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Uncertainty in climate science:
opportunities for reframing the debate
Myles Allen
Department of Physics, University of Oxford
[email protected]
University of Oxford
What they were able to agree on
“…recognizing the scientific view that the increase
of global temperature should be below 2 degrees
“…deep cuts in global emissions are required … to
hold the increase in global temperature below 2
degrees Celsius.”
– Copenhagen Accord, 2010
University of Oxford
Why 2oC? Vulnerability of critical components of
the global climate system
Lenton and Schellnhuber (2007)
University of Oxford
Vulnerability versus target of 2oC above preindustrial temperatures (<1.5oC above present)
Lenton and Schellnhuber (2007)
University of Oxford
And the not-so-good news: the impact of
national pledges following Copenhagen
Rogelj et al, 2010
University of Oxford
How not to avoid dangerous climate change
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.
There is none: not because these
targets are too ambitious, but
because the problem is ill-posed.
Kyoto/Copenhagen vision of
emission and/or concentration
targets has become part of the
Kyoto and Wallace’s Technotrousers: Prins & Rayner, 2008
University of Oxford
Asking a different question: the story of the
trillionth tonne of carbon
Generate idealised CO2
emission scenarios varying:
– Initial rate of exponential
– Year in which growth begins
to slow
– Rate of turnaround.
– Maximum rate of decline.
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
University of Oxford
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.
University of Oxford
Cumulative emissions determine peak warming:
peak emissions determine peak warming rate
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.
University of Oxford
Why this matters
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.
University of Oxford
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
University of Oxford
A regulatory alternative to a global emission cap
or carbon tax: SAFE carbon
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%.
Simplest option: S=C/C0:
– C = Cumulative emissions from the time policy is adopted.
– C0= Atmospheric capacity at the time policy is adopted.
If all carbon sources were SAFE, we would never
exceed the atmospheric capacity.
University of Oxford
What SAFE carbon means in practice:
connecting A to B
University of Oxford
Anchoring S to cumulative emissions decouples
consumption from mitigation policy
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.
University of Oxford
Policy implications of cumulative warming
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
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.
University of Oxford