University of Oxfordtrillionthtonne.org Uncertainty in climate science: opportunities for reframing the debate Myles Allen Department of Physics, University of Oxford

University of Oxfordtrillionthtonne.org 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

University of Oxfordtrillionthtonne.org Why 2 o C? Vulnerability of critical components of the global climate system Lenton and Schellnhuber (2007)

University of Oxfordtrillionthtonne.org Vulnerability versus target of 2 o C above pre- industrial temperatures (<1.5 o C above present) Lenton and Schellnhuber (2007)

University of Oxfordtrillionthtonne.org And the not-so-good news: the impact of national pledges following Copenhagen Rogelj et al, 2010

University of Oxfordtrillionthtonne.org How not to avoid dangerous climate change Desperate search for the “scientific case” that 2 o C means –Annex 1 emissions must drop by 25-40% by 2020, or –Long-term concentrations must stabilise at ppm. 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

University of Oxfordtrillionthtonne.org Asking a different question: the story of the trillionth tonne of carbon Generate idealised CO 2 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.

University of Oxfordtrillionthtonne.org 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 Oxfordtrillionthtonne.org Emissions in 2020 & 2050 only matter for peak warming insofar as they determine total emissions Colours show most likely peak CO 2 -induced warming under various idealised scenarios.

University of Oxfordtrillionthtonne.org 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 Oxfordtrillionthtonne.org Why this matters In effect, CO 2 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 2 o C, with a 1-σ range of o C. Postponing emissions peak to 2020 does not “commit us to 2 o C”, it commits us to potentially unfeasible rates of emission reductions after 2020 if we are still to avoid 2 o C. CO 2 emission rates matter for rates of warming, but shorter-lived agents matter much more.

University of Oxfordtrillionthtonne.org Conventional and unconventional reserves The heart of the problem: how fossil fuel reserves relate to atmospheric capacity Past emissions Conventional oil and gas Conventional oil, gas and coal

University of Oxfordtrillionthtonne.org 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/C 0 : –C = Cumulative emissions from the time policy is adopted. –C 0 = Atmospheric capacity at the time policy is adopted. If all carbon sources were SAFE, we would never exceed the atmospheric capacity.

University of Oxfordtrillionthtonne.org What SAFE carbon means in practice: connecting A to B A B

University of Oxfordtrillionthtonne.org 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 S tied to cumulative emissions, not time S rises automatically to give the same emissions independent of fossil fuel consumption.

University of Oxfordtrillionthtonne.org Policy implications of cumulative warming commitment There is no “fair exchange rate” between CO 2 and methane: CO 2 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 2 o C above pre-industrial.