Uncertainty in temperature projections reduced using carbon cycle and climate observations

The future behaviour of the carbon cycle is a major contributor to uncertainty in temperature projections for the twenty-first century. Using a simplified climate model, we show that, for a given emission scenario, it is the second most important contributor to this uncertainty after climate sensitivity, followed by aerosol impacts. Historical measurements of carbon dioxide concentrations have been used along with global temperature observations to help reduce this uncertainty. This results in an increased probability of exceeding a 2C global-mean temperature increase by 2100 while reducing the probability of surpassing a 6C threshold for non-mitigation scenarios such as the Special Report on Emissions Scenarios A1B and A1FI scenarios, as compared with projections from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Climate sensitivity, the response of the carbon cycle and aerosol effects remain highly uncertain but historical observations of temperature and carbon dioxide imply a trade-off between them so that temperature projections are more certain than they would be considering each factor in isolation. As well as pointing out the promise from the formal use of observational constraints in climate projection, this also highlights the need for an holistic view of uncertainty. © 2013 Macmillan Publishers Limited. All rights reserved.