009
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Emissions in 2020 & 2050 only matter for peak
warming insofar as they determine total emissions

Colours show most likely peak CO2-induced warming under
various idealised scenarios.
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Cumulative emissions determine peak warming:
peak emissions determine peak warming rate
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BUT, limiting cumulative emissions to ~1 TtC effectively limits
peak emission rate to <12 GtC/year for plausible, smooth
emission trajectories.
Emission rates and consequent rates of warming only really
relevant to shorter-lived anthropogenic forcings.
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Why this matters
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In effect, CO2 accumulates in the atmosphere. Most
other greenhouse gases do not.
We need to limit cumulative emissions of carbon
dioxide to avoid dangerous climate change.
One trillion tonnes of carbon (1 TtC) implies a most
likely warming of 2oC, with a 1-σ range of 1.6-2.6oC.
Postponing emissions peak to 2020 does not
“commit us to 2oC”, it commits us to potentially
unfeasible rates of emission reductions after 2020 if
we are still to avoid 2oC.
CO2 emission rates matter for rates of warming, but
shorter-lived agents matter much more.
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The heart of the problem: how fossil fuel
reserves relate to atmospheric capacity
Conventional and unconventional reserves
Conventional oil, gas and coal
Conventional oil and gas
Past emissions
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A regulatory alternative to a global emission cap
or carbon tax: SAFE carbon
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Sequestered Adequate Fraction of Extracted (SAFE)
carbon: carbon from a supply that ensures we never
exceed the atmospheric capacity.
So, what is an “Adequate Fraction”?
– S = net carbon sequestered / carbon extracted
– In the very long term, S→100%.
– At present, S=0%.
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Simplest option: S=C/C0:
– C = Cumulative emissions from the time policy is adopted.
– C0= Atmospheric capacity at the time policy is adopted.
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If all carbon sources were SAFE, we would never
exceed the atmospheric capacity.
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What SAFE carbon means in practice:
connecting A to B
B
A
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Anchoring S to cumulative emissions decouples
consumption from mitigation policy
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A1: medium population, high growth, fossil fuels dominant.
A1T-R: A1T with 25% higher renewable growth after 2020,
doubling nuclear capacity 2050-2100.
S tied to cumulative emissions, not time
S rises automatically to give the same emissions independent
of fossil fuel consumption.
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Policy implications of cumulative warming
commitment
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There is no “fair exchange rate” between CO2 and
methane: CO2 accumulates, methane does not.
We need separate controls on
– Short-lived gases, to avoid dangerous rates of warming
– Long-lived gases, to avoid dangerous peak warming
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In place of a single, overarching cap-and-trade
system, every sector (including the fossil fuel
industry) needs to produce a road-map of how they
are going to stop causing global warming before
temperatures reach 2oC above pre-industrial.
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