‘Save the trillionth tonne’: Scientists present new challenges for climate change policy

28 April 2009

Two studies published in Nature on 30 April show that the risk of dangerous climate change is primarily determined by the accumulation of carbon dioxide emissions over time, not by short-term emission rates.
 
A study (1) led by Dr Myles Allen from Oxford University shows that total cumulative emissions of one trillion tonnes of carbon (1 Tt C, or 3,670 billion tonnes of CO2­) over the entire ‘anthropocene’ period 1750-2500 causes a most likely peak warming of 2oCelsius above pre-industrial temperatures. Most of the world’s governments are committed to avoiding warming in excess of 2oCelsius. Of this budget, emissions to 2008 have already consumed approximately half (0.5 Tt C).
 
The other study (2), led by Dr Malte Meinshausen from the Potsdam Institute for Climate Impacts Research, finds that a total emission budget of about 0.9 Tt C gives a best-estimate peak warming by 2100 of 2oCelsius, including the effects of other human influences on climate. This budget drops to less than ¾ Tt C (equivalent to 1,000 billion tonnes of CO2 between 2000 and 2050) if the risk of temperatures exceeding 2oCelsius is limited to one-in-four.
 
These results present a challenge for climate policy, which the study authors detail in a Commentary (3) published in Nature Reports Climate Change, since policy has traditionally focussed on concentration targets or emission rates in 2020 or 2050 without placing these in the context of the need to limit cumulative CO2 emissions.
 
‘Mother Nature doesn’t care about dates,’ remarks Dr Myles Allen (an author of studies (1) and (2)) of the Oxford University Department of Physics. ‘To avoid dangerous climate change we will have to limit the total amount of carbon we inject into the atmosphere, not just the emission rate in any given year. Climate policy needs an exit strategy: as well as reducing carbon emissions now, we need a plan for phasing out net emissions entirely.’
 
Dr Chris Huntingford (an author of study (1)), from the NERC Centre for Ecology and Hydrology, explains: ‘Research often reveals new complexities, but this analysis could actually simplify matters for policymakers: the relationship between total emissions and future warming can be inferred largely from quantities we can observe, and is remarkably insensitive to the timing of future emissions.’
 
Dr Jason Lowe (an author of study (1)), of the Met Office, notes: ‘This accumulation of emissions over time only applies to very long-lived greenhouse gases like CO2, which helps us understand why we need both long-term and short-term targets. Dr Reto Knutti (an author of study (2)), of ETH Zurich, adds: ‘A tonne of carbon is a tonne of carbon, whether released today or in fifty years time.’
 
Dr Malte Meinshausen (an author of studies (1) and (2)), of the Potsdam Institute for Climate Impacts Research, cautions: ‘These cumulative budgets imply that substantial reductions in global emissions need to begin soon, before 2020. If we wait any longer, the required phase-out of carbon emissions will involve tremendous economic costs and technological challenges – far beyond what can be considered politically feasible today.’
 
Dr David Frame (an author of studies (1) and (2)), from the Oxford University Smith School of Enterprise and the Environment, agrees; but notes these results could also be seen as an opportunity: ‘By treating the effective ‘CO2 capacity’ of the atmosphere as an exhaustible resource, we could provide governments and industry with simple, clear and tractable guidance for long term planning.’

Myles Allen, Malte Meinshausen, Bill Hare and Dave Frame will discuss their research and its implications. A Q&A session will follow.
Dial in details: For reporters calling from the United Kingdom: 0845 359 0170. For reporters calling from outside the UK: +44 20 3003 2666. Conference password: Nature Press Briefing

For more information contact:Dr Myles Allen, Oxford University, myles.allen@physics.ox.ac.ukDr Chris Huntingford, CEH Wallingford, +44 1491 838800 chg@ceh.ac.ukDr Jason Lowe, Met Office, +44 118 378 5612, jason.lowe@metoffice.gov.ukDr Reto Knutti, ETH Zurich, +41 44 632 35 40, reto.knutti@env.ethz.chDr Malte Meinshausen, Potsdam Institute for Climate Impact Research, +49 163 175 0084, malte.meinshausen@pik-potsdam.deDr David Frame, Oxford University, +44 1865 614913, david.frame@smithschool.ox.ac.uk
 
To request a figure illustrating the results or for other enquiries contact the University of Oxford Press Office on +44 (0)1865 283877 or email press.office@admin.ox.ac.uk