In December, national delegations from almost 200 nations will meet in Paris to hammer out an agreement on greenhouse gas emission (GHG) reductions. Like Kyoto (1997) and Copenhagen (2009) before it, Paris 2015 will rely on GHG emission reduction pledges to achieve their climate goals, this time to limit global warming to less than 2 °C.

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To forge a strong climate accord in Paris, nations must agree on a common goal in everyone's self-interest, say David J. C. MacKay and colleagues.

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Policy makers have called for a 'fair and ambitious' global climate agreement. Scientific constraints, such as the allowable carbon emissions to avoid exceeding a 2 °C global warming limit with 66% probability, can help define ambitious approaches to climate targets. However, fairly sharing the mitigation challenge to meet a global target involves human values rather than just scientific facts.