university of oxfordtrillionthtonne.org uncertainty in climate science: opportunities for reframing...
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University of Oxford
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Uncertainty in climate science:opportunities for reframing the debate
Myles AllenDepartment of Physics, University of Oxford
University of Oxford
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What they were able to agree on
“…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
Lenton and Schellnhuber (2007)
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Vulnerability versus target of 2oC above pre-industrial temperatures (<1.5oC above present)
Lenton and Schellnhuber (2007)
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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
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 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
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.
Simulate response using simple coupled climate carbon-cycle models.
Identify properties of emission scenarios that determine peak warming.
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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
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
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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Conventional and unconventional reserves
The heart of the problem: how fossil fuel reserves relate to atmospheric capacity
Past emissions
Conventional oil and gasConventional oil, gas and coal
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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.
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What SAFE carbon means in practice: connecting A to B
A
B
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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.
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Policy implications of cumulative warming commitment
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.